氧中心自由基类型

Radical asymmetric cyclization has emerged as a powerful strategy for constructing ring structures. Despite significant progress in carbon-centered radical cyclization, the catalytic asymmetric cyclization of oxygen-centered radicals, key species in many biological processes, remains challenging owing to their high oxidizing power and electrophilicity. Herein, we report an enantioconvergent oxygen-centered radical cyclization via Cu-catalyzed asymmetric C(sp3)–O oxidative coupling between the tertiary C(sp3)–H bond and the oxime O–H bond of racemic γ-ketoximes. This radical reaction proceeds efficiently under aerobic and mild conditions, affording a wide range of valuable isoxazolines bearing a fully substituted stereocenter in good yields with excellent enantioselectivity. Mechanistic studies were conducted to elucidate the origin of enantiocontrol, and the synthetic utility of the method was demonstrated through the late-stage transformation of isoxazolines into polyhydroxy building blocks.

The tetraoxido ruthenium(VIII) radical cation, [RuO4]+, should be a strong oxidizing agent, but has been difficult to produce and investigate so far. In our X-ray absorption spectroscopy study, in combination with quantum-chemical calculations, we show that [RuO4]+, produced via oxidation of ruthenium cations by ozone in the gas phase, forms the oxygen-centered radical ground state. The oxygen-centered radical character of [RuO4]+ is identified by the chemical shift at the ruthenium M3 edge, indicative of ruthenium(VIII), and by the presence of a characteristic low-energy transition at the oxygen K edge, involving an oxygen-centered singly-occupied molecular orbital, which is suppressed when the oxygen-centered radical is quenched by hydrogenation of [RuO4]+ to the closed-shell [RuO4H]+ ion. Hydrogen-atom abstraction from methane is calculated to be only slightly less exothermic for [RuO4]+ than for [OsO4]+.

Tricyclic lactone compounds featuring a vinyl bromide tether were investigated for intramolecular radical cyclization. α-Aminoalkyl radicals, generated through the translocation of an initially generated vinyl radical, underwent cyclization or cyclization/1,5-hydrogen transfer, C─O bond fragmentation, and further cyclization to construct the nitrogen-containing heterocyclic rings. When two diastereomers with substituents in opposite stereochemical configurations underwent radical reactions separately, they yielded distinct products. Density functional theory (DFT) calculation studies were carried out to unravel mechanistic insights in this process.

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The oxygen-centered radical bound to the trinuclear copper center was detected as an intermediate during the reoxidation process of the reduced Rhus vernicifera laccase with dioxygen and characterized by using absorption, stopped-flow, and electron paramagnetic resonance (EPR) spectroscopies and by super conducting quantum interface devices measurement. The intermediate bands appeared at 370 nm (ε ∼ 1000), 420 nm (sh), and 670 nm (weak) within 15 ms, and were observable for ∼2 min at pH 7.4 but for less than 5 s at pH 4.2. The first-order rate constant for the decay of the intermediate has been determined by stopped-flow spectroscopy, showing the isotope effect,kH /kD of 1.4 in D2O. The intermediate was found to decay mainly from the protonated form by analyzing pH dependences. The enthalpy and entropy of activation suggested that a considerable structure change takes place around the active site during the decay of the intermediate. The EPR spectra at cryogenic temperatures (<27 K) showed two broad signals with g ∼ 1.8 and 1.6 depending on pH. We propose an oxygen-centered radical in magnetic interaction with the oxidized type III copper ions as the structure of the three-electron reduced form of dioxygen.

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Removing organic micropollutants from water through photocatalysis is hindered by catalyst instability and substantial residuals from incomplete mineralization. Here, a novel water treatment paradigm, the unified heterogeneous self‐Fenton process (UHSFP), which achieved an impressive 32% photon utilization efficiency at 470 nm, and a significant 94% mineralization of organic micropollutants—all without the continual addition of oxidants and iron ions is presented. In UHSFP, the active species differs fundamentally from traditional photocatalytic processes. One electron acceptor unit of photocatalyst acquires only one photogenerated electron to convert into oxygen‐centered organic radical (OCOR), then spontaneously completing subsequent processes, including pollutant degradation, hydrogen peroxide generation, activation, and mineralization of organic micropollutants. By bolstering electron‐transfer capabilities and diminishing catalyst affinity for oxygen in the photocatalytic process, the generation of superoxide radicals is effectively suppressed, preventing detrimental attacks on the catalyst. This study introduces an innovative and cost‐effective strategy for the efficient and stable mineralization of organic micropollutants, eliminating the necessity for continuous chemical inputs, providing a new perspective on water treatment technologies.

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The stability of various alkoxy/aryloxy/peroxy radicals, as well as TEMPO and triplet dioxygen (3O2) has been explored at a variety of theoretical levels. Good correlations between RSEtheor and RSEexp are found for hybrid DFT methods, for compound schemes such as G3B3‐D3, and also for DLPNO‐CCSD(T) calculations. The effects of hydrogen bonding interactions on the stability of oxygen‐centered radicals have been probed by addition of a single solvating water molecule. While this water molecule always acts as a H‐bond donor to the oxygen‐centered radical itself, it can act as a H‐bond donor or acceptor to the respective closed‐shell parent.

In this work, the mechanism of the activation of peroxides by quinones has been investigated through quantum chemical calculations. Hydrogen peroxide (H2O2), peroxomonosulfate (PMS), peracetic acid (PAA), and CH3OOH were chosen as the model peroxides and p-benzoquinone (p-BQ) and tetrachloro-1,4-benzoquinone (TCBQ) as the model quinones. The nucleophilic attack of peroxides can occur on the carbonyl and olefinic carbons of quinones. For p-BQ, the nucleophilic attack of HO2-, CH3OO-, PMS, and PAA might prefer to occur on the carbonyl carbons, which have more positive atomic charges. Then, further transformation could not be induced from the addition of HO2- and CH3OO- to p-BQ. Comparatively, singlet oxygen (1O2) could be generated in the cases of PMS and PAA. For TCBQ, the chlorine atoms cause the olefinic carbons to carry more positive atomic charges, and then, HO2- preferred to add to the olefinic carbons, which might induce the formation of the hydroxyl radical (•OH). The activation of PMS by TCBQ was similar to that by p-BQ, with the kinetical feasibility of 1O2 formation. These findings may provide some theoretical insights into the reaction of peroxides with quinones, especially into the interconnection between the substitutes and the formation of oxygen-centered radicals (e.g., •OH) and 1O2.

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Neutrophils and macrophages undergo a respiratory burst and an increase in the activity of the hexose monophosphate pathway in response to particulate or soluble agents. The increase in oxygen consumption was found to be associated with the production of oxygen-centered radicals. The ESR technique of spin trapping showed that besides a superoxide spin adduct, a hydroxyl spin adduct is also produced. ESR is considered to be the least ambiguous technique for the detection of free radicals. The spin-trapping agents used for oxygen-centered radical detection are usually nitrones. The most commonly used nitrone is 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), which reacts with O2-. to form 5,5-dimethyl-2-hydroperoxypyrroline-N-oxide (DMPO-OOH) and with OH. to form 5,5-dimethyl-2-hydroxypyrroline-N-oxide (DMPO-OH). Although spin-adduct formation is considered to be the most direct technique for the detection of free radicals, some disadvantages are encountered. There has been considerable interest in the isolation of the O2-. generating activity from phagocytic cells. The enzyme can be extracted with deoxycholate and gel filtration indicates that it is a high molecular weight complex. Maximum activity was between pH 7.0 and pH 7.5. The Km value was 15.8 microM for NADPH and 434 micron for NADH, indicating that NADPH is the preferred substrate.

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Synthetic side-on peroxide-bound dicopper(II) (SP) complexes are important for understanding the active site structure/function of many copper-containing enzymes. This work highlights the formation of new {CuII(μ-η2:η2-O22-)CuII} complexes (with electronic absorption and resonance Raman (rR) spectroscopic characterization) using tripodal N3ArOH ligands at -135 °C, which spontaneously participate in intramolecular phenolic H-atom abstraction (HAA). This results in the generation of bis(phenoxyl radical)bis(μ-OH)dicopper(II) intermediates, substantiated by their EPR/UV-vis/rR spectroscopic signatures and crystal structural determination of a diphenoquinone dicopper(I) complex derived from ligand para-C═C coupling. The newly observed chemistry in these ligand-Cu systems is discussed with respect to (a) our Cu-MeAN (tridentate N,N,N',N',N″-pentamethyldipropylenetriamine)-derived model SP species, which was unreactive toward exogenous monophenol addition (J. Am. Chem. Soc. 2012, 134, 8513-8524), emphasizing the impact of intramolecularly tethered ArOH groups, and (b) recent advances in understanding the mechanism of action of the tyrosinase (Ty) enzyme.

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Perfluoroalkyl iodides generally formed electron donor-acceptor (EDA) complexes by halogen bonding with a nitrogen atom containing Lewis bases. Since the electronegativity of the oxygen atom is stronger than that of the nitrogen atom, the resulting Rf-I···O-type halogen bonding EDA complex is less inclined to undergo electron transfer. Here, we reported rare ternary EDA complexes among perfluoroalkyl iodide, oxygen atom, and base. Mechanism experiments and density functional theory theoretical (DFT) calculations indicated that a base-promoted proton-coupled electron transfer (PCET) process was involved in this photochemical reaction. The intracomplex electron transfer event generated two radical species, perfluoroalkyl radical and TEMPO radical, enabling the subsequent oxy-perfluoroalkylation of olefins.

The photolytic radical-induced vicinal azidooxygenation of synthetically important and diverse functionalized substrates including unactivated alkenes is reported. The photoredox-catalyst/additive-free protocol enables intermolecular oxyazidation by simultaneously incorporating two new functionalities; C–O and C–N across the C=C double bond in a selective manner. Mechanistic investigations reveal the intermediacy of the azidyl radical facilitated via the photolysis of λ3-azidoiodane species and cascade proceeding to generate an active carbon-centered radical. The late-stage transformations of azido- and oxy-moieties were amply highlighted by assembling high-value drug analogs and bioactive skeletons.

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The selective autoxidation for the synthesis of valuable oxygenates has provoked keen interest from both academic and industrial sectors. Although the generation of reactive oxygen species via oxygen attack on C-H bonds near ester linkages is well-established, research into aliphatic ester oxidation has primarily focused on combustion, neglecting their potential utility in oxidation processes. Herein, we demonstrated a protocol for producing propylene oxide through the autoxidation of ethyl acetate in tandem with propylene epoxidation. The ethoxy radical, generated by ester C(acyl)-O bond cleavage in situ, subsequently underwent proton-coupled electron transfer with the Co(OAc)(μ-H2O)2Ni, followed by the formation of the peracetic acid optimally suited for the epoxidation reaction. Our research not only eliminates the need for co-substrates in the epoxidation process but also fills the application gap in the bulk-ester autoxidation, offering insights into the effective utilization of oxy-intermediates in autoxidation reactions.

The first theoretical investigation was provided by our DFT calculation on I2-catalyzed aerobic oxidation of 2-picolyl ketone leading to 1,2-dicarbonyl compound with 1,2-diaminobenzene. Initially, the 2-picolyl ketone is coordinated with molecular iodine to realize iodination. Promoted by base, one hydrogen iodide is assembled and cleaved producing an N-iodo species. Then, the homolytic cleavage of N−I bond gives C-centered radical, which is bonded with additional molecular oxygen forming peroxide radical. Subsequently, a second C-centered radical couples with peroxide radical yielding peroxide dimer, which undergoes homolytic O−O bond cleavage giving two oxy-radical. After removal of second hydrogen iodide, the reaction of iodine radical I• with oxy-radical forms 1,2-dicarbonyl compound. Via three steps of condensation between 1,2-diaminobenzene and 1,2-dicarbonyl compound, the product quinoxaline is finally yielded. The condensation in last step is rate-limiting for I2 -catalyzed aerobic oxidation of 2-picolyl ketone with 1,2-diaminobenzene to quinoxaline

Pyrite, a ubiquitous sulfide mineral, exerts a strong influence on the fate of coexisting As(III) and As(V) species in natural settings, such as gold deposits, sedimentary basins, and hydrothermal systems. However, the As(III) adsorption and oxidation mechanisms on pyrite at neutral pH remain contested. Through oxic and anoxic kinetic experiments using pyrite with varying oxidation degrees, we demonstrate that As(III) adsorbs preferentially to the Fe(III) (oxy)(hydr)oxide coatings rather than to pyrite sites. Contrary to prevailing assumptions, the HO• radicals contribute minimally to As(III) oxidation at circumneutral pH. Spectroscopy and molecular simulations revealed that pyrite-generated H2O2 oxidizes As(III) via an inner-sphere electron transfer process. This heterogeneous oxidation likely proceeds through a ternary surface complexation involving arsenite and Fe sites. These findings challenge both the conventional radical-dominated pathway and the assumed mechanism of natural arsenopyrite formation. By elucidating the dominant As(III) sorption/oxidation pathway on pyrite surfaces, our findings reshape our current understanding of arsenic geochemistry. They also inform risk assessment and remediation strategies for arsenic-impacted environments.

Cleavage of carbon-carbon bonds remains a challenging task in organic synthesis. Traditional methods for splitting Csp2=Csp2 bonds into two halves typically involve non-redox (metathesis) or oxidative (ozonolysis) mechanisms, limiting their synthetic potential. Disproportionative deconstruction of alkenes, which yields one reduced and one oxidized fragment, remains an unexplored area. In this study, we introduce a redox-neutral approach for deleting a Csp2 carbon unit from substituted arylalkenes, resulting in the formation of an arene (reduction) and a carbonyl product (oxidation). This transformation is believed to proceed through a mechanistic sequence involving visible-light-promoted anti-Markovnikov hydration, followed by photoredox cleavage of Csp3-Csp3 bond in the alcohol intermediate. A crucial consideration in this design is addressing the compatibility between the highly reactive oxy radical species in the latter step and the required hydrogen-atom-transfer (HAT) reagent for both steps. We found that ethyl thioglycolate serves as the optimal hydrogen-atom shuttle, offering remarkable chemoselectivity among multiple potential HAT events in this transformation. By using D2O, we successfully prepared dideuteromethylated (-CD2H) arenes with good heavy atom enrichment. This work presents a redox-neutral alternative for alkene deconstruction, with considerable potential in late-stage modification of complex molecules. Cleavage of carbon-carbon bonds remains a challenging task in organic synthesis. Here the authors report a redox-neutral and catalytic photoredox cleavage of arylalkenes, producing two distinct fragments.

Coupling by metal–carbene transfer enables the formation of several different bonds at the carbenoid site, enabling prochiral Csp3 centers that are fundamental three-dimensional substructures for medicines to be forged with increased efficiency. However, strategies using bulk chemicals are rare because of the challenge of breaking two unactivated geminal bonds. Herein, we report the reactivity of ethers to form metal–carbene intermediate by cleavage of α-Csp3–H/Csp3–O bonds, which achieve selective coupling with arylmagnesium bromides and chlorosilanes. These couplings are catalysed by cyclic (alkyl)(amino)carbene-chromium complex and enable the one-step formation of 1,n-arylsilyl alcohols and α-arylated silanes. Mechanistic studies indicate that the in-situ formed low-valent Cr might react with iodobenzene to form phenyl radical species, which abstracts the α-H atom of ether in giving α-oxy radical. The latter combines with Cr by breaking α-Csp3–O bond to afford metal–carbene intermediate, which couples with aryl Grignard and chlorosilane to form two σ-bonds. Harnessing carbenoid intermediates during organic transformations is an essential strategy for catalysis but strategies using bulk chemicals are rare due to the challenge of breaking two unactivated geminal bonds. Here, the authors report the reactivity of readily available ethers to form a metal–carbene intermediate via radical-relay bond cleavage.

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With the aim of developing effective anti-inflammatory drugs, we have been investigating the biochemical effects of shikonin of “Shikon” roots, which is a naphthoquinone with anti-inflammatory and antioxidative properties. Shikonin scavenged reactive oxygen species like hydroxyl radical, superoxide anion (O2•−) and singlet oxygen in previous studies, but its reactivity with reactive oxygen species is not completely understood, and comparison with standard antioxidants is lacking. This study aimed elucidation of the reactivity of shikonin with nitric oxide radical and reactive oxygen species such as alkyl-oxy radical and O2•−. By using electron paramagnetic resonance spectrometry, shikonin was found unable of reacting with nitric oxide radical in a competition assay with oxyhemoglobin. However, shikonin scavenged alkyl-oxy radical from 2,2'-azobis(2-aminopropane) dihydrochloride with oxygen radical absorbance capacity, ORAC of 0.25 relative to Trolox, and showed a strong O2•−-scavenging ability (42-fold of Trolox; estimated reaction rate constant: 1.7 × 105 M−1s−1) in electron paramagnetic resonance assays with CYPMPO as spin trap. Concerning another source of O2•−, the phagocyte NADPH oxidase (Nox2), shikonin inhibited the Nox2 activity by impairing catalysis when added before enzyme activation (IC50: 1.1 µM; NADPH oxidation assay). However, shikonin did not affect the preactivated Nox2 activity, although having potential to scavenge produced O2•−. In conclusion, shikonin scavenged O2•− and alkyl-oxy radical, but not nitric oxide radical.

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Abstract The incorporation of the gem‐difluoromethylene (CF2) group into organic frameworks is highly sought due to the influence of this unit on the physicochemical and pharmacological properties of molecules. Herein we report an operationally simple, mild, and switchable protocol to access various gem‐difluoro compounds that employs chlorodifloroacetic anhydride (CDFAA) as a low‐cost and versatile fluoroalkylating reagent. Detailed mechanistic studies revealed that electron‐transfer photocatalysis triggers mesolytic cleavage of a C−Cl bond generating a gem‐difluoroalkyl radical. In the presence of alkene, this radical species acts as a unique intermediate that, under solvent‐controlled reaction conditions, delivers a wide range of gem‐difluorinated γ‐lactams, γ‐lactones, and promotes oxy‐perfluoroalkylation. These protocols are flow‐ and batch‐scalable, possess excellent chemo‐ and regioselectivity, and can be used for the late‐stage diversification of complex molecules.

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We have studied the time course of the absorption of bovine liver catalase after pulse radiolysis with oxygen saturation in the presence and absence of superoxide dismutase. In the absence of superoxide dismutase, catalase produced Compound I and another species. The formation of Compound I is due to the reaction of ferric catalase with hydrogen peroxide, which is generated by the disproportionation of the superoxide anion (O-2). The kinetic difference spectrum showed that the other species was neither Compound I nor II. In the presence of superoxide dismutase, the formation of this species was found to be inhibited, whereas that of Compound I was little affected. This suggests that this species is formed by the reaction of ferric catalase with O-2 and is probably the oxy form of this enzyme (Compound III). The rate constant for the reaction of O-2 and ferric catalase increased with a decrease in pH (cf. 4.5 X 10(4) M-1 s-1 at pH 9 and 4.6 X 10(6) M-1 s-1 at pH 5.). The pH dependence of the rate constant can be explained by assuming that HO2 reacts with this enzyme more rapidly than O-2.

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A dual photochemical/nickel-mediated decarboxylative strategy for the assembly of C(sp3)–C(sp2) linkages is disclosed. Under light irradiation at 390 nm, commercially available and inexpensive Hantzsch ester (HE) functions as a potent organic photoreductant to deliver catalytically active Ni(0) species through single-electron transfer (SET) manifolds. As part of its dual role, the Hantzsch ester effects a decarboxylative-based radical generation through electron donor–acceptor (EDA) complex activation. This homogeneous, net-reductive platform bypasses the need for exogenous photocatalysts, stoichiometric metal reductants, and additives. Under this cross-electrophile paradigm, the coupling of diverse C(sp3)-centered radical architectures (including primary, secondary, stabilized benzylic, α-oxy, and α-amino systems) with (hetero)aryl bromides has been accomplished. The protocol proceeds under mild reaction conditions in the presence of sensitive functional groups and pharmaceutically relevant cores.

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Alcoholic liver disease is the result of cascade events, which clinically first lead to alcoholic fatty liver, and then mostly via alcoholic steatohepatitis or alcoholic hepatitis potentially to cirrhosis and hepatocellular carcinoma. Pathogenetic events are linked to the metabolism of ethanol and acetaldehyde as its first oxidation product generated via hepatic alcohol dehydrogenase (ADH) and the microsomal ethanol-oxidizing system (MEOS), which depends on cytochrome P450 2E1 (CYP 2E1), and is inducible by chronic alcohol use. MEOS induction accelerates the metabolism of ethanol to acetaldehyde that facilitates organ injury including the liver, and it produces via CYP 2E1 many reactive oxygen species (ROS) such as ethoxy radical, hydroxyethyl radical, acetyl radical, singlet radical, superoxide radical, hydrogen peroxide, hydroxyl radical, alkoxyl radical, and peroxyl radical. These attack hepatocytes, Kupffer cells, stellate cells, and liver sinusoidal endothelial cells, and their signaling mediators such as interleukins, interferons, and growth factors, help to initiate liver injury including fibrosis and cirrhosis in susceptible individuals with specific risk factors. Through CYP 2E1-dependent ROS, more evidence is emerging that alcohol generates lipid peroxides and modifies the intestinal microbiome, thereby stimulating actions of endotoxins produced by intestinal bacteria; lipid peroxides and endotoxins are potential causes that are involved in alcoholic liver injury. Alcohol modifies SIRT1 (Sirtuin-1; derived from Silent mating type Information Regulation) and SIRT2, and most importantly, the innate and adapted immune systems, which may explain the individual differences of injury susceptibility. Metabolic pathways are also influenced by circadian rhythms, specific conditions known from living organisms including plants. Open for discussion is a 5-hit working hypothesis, attempting to define key elements involved in injury progression. In essence, although abundant biochemical mechanisms are proposed for the initiation and perpetuation of liver injury, patients with an alcohol problem benefit from permanent alcohol abstinence alone.

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We studied the mechanism of formation of oxygen radicals during ferrous ion-induced decomposition of linoleic acid hydroperoxide using the spin trapping and chemiluminescence methods. The formation of the superoxide anion (O2*-) was verified in the present study. The hydroxyl radical is also generated through Fenton type decomposition of hydrogen peroxide produced on disproportionation of O2*-. A carbon-centered radical was detected using 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO) as a spin trap. Alkoxyl radical formation is essential for the conversion of linoleic acid hydroperoxide into the peroxyl radical by ferrous ion. It is likely that the alkoxyl radical [R1CH(O*)R2] is converted into the hydroxylcarbon radical [R1C*(OH)R2] in water, and that this carbon radical reacts with oxygen to give the alpha-hydroxyperoxyl radical [R1R2C(OH)OO*], which decomposes into the carbocation [R1C+(OH)R2] and O2*-.

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Oxo-octadecadienoic acids (OxoODEs) act as peroxisome proliferator-activated receptor (PPAR) agonists biologically, and are known to be produced in the lipoxygenase/linoleate system. OxoODEs seem to originate from the linoleate alkoxyl radicals that are generated from (E/Z)-hydroperoxy octadecadienoic acids ((E/Z)-HpODEs) by a pseudoperoxidase reaction that is catalyzed by ferrous lipoxygenase. However, the mechanism underlying the conversion of alkoxyl radical into OxoODE remains obscure. In the present study, we confirmed that OxoODEs are produced in the lipoxygenase/linoleate system in an oxygen-dependent manner. Interestingly, we revealed a correlation between the (E/Z)-OxoODEs content and the (E/E)-HpODEs content in the system. (E/E)-HpODEs could have been derived from (E/E)-linoleate peroxyl radicals, which are generated by the reaction between a free linoleate allyl radical and an oxygen molecule. Notably, the ferrous lipoxygenase-linoleate allyl radical (LOx(Fe2+)-L·) complex, which is an intermediate in the lipoxygenase/linoleate system, tends to dissociate into LOx(Fe2+) and a linoleate allyl radical. Subsequently, LOx(Fe2+) converts (E/Z)-HpODEs to an (E/Z)-linoleate alkoxyl radical through one-electron reduction. Taken together, we propose that (E/Z)-OxoODEs and (E/E)-HpODEs are produced through radical-radical dismutation between (E/Z)-linoleate alkoxyl radical and (E/E)-linoleate peroxyl radical. Furthermore, the production of (E/Z)-OxoODEs and (E/E)-HpODEs was remarkably inhibited by a hydrophobic radical scavenger, 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO). On the contrary, water-miscible radical scavengers, 4-hydroxyl-2,2,6,6-tetramethylpiperidine 1-oxyl (OH-TEMPO) and 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrroline-N-oxyl (CmΔP) only modestly or sparingly inhibited the production of (E/Z)-OxoODEs and (E/E)-HpODEs. These facts indicate that the radical-radical dismutation between linoleate alkoxyl radical and linoleate peroxyl radical proceeds in the interior of micelles.

Hydroethidine (HE) and hydropropidine (HPr+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HPr}^{+}$$\end{document}) are fluorogenic probes used for the detection of the intra- and extracellular superoxide radical anion (O2∙-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {O}_{ {2}}^{\bullet -}$$\end{document}). In this study, we provide evidence that HE and HPr+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HPr}^{+}$$\end{document} react rapidly with the biologically relevant radicals, including the hydroxyl radical, peroxyl radicals, the trioxidocarbonate radical anion, nitrogen dioxide, and the glutathionyl radical, via one-electron oxidation, forming the corresponding radical cations. At physiological pH, the radical cations of the probes react rapidly with O2∙-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {O}_{ {2}}^{\bullet -}$$\end{document}, leading to the specific 2-hydroxylated cationic products. We determined the rate constants of the reaction between O2∙-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {O}_{ {2}}^{\bullet -}$$\end{document} and the radical cations of the probes. We also synthesized N-methylated analogs of HPr+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HPr}^{+}$$\end{document} and HE which were used in mechanistic studies. Methylation of the amine groups was not found to prevent the reaction between the radical cation of the probe and the superoxide, but it significantly increased the lifetime of the radical cation and had a substantial effect on the profiles of the oxidation products by inhibiting the formation of dimeric products. We conclude that the N-methylated analogs of HE and HPr+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HPr}^{+}$$\end{document} may be used as a scaffold for the design of a new generation of probes for intra- and extracellular superoxide.

Clinical measurement of neopterin has been extensively used as a marker of inflammation but the in vivo mechanism generating neopterin is poorly understood. Neopterin is described as the oxidation product of 7,8-dihydroneopterin, a potent antioxidant generated by monocyte/macrophages in response to interferon-γ. While peroxyl and hydroxyl scavenging generates dihydroxanthopterin, hypochlorite efficiently oxidises 7,8-dihydroneopterin into neopterin, but this reaction alone does not explain the high levels of neopterin seen in clinical data. Here, we examine whether superoxide scavenging by 7,8-dihydroneopterin generates neopterin. U937 cells incubated with oxLDL showed a time dependent increase superoxide and 7,8-dihydroneopterin oxidation to neopterin. Neopterin generation in oxLDL or phorbol ester treated U937 cells or human monocytes was inhibited by apocynin and PEG-SOD. Addition of the myeloperoxidase inhibitor 4-aminobenzoic acid hydrazide (ABAH) had no effect of the superoxide generation or neopterin formation. 7,8-Dihydroneopterin reacted with superoxide/hydroxy radical mixtures generated by X-ray radiolysis to give neopterin. Formation of neopterin by superoxide derived from the xanthine/xanthine oxidase system was inhibited by superoxide dismutase. Neopterin formation was inhibited by apocynin in phorbol ester treated human carotid plaque rings in tissue culture. These results indicate that 7,8-dihydroneopterin scavenges superoxide and is subsequently oxidised into neopterin in cellular and cell-free experimental systems.

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Abstract The present study reports radiation-chemical yields of 2.5-diaminoimidazolone (Iz) derivatives in X-irradiated phosphate-buffered solutions of guanosine and double-stranded DNA. Various gassing conditions (air, N20/O2 (4:1), N2O, vacuum) were employed to elucidate the contribution of several alternative pathways leading to Iz in reactions initiated by hydroxyl radical attack on guanine. In all systems, Iz was identified as the second by abundance guanine degradation product after 8-oxoguanine, formed in 1:5 (guanosine) and 1:3.3 (DNA) ratio to the latter in air-saturated solutions. Experimental data strongly suggest that the addition of molecular oxygen to the neutral guanine radical G(-H)• plays a major in Iz production in oxygenated solutions of double-stranded DNA while in other systems it may compete with recombination of G(-H)• with superoxide and/or alkyl peroxyl radicals. The production of Iz through hydroxyl radical attack on 8-oxoguanine was also shown to take place although the chemical yield of Iz (ca 6%) in this process is too low to compete with the other pathways. The linearity of Iz accumulation with dose also indicates a negligible contribution of this channel to its yield in all systems.

Abstract During the synthesis of a known drug, we synthesized a novel compound impromptu, which we have named resorcimoline. This compound exhibited significant antioxidative activity. In this report, we present the concentration-dependent free radical scavenging activity of resorcimoline against various free radical species. The scavenging activity of resorcimoline was evaluated against nine free radicals using electron spin resonance spectroscopy with a spin-trapping method. These free radicals were hydroxyl radical, superoxide anion, tert-butyl peroxyl radical, tert-butoxyl radical, ascorbyl free radical, singlet oxygen, nitric oxide, 2,2-diphenyl-1-picrylhydrazyl, and tyrosyl radical. Sigmoid concentration-response curves were fitted to estimate the reaction rate constants of resorcimoline for the free radicals, and these were compared with those of edaravone, the only current clinically approved free radical scavenger. The antioxidative activity of resorcimoline against lipid peroxidation within tissue was assessed using the thiobarbituric acid reactive substance (TBARS) assay. The cytotoxicity and stability of resorcimoline were also evaluated. Resorcimoline demonstrated significant concentration-dependent scavenging activity against all tested free radicals. Notably, the reaction rate constants for superoxide anion and nitric oxide were significantly higher than those of edaravone, while the rate constant for hydroxyl radical was significantly lower. The TBARS assay revealed that resorcimoline inhibited tissue lipid peroxidation in a concentration-dependent manner. Moreover, resorcimoline exhibited no cytotoxicity at concentrations up to 100 μM and remained stable at room temperature under ambient light for 7 d. These findings indicate that resorcimoline’s direct free radical scavenging activity could contribute to its potential clinical antioxidative effects.

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Abstract Sepsis is caused by infections associated with life-threatening multiple organ failure (MOF). Septic MOF appears to be closely related to circulatory failure due to immunothrombosis. This process involves the production of reactive oxygen spices (ROS) in inflammatory sites. Therefore, the detoxification of the systemic excess ROS is important for the improvement of the process in septic pathogenesis. Histidine-rich glycoprotein (HRG), a plasma glycoprotein, ameliorates a septic condition through the suppression of both excess ROS production from neutrophils and immunothrombosis. Hydroxyl radical is known as the most important species among ROS in pathogenesis; however, the direct influence of HRG on hydroxyl radical formation and ROS activity is poorly understood. In this study, we showed that HRG, in a concentration-dependent manner, efficiently inhibited the production of hydroxyl radical induced by the Fenton’s reaction through chelation of the divalent iron. HRG also exhibited antioxidant activity against peroxyl radical by oxidation of HRG itself as a substrate; however, it did not show superoxide dismutase and catalase-like activities. Additionally, HRG enhanced glutathione peroxidase, a well-known antioxidant enzyme, activity. These results suggest that HRG may play a unique role in suppression of the production of hydroxyl radicals and subsequent tissue damage at inflammatory sites. Marked reduction in plasma HRG in sepsis might lose such an important protective mechanism. Thus, the present study provides evidence that inhibition of ROS and ROS-production systems by HRG may contribute to antiseptic effects in vivo and that HRG could be potential therapy for ROS-related diseases.

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Alcohols are promising sacrificial agents for improving the photocatalytic production of hydrogen peroxide (H2O2) through reaction with holes, inhibiting the electron-hole recombination. However, the side effects of alcohols on oxygen reduction and reactive oxygen species (ROS) generation remain controversial due to intrinsic formation of organic peroxides under irradiation in the presence of air or oxygen. To address this, we employ electron paramagnetic resonance (EPR), in-situ Fourier-transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance (NMR) to directly monitor intermediate species and eliminate false-positive results on H2O2 production. Our results demonstrate that varying oxygen concentrations, alcohol types, and alcohol dosages significantly influence H2O2 yields. Organic radicals were identified as critical intermediates, including hydroperoxyl radicals, alkoxyl radicals, and peroxyl radicals, contributing to H2O2 regeneration. Furthermore, the concept of sacrificial agents was expanded to phenolic compounds (PCs) and humic acid (HA) in the natural water environment. A strong linear correlation (R2= 0.91) was observed between the para-substituent Hammett constant (σp) of these "RHOH" structured PCs and the apparent rate constants of forming H2O2 (kf). Spectroscopy and molecular-level experiments were conducted to identify and quantify the generation of H2O2 and hydroxyl radical (•OH) during the HA photolysis. This study reveals an overlooked H2O2 re-form pathway in system containing sacrificial agents and provides valuable insights into synergistic mechanisms involved in photocatalytic of H2O2 generation and pollutant degradation, which was beneficial for the strategic development of the H2O2 energy generation synergistically water pollution treatment.

Advanced Oxidation Processes (AOPs) offer promising methods for disinfection by generating radical species like hydroxyl radicals, superoxide anion radicals, and hydroxy peroxyl, which can induce oxidative stress and deactivate bacterial cells. Photocatalysis, a subset of AOPs, activates a semiconductor using specific electromagnetic wavelengths. A novel material, Cu/Cu2O/CuO nanoparticles (NPs), was synthesized via a laser ablation protocol (using a 1064 nm wavelength laser with water as a solvent, with energy ranges of 25, 50, and 80 mJ for 10 min). The target was sintered from 100 °C to 800 °C at rates of 1.6, 1.1, and 1 °C/min. The composite phases of Cu, CuO, and Cu2O showed enhanced photocatalytic activity under visible-light excitation at 368 nm. The size of Cu/Cu2O/CuO NPs facilitates penetration into microorganisms, thereby improving the disinfection effect. This study contributes to synthesizing mixed copper oxides and exploring their activation as photocatalysts for cleaner surfaces. The electronic and electrochemical properties have potential applications in other fields, such as capacitor materials. The laser ablation method allowed for modification of the band gap absorption and enhancement of the catalytic properties in Cu/Cu2O/CuO NPs compared to precursors. The disinfection of E. coli with Cu/Cu2O/CuO systems serves as a case study demonstrating the methodology’s versatility for various applications, including disinfection against different microorganisms, both Gram-positive and Gram-negative.

Sonicated emulsive water microdroplets (SEWMs) accelerate and enable a variety of catalyst-free chemical transformations. However, significant unanswered questions remain regarding the chemical intermediates they form and their possible redox origin. In this study, we identified dissolved O2 as the primary originator of reactive oxygen species (ROS) such as OH• and H2O2. We uncovered the role of dissolved O2 redox by using a combination of microelectrochemical methods to detect H2O2, isotopic methods to identify the source of H2O2, and a combination of electron spin resonance and the DMPO spin trap to detect radicals such as OH•. Notably, we found that H2O2 production is correlated with O2 content via a reduction pathway enabled by a sufficiently large reducing power that can additionally generate H2 and even perform Pb electroless deposition on Au and Cu metal substrates. Building on our findings, continuous O2 bubbling of SEWMs showed accumulation of H2O2 up to ∼88 mM in the aqueous phase within 1 h of sonication, demonstrating the scale-up promise of this method. Distinct to sonochemistry of a single phase, this study advances our understanding of the confluence of redox and chemical reaction mechanisms within SEWMs as a biphasic system. This insight paves the way for improving their reaction kinetics, yield, and selectivity, positioning these attractive redox microreactors as alternatives to traditional electrolyzers.

Pentachlorophenol (PCP) caused water quality problems owe to its past widespread application and stability, harmful to human health. Photocatalysis, which was mainly involved in the reactive oxygen species (ROS) reaction, has large potential as water treatment process. However, the roles of ROS on the degradation process of PCP are not yet clearly defined. The main objectives of this work were to investigate the roles of ROS involved in the whole degradation of PCP and main toxic intermediates and elucidate the degradation mechanisms. Tetrachloro-1,4-benzo/hydroquinone (TCBQ/TCHQ), trichlorohydroxy-1,4-benzoquinone (OH-TrCBQ) and 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (OH-DCBQ) were identified as main intermediates. The roles of generated ROS including OH, O2- and H2O2 were systematically explored for the degradation of PCP and its main intermediates using radical quenchers. The results showed that, OH played the dominant role for the degradation of PCP, O2- played more contributing roles for the degradation of TCBQ, H2O2 exhibited major contribution for the degradation of OH-TrCBQ and OH-DCBQ. These results offered us an insight into the degradation mechanism of PCP involved with ROS. It can also serve as the basis for controlling and blocking the generation of highly toxic substances through regulating the ROS generation during the PCP degradation.

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The sulfite (S(IV))-based advanced oxidation process (AOP) has emerged as an appealing alternative to the traditional persulfate-based AOP for the elimination of organic contaminants from diverse water matrices. In this work, a silica reinforced ZIF-67(Co) catalyst (CZS) is fabricated, characterized and tested in the activation of S(IV) for the sulfamethoxazole (SMX) degradation. The prepared CZS demonstrates superior stability and catalytic ability for the degradation of SMX compared to ZIF-67(Co) across a broad pH range. Unlike the conventional radical-dominated oxidation systems, the CZS/S(IV) system for SMX degradation operates through a non-radical mechanism, featuring high-valent Co(IV) and singlet oxygen (1O2) as the predominated reactive species. The hydroxylated Co species exposed on the CZS surface is identified as the pivotal active site, realizing the S(IV) activation through a complexation-electron transfer process, resulting in the production of various reactive intermediates. Co(II) undergoes the conversion to Co(IV) by generated HSO5-, and 1O2 predominantly originates from the intermediate SO4•-. Profiting from the highly selective oxidation capacities of Co(IV) and 1O2, the established oxidative system demonstrates a remarkable interference resistance and exhibits an exceptional decontamination performance under real-world water conditions. In short, this work provides a sustainable S(IV)-based oxidation strategy for environmental remediation via non-radical mechanism.

Different reactive oxygen species (ROS) tend to attack specific sites on pollutants, leading to the formation of intermediates with different toxic effects. Therefore, regulating the directional transformation of ROS is a new effective approach for safe degradation of refractory organic compounds in wastewater. However, the regulation mechanism and transformation path of ROS remain unclear. In this work, the dissolved oxygen (DO) content was controlled by aeration to generate different ROS through the activation of O2 on the calcined CuCoFe-LDH (CuCoFe-300). ROS quantitative experiments and electron paramagnetic resonance proved that O2 was mainly activated to superoxide radical (•O2-) and singlet oxygen (1O2) under low DO concentration (0.231 mmol/L) (O2 → •O2- → 1O2). With the increasing of DO concentration (0.606 mmol/L), O2 was inclined to convert into hydroxyl radicals (•OH) (O2 → •O2- → H2O2 → •OH). The density functional theory and function model of active sites utilization and DO concentration built a solid proof for ROS conversion mechanism that increasing the DO concentration promotes the increase of active sites utilization on the CuCoFe-300 system. That is, the •O2- was more prone to convert to •OH, not 1O2 in thermodynamics under high active sites utilization condition. Hence, the ROS generation was controlled by regulating DO concentration, and the nontoxic degradation pathway of ciprofloxacin was well-designed. This work is dedicated to the in-depth exploration of the mechanism between DO concentration and ROS conversion, which provides an extremely flexible, low energy consumption, and environmentally friendly wastewater treatment method in a new perspective.

Photochemically produced reactive intermediates (PPRIs) by natural photosensitizers such as chromophoric dissolved organic matter (CDOM) play numerous key roles in aquatic biogeochemical processes. PPRI productions rely on both the intensity and the spectrum of incident sunlight. While the impacts of sunlight intensity on PPRI productions are well-studied, there remains insufficient understanding of the spectrum-dependence of PPRI productions. Here we designed a high sample throughput reactor equipped with monochromatic LED lights for systematic assessments of wavelength-dependent productions of four important PPRI species, i.e., triplet-state excited CDOM (3CDOM*), singlet oxygen (1O2), hydrogen peroxide (H2O2), and hydroxyl radical (•OH), in CDOM solutions. The quantum yields of PPRIs followed the order: 3CDOM* > 1O2 ≫ H2O2 > •OH. Moreover, PPRI quantum yields decreased with the light wavelength increasing from 375 to 490 nm and sharply decreased to zero above 490 nm, while the shapes of quantum yield spectra differed among PPRI species. Simulations on PPRI productions under varying season, latitude, altitude, and cloud cover conditions show that the sunlight spectrum plays a role as equally important as intensity in determining PPRI productions and PPRI-mediated transformations of aquatic nutrients and micropollutants. Therefore, incorporating the spectrum dependence of PPRI productions will advance our understandings of PPRI-driven biogeochemical processes and pollutant dynamics under varying spatial-temporal and climatic conditions. Regarding this, the high sample throughput LED reactor sheds light on a new approach for the facile characterization of PPRI quantum yield spectrum.

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Photochemically produced reactive oxygen species in wastewater lagoons upon sunlight exposure are important in the attenuation of emerging contaminants (ECs). The production of reactive radicals in wastewater lagoons depends on both environmental factors and the composition of effluent organic matter (EfOM) in the wastewater. Knowing the steady state concentrations of these reactive species produced in a particular lagoon wastewater is critical to the prediction of the persistence and attenuation of ECs in that sunlit wastewater treatment lagoon. This study quantified the formation of four photochemically produced reactive intermediates (PPRIs): hydroxyl radical, carbonate radical, singlet oxygen, and triplet excited state EfOM in 11 samples collected from a municipal wastewater lagoon over a full year. The temporal distribution of these key PPRIs in the lagoon under investigation was determined in relation to sunlight irradiance, wastewater composition and temperature. Greater sunlight intensity led to greater PPRI production over the year. Increasing wastewater temperature from 12 to 25 °C led to greater production of singlet oxygen, a moderate decrease in hydroxyl radical and increase in triplet excited state EfOM, and minimal impact on carbonate radical production. The optical properties of the lagoon wastewater of Napierian absorption coefficient (A300) and E2:E3 ratio could be used as indicators of the formation of singlet oxygen (Pearson's r = 0.79) and triplet excited EfOM (Pearson's r = 0.76) produced upon solar irradiation. The concentration of carbonate radical formed was strongly correlated to the nitrate level in the wastewater (Pearson's r = 0.85). The findings could be used for modelling the seasonal sunlight-induced photolysis process of ECs during lagoon-based wastewater treatment, with a view to optimising the treatment process, predicting the efficacy of EC removal, and risk assessment of the treated water.

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Spinel is a kind of desirable catalyst to activate peroxymonosulfate (PMS) for chemical oxidation of organic contaminants in wastewater treatment. However, apart from classic sulfate radical based AOPs (SR-AOPs), the generation and oxidative pathways of singlet oxygen (1O2) by Co/Mn spinels have been little explored in PMS catalysis. In this study, spinel-type oxide Co2Mn1O4 was successfully synthesized, and used as highly effective catalyst in PMS activation for heterogeneous degradation of TCS (up to 96.4% within 30 min) at initial pH of 6.8, which was also slightly impacted by coexisting ions. Based on radical scavengers and electron paramagnetic resonance (EPR) experiments, sulfate radicals and singlet oxygen (1O2) were unveiled to be the dominant reactive oxygen species (ROS) in Co2Mn1O4/PMS system. Co2Mn1O4 catalyst exhibited reversible redox properties based on the results of cyclic voltammetry (CV). More importantly, the generation of 1O2 might not only promote the TCS removal rate directly, but also facilitate the metal redox cycle in spinel structure in Co2Mn1O4/PMS system. Finally, degradation pathways of TCS in Co2Mn1O4/PMS system were proposed, which involved the breakage of ether bond and cycloaddition reaction.

Oxidative post-translational modification of proteins by molecular oxygen (O2)- and nitric oxide (•NO)-derived reactive species is a usual process that occurs in mammalian tissues under both physiological and pathological conditions and can exert either regulatory or cytotoxic effects. Although the side chain of several amino acids is prone to experience oxidative modifications, tyrosine residues are one of the preferred targets of one-electron oxidants, given the ability of their phenolic side chain to undergo reversible one-electron oxidation to the relatively stable tyrosyl radical. Naturally occurring as reversible catalytic intermediates at the active site of a variety of enzymes, tyrosyl radicals can also lead to the formation of several stable oxidative products through radical-radical reactions, as is the case of 3-nitrotyrosine (NO2Tyr). The formation of NO2Tyr mainly occurs through the fast reaction between the tyrosyl radical and nitrogen dioxide (•NO2). One of the key endogenous nitrating agents is peroxynitrite (ONOO-), the product of the reaction of superoxide radical (O2•-) with •NO, but ONOO--independent mechanisms of nitration have been also disclosed. This chemical modification notably affects the physicochemical properties of tyrosine residues and because of this, it can have a remarkable impact on protein structure and function, both in vitro and in vivo. Although low amounts of NO2Tyr are detected under basal conditions, significantly increased levels are found at pathological states related with an overproduction of reactive species, such as cardiovascular and neurodegenerative diseases, inflammation and aging. While NO2Tyr is a well-established stable oxidative stress biomarker and a good predictor of disease progression, its role as a pathogenic mediator has been laboriously defined for just a small number of nitrated proteins and awaits further studies.

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Photoacoustic imaging is attracting a great deal of interest owing to its distinct advantages over other imaging techniques such as fluorescence or magnetic resonance image. The availability of photoacoustic probes for reactive oxygen and nitrogen species (ROS/RNS) could shed light on a plethora of biological processes mediated by these key intermediates. Tetramethylbenzidine (TMB) is a non-toxic and non-mutagenic colorless dye that develops a distinctive blue color upon oxidation. In this work, we have investigated the potential of TMB as an acoustogenic photoacoustic probe for ROS/RNS. Our results indicate that TMB reacts with hypochlorite, hydrogen peroxide, singlet oxygen, and nitrogen dioxide to produce the blue oxidation product, while ROS, such as the superoxide radical anion, sodium peroxide, hydroxyl radical, or peroxynitrite, yield a colorless oxidation product. TMB does not penetrate the Escherichia coli cytoplasm but is capable of detecting singlet oxygen generated in its outer membrane.

Tetrabromobisphenol A (TBBPA) has attracted considerable attention due to its ubiquitous presence in different environmental compartments worldwide. However, information on its aerobic biodegradability in coastal environments remains unknown. Here, the aerobic biodegradation of TBBPA using a Pseudoalteromonas species commonly found in the marine environment was investigated. We found that extracellular biogenic siderophore, superoxide anion radical (O2•-), hydrogen peroxide (H2O2), and hydroxyl radical (•OH) were involved in TBBPA degradation. Upregulation of genes (nqrA and lodA) encoding Na+-translocating NADH-quinone oxidoreductase and l-lysine-ε-oxidase supported the extracellular O2•- and H2O2 production. The underlying mechanism of TBBPA biodegradation presumably involves both O2•- reduction and •OH-based advanced oxidation process (AOP). Furthermore, TBBPA intermediates of tribromobisphenol A, 4-isopropylene-2,6-dibromophenol, 4-(2-hydroxyisopropyl)-2,6-dibromophenol, 2,4,6-tribromophenol (TBP), 4-hydroxybenzoic acid, and 2-bromobenzoic acid were detected in the culture medium. Debromination and β-scission pathways of TBBPA biodegradation were proposed. Additionally, membrane integrity assays revealed that the increase of intracellular catalase (CAT) activity and the extracellular polymeric substances (EPS) might account for the alleviation of oxidative damage. These findings could deepen understanding of the biodegradation mechanism of TBBPA and other related organic pollutants in coastal and artificial bioremediation systems.

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O2 to water via Co(III)-O-O-Co(III) peroxo intermediates.

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The non‐enzymatically catalyzed oxidation of dopamine (DA) and the resultant formation of powerful oxidants such as the hydroxyl radical (•OH) through ‘Fenton chemistry’ in the presence of iron within dopaminergic neurons are thought to contribute to the damage of cells or even lead to neuronal degenerative diseases such as Parkinson's disease. An understanding of DA oxidation as well as the transformation of the intermediates that are formed in the presence of iron under physiological conditions is critical to understanding the mechanism of DA and iron induced oxidative stress. In this study, the generation of H2O2 through the autoxidation and iron‐catalyzed oxidation of DA, the formation of the dominant complex via the direct reaction with Fe(II) and Fe(III) in both oxygen saturated and deoxygenated conditions and the oxidation of Fe(II) in the presence of DA at physiological pH 7.4 were investigated. The oxidation of DA resulted in the generation of significant amounts of H2O2 with this process accelerated significantly in the presence of Fe(II) and Fe(III). At high DA:Fe(II) ratios, the results from this study suggest that DA plays a protective role by complexing Fe(II) and preventing it from reacting with the generated H2O2. However, the accumulation of H2O2 may result in cellular damage as high intracellular H2O2 concentrations will result in the oxidation of remaining Fe(II) mainly through the peroxidation pathway. At low DA:Fe(II) ratios however, it is likely that DA will act as a pro‐oxidant by generating H2O2 which, in the presence of Fe(II), will result in the production of strongly oxidizing •OH radicals.

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Significance In the bulk phase, hydroxyl radical from the one-electron transfer and high-valent iron-oxo species from the O-atom transfer compete to be the reactive intermediates in the Fenton and related reactions. In the confined space at a nanoscale, however, the behavior of the Fenton reaction is elusive. Herein, we report an unprecedented singlet oxygen mediated Fenton’s reaction occurred inside carbon nanotube with inner diameter of ∼7 nm, showing exotic catalytic activities, unforeseen adsorption-dependent selectivity, and pH stability for the oxidation of organic compounds. Our results suggest the use of Fenton’s reaction in more scenarios than ever explored. For several decades, the iron-based Fenton-like catalysis has been believed to be mediated by hydroxyl radicals or high-valent iron-oxo species, while only sporadic evidence supported the generation of singlet oxygen (1O2) in the Haber–Weiss cycle. Herein, we report an unprecedented singlet oxygen mediated Fenton-like process catalyzed by ∼2-nm Fe2O3 nanoparticles distributed inside multiwalled carbon nanotubes with inner diameter of ∼7 nm. Unlike the traditional Fenton-like processes, this delicately designed system was shown to selectively oxidize the organic dyes that could be adsorbed with oxidation rates linearly proportional to the adsorption affinity. It also exhibited remarkably higher degradation activity (22.5 times faster) toward a model pollutant methylene blue than its nonconfined analog. Strikingly, the unforeseen stability at pH value up to 9.0 greatly expands the use of Fenton-like catalysts in alkaline conditions. This work represents a fundamental breakthrough toward the design and understanding of the Fenton-like system under nanoconfinement, might cause implications in other fields, especially in biological systems.

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Polymorphonuclear neutrophils (PMN) kill Cryptococcus neoformans (Cn) by oxidative mechanisms, but the roles of various reactive oxygen intermediates (ROIs) are not known. We used a mannitol low-producing Cn mutant (Cn MLP) and its wild-type parent (Cn H99) to examine the role of ROIs distal to H2O2 in PMN killing and to determine whether mannitol produced by Cn protects the fungus against ROIs. At PMN:Cn cell ratios of 1:1, 10:1, and 100:1, PMN killed significantly more Cn MLP than Cn H99 cells after 2 and 4 h (p less than 0.05). Superoxide dismutase and the hydroxyl radical (OH.) scavengers mannitol and DMSO inhibited killing of both strains (p less than 0.05), but catalase did not. Cn H99 and Cn MLP stimulated PMN to produce similar amounts of O2- and H2O2. In contrast, Cn MLP stimulated greater luminol-dependent chemiluminescence than did Cn H99 (p less than 0.05). Finally, H2O2 alone killed similar numbers of Cn H99 and Cn MLP cells, but oxidants generated by FeSO4 (1 microM), H2O2 (10 microM), and iodide (1 to 3 microM) killed significantly more Cn MLP than Cn H99 cells in 1 h (p less than 0.05). Mannitol, DMSO, and catalase completely inhibited killing of both Cn strains by this cellfree system, but superoxide dismutase did not. These results suggest that 1) distal ROIs such as OH. and HOCI are key effector molecules against Cn, and 2) mannitol produced by Cn may protect against oxidative killing by scavenging distal ROIs.

The cytochrome P-450-mediated activation of phenacetin (PHEN) to reactive intermediates by two hypothetical mechanisms has been studied by use of SV 6-31G ab initio energy and spin distribution calculations. In our calculations, the cytochrome P-450 enzyme system has been substituted by a singlet oxygen atom in order to reduce the computational efforts and to fulfill the requirements as to spin conservation. Both mechanisms are based on the currently increasingly accepted view that radical intermediates, formed via sequential one-electron steps, play a crucial role in the metabolic activation of substrates by cytochrome P-450. The first pathway is proposed to involve an initial abstraction of an electron and a proton from the alpha-methylene carbon atom in the ethoxy side chain and can explain the O-deethylation products paracetamol and acetaldehyde. In the second pathway, an initial abstraction of an electron and a proton from the nitrogen atom in the acetylamino side chain is proposed. The calculated spin densities of the formed nitrogen radical indicate that the unpaired electron is primarily localized at the nitrogen atom and to a smaller extent at the ortho- and paracarbon atoms relative to the acetylamino group. Radical recombination reactions between a hydroxyl radical and the spin delocalization-radicalized reactive centers of the nitrogen radical can explain the formation of the metabolites N-hydroxy-PHEN, 2-hydroxy-PHEN, and the arylating metabolite N-acetyl-p-benzoquinone imine (NAPQI), which forms a 3-(S-glutathionyl)paracetamol conjugate in the presence of glutathione. NAPQI is proposed to be formed via intermediate formation of a hemiketal. Proposals are made for the decomposition of this hemiketal into NAPQI that are consistent with currently available experimental data on 14C- and 18O-labeled PHEN.

SummaryThe present study examined the ability of human monocytes to produce reactive oxygen intermediates after a contact with tumour cells. Monocytes generated oxygen radicals, as measured by luminol-enhanced chemiluminescence and superoxide anion production, after stimulation with the tumour, but not with untransformed, cells. The use of specific oxygen radical scavengers and inhibitors, superoxide dismutase, catalase, dimethyl sulphoxide and deferoxamine as well as the myeloperoxidase inhibitor 4-aminobenzoic acid hydrazide, indicated that chemiluminescence was dependent on the production of superoxide anion and hydroxyl radical and the presence of myeloperoxidase. The tumour cell-induced chemiluminescent response of monocytes showed different kinetics from that seen after activation of monocytes with phorbol ester. These results indicate that human monocytes can be directly stimulated by tumour cells for reactive oxygen intermediate production. Spontaneous monocyte-mediated cytotoxicity towards cancer cells was inhibited by superoxide dismutase, catalase, deferoxamine and hydrazide, implicating the role of superoxide anion, hydrogen peroxide, hydroxyl radical and hypohalite. We wish to suggest that so-called ‘spontaneous’ tumoricidal capacity of freshly isolated human monocytes may in fact be an inducible event associated with generation of reactive oxygen intermediates and perhaps other toxic mediators, resulting from a contact of monocytes with tumour cells.

Freshly isolated human blood monocytes were spontaneously cytotoxic toward K562 tumor cells. During culture of the monocytes in vitro cytotoxicity decreased during the first 48 h but tumoricidal competence was restored after 3 to 4 days in vitro. These changes were accompanied by changes in both reactive oxygen intermediate generating capacity and reactive nitrogen intermediate production. Lucigenin-dependent chemiluminescence stimulated with either FMLP or PMA declined during the first 2 days in culture and was negligible during the later days in culture. Superoxide radical production in response to either FMLP or PMA remained fairly constant for the first few days in vitro and then declined. NO2- concentration in monocyte-conditioned medium was fairly constant during the first few days in vitro but increased after 6 days. The return to tumoricidal competence after 3 to 4 days in culture was decreased by the addition of NG-monomethyl-L-arginine. These results indicate that reactive oxygen intermediates are employed by monocytes in the killing of tumor cells. However, after maturation of monocytes to macrophages, this mechanism becomes less important and reactive nitrogen intermediates are employed in mediating macrophage cytotoxicity.

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The selective activation of C-O ether bonds is an essential tool in organic synthesis and natural polymer depolymerization. However, the direct cleavage of the ether bond is still challenging work, especially breaking this inert and redox-neutral bond to provide one active carbon radical and another oxygen-centered fragment with oxidation capacity that can participate in the controllable radical reaction. We herein report that commercial 2H-MoS2 with negligible acidity can efficiently catalyze the benzylation of arenes with benzyl ethers, and a new Radical-Friedel-Crafts mechanism is proposed, which is quite different from the strong acid-catalyzed Friedel-Crafts mechanism. With dibenzyl ether as the model benzylation reagent, 2H-MoS2 can achieve the homolytic cleavage of the Bn-OR bond to generate the benzyl carbon radical and RO˙ species, identified by EPR measurement and radical trap experiments. The following radical-involved benzylation is confirmed by the Hammett results and a plausible pathway is proposed to clarify the Radical-Friedel-Crafts process. Heterogeneous 2H-MoS2 can be consecutively used four times without regeneration and it offers 94-95% yields of 2-benzyl-1,4-dimethylbenzene from dibenzyl ether and p-xylene in 30 min at 140 °C. Furthermore, this mechanism can provide some inspiration to activate the ether bond and to utilize ether as an oxidant in C-H bond activation.

Selective ether bond activation is essential in organic synthesis and natural polymer depolymerization. Herein, we report that MoO2 with negligible acidity but characterized metallic-basic bifunctional properties can efficiently catalyze the benzylation of arenes with benzyl ethers as a benzylation reagent via a radical-Friedel-Crafts mechanism. Through combining catalyst characterizations, control experiments, thermomechanical analysis, intermediates capture, and density functional theory (DFT) calculations, multiple Mo sites on the MoO2 surface facilitate the initial transfer of oxygen-centered groups from adsorbed dibenzyl ether (DBE*). This process involved the homolytic cleavage of Bn-OBn ether bonds, promoted by adjacent-group auxiliary activation of benzyl aromatic rings adsorbed on the MoO2 surface. The generated benzyl radical (Bn˙) attacks the aromatic ring of the arene substrates, while the first-exfoliated oxygen-centered group (BnO*) and its potential decomposition fragment, the oxidative MoO* species, abstract a H atom from the above addition-intermediate to restore aromaticity and achieve benzylation. The remaining alcohol (BnOH) can also participate in the benzylation mediated by MoO2. This study could provide some inspiration on C-O bond activation and ether utilization mediated by a Mo-based catalyst.

Herein, we describe the evolution of our syntheses of the pleurotinoid natural products pleurotin (1), pleurogrisein (3), and 4-hydroxypleurogrisein (4). An approach based on a proximity-induced intramolecular Diels-Alder cycloaddition of a transient ortho-quinone dimethide (e.g., 6, Scheme 1) was inferior to an alternative construction featuring Gao's titanium(IV)-mediated photoenolization Diels-Alder coupling of ortho-tolualdehyde 20 with functionalized hydrindenone 22. While this pairing exhibited the desired stereoface selectivity and produced cis-fused hydrindanone 23, the successful realization of our syntheses of 1, 3, and 4 required a post-Diels-Alder epimerization of the unactivated stereocenter at C-5 in compound 23. Ultimately, it was possible to generate a reactive oxygen-centered radical via a reductive homolytic cleavage of the N-O bond in 23 and capitalize on its ability to break the C5-H bond in an intramolecular 1,5-hydrogen atom transfer (HAT). The carbon radical arising from this pivotal 1,5-HAT was subsequently trapped in situ by an exogenous thiol in a kinetically controlled HAT reaction to establish the natural configuration at C-5. The successful flipping of the cis-hydrindane in 23 to the challenging trans configuration in 24 provided a firm foundation for a formal synthesis of pleurotin (1), as well as syntheses of pleurogrisein (3) and 4-hydroxypleurogrisein (4).

The boron monoxide radical has emerged as a fascinating molecule and a short-lived intermediate, previously observed only under matrix isolation conditions. In this study, we report the successful synthesis and characterization of a stable organic boron monoxide radical, achieved through the reaction of a diboron (6) dianion with nitric oxide (NO). This oxygen-centered radical is uniquely functionalized and stabilized by a triaryl-substituted boryl group. Comprehensive characterization was performed using various spectroscopic and structural techniques, including electron paramagnetic resonance (EPR) spectroscopy and single-crystal X-ray diffraction analysis. Remarkably, this oxygen-centered radical is stabilized by the K cation and exhibits significant stability under argon at room temperature, showing no self-dimerization even when heated or exposed to UV light. However, it can dimerize to form peroxide species when the K cation is fully encapsulated. Furthermore, it can mimic transition-metal complexes by mediating NO coupling to form a boryl hyponitrite (N2O2 2–) derivative. Finally, this boron monoxide radical also shows promising catalytic potential in Sn–Sn coupling reactions.

Structures containing N–O bonds are well-established precursors of nitrogen- and/or oxygen-centered radicals under visible-light conditions in modern organic synthesis. Whereas both heterolytic and homolytic scissions of N–O bonds have been extensively documented, intrinsic limitations related to substrate structure somewhat restrict their broader application. This paper highlights a novel strategy that synergistically combines a radical-generation process that is independent of the substrate’s redox potential with a radical-induced β-fragmentation of the N–O bond. Subsequent manipulation of the generated nitrogen- or oxygen-centered radicals leads to the successful development of group-transfer carboamination of alkenes, ring-opening functionalization of heterocycles, and efficient trifunctionalization of nonactivated alkenes. 1 Introduction 2 Carboamination of Nonactivated Alkenes 3 Radical-Addition-Induced Ring-Opening Functionalization of 4-Isoxazolines 4 Multisite Functionalization of Alkenes by Merging Cycloaddition and Ring-Opening Functionalization 5 Conclusion

With advances in organoboron chemistry, boron-centered functional groups have become increasingly attractive. In particular, alkylboron species are highly versatile reagents for organic synthesis, but the direct generation of alkyl radicals from commonly used, bench-stable boron species has not been thoroughly investigated. Herein, we describe a method for activating C–B bonds by nitrogen- or oxygen-radical transfer that is applicable to alkylboronic acids and esters and can be used for both Michael addition reactions and Minisci reactions to generate alkyl or arylated products.

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The generation of superoxide radical anion in biological systems is one of the major initiating events in the redox biology of NADPH oxidases and mitochondrial redox signalling. However, the pallette of chemical tools for superoxide detection is very limited, hampering progress in understanding the chemical biology of superoxide. Although EPR spin trapping is regarded as the most rigorous technique for superoxide detection, rapid reduction of the EPR-active superoxide spin adducts to EPR-silent hydroxylamines, or to hydroxyl radical adducts by bioreductants, significantly limits the applicability of this technique in biological systems. To overcome these limitations, in this work, we report the synthesis and characterization of a new mesoporous silica functionalized with a phosphorylated cyclic spin trap (DIPPMPO nitrone). The DIPPMPO-grafted silica is a versatile spin-trap agent enabling the identification of a wide range of carbon or oxygen-centered transient radicals in organic and in aqueous media. Moreover, superoxide was efficiently trapped under in vitro conditions in both cell-free and cellular systems. The generated superoxide adduct exhibited an exceptional half-life of 3.5 h and a resistance toward bioreductant agents such as glutathione for several hours.

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Radical reactions in water or aqueous media are important for organic synthesis, realizing high-yielding processes under non-toxic and environmentally friendly conditions. This overview includes (i) a general introduction to organic chemistry in water and aqueous media, (ii) synthetic approaches in, on, and with water as well as in heterogeneous phases, (iii) reactions of carbon-centered radicals with water (or deuterium oxide) activated through coordination with various Lewis acids, (iv) photocatalysis in water and aqueous media, and (v) synthetic applications bioinspired by naturally occurring processes. A wide range of chemical processes and synthetic strategies under different experimental conditions have been reviewed that lead to important functional group translocation and transformation reactions, leading to the preparation of complex molecules. These results reveal how water as a solvent/medium/reagent in radical chemistry has matured over the last two decades, with further discoveries anticipated in the near future.

Photocatalysis is a promising method for in situ water pollution remediation but faces challenges due to the limited natural light intensity. Herein, we achieved highly-efficient photocatalytic removal of organic pollutants even under ultra-low light intensities of only 0.1 mW cm−2. This was accomplished by developing and effectively stabilizing novel reactive species, oxygen-centered organic radicals (OCORs), which have an impressive half-life of up to seven minutes in water. With lifetimes that are 8 to 11 orders of magnitude longer than for traditional transient radicals, OCORs can effectively wait for pollutants to diffuse, enabling them to remove organic pollutants through polymerization and degradation pathways. The mechanism behind the stability of OCORs lies in the enhanced electron-withdrawing ability of the electron acceptor and the extended conjugation of the catalyst, which effectively prevent back electron transfer. This study provides a theoretical foundation for practical applications of photochemistry in pollution remediation.

Development of environmentally benign methods for CP bond formation is of great significance due to the extensive applications of organophosphorus compounds in pharmaceuticals and agrochemicals. Herein, we report a polyoxovanadate‐based Cu–organic framework, [Cu 3 (pty) 2 ][V 8 O 23 ]·H 2 O ( Cu‐POV , pty =  4′‐(pyridin‐4‐yl)‐2,2′:6′,2″‐terpyridine), for catalyzing the cross‐dehydrogenative coupling of N‐aryl tetrahydroisoquinolines with diarylphosphine oxides to form CP bonds using molecular oxygen (O 2 ) as an oxidant in green ethanol medium. The outstanding efficiency of Cu‐POV stems from a synergistic mechanism involving its structural Cu II and V V centers: the two activate the N‐aryl tetrahydroisoquinolines through single‐electron transfer and subsequently react with O 2 to generate the key superoxide radical species. The catalyst can be recycled at least six times without compromising performance, and applied in gram‐scale reaction with a turnover number of 2025.

Conspectus Conjugated porous polymers (CPPs), featuring π-conjugation systems, freedom in molecular structural design, and intrinsic porosity, have emerged as a modular platform for visible-light-driven organic synthesis. At present, their photocatalytic efficiency is limited by incomplete absorption of visible light, inefficient charge separation, and inadequate management of oxygen-active species, urging the field to explore solutions. Light absorption can be strengthened by molecular engineering strategies, e.g., extension of π-conjugation, adjustment of donor–acceptor units, and incorporation of chromophores, e.g., triazine and phenothiazine, that redshift and thus broaden the absorption. Charge separation can intensify by integration of donor–acceptor segments and π-bridged linkers to cut exciton binding energy and extend lifetime of carriers; migration of charge carriers can be more directed by introduction of polar substituents and localized dipoles. Along with modifying the bandgap structure, modulation of the catalytic microenvironment can shape selective substrate activation, for instance, framework rigidification, control of electronic structure of active sites, and spatial confinement of intermediates. In terms of handling oxygen-active species, we can regulate charge distribution and electronic structure within the conjugated backbone. This regulation enhances formation of reactive intermediates such as superoxide, hydroxyl radical, and other essential oxygen-derived species to drive oxidative photocatalytic processes. Together, these approaches establish a coherent design scheme to develop high-performance, metal-free photocatalysts for diverse organic synthesis and sets a foundation for future sustainable catalysis and synthesis of photoresponsive materials.

Oxygen vacancy-enriched N/P co-doped cobalt ferrite (NPCFO) was synthesized using ionic liquid as N and P sources, and then the catalytic performance and mechanism of NPCFO upon peroxymonosulfate (PMS) activation for the degradation of organic pollutants were investigated. The as-synthesized NPCFO-700 exhibited excellent catalytic performance in activating PMS, and the degradation rate constant of 4-chlorophenol (4-CP) increased with the increase of OV concentration in NPCFO-x. EPR analysis confirmed the existence of ·OH, SO4·-, and 1O2 in the NPCFO-700/PMS system, in which OV could induce the generation of 1O2 by PMS adsorption and successive capture, and also served as electronic transfer medium to accelerate the redox cycle of M2+/M3+ (M denotes Co or Fe) for the generation of radical to synergistically degrade organic pollutants. In addition, the contribution of free radical and nonradical to 4-CP degradation was observed to be strongly dependent on solution pH, and SO4·- was the major ROS in 4-CP degradation under acid and alkaline condition, while 1O2 was involved in the degradation of 4-CP under neutral condition due its selective oxidation capacity, as evidenced by the fact that such organic pollutants with ionization potential (IP) below 9.0 eV were more easily attacked by 1O2. The present study provided a novel insight into the development of transition metal-based heterogeneous catalyst containing massive OV for high-efficient PMS activation and degradation of organic pollutants.

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During the last decade several research groups have been developing electrochemical procedures to access highly functionalized organic molecules. Among the most exciting advances, the possibility of using free radical chemistry has attracted the attention of the most important synthetic groups. Nowadays, electrochemical strategies based on these species with a synthetic purpose are published continuously in scientific journals, increasing the alternatives for the synthetic organic chemistry laboratories. Free radicals can be obtained in organic electrochemical reactions; thus, this review reassembles the last decade's (2010–2020) efforts of the electrosynthetic community to generate and take advantage of the C‐, O‐, and N‐centered radicals’ reactivity. The electrochemical reactions that occur, as well as the proposed mechanism, are discussed, trying to give clear information about the used conditions and reactivity of these reactive intermediate species.

Singlet oxygen and superoxide anion radicals represent two key active species in photocatalytic reactions. However, most conventional catalysts depend exclusively on one of these species or necessitate altered reaction conditions to access different active intermediates. In this work, we designed and synthesized three novel two-dimensional polyimide-based covalent organic frameworks (COFs), in which the electron-donating phthalocyanine and electron-accepting benzothiadiazole units are closely stacked, forming an alternating π-column architecture. These donor-acceptor COFs exhibit pronounced spatial charge separation, which not only promotes charge-carrier separation and electron transport but also narrows the energy gap between singlet and triplet excited states, enhances intersystem crossing (ISC) efficiency, and enables the simultaneous and efficient execution of both electron-transfer and energy-transfer processes. Consequently, this study not only expands the potential for functionalizing highly stable 2D COFs but also offers new perspectives on the conversion of solar energy into chemical energy using COF-based systems.

The photoinduced generation of a superoxide anion on the surface of a semiconductor photocatalyst is usually attributed to the reduction of O2 with conduction-band electrons. In the current work, the reaction of TiO2 with O2 giving rise to TiO4 in superoxide and peroxide states has been investigated with ab initio (CAS, CCSD) and DFT (B3LYP) calculations. The ground triplet state and two substates (open-shell singlet (OSS) and closed-shell singlet (CSS)) of a doubly degenerate excited singlet state (a1Δg) are considered as reactive states of oxygen, participating in spontaneous or photoinduced processes, respectively. The triplet and OSS singlet states of TiO4 contain O2- as structural units and can be defined as titanium superoxides. Both states have energy less than the level of the initial pair TiO2+O2 by about 30 kcal/mol. The CSS state of TiO4 has a diperoxide structure Ti4+(O22-)2 and also lies in energy below the initial pair TiO2+3O2. Titanium superoxide is considered to be the carrier of an "exceptionally stable" and "long-lived" superoxide anion, which was earlier synthesized or detected on the surface of TiO2. The low-energy location of the conical intersections on the way from reagents to 3TiO4 allows us to explain the literature data on the spontaneous generation of the "long-lived" superoxide anion on the TiO2 surface.

Abstract Recent experimental investigations demonstrated the generation of singlet oxygen during charging at high potentials in lithium/oxygen batteries. To contribute to the understanding of the underlying chemical reactions a key step in the mechanism of the charging process, namely, the dissociation of the intermediate lithium superoxide to oxygen and lithium, was investigated. Therefore, the corresponding dissociation paths of the molecular model system lithium superoxide (LiO2) were studied by CASSCF/CASPT2 calculations. The obtained results indicate the presence of different dissociation paths over crossing points of different electronic states, which lead either to the energetically preferred generation of triplet oxygen or the energetically higher lying formation of singlet oxygen. The dissociation to the corresponding superoxide anion is energetically less preferred. The understanding of the detailed reaction mechanism allows the design of strategies to avoid the formation of singlet oxygen and thus to potentially minimize the degradation of materials in alkali metal/oxygen batteries. The calculations demonstrate a qualitatively similar but energetically shifted behavior for the homologous alkali metals sodium and potassium and their superoxide species. Fundamental differences were found for the covalently bound hydroperoxyl radical.

Near-infrared light excitable triplet-triplet annihilation upconversion (NIR TTA-UC) materials have attracted interest in a variety of emerging applications such as photoredox catalysis, optogenetics, and stereoscopic 3D printing. Currently, the practical application of NIR TTA-UC materials requires substantial improvement in photostability. Here, we found that the new annihilator of π-expanded diketopyrrolopyrrole (π-DPP) cannot activate oxygen to generate superoxide anion via photoinduced electron transfer, and its electron-deficient characteristics prevent the singlet oxygen-mediated [2 + 2] cycloaddition reaction; thus, π-DPP exhibited superior resistance to photobleaching. In conjunction with the NIR photosensitizer PdTNP, the upconversion efficiency of π-DPP is as high as 8.9%, which is eight times of the previously reported PdPc/Furan-DPP. Importantly, after polystyrene film encapsulation, less than 10% photobleaching was observed for this PdTNP/π-DPP-based NIR TTA-UC material after four hours of intensive NIR light exposure. These findings provide a type of annihilator with extraordinary photostability, facilitating the development of NIR TTA-UC materials for practical photonics.

Six polypyridyl Ru(II) complexes were designed for single‐molecule photodynamic and sonodynamic therapy (PDT/SDT) synergistic multimodal anticancer toward cisplatin‐resistant NSCLC. They demonstrated lowest 3ES with distinct intraligand transition nature, which is beneficial for singlet oxygen generation. Remarkable quantum yields of both singlet oxygen and superoxide anion under either 808 nm laser irradiation or ultrasonic treatment and could induce apoptosis and ferroptosis of A549R cells. Cytotoxicity experiments clearly demonstrated a synergistic effect between PDT and SDT. The relationship between the structures of these complexes and their cellular biological mechanisms has been explored in detail. Using a single‐molecule sensitizer to achieve synergistic PDT/SDT may provide valuable insights for the treatment of drug‐resistant tumors that located deeply and in hypoxic microenvironment.

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Type I photosensitizers (PSs) with the ability to generate reactive oxygen species (ROS) containing superoxide anion and hydroxyl radical have promising application potential for treating hypoxia tumors, but the deep mechanism of type II ROS converts to the type I ROS in the PSs is still unclear, it is urgent to reveal influencing factors about inducing type I ROS generation. Herein, six PSs with aggregation‐induced emission properties, which were fabricated with the same electronic acceptor but different electronic donors and “π‐bridge”, have been successfully prepared to explore the influencing mechanism of generating superoxide anion and hydroxyl radical from organic PSs. Experimental results discovered two factors containing molecular structure and aggregated environment could decide the ROS efficiency and types of PSs. On the level of designing molecular structure, we discovered that “π‐bridge” with a lower energy level of the lowest triplet state could be beneficial for triggering the production of superoxide anion, and electronic donor of triphenylamine was an important factor in producing hydroxyl radical than another donor of dimethylamine. On the level of designing aggregates of PS‐based polymeric nanoparticles, bovine serum albumin could improve largely the generation efficiency of superoxide anion. Due to the satisfactory ROS efficiency and better biocompatibility, synthetic PSs showed excellent photodynamic therapy outcomes in vitro/vivo.

Type-I photodynamic therapy (PDT) with less oxygen consumption shows great potential for overcoming the vicious hypoxia typically observed in solid tumors. However, the development of type-I PDT is hindered by insufficient radical generation and the ambiguous design strategy of type-I photosensitizers (PSs). Therefore, developing highly efficient type-I PSs and unveiling their structure-function relationship are still urgent and challenging. Herein, we develop two phenanthro[9,10-d]imidazole derivatives (AQPO and AQPI) with aggregation-induced emission (AIE) characteristics and boost their reactive oxygen species (ROS) generation efficiency by reducing singlet-triplet splitting (ΔEST). Both AQPO and AQPI show ultrasmall ΔEST values of 0.09 and 0.12 eV, respectively. By incorporating electron-rich anisole, the categories of generated ROS by AIE PSs are changed from type-II (singlet oxygen, 1O2) to type-I (superoxide anion radical, O2•- and hydroxyl radical, •OH). We demonstrate that the assembled AQPO nanoparticles (NPs) achieve a 3.2- and 2.9-fold increase in the O2•- and •OH generation efficiencies, respectively, compared to those of AQPI NPs (without anisole) in water, whereas the 1O2 generation efficiency of AQPO NPs is lower (0.4-fold) than that of AQPI NPs. The small ΔEST and anisole group endow AQPO with an excellent capacity for type-I ROS generation. In vitro and in vivo experiments show that AQPO NPs achieve an excellent hypoxia-overcoming PDT effect by efficiently eliminating tumor cells upon white light irradiation with good biosafety.

Carbon dots (CDs) have emerged as promising nanomaterials for bioimaging-guided photodynamic therapy (PDT). However, designing red-emissive CDs (RCDs) with tunable type I and type II reactive oxygen species (ROS) generation to simultaneously meet PDT applications in aerobic and hypoxic scenarios still remain major challenges. Herein, three types of RCDs with maximum emission at approximately 680 nm are successfully prepared. It is noteworthy that they exhibit the adjustable ROS production with equal superoxide anion (via type I PDT) and incremental singlet oxygen (via type II PDT). Detailed structural and optical characterizations along with theoretical calculation reveal that the unique type I/II ROS formation mainly depends on the core sizes and surface states of RCDs, which determine their identical redox potentials and tapering energy gaps between singlet- and triplet states, respectively. Additionally, due to the inherent mitochondria targeting capability, RCDs enable themselves to induce cell programmed death via activating mitochondrion-mediated apoptotic pathways. This work exploits the unprecedented RCDs with tunable type I and type II ROS generation that could ensure highly efficient tumor eradication both in vitro and in vivo, even under the harsh tumor microenvironment, providing a new prospect for CDs as nanophotosensitizers to conquer the limitations of single type PDT.

The oxygen molecule in its ground triplet state (3O2) is a strong electron acceptor. Electron transfer to 3O2 to form a superoxide anion is an important elementary step in many chemical and biological processes. If this transfer occurs from a spin 1/2 paramagnetic particle where the total spin of the reactants is equal to 3/2, the reaction is spin-forbidden. In liquids, the significant dipole-dipole electron spin interaction in 3O2 is supposed to mix the non-reactive quartet and reactive doublet states at a time scale of ∼10 ps, thus avoiding the barrier. To elucidate the role of spin effects in the electron transfer to 3O2, we studied this reaction over a range of more than three orders of magnitude of the relative diffusion coefficient (D) of the reactants. It was found that spin effects during electron transfer to 3O2 become insignificant when D < 10-9 m2 s-1. In the range of intermediate D values (10-9 m2 s-1 < D < 10-8 m2 s-1) - which corresponds to some reactions of oxygen with small radicals in aqueous solutions - the effective spin factor decreases with increasing D value. If D > 10-8 m2 s-1, the electron transfer is spin-selective with the spin factor of 1/3 as determined by the spin statistics. At such D values, the reaction encounter time may exceed the expected quartet-doublet mixing time by almost an order of magnitude. The reduced rate of quartet-doublet transitions within the encounter complex in the reaction with 3O2 has been explained by the spin-exchange interaction and chemical Zeno effect.

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The collision complex between the ground electronic state of an organic molecule, M, and ground state oxygen, O2(X3Σg-), can absorb light to produce an intermolecular charge transfer (CT) state, often represented simply as the M radical cation, M+˙, paired with the superoxide radical anion, O2-˙. Aspects of this transition have been the subject of numerous studies for ∼70 years, many of which address fundamental concepts in chemistry and physics. We now examine the extent to which the combination of Molecular Dynamics simulations and electronic structure response methods can model transitions to the toluene-O2 CT state. To account for the experimental spectra, we consider (a) the distribution of toluene-O2 geometries that contribute to the transitions, (b) a quantitative description of intermolecular CT, and (c) oxygen-induced local transitions in toluene that complement the CT transitions, specifically transitions that populate toluene triplet states. We find that the latter oxygen-induced local transitions play a prominent role on the long wavelength side of the spectrum commonly attributed to the intermolecular CT transition. Our calculations provide a new perspective on the seminal discussion between R. S. Mulliken and D. F. Evans on the nature of O2-dependent transitions in organic molecules, and bode well for modeling transitions to excited states with CT character in noncovalent weakly-bonded molecular complexes.

Photodynamic therapy (PDT) using oxygen-dependent type II photosensitizers is frequently limited by the hypoxic microenvironment of solid tumors. Type I photosensitizers show oxygen-independent reactive oxygen species (ROS) generation upon light irradiation but still face the challenges of aggregation-caused quenching (ACQ) and low efficiency to produce ROS. Herein, we first prepare an efficient type I photosensitizer from a perylene derivative via intramolecular donor-acceptor binding and sulfur substitution, which significantly enhance intersystem crossing between singlet and triplet states and electron transfer capability. After reaction with a type II photosensitizer, the covalent organic framework (COF) nanophotosensitizer is formed with alternated type I and II photosensitizer motifs in the same layer and staggered AB stacking between layers to avoid ACQ. The nanophotosensitizer exhibits high-efficiency generation of singlet oxygen (1O2) and superoxide anion radicals (O2•-) via type I and II mechanism under normoxia upon exposure to light irradiation. Under hypoxia, massive O2•- can be produced continuously. The potent ROS generation capability results in efficient cellular apoptosis and immunogenic cell death (ICD) efficiently. After combination with immune checkpoint inhibitors, tumor immunosuppressive microenvironment is reversed, which effectively ablates bulky hypoxic primary tumors and suppresses metastases via photodynamic immunotherapy. The COF nanophotosensitizers with staggered type I and II photosensitizer motifs represent a promising strategy to boost photodynamic immunotherapy of hypoxic tumors.

The development of photosensitizers that function effectively in hypoxic environments and enable deep-tissue treatment remains a significant challenge in photodynamic therapy (PDT). Here, we report two novel Ir(III) complexes functionalized with fluorescein designed as efficient Type I photosensitizers for both light-driven PDT and X-ray-induced PDT (X-PDT). By populating the triplet state of the fluorescein ligands, these complexes facilitate the generation of reactive oxygen species (ROS) through electron transfer, producing superoxide anion radicals (O2•-) and hydroxyl radicals (•OH) under irradiation. The complexes exhibit pronounced phototoxicity against cancer cells, particularly under hypoxic conditions, where oxygen-dependent Type II photosensitizers are less effective. Remarkably, these complexes also demonstrate direct X-ray activation, offering a solution for deep-tissue cancer treatment. The lead complex, PS1, outperforms existing systems by efficiently generating both singlet oxygen O2(1Δg) and free radicals, enabling synergistic Type I and II PDT effects. This work represents a major advancement in the design of oxygen-independent PDT agents by using fluorescein's triplet state, with potential applications in deep-tissue and hypoxic tumor environments.

Based on the cyanine dye scaffold, two photosensitizers (PSs) (C1 and C2) were successfully synthesized. These PSs exhibited a maximum absorption wavelength of 660 nm, falling within the near-infrared window. Compared to the commercially available photosensitizer ICG, the newly developed PSs demonstrated enhanced reactive oxygen species (ROS) generation under lower light doses. Notably, these PSs can simultaneously produce both singlet oxygen (1O2) and superoxide anion radicals (O2˙−), classifying them as dual type I/II PSs. Compared with C1, the 1O2 quantum yield of C2 was higher, as high as 10.6 times that of ICG. Theoretical calculations revealed that the molecules of C2 possessed a small singlet–triplet energy gap (ΔES–T), which facilitated more efficient intersystem crossing (ISC) from the singlet to triplet state, thereby promoting greater ROS generation. Experiments showed that C1 and C2 were located in mitochondria and could cause a decrease in mitochondrial membrane potential, leading to cell death. Animal experiments have shown that C2 effectively suppressed tumor growth without side effects.

Humic acid (HA) as a vital component of dissolved organic matter can be photochemically driven to produce reactive oxygen species (ROS), which are essential for aquatic pollutant degradation. However, the role of organic acids (e.g., oxalic acid) in HA-induced photochemical ROS production remains poorly understood. Herein, we found that the addition of 100 μM oxalic acid to 16 mgC L-1 HA enhanced hydroxyl radical (•OH) formation rates by 6.5-fold. By combining electron paramagnetic resonance, photoelectrochemical measurements, laser flash photolysis, and density functional theory calculations, this study revealed that oxalic acid interacted with triplet-state HA to accelerate electron transfer, promoting the formation of carbon dioxide radical anion and HA-derived reducing intermediates (e.g., ketyl- and semiquinone-like radicals). These intermediates readily reacted with O2 to generate superoxide radical and hydrogen peroxide, which ultimately promoted •OH formation. Furthermore, a dual-pathway involving •OH and reductive free radicals induced both oxidative and reductive degradation of halogenated pollutants. Overall, these findings provide new insights into the inter-fractional interactions of natural organic matter under solar irradiation, and underscore the previously unappreciated contribution of reductive free radicals to ROS formation and pollutant transformation in acidic wastewaters.

Photodynamic therapy (PDT) requires photosensitizers (PSs) that efficiently generate reactive oxygen species (ROS) or otherwise induce cell damage, even under the hypoxic conditions typical of solid tumor environments. Here, we report a heavy-atom-free octupolar donor-acceptor dye, 3CN, designed with a triphenylamine core and three cyanovinyl-benzothiazole acceptors. This dye harnesses aggregation-induced intersystem crossing (AI-ISC) to promote efficient triplet state population. In aqueous solutions, 3CN spontaneously self-assembles into stable nanoaggregates (3CNAgg) that exhibit excellent colloidal stability and prolonged triplet-state lifetimes. These aggregates demonstrate superior Type I photosensitization capabilities, generating the superoxide anion radical (O2˙-) with significantly greater efficiency under white light irradiation than the reference PS, rose bengal. In encapsulated formulation, 3CNEn, exhibits remarkable therapeutic efficacy with IC50 values of 8.6 μM against triple-negative human breast cancer cells (MDA-MB-231), while maintaining potent photocytotoxicity even under severely hypoxic conditions (1% O2). In vivo studies using mouse models bearing MDA-MB-231 tumors demonstrate 100% inhibition of tumor growth with minimal dark cytotoxicity and inherent tumor-targeting properties.

Photodynamic therapy (PDT) still confronts substantial challenges in treating hypoxic tumors within deep-seated tissues, including the lack of oxygen-independent Type I photosensitizers and design principles. In this work, a series of two-photon photosensitizers of naphthalimide-based derivatives are designed by introducing furan/thiophene at the 4-position of NS (1S) and thio/selenocarbonyl modifications and synthesis routes are suggested. Theoretical studies by density functional theory (DFT) suggested that the compounds 1Se-furan2 and 1Se-furan3 exhibit exceptional photodynamic properties including large two-photon absorption cross sections (262.02/183.16 GM in the 650-900 nm therapeutic window), prolonged triplet state lifetimes (828/4487 μs), optimal lipophilicity (logP = 4.53/4.38), and exclusive superoxide anion radical generation via the Type I mechanism with low oxygen dependence. More importantly, the synergistic regulation mechanism of the two-photon response characteristics and type I/II reaction pathways of naphthalimide by heterocyclic substitution and thio/selenocarbonyl modifications is elucidated. A theoretical framework is also presented for the development of two-photon photosensitizers that preferentially undergo Type I reactions, thereby providing stronger tissue penetration and reducing the risk of photodamage.

Cyclometallated rhodium(iii) complexes have been underexplored as photosensitisers due to their low-lying d-d excited states, which result in weak visible-light absorption and non-emissive properties, coupled with a modest heavy atom effect that limits reactive oxygen species (ROS) generation. In this work, a series of cyclometallated rhodium(iii) polypyridine complexes appended with two rhodamine units [Rh(N^C)2(bpy-diRho)](PF6)3 was rationally designed as Type I photosensitisers. These complexes exhibited intense absorption in the visible region and moderate rhodamine fluorescence in solution upon photoexcitation. Time-resolved transient absorption spectroscopy revealed a long-lived rhodamine-based triplet excited state as the lowest-lying excited state in this hybrid system, which is attributed to the presence of the rhodium(iii) centre and is responsible for ROS photosensitisation. Notably, these rhodium(iii) complexes efficiently generated superoxide anion (O2˙-) and hydroxyl (HO˙) radicals via the Type I pathway upon photoirradiation, likely via intramolecular electron transfer between the two adjacent excited rhodamine units within the complex to form radical cation and anion. Cellular colocalisation studies demonstrated that these complexes predominantly accumulated in mitochondria, where the photosensitised ROS triggered significant mitochondrial dysfunction, resulting in their outstanding photocytotoxicity under both normoxic and CoCl2-induced hypoxic conditions. Further mechanistic investigations revealed that the photoinduced mitochondrial ROS generation triggered cancer cell death via gasdermin D-mediated pyroptosis. This rhodium(iii)-dirhodamine system further explores the utilisation of rhodium(iii) complexes as phototheranostic agents and underscores their potential in this role.

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In recent years, the development of photocatalysts based on noncovalent strategies has shown an important role in medical and organic materials. Herein, an organic fluorescent dye benzothiazole derivative (2‐(N,N‐diethylanilin‐4‐yl)‐4,6‐bis(3‐methylpyrazol‐1‐yl)‐1,3,5‐triazine [MPBT]) was designed and synthesized. It was encapsulated in the cavity of cucurbit[8]uril (CB[8]) to form a supramolecular dimer through host–guest interaction, which converted the dye into a highly efficient photocatalyst. With the formation of 2MPBT‐CB[8] supramolecular dimer, the emergence of host‐enhanced charge transfer interactions could significantly facilitate singlet to triplet through intersystem crossing. At the same time, the alternating structure of 2MPBT‐CB[8] facilitated the triplet states for further energy transfer and electron transfer. In addition, the electron transfer process with electron donor generated cationic free radical and photocatalyst negative ion free radical (), which in turn reacted with oxygen (O2) to form superoxide anion radical (). The generated could be used to catalyze the oxidative hydroxylation of aryl boronic acid. Therefore, the 2MPBT‐CB[8] had become a highly efficient photocatalyst for the oxidative hydroxylation of aryl boronic acid. This strategy of supramolecular dimerization provides a new strategy for the development of new photocatalysts based on noncovalent interactions.

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A 1:1 complex between 3,3'-dihydroxyflavone (DHF) and La(III) (DHF-La(III)) is formed in methanolic solution with the relatively high apparent stability constant value of 2.3×10(6) and a calculated standard entropy change of 88.2 J mol(-1) K(-1), both at 25 °C. The photophysical properties of the complex and the free flavonoid are discussed in comparison to the well known related compound 3-hydroxyflavone. The ligand photogenerates O2((1)Δg) by energy transfer from its excited triplet state ((3)DHF(*)) to dissolved ground state oxygen, with a quantum yield of 0.13. (3)DHF(*) is quenched by La(III) with a rate constant close to the diffusion-controlled value. The respective abilities of the free flavonoid and DHF-La(III) as quenchers of the riboflavin-photogenerated reactive oxygen species singlet molecular oxygen (O2((1)Δg)) and superoxide radical anion (O2(-)) have been investigated. Both individual compounds were photoirradiated with visible light in the presence of the flavin as the only light-absorbing compound. A detailed kinetics and mechanistic study employing polarographic monitoring of oxygen uptake and time resolved detection of O2((1)Δg) phosphorescence indicates that DHF and the complex react with O2((1)Δg) and O2(-) by a non simple mechanism. The former deactivates O2((1)Δg) in a predominant physical fashion, a fact that constitutes a desirable property for antioxidants. It was found that metal chelation greatly enhances the ability of DHF as an overall O2((1)Δg) quencher.

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Kinetics and mechanism of photoprocesses generated by visible light‐irradiation of the system riboflavin (Rf, vitamin B2) plus Thiamine (Th) and Thiamine pyrophosphate (ThDP), representing vitamin B1, was studied in pH 7 water. A weak dark complex vitamin B2–vitamin B1, with a mean value of 4 ± 0.4 m−1 is formed. An intricate mechanism of competitive reactions operates upon photoirradiation, being the light only absorbed by Rf. Th and ThDP quench excited singlet and triplet states of Rf, with rate constants in the order of 109 and 106 m−1 s−1, respectively. With Vitamin B1 in a concentration similar to that of dissolved molecular oxygen in water, the quenching of triplet excited Rf by the latter is highly predominant, resulting in the generation of O2(1Δg). Superoxide radical anion was not detected under work conditions. A relatively slow O2(1Δg)‐mediated photodegradation of Th and ThDP was observed. Nevertheless, Th and especially ThDP behave as efficient physical deactivators of O2(1Δg). The thiazol structure in vitamin B1 appears as a good scavenger of this reactive oxygen species. This characteristic, that presents at vitamin B1 as a potential photoprotector of biological entities against O2(1Δg) attack, was been experimentally confirmed employing the protein lisozime as a photo‐oxidizable target.

Riboflavin (Rf) is an endogenous photosensitizer, which can participate in Type I and Type II processes. We have recently shown that the yield of the triplet excited states of Rf is enhanced in the presence of pectin‐coated silver nanoparticles (Pec@AgNP) due to formation of a complex between Rf and Pec@AgNP (Rf‐Pec@AgNP). Consequently, under aerobic conditions, the amounts of singlet molecular oxygen and superoxide radical anion generated are also larger in the presence of the nanoparticles. This result made us suspect that the nanoparticles could have a beneficial effect in Rf‐based PDT. To prove this hypothesis, we here compared the photodamage in HeLa cells incubated with Rf in the presence and in the absence of Pec@AgNP applying several optical assays. We used fluorescence imaging of irradiated HeLa cells incubated with Annexin V and propidium iodide to evaluate the occurrence of apoptosis/necrosis, the reduction of the tetrazolium dye MTT to formazan and neutral red uptake to prove cell viability, as well as synchrotron infrared microscopy of single cells to evaluate possible structural changes of DNA and nuclear proteins. The enhanced photodamage observed in the presence of Pec@AgNP seems to indicate that Rf enters into the cells complexed with the nanoparticles.

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Although the activation of inert C-H bonds by metal-oxo complexes has been widely studied, important questions remain, particularly regarding the role of oxygen spin population (i.e., unpaired electrons on the oxo ligand) in facilitating C-H bond cleavage. In order to shed light on this issue, we have utilized 17O electron nuclear double resonance spectroscopy to measure the oxygen spin populations of three compound I intermediates in heme enzymes with different reactivities toward C-H bonds: chloroperoxidase, cytochrome P450, and a selenolate (selenocysteinyl)-ligated cytochrome P450. The experimental data suggest an inverse correlation between oxygen spin population and electron donation from the axial ligand. We have explored the implications of this result using a Hückel-type molecular orbital model and constrained density functional theory calculations. These investigations have allowed us to examine the relationship between oxygen spin population, oxygen charge, electron donation from the axial ligand, and reactivity.

Abstract Reactive oxygen species (ROS) is a kind of single electron reduction product of oxygen in vivo. Because they contain unpaired electrons, ROS has high chemical reactivity. Various researches of ROS are focused on DNA damage, cell apoptosis, oxidative stress. Actually, ROS is also closed related to immune regulation. Based on this, we review the research in immune regulated ROS production, regulation of ROS on inflammatory factors and inflammatory response, antigen presentation and macrophage polarization for better understanding of its role.

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The reactivity of the molybdenum oxide cluster anion (MoO 3 ) 5 O - , bearing an unpaired electron at a bridging oxygen atom (O b •- ), towards methane under thermal collision conditions has been studied by mass spectrometry and density functional theory calculations. This reaction follows the mechanism of hydrogen atom transfer (HAT) and is facilitated by the O b •- radical center. The reactivity of (MoO 3 ) 5 O - can be traced back to the appropriate orientation of the lowest unoccupied molecular orbitals (LUMO) that is essentially the 2p orbital of the O b •- atom. This study not only makes up the blank of thermal methane activation by the O b •- radical on negatively charged clusters but also yields new insights into methane activation by the atomic oxygen radical anions.

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Abstract Carbon materials hold significant promise in electrocatalysis, particularly in electrochemical CO2 reduction reaction (eCO2RR) and two‐electron oxygen reduction reaction (2e− ORR). The pivotal factor in achieving exceptional overall catalytic performance in carbon catalysts is the strategic design of specific active sites and nanostructures. This work presents a comprehensive overview of recent developments in carbon electrocatalysts for eCO2RR and 2e− ORR. The creation of active sites through single/dual heteroatom doping, functional group decoration, topological defect, and micro‐nano structuring, along with their synergistic effects, is thoroughly examined. Elaboration on the catalytic mechanisms and structure‐activity relationships of these active sites is provided. In addition to directly serving as electrocatalysts, this review explores the role of carbon matrix as a support in finely adjusting the reactivity of single‐atom molecular catalysts. Finally, the work addresses the challenges and prospects associated with designing and fabricating carbon electrocatalysts, providing valuable insights into the future trajectory of this dynamic field.

In the antioxidant activity of quercetin (Q), stabilization of the energy in the quercetin radical (Q•) by delocalization of the unpaired electron (UE) in Q• is pivotal. The aim of this study is to further examine the delocalization of the UE in Q•, and to elucidate the importance of the functional groups of Q for the stabilization of the UE by combining experimentally obtained spin resonance spectroscopy (ESR) measurements with theoretical density functional theory (DFT) calculations. The ESR spectrum and DFT calculation of Q• and structurally related radicals both suggest that the UE of Q• is mostly delocalized in the B ring and partly on the AC ring. The negatively charged oxygen groups in the B ring (3′ and 4′) of Q• have an electron-donating effect that attract and stabilize the UE in the B ring. Radicals structurally related to Q• indicate that the negatively charged oxygen at 4′ has more of an effect on concentrating the UE in ring B than the negatively charged oxygen at 3′. The DFT calculation showed that an OH group at the 3-position of the AC ring is essential for concentrating the radical on the C2–C3 double bond. All these effects help to explain how the high energy of the UE is captured and a stable Q• is generated, which is pivotal in the antioxidant activity of Q.

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To achieve high efficiency and low consumption for water treatment in the Fenton reaction, we use the surface oxygen vacancies (OVs) as the electron temporary residences to construct a dual-reaction-center (RDC) Fenton-like catalyst with abundant surface electron-rich/poor areas consisting of OV-rich Co-ZnO microparticles (OV-CoZnO MPs). The lattice-doping of Co into ZnO wurtzite results in the formation of OVs with unpaired electrons (electron-rich OVs) and electron-deficient Co3+ sites according to the structural and electronic characterizations. Both experimental and theoretical calculations prove that the electron-rich OVs are responsible for the capture and reduction of H2O2 to generate hydroxyl radicals, which quickly degrades pollutants, while a large amount of pollutants are adsorbed at the electron-deficient Co3+ sites and act as electron donors for the system, accompanied by their own oxidative degradation. The electrons obtained from the pollutants in the electron-deficient sites are transferred to the OVs through the internal bond bridge to achieve the balance of electron gain/loss. Through this process, pollutants are efficiently converted and degraded by multiple pathways in a wide range of pH (4.5-9.5). The reaction rate of the OV-CoZnO MPs/H2O2 system is increased by ~17 times compared with the non-DRC system. This discovery provides a sustainable strategy for pollutant utilization, which shows new implications for solving the troublesome issues of the Fenton reaction and for developing novel environmental remediation technologies.

Electron paramagnetic resonance (EPR) spectroscopy using sterically hindered amine is extensively applied to detect singlet oxygen (1O2) possibly generated in advanced oxidation processes. However, EPR-detectable 1O2 signals were observed in not only the 1O2-dominated hydrogen peroxide (H2O2)/hypochlorite (NaClO) reaction but surprisingly also the 1O2-absent Fe(II)/H2O2, UV/H2O2, and ferrate [Fe(VI)] process with even stronger intensities. By taking advantage of the characteristic reaction between 1O2 and 9,10-diphenyl-anthracene and near-infrared phosphorescent emission of 1O2, 1O2 was excluded in the Fe(II)/H2O2, UV/H2O2, and Fe(VI) process. The false detection of 1O2 was ascribed to the direct oxidation of hindered amine to piperidyl radical by reactive species [e.g., •OH and Fe(VI)/Fe(V)/Fe(IV)] via hydrogen transfer, followed by molecular oxygen addition (forming a piperidylperoxyl radical) and back reaction with piperidyl radical to generate a nitroxide radical, as evidenced by the successful identification of a piperidyl radical intermediate at 100 K and theoretical calculations. Moreover, compared to the highly oxidative species (e.g., •OH and high-valence Fe), the much lower reactivity of 1O2 and the profound nonradiative relaxation of 1O2 in H2O resulted it too selective and inefficient in organic contaminant destruction. This study demonstrated that EPR-based 1O2 detection could be remarkably misled by common oxidative species and thereby jeopardize the understandings on 1O2.

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Nitric acid (HNO3) serves as a vital industrial raw material and ranks among the most widely manufactured chemicals. Recently, photocatalytic systems have been explored as an environmentally friendly pathway for direct N2 utilization in oxidation reactions. However, progress has been hindered by the high activation barrier of the N≡N bond and severe electron-hole recombination in catalysts. Here, we highlight that polyoxometalates (POMs)-based metal-organic frameworks (MOFs) (Mo72Cr30/UiO-66) enable highly efficient HNO3 synthesis, owing to a dual-site mechanism. The results reveal that oxygen vacancies on UiO-66 capture electrons from Mo72Cr30, inducing adjacent metals to form Zr3+ with unpaired electrons, which transfer to N2 antibonding orbitals, thereby lowering the activation barrier. Furthermore, the holes enrich on the Mo72Cr30 surface to oxidize water for generating strong oxidative reactive oxygen species hydroxyl radical (•OH), thus facilitating HNO3 production. The distinct site synergy of Mo72Cr30 and UiO-66 for N2 activation results in an exceptional activity of 646.3 μg g-1 h-1, which far surpasses the performance of Mo72Cr30 and UiO-66 alone by approximately 18-fold and 6-fold, respectively. Our study offers a novel design vision for photocatalytic materials and presents a promising approach for advancing sustainable artificial N2 fixation.

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Lipid peroxidation (LPO), the process of membrane lipid oxidation, is a potential new form of cell death for cancer treatment. However, the radical chain reaction involved in LPO is comprised of the initiation, propagation (the slowest step), and termination stages, limiting its effectiveness in vivo. To address this limitation, we introduce the radical chain transfer reaction into the LPO process to target the propagation step and overcome the sluggish rate of lipid peroxidation, thereby promoting endogenous lipid peroxidation and enhancing therapeutic outcomes. Firstly, radical chain transfer agent (CTA-1)/Fe nanoparticles (CTA-Fe NPs-1) was synthesized. Notably, CTA-1 convert low activity peroxyl radicals (ROO·) into high activity alkoxyl radicals (RO·), creating the cycle of free radical oxidation and increasing the propagation of lipid peroxidation. Additionally, CTA-1/Fe ions enhance reactive oxygen species (ROS) generation, consume glutathione (GSH), and thereby inactivate GPX-4, promoting the initiation stage and reducing termination of free radical reaction. CTA-Fe NPs-1 induce a higher level of peroxidation of polyunsaturated fatty acids in lipid membranes, leading to highly effective treatment in cancer cells. In addition, CTA-Fe NPs-1 could be enriched in tumors inducing potent tumor inhibition and exhibit activatable T1-MRI contrast of magnetic resonance imaging (MRI). In summary, CTA-Fe NPs-1 can enhance intracellular lipid peroxidation by accelerating initiation, propagation, and inhibiting termination step, promoting the cycle of free radical reaction, resulting in effective anticancer outcomes in tumor-bearing mice.

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Updating the facile chemiluminescence oxygen‐aftereffect method, most suitable for determining the rate constant (kt) of the peroxy‐radical self‐reaction (main chemiluminescence channel), pertained to considering the sensitivity of such a method toward a disturbing influence of the peroxy radicals of the initiator of the chain oxidation process. Such a disturbance may derive from the side chemiluminescent reaction, which involves peroxy radicals of both hydrocarbon and initiator. To examine the applicability and limitations of the chemiluminescence method under present scrutiny, cyclohexene was used as the model oxidizable hydrocarbon substrate. Computer simulations of the reaction and chemiluminescence kinetics have demonstrated the validity of the considered methodology at the value of the rate constant of the propagation of the overall chain process by peroxy radicals of the initiator higher than 1 m−1 s−1. Despite that the chemiluminescence time profile and the stationary level of the total chemiluminescence intensity depend on the kinetics of the side chemiluminescence channel and on the ratio of the excited‐state generation yields in the mentioned reaction channel and in the main chemiluminescence process, the value of kt assessed by the oxygen‐aftereffect method has been found independent of variation of these characteristics.

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Use of coantioxidant systems is a prospective way to increase the effectiveness of antioxidant species in tissue repair and regeneration. In this paper, we introduce a novel scheme of a reactive oxygen species (ROS) trap and neutralization during self-assembly of supramolecular melamine barbiturate material. The performed reaction chain mimics the biological process of ROS generation in key stages and enables one to obtain stable hydroperoxyl and organic radicals in a melamine barbiturate structure. Melamine barbiturate also neutralizes hydroxyl radicals, and the effectiveness of the radical trap is controlled with ROS scavenger incorporation. The number of radicals dramatically increases during light-inducing and depends on pH. The proposed scheme of the ROS trap and neutralization opens a way to the use of supramolecular assemblies as a component of coantioxidant systems and a source of organic radicals.

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Recruitment of H2O as the final donor of electrons for light-governed reactions in photosynthesis has been an utmost breakthrough, bursting the evolution of life and leading to the accumulation of O2 molecules in the atmosphere. O2 molecule has a great potential to accept electrons from the components of the photosynthetic electron transfer chain (PETC) (so-called the Mehler reaction). Here we overview the Mehler reaction mechanisms, specifying the changes in the structure of the PETC of oxygenic phototrophs that probably had occurred as the result of evolutionary pressure to minimize the electron flow to O2. These changes are warranted by the fact that the efficient electron flow to O2 would decrease the quantum yield of photosynthesis. Moreover, the reduction of O2 leads to the formation of reactive oxygen species (ROS), namely, the superoxide anion radical and hydrogen peroxide, which cause oxidative stress to plant cells if they are accumulated at a significant amount. From another side, hydrogen peroxide acts as a signaling molecule. We particularly zoom in into the role of photosystem I (PSI) and the plastoquinone (PQ) pool in the Mehler reaction.

A rapid oxygen-initiated and -regulated controlled radical polymerization was conducted under ambient temperature and atmosphere. The reaction between triethylborane and oxygen provides ethyl radicals, which initiate and mediate the radical polymerization. The controlled radical polymerization was achieved using RAFT chain transfer agents (CTA) without any process of removing oxygen, providing well-defined polymers with almost full conversion (>95 %) in a short period (15 min). High-throughput screening was used to discover the suitable conditions for various CTA and monomers. To show the versatility of this method, a polymer library containing 25 well-defined polymers with different compositions (block and statistical copolymers) and molecular weights were synthesized in 1 h via high-throughput synthesis technique. A polymer-painting technique was developed using this method, forming films with spatial control and excellent control in molecular weight and dispersity.

Copper complexes act as catalysts for redox reactions to generate reactive oxygen species that destroy biomolecules and, therefore, are utilized to design drugs including antitumor and antibacterial medicines. Especially, catalytic reaction for hydrogen peroxide decomposition is important because it includes the process for generating highly toxic hydroxyl radical, i.e., Fenton-like reaction. Considering that multicoppers/hydrogen peroxide species are the important intermediates for the redox reaction, herein a polymer having copper complexes in the side chains is designed to facilitate the formation of the intermediates by building locally concentrated state of the copper complexes. The polymer increases their catalytic activities for hydrogen peroxide decomposition and promotes reactive oxygen species' generation, eventually leading to higher antibacterial activity. This reveals the virtue of building a locally concentrated state of catalysts on polymers toward drug design with low amounts of transition metals.

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