Multi‐faceted Reactivity of Alkyltellurophenols Towards Peroxyl Radicals: Catalytic Antioxidant Versus Thiol‐Depletion Effect

Hydroxyaryl alkyl tellurides are effective antioxidants both in organic solution and aqueous biphasic systems. They react by an unconventional mechanism with ROO^. radicals with rate constants as high as 10⁷ M ^?1 s^?1 at 303 K, outperforming common phenols. The reactions proceed by oxygen atom transfer to tellurium followed by hydrogen atom transfer to the resulting RO^. radical from the phenolic OH. The reaction rates do not reflect the electronic properties of the ring substituents and, because the reactions occur in a solvent cage, quenching is more efficient when the OH and TeR groups have an ortho arrangement. In the presence of thiols, hydroxyaryl alkyl tellurides act as catalytic antioxidants towards both hydroperoxides (mimicking the glutathione peroxidases) and peroxyl radicals. The high efficiency of the quenching of the peroxyl radicals and hydroperoxides could be advantageous under normal cellular conditions, but pro‐oxidative (thiol depletion) when thiol concentrations are low.     

Mechanism of the reaction of radicals with peroxides and dimethyl sulfoxide in aqueous solution

The reactions of methyl and methylperoxyl radicals derived from dimethyl sulfoxide (DMSO) with hydrogen peroxide, peroxymonocarbonate (HCO4 (-)), and persulfate were studied. The major reaction observed for the hydroperoxides was the abstraction of the hydrogen atom by the radicals. The radicals interact with a lone pair of electrons on the peroxide to produce methanol and formaldehyde. Furthermore, the results indicate that in RO2H and RO2R', electron-withdrawing groups cause a considerable increase in the reactivity of the peroxides towards the radicals and not only towards nucleophiles. The HO2 (.)/O2 (.-) and CO3 (.-) radicals react with DMSO to produce methyl radicals. Thus, the formation of the (.)CH3 radicals in the presence of DMSO is not proof of the formation of the (.)OH radicals in the system. These reactions must be considered when radical processes, such as in biological and catalytic systems, are studied. Especially, the plausible role of HCO4 (-) ions in biological systems as a source of oxidative stress cannot be overlooked.     

The atmospheric chemistry of hydrogen peroxide: a review

This paper reviews the atmospheric chemistry of hydrogen peroxide, taking into account the formation processes of both gas-phase and aqueous H2O2, and the reactions involving hydrogen peroxide in the gas phase and in atmospheric hydrometeors. Gas-phase hydrogen peroxide mainly forms upon dismutation of the hydroperoxyl radical, a product of the reactions between atmospheric hydrocarbons, hydroxyl radicals, nitric oxide, and oxygen. Aqueous hydrogen peroxide originates from the dissolution of the gaseous one, the reduction of molecular oxygen, a series of reactions involving dissolved ozone, and the irradiation of anthraquinones, aromatic carbonyls, and semiconductor oxides. The reactions involving aqueous H2O2 are very important in the context of the chemistry of the atmosphere. They include oxidation of S(IV) to S(VI), photolysis, the Fenton reaction in the presence of Fe(II), and possibly the formation of peroxynitrous acid. Within this framework, the correlation of hydrogen peroxide with other atmospheric components and the time trends of hydrogen peroxide in the atmosphere are easily accounted for.     

Sonochemistry of aqueous solutions of amino acids and peptides. A spin trapping study

The sonolysis of argon-saturated neutral aqueous solutions of several amino acids and peptides was investigated by ESR and spin trapping. The water-soluble non-volatile spin trap, 3,5-dibromonitrosobenzene sulfonate, was found to be particularly useful for ESR and spin trapping investigations of sonochemical reactions. By comparison with analogous experiments in which hydroxyl radicals were generated by u.v.-photolysis of solutions containing hydrogen peroxide, the amino acid and peptide radicals produced by sonolysis could be identified. These observations can be explained by the reactions of hydrogen atoms and hydroxyl radicals which are the primary products of the sonolysis of water.     

Transition metal complexes with “non-innocent” ligands in the activation of hydrogen sulfide

The reactions of hydrogen sulfide with transition metal complexes containing redox-active ligands are studied. A combination of electrochemical and spectral data indicates that the one-electron process affording the hydrogen sulfide radical and monoanionic complexes is an elementary act for the most part of the reactions studied. The accessibility of the metal center in the Co, Ni, Zn, and Pt complexes allows hydrogen sulfide to preliminary coordinate to the metal followed by the inner-sphere electron transfer in the hydrogen sulfide-metal-organic ligand system. Active intermediates (radical cation, thiyl radical, and proton) formed due to oxidation react with aromatic substrates. The substitution reaction in the aromatic ring produces a mixture of isomeric thiols and dimerization products of organylthiyl radicals (disulfides).     

Water Activation for Boosting Electrochemiluminescence

In conventional luminol electrochemiluminescence (ECL) systems, hydrogen peroxide and dissolved oxygen are employed as typical co-reactants to produce reactive oxygen species (ROS) for efficient ECL emission. However, the self-decomposition of hydrogen peroxide and limited solubility of oxygen in water inevitably restrict the detection accuracy and luminous efficiency of luminol ECL system. Inspired by ROS-mediated ECL mechanism, for the first time, we used cobalt-iron layered double hydroxide as co-reaction accelerator to efficiently activate water to generate ROS for enhancing luminol emission. Experimental investigations verify the formation of hydroxyl and superoxide radicals in the process of electrochemical water oxidation, which subsequently react with luminol anion radicals to trigger strong ECL signals. Finally, the detection of alkaline phosphatase has been successfully achieved with impressive sensitivity and reproducibility for practical sample analysis.     

Kinetics of the reaction between nitroxide and thiyl radicals: nitroxides as antioxidants in the presence of thiols

Cyclic nitroxides effectively protect cells, tissues, isolated organs, and laboratory animals from radical-induced damage. The present study focuses on the kinetics and mechanisms of the reactions of piperidine and pyrrolidine nitroxides with thiyl radicals, which are involved in free radical "repair" equilibria, but being strong oxidants can also produce cell damage. Thiyl radicals derived from glutathione, cysteine, and penicillamine were generated in water by pulse radiolysis, and the rate constants of their reactions with 2,2,6,6-tetramethylpiperidine-1-oxyl (TPO), 4-OH-TPO, and 3-carbamoyl-proxyl were determined to be (5-7) x 10 (8) M (-1) s (-1) at pH 5-7, independent of the structure of the nitroxide and the thiyl radical. It is suggested that the reaction of nitroxide (>NO (*)) with thiyl radical (RS (*)) yields an unstable adduct (>NOSR). The deprotonated form of this adduct decomposes via heterolysis of the N-O bond, yielding the respective amine (>NH) and sulfinic acid (RS(O)OH). The protonated form of the adduct decomposes via homolysis of the N-O bond, forming the aminium radical (>NH (*+)) and sulfinyl radical (RSO (*)), which by subsequent reactions involving thiol and nitroxide produce the respective amine and sulfonic acid (RS(O) 2OH). Nitroxides that are oxidized to the respective oxoammonium cations (>N (+)O) are recovered in the presence of NADH but not in the presence of thiols. This suggests that the reaction of >N (+)O with thiols yields the respective amine. Two alternative mechanisms are suggested, where >N (+)O reacts with thiolate (RS (-)) directly generating the adduct >NOSR or indirectly forming >NO (*) and RS (*), which subsequently together yield the adduct >NOSR. Under physiological conditions the adduct is mainly deprotonated, and therefore nitroxides can detoxify thiyl radicals. The proposed mechanism can account for the protective effect of nitroxides against reactive oxygen- and nitrogen-derived species in the presence of thiols.     

The Cu(I)–glutathione complex: factors affecting its formation and capacity to generate reactive oxygen species

Cu²⁺ ions and reduced glutathione (GSH) swiftly interact to form the physiologically occurring Cu(I)–[GSH]₂ complex. Prompted by the recently reported ability of this complex to generate superoxide radicals from molecular oxygen, the present study addressed how the concentration of Cu²⁺ and GSH, the pH, and the temperature affect the formation of the Cu(I)–[GSH]₂ complex and its capacity to generate superoxide radicals and hydrogen peroxide. Increasing concentrations of Cu²⁺ and GSH, added at a fixed molar ratio of 1:3, led to a proportionally greater production of superoxide anions, hydrogen peroxide, and oxidized glutathione (GSSG). GSSG formation was found to closely reflect the formation of Cu(I)–[GSH]₂. Biologically relevant changes in pH (e.g., from 6.8 to 7.7) and temperature (from 22 to 37 °C) did not affect the formation of the Cu(I)–[GSH]₂, as assessed by GSSG production. However, production of superoxide radicals increased as the pH values were incremented. An opposite effect was observed regarding hydrogen peroxide production. The ability of a freshly prepared Cu(I)–[GSH]₂ complex (assayed within a minute from its formation) to generate superoxide radicals was incremented by as the temperature was increased. Such ability, however, correlated inversely with the temperature when, before assaying for superoxide, the earlier referred preparation was incubated during 30 min in the presence of oxygen. Under the latter condition, hydrogen peroxide linearly accumulated in time, suggesting that an increased autodismutation underlies the apparent time-dependent “aging” of the capacity of the complex to generate superoxide.     

EPR study of radicals generated from bioactive S‐nitrosothiols and related thiols

Thermal or photolytic reactions of bioactive S‐nitrosothiols and related thiols in the presence of radical generators in deaerated DMSO or aqueous solutions under argon or saturated with nitric oxide (NO) produced nitroxides and an oxyaminyl radical, which were well characterized by EPR spectra. Nitroxides containing a thiyl substituent were obtained. Possible mechanisms are proposed. Bioactive S‐nitrosothiols such as S‐nitrosoglutathione, S‐nitroso‐N‐acetylpenicillamine and related thiols such as glutathione and N‐acetylpenicillamine were used for the investigation. Radical generators utilized as transient radical sources were 2,2^′‐azobisisobutyronitrile, 2,2^′‐azobis(2‐methylpropionamidine) dihydrochloride, tert‐butyl peroxide and benzoyl peroxide. Copyright © 2002 John Wiley & Sons, Ltd.     

Acid‐Mediated Formation of Radicals or Baeyer–Villiger Oxidation from Criegee Adducts

The acid‐mediated reaction of ketones with hydroperoxides generates radicals, a process with reaction conditions similar to those of the Baeyer–Villiger oxidation but with an outcome resembling the formation of hydroxyl radicals via ozonolysis in the atmosphere. The Baeyer–Villiger oxidation forms esters from ketones, with the preferred use of peracids. In contrast, alkyl hydroperoxides and hydrogen peroxide react with ketones by condensation to form alkenyl peroxides, which rapidly undergo homolytic O? O bond cleavage to form radicals. Both reactions are believed to proceed via Criegee adducts, but the electronic nature of the peroxide residue determines the subsequent reaction pathways. DFT calculations and experimental results support the idea that, unlike previously assumed, the Baeyer–Villiger reaction is not intrinsically difficult with alkyl hydroperoxides and hydrogen peroxide but rather that the alternative radical formation is increasingly favored.     

Studies on some radical transfer reactions. Part I. Hydrogen atom abstraction from some organic substrates by ȮH radicals

Polymerization of methyl methacrylate was carried out in aqueous and nonaqueous media in the presence of some sulfonated and carboxylic organic compounds, hydroxyl radicals generated from hydrogen peroxide being used as initiators of polymerization. The occurrence of radical transfer reactions by way of hydrogen atom abstraction from the organic substrates by the ?H radicals was demonstrated by the detection of sulfonate and carboxyl endgroups in the respective polymers. It was found that the radical transfer reactions were more favored in aqueous media than in nonaqueous systems.     

Kinetics and thermodynamics of the exchange reactions of peroxy radicals with hydroperoxides

The enthalpies and equilibrium constants of the exchange reactions of peroxy radicals with hydroperoxides of various structures are calculated. The experimental data on the reactions of hydrogen atom abstraction by the peroxy radicals from the hydroperoxides are analyzed, and the kinetic parameters characterizing these reactions are calculated using the intersecting parabolas method. The activation energies and rate constants for nine reactions of H atom abstraction by a peroxy radical from the OOH group of a peroxide are calculated using the above parameters. The geometric parameters of the transition states for the reactions are calculated. The low triplet repulsion plays an important role in the fast occurrence of the reactions. The polar interaction in the transition state is manifested in the reactions of the peroxy radicals with hydroperoxides containing a polar group.     

Kinetics of Homolytic Substitutions by Hydrogen Atoms at Thiols and Sulfides

Thermodynamic and kinetic data in the temperature range 300–1500 K are calculated for 94 homolytic substitution reactions by a hydrogen atom at thiols and sulfides with the CBS‐QB3//BMK/6‐311G(2d,d,p) method. The studied reactions were found to proceed according to a one‐step mechanism. A group additivity (GA) method is presented to model the Arrhenius parameters of this reaction family. The required GA values were derived from data obtained for a set containing 58 reactions. By using the developed GA scheme, rate coefficients at 300 K for 26 substitution reactions by the hydrogen atom are reproduced within a factor of 2.2. Mean absolute deviations on the activation energy and pre‐exponential factor are limited to 1.1 kJ mol^?1 and 0.19, respectively. Rate coefficients for the reverse reactions, that is, substitution reactions by C‐ and S‐centered radicals with expulsion of a hydrogen atom, are reproduced within a factor of 6 by using thermodynamic consistency. At 1000 K, group additive and calculated rate coefficients for the forward and reverse reactions agree within a factor of 1.8 and 4, respectively. Experimental rate coefficients in the temperature range 300–400 K are reproduced within a factor of 5. Discrepancies between calculated and experimental data are discussed.     

Photooxidative degradation of cellulose: Reactions of the cellulosic free radicals with oxygen

Photooxidative degradation of cellulose resulted in decreases of degree of polymerization (DP) and α-cellulose content, concurrently producing chromophoric groups; namely, carbonyl, carboxyl, and hydroperoxide groups within the polymer. Electron spin resonance (ESR) studies revealed that cellulosic carbon free radicals readily reacted with oxygen molecules at 143–160 K to produce peroxy radicals, whereas cellulosic oxygen free radicals were inert toward oxygen molecules throughout the photooxygenation reactions. At 77 K it is feasible that only photoexcited oxygen molecules reacted with cellulosic carbon free radicals to produce peroxide radicals. These radicals were themselves stabilized at 273 K by abstraction of hydrogen atoms from cellulose to produce polymer hydroperoxides. Simultaneously, new radical sites, which exhibited three-line ESR spectra, were generated in cellulose.     

Intramolecular reactions of free radicals formed from artemisinin

Two cyclic alkoxyl radicals are formed as a result of peroxide bridge scission in artemisinin. Intramolecular reactions of these radicals induce the cascade of reactions of isomerization, decyclization, and decomposition of formed free radicals. It includes 14 reactions of intramolecular free radical hydrogen transfer, 17 reactions of decyclization of alkoxyl and alkyl radicals, and 4 reactions of decomposition of alkoxyl, acyl, and carboxyl radicals. The enthalpies of these 35 reactions are calculated. Using intersection parabolas method, activation energies and rate constants of all these reactions are calculated. The most rapid reactions are selected for every intermediate free radical. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 554–565, 2005     

ESR studies of vinyl polymerization. II. Initial reactions of vinyl esters with redox systems in aqueous media

ESR measurements of transient radicals during redox polymerization of various vinyl esters in aqueous solutions have been made by using the rapid-mixing flow method. The initiation was by means of hydroxyl and amino radicals from the systems titanous chloride-hydrogen peroxide and titanous chloride-hydroxylamine, respectively. The well resolved hyperfine structures obtained at monomer concentrations of about 0.05 mole/1. are unambiguously assigned to the monomer radicals formed by addition of initiator radicals to monomers. At higher monomer concentrations, additional weak signals attributed to the growing polymer radicals were observed. The effect of reaction conditions on the signal intensity has been studied in particular for vinyl acetate. The coupling constants of monomer radicals from various vinyl esters (acetate, propionate, butyrate, crotonate, and isopropenyl acetate) were obtained and the spin densities calculated. From the ESR spectra, the monomer radicals have a conformation with the substituent R (R = HO or NH₂) of R? CH₂? CH(OCOR′) locked in a position above or below the radical plane. This is tentatively interpreted as due to formation of intramolecular hydrogen bonds to ring structures or complexes with titanium ions. In addition, hydrogen abstraction reactions of some model compounds for poly(vinyl acetate) have been briefly studied in relation to chain transfer and grafting reactions.     

THE PEROXIDASE/OXIDASE ACTIVITY OF SOYBEAN LIPOXYGENASE - I. TRIPLET EXCITED CARBONYLS FROM THE REACTION WITH ISOBUTANAL AND THE EFFECT OF GLUTATHIONE

Soybean lipoxygenase shows a secondary peroxidase/oxidase activity: The aerobic reaction with isobutanal, enhanced by hydrogen peroxide as a cosubstrate, yields acetone, exhibits chemiluminescence and consumes oxygen (phi cl = 1.3 x 10(-9) photons/O2 molecule consumed). 9,10-Dibromoanthracene-2-sulfonate increases the photoemission (kET tau 0 = 2 x 10(4) M-1; phi cl = 0.9 x 10(-7) photons/O2), whereas it is diminished by sorbate, tryptophan, 2-methyl-1,4-naphthoquinone, glutathione, and superoxide dismutase. In the presence of hydrogen peroxide the lipoxygenase reaction with glutathione yields yet another excited state. From the well-known reactions promoted by horseradish-peroxidase, these features are concluded to indicate the novel activity of soybean lipoxygenase. With isobutanal as a substrate lipoxygenase acts as an oxidase and as a peroxidase. The mechanism suggested leads to photoemissive triplet excited acetone as expected from the cleavage of an intermediate dioxetane.     

Integration of anodic and cathodic processes for the synergistic electrochemical production of peracetic acid

Efficient in-situ production of peracetic acid is an unreached milestone of electrochemical engineering. Previous attempts in the production of peracetic acid were focused either on the cathodic production of hydrogen peroxide and its further addition to acetic acid solutions or on the oxidation of a suitable raw material (v. g. acetic acid, acetaldehyde, ethanol). In the present work, the oxidation of acetic acid by a boron doped diamond (BDD) anode was integrated with the cathodic production of hydrogen peroxide using a carbon felt gas diffusion electrode. A marked synergistic effect (synergy coefficient of 192.0 ± 13.1%) is observed when the oxidation of acetic acid by hydroxyl radicals is performed together with the cathodic production of hydrogen peroxide. A maximum PAA production efficiency of 19.87% was obtained, a value much higher than previous works based on the oxidation of acetic acid by BDD anodes and approximately double the optimal value reported in studies based on the production of hydrogen peroxide.     

Chemical studies of the antioxidant mechanism of theaflavins: radical reaction products of theaflavin 3,3′-digallate with hydrogen peroxide

The objective of the current study is to characterize the reaction products of theaflavin 3,3′-digallate, one of the major characteristic polyphenols of black tea, with hydroxyl radicals generated by hydrogen peroxide, with the aim of gaining insights into specific mechanisms of antioxidant reactions in physiological systems. Two major reaction products were isolated and identified using high-field 1D and 2D NMR spectral analysis. Both of them are A-ring fission products. The observation of these compounds indicates that the A ring rather than the benzotropolone moiety is the initial site for formation of reaction products in the hydrogen peroxide oxidant system.     

Suppression of thiol exchange reaction in the determination of reduced-form thiols by high-performance liquid chromatography with fluorescence detection after derivatization with fluorogenic benzofurazan reagent, 7-fluoro-2,1,3-benzoxadiazole-4-sulfonate and 4-aminosulfonyl-7-fluoro-2,1,3-benzoxadiazole

The derivatization of the reduced-form thiols with SBD-F (7-fluoro-2,1,3-benzoxadiazole-4-sulfonate) and ABD-F (4-aminosulfonyl-7-fluoro-2,1,3-benzoxadiazole) was studied. The yields of the derivatives of the reduced-form thiols (cysteine, homocysteine, reduced-form glutathione) with SBD-F at 60 degrees C for 45 min in the borate buffer (pH 9.3) were significantly decreased in the presence of the oxidized-form thiols (cystine, homocystine, oxidized-form glutathione) because of the thiol exchange reaction between the reduced-form and the oxidized-form thiols. The use of ABD-F at low temperature enabled the suppression of these thiol exchange reactions, and the recommended conditions were below 5 degrees C for 90 min in borate buffer (pH 9.3). These results suggest that ABD-F is a preferred derivatization reagent for the accurate determination of the reduced-form thiols in samples containing the oxidized-form thiols. In addition, it was also suggested that the derivatization of the reduced-form thiols should also be performed at low temperature when derivatization reagents such as o-phthalaldehyde (OPA) and monobromobimane (BrB) are used.