The glutathione free radical equilibrium, GS. + GS−⇌ GSS.−G, mediating electron transfer to FE(III) -cytochrome c

OH-Radical Specific Addition to Glutathione S-Atom at the Air-Water Interface: Relevance to the Redox Balance of the Lung Epithelial Lining Fluid

Antioxidants in epithelial lining fluids (ELF) prevent inhaled air pollutants from reaching lung tissue. This process, however, may upset ELF's redox balance, which is deemed to be expressed by the ratio of the major antioxidant glutathione (GSH) to its putative oxidation product GSSG. Previously, we found that at physiological pH O3(g) rapidly oxidizes GS(2-)(aq) (but not GSH(-)) to GSO3(-) rather than GSSG. Here, we report that in moderately acidic pH ≤ 5 media ·OH(g) oxidizes GSH(-)(aq) to sulfenic GSOH(-), sulfinic GSO2(-), and sulfonic GSO3(-) acids via ·OH specific additions to reduced S-atoms. The remarkable specificity of ·OH on water versus its lack of selectivity in bulk water implicates an unprecedented steering process during [OH···GSH] interfacial encounters. Thus, both O3 and ·OH oxidize GSH to GSOH(-) under most conditions, and since GSOH(-) is reduced back to GSH in vivo by NADPH, redox balance may be in fact signaled by GSH/GSOH ratios.

Redox-implications associated with the formation of complexes between copper ions and reduced or oxidized glutathione

Binding of copper by reduced glutathione (GSH) is generally seen as a mechanism to lower, if not abolish, the otherwise high electrophilicity and redox activity of its free ions. In recent years, however, this concept has been contradicted by new evidence revealing that, rather than stabilizing free copper ions, its binding to GSH leads to the formation of a Cu(I)-[GSH]2 complex capable of reducing molecular oxygen into superoxide. It is now understood that, under conditions leading to the removal of such radicals, the Cu(I)-[GSH]2 complex is readily oxidized into Cu(II)-GSSG. Interestingly, in the presence of a GSH excess, the latter complex is able to regenerate the superoxide-generating capacity of the complex it originated from, opening the possibility that a GSH-dependent interplay exists between the reduced and the oxidized glutathione forms of these copper-complexes. Furthermore, recent evidence obtained from experiments conducted in non-cellular systems and intact mitochondria indicates that the Cu(II)-GSSG complex is also able to function in a catalytic manner as an efficient superoxide dismutating- and catalase-like molecule. Here we review and discuss the most relevant chemical and biological evidence on the formation of the Cu(I)-[GSH]2 and Cu(II)-GSSG complexes and on the potential redox implications associated with their intracellular occurrence.

Glutathione--hydroxyl radical interaction: a theoretical study on radical recognition process

Non-reactive, comparative (2 × 1.2 μs) molecular dynamics simulations were carried out to characterize the interactions between glutathione (GSH, host molecule) and hydroxyl radical (OH(•), guest molecule). From this analysis, two distinct steps were identified in the recognition process of hydroxyl radical by glutathione: catching and steering, based on the interactions between the host-guest molecules. Over 78% of all interactions are related to the catching mechanism via complex formation between anionic carboxyl groups and the OH radical, hence both terminal residues of GSH serve as recognition sites. The glycine residue has an additional role in the recognition of OH radical, namely the steering. The flexibility of the Gly residue enables the formation of further interactions of other parts of glutathione (e.g. thiol, α- and β-carbons) with the lone electron pair of the hydroxyl radical. Moreover, quantum chemical calculations were carried out on selected GSH/OH(•) complexes and on appropriate GSH conformers to describe the energy profile of the recognition process. The relative enthalpy and the free energy changes of the radical recognition of the strongest complexes varied from -42.4 to -27.8 kJ/mol and from -21.3 to 9.8 kJ/mol, respectively. These complexes, containing two or more intermolecular interactions, would be the starting configurations for the hydrogen atom migration to quench the hydroxyl radical via different reaction channels.

Oxidation of glutathione by hexachloroiridate(IV), dicyanobis(bipyridine)iron(III), and tetracyano(bipyridine)iron(III)

The aqueous oxidations of glutathione (GSH) by [IrCl(6)](2-), [Fe(bpy)(2)(CN)(2)](+), and [Fe(bpy)(CN)(4)](-) are described. All three reactions are highly susceptible to catalysis by traces of copper ions, but this catalysis can be fully suppressed with suitable chelating agents. The direct oxidation by [IrCl(6)](2-) yields [IrCl(6)](3-) and GSO(3)(-); some GSSG is also obtained in the presence of O(2). The two Fe(III) oxidants are reduced to their corresponding Fe(II) complexes with nearly quantitative formation of GSSG. The kinetics of these reactions have been studied at 25 °C and μ = 0.1 M between pH 1 and 11. All three reactions have rate laws that are first order in [M(ox)] and [GSH](t) and show a general increase in rate with increasing pH. Detailed studies of the pH dependence enable the rate law to be elaborated with terms for reaction of the individual protonation states of GSH. These pH-resolved rate constants are interpreted with a mechanism having rate-limiting outer-sphere electron-transfer from the various thiolate forms of GSH.

Utilization of the ferrous sulfate (Fricke) dosimeter for evaluating the radioprotective potential of cystamine: experiment and Monte Carlo simulation

Cystamine, an organic disulfide (RSSR), is among the best of the known radiation-protective compounds and has been used to protect normal tissues in clinical radiation therapy. Recently, it has also proved to be beneficial in the treatment of disorders of the central nervous system in animal models. However, the underlying mechanism of its action at the chemical level is not yet well understood. The present study aims at using the ferrous sulfate (Fricke) dosimeter to quantitatively evaluate, both experimentally and theoretically, the radioprotective potential of this compound. The well-known radiolysis of the Fricke dosimeter by (60)Co γ rays or fast electrons, based on the oxidation of ferrous ions to ferric ions by the oxidizing species (•)OH, HO(2)(•), and H(2)O(2) produced in the radiolytic decomposition of water, forms the basis for our method. The presence of cystamine in Fricke dosimeter solutions during irradiation prevents the radiolytic oxidation of Fe(2+) and leads to decreased ferric yields (or G values). The observed decrease in G(Fe(3+)) increases upon increasing the concentration of the disulfide compound over the range 0-0.1 M under both aerated and deaerated conditions. To help assess the basic radiation-protective mechanism of this compound, a full Monte Carlo computer code is developed to simulate in complete detail the radiation-induced chemistry of the studied Fricke/cystamine solutions. Benefiting from the fact that cystamine is reasonably well characterized in terms of radiation chemistry, this computer model proposes reaction mechanisms and incorporates specific reactions describing the radiolysis of cystamine in aerated and deaerated Fricke solutions that lead to the observable quantitative chemical yields. Results clearly indicate that the protective effect of cystamine originates from its radical-capturing ability, which allows this compound to act by competing with the ferrous ions for the various free radicals--especially (•)OH radicals and H(•) atoms--formed during irradiation of the surrounding water. Most interestingly, our simulation modeling also shows that the predominant pathway in the oxidation of cystamine by (•)OH radicals involves an electron-transfer mechanism, yielding RSSR(•+) and OH(-). A very good agreement is found between calculated G(Fe(3+)) values and experiment. This study concludes that Monte Carlo simulations represent a very efficient method for understanding indirect radiation damage at the molecular level.

Thiyl radicals react with nitric oxide to form S-nitrosothiols with rate constants near the diffusion-controlled limit

A possible route to S-nitrosothiols in biology is the reaction between thiyl radicals and nitric oxide. D. Hofstetter et al. (Biochem. Biophys. Res. Commun.360:146-148; 2007) claimed an upper limit of (2.8+/-0.6)x10(7) M(-1)s(-1) for the rate constant between thiyl radicals derived from glutathione and nitric oxide, and it was suggested that under physiological conditions S-nitrosation via this route is negligible. In the present study, thiyl radicals were generated by pulse radiolysis, and the rate constants of their reactions with nitric oxide were determined by kinetic competition with the oxidizable dyes 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) and a phenothiazine. The rate constants for the reaction of nitric oxide with thiyl radicals derived from glutathione, cysteine, and penicillamine were all in the range (2-3) x10(9) M(-1)s(-1), two orders of magnitude higher than the previously reported estimate in the case of glutathione. Absorbance changes on reaction of thiyl radicals with nitric oxide were consistent with such high reactivity and showed the formation of S-nitrosothiols, which was also confirmed in the case of glutathione by HPLC/MS. These rate constants imply that formation of S-nitrosothiols in biological systems from the combination of thiyl radicals with nitric oxide is much more likely than claimed by Hofstetter et al.

The roles of thiol-derived radicals in the use of 2',7'-dichlorodihydrofluorescein as a probe for oxidative stress

2',7'-Dichlorodihydrofluorescein (DCFH2) is one of the most widely used probes for detecting intracellular oxidative stress, but requires a catalyst to be oxidized by hydrogen peroxide or superoxide and reacts nonspecifically with oxidizing radicals. Thiyl radicals are produced when many radicals are "repaired" by thiols, but are oxidizing agents and thus potentially capable of oxidizing DCFH2. The aim of this study was to investigate the reactivity of thiol-derived radicals toward DCFH2 and its oxidized, fluorescent form 2',7'-dichlorofluorescein (DCF). Thiyl radicals derived from oxidation of glutathione (GSH) or cysteine (CysSH) oxidized DCFH2 with rate constants at pH 7.4 of approximately 4 or approximately 2x10(7) M(-1) s(-1), respectively. Both the rates of oxidation and the yields of DCF were pH-dependent. Glutathione-derived radicals interacted with DCF, resulting in the formation of DCFH* absorbing at 390 nm and loss of fluorescence; in contrast, cysteine-derived radicals did not cause any depletion of DCF fluorescence. We postulate that the observed apparent difference in reactivity between GS* and CysS* toward DCF is related to the formation of carbon-centered, reducing radicals from base-catalyzed isomerization of GS*. DCF formation from interaction of DCFH2 with GS* was inhibited by oxygen in a concentration-dependent manner over the physiological range. These data indicate that in applying DCFH2 to measure oxidizing radicals in biological systems, we have to consider not only the initial competition between thiols and DCFH2 for the oxidizing radicals, but also subsequent reactions of thiol-derived radicals, together with variables--including pH and oxygen concentration--which control thiyl radical chemistry.

The oxidizing power of the glutathione thiyl radical as measured by its electrode potential at physiological pH

The oxidizing power of the thiyl radical (GS*) produced on oxidation of glutathione (GSH) was determined as the mid-point electrode potential (reduction potential) of the one-electron couple E(m)(GS*,H+/GSH) in water, as a function of pH over the physiological range. The method involved measuring the equilibrium constants for electron-transfer equilibria with aniline or phenothiazine redox indicators of known electrode potential. Thiyl and indicator radicals were generated in microseconds by pulse radiolysis, and the position of equilibrium measured by fast kinetic spectrophotometry. The electrode potential E(m)(GS*,H+/GSH) showed the expected decrease by approximately 0.06 V/pH as pH was increased from approximately 6 to 8, reflecting thiol/thiolate dissociation and yielding a value of the reduction potential of GS*=0.92+/-0.03 V at pH 7.4. An apparently almost invariant potential between pH approximately 3 and 6, with potentials significantly lower than expected, is ascribed at least in part to errors arising from radical decay during the approach to the redox equilibrium and slow electron transfer of thiol compared to thiolate.

Free radical scavenging reactions of sulfasalazine, 5-aminosalicylic acid and sulfapyridine: mechanistic aspects and antioxidant activity

Reactions of sulfasalazine (SAZ) and its metabolites, 5-aminosalicylic acid (5-ASA) and sulfapyridine (SP), with various oxidizing and reducing free radicals (hydroxyl, haloperoxyl, one-electron oxidizing, lipid peroxyl, glutathiyl, superoxide, tryptophanyl, etc.) have been studied to understand the mechanistic aspects of its action against free radicals produced during inflammation. Nanosecond pulse radiolysis technique coupled with transient spectrophotometry has been used for in situ generation of free radicals and to follow their reaction pathways. The transients produced in these reactions have been assigned and radical scavenging rate constants have been measured. In addition to scavenging of various primary and secondary free radicals by SAZ, 5-ASA and SP, 5-ASA has also been observed to efficiently scavenge radicals of biomolecules. 5-ASA has been found to be the active moiety of SAZ involved in the scavenging of oxidizing free radicals whereas reduction of SAZ produced molecular radical anion. The study suggests that free radical scavenging activity of 5-ASA may be a major path of pharmacological action of SAZ against inflammatory bowel diseases (IBD).

Mechanistic studies of S-nitrosothiol formation by NO*/O2 and by NO*/methemoglobin

The mechanisms of formation of S-nitrosothiols under physiological conditions and, in particular, of generation of SNO-Hb (the hemoglobin form in which the cysteine residues beta93 are S-nitrosated) are still not completely understood. In this paper, we investigated whether, in the presence of O2, NO* is more efficient to nitrosate protein-bound thiols such as Cysbeta93 or low molecular weight thiols such as glutathione. Our results show that when substoichiometric amounts of NO* are mixed slowly with the protein solution, NO*, O2, and possibly NO2* and/or N2O3 accumulate in hydrophobic pockets of hemoglobin. Since the environment of the cysteine residue beta93 is rather hydrophobic, these conditions facilitate SNO-Hb production. Moreover, we show that S-nitrosation mediated by reaction of NO* with the iron(III) forms of Hb or Mb is significantly more effective when it can take place intramolecularly, as in metHb. Intermolecular reactions lead to lower S-nitrosothiol yields because of the concurring hydrolysis to nitrite.

Thiyl radicals in biosystems: inhibition of the prostaglandin metabolism by the cis-trans-isomerization of arachidonic acid double bonds

This paper describes parallel and comparative experiments on the enzymatic cyclooxygenase (COX) driven conversion of arachidonic acid (AA, all-cis-5,8,11,14-eicosatetraenoic acid) into prostaglandins by using pure arachidonic acid and AA samples containing relatively small amounts of thiyl radical induced trans-isomers. The experiments were performed in a liquid aqueous model system using COX-1 as well as by the in vitro feeding of VD(3)-differentiated and LPS-stimulated promyelocytic HL-60 cells using the cell's own COX-2. In the model solution, all the different test methods used (oxygen consumption, ROS induced luminescence, and TMPD oxidation) indicated the greatly disproportionate, non-stoichiometric inhibition of the prostaglandin metabolism by the trans-isomers. Accordingly, measurements performed in the cell system gave comparable results: both luminescence ROS detection and the ELISA test on PGE(2) expression resulted in the strong inhibition of the prostaglandin metabolism. We interpret these findings as enzyme blocking caused by just one mono-trans-isomerized double bond of AA.

Kinetics of the reactions of nitrogen dioxide with glutathione, cysteine, and uric acid at physiological pH

Nitrogen dioxide (NO(2)(*)) is a key biological oxidant. It can be derived from peroxynitrite via the interaction of nitric oxide with superoxide, from nitrite with peroxidases, or from autoxidation of nitric oxide. In this study, submicromolar concentrations of NO(2)(*) were generated in < 1 micros using pulse radiolysis, and the kinetics of scavenging NO(2)(*) by glutathione, cysteine, or uric acid were monitored by spectrophotometry. The formation of the urate radical was observed directly, while the production of the oxidizing radical obtained on reaction of NO(2)(*) with the thiols (the thiyl radical) was monitored via oxidation of 2,2'-azino-bis-(3-ethylthiazoline-6-sulfonic acid). At pH 7.4, rate constants for reaction of NO(2)(*) with glutathione, cysteine, and urate were estimated as approximately 2 x 10(7), 5 x 10(7), and 2 x 10(7) M(-1) s(-1), respectively. The variation of these rate constants with pH indicated that thiolate reacted much faster than undissociated thiol. The dissociation of urate also accelerated reaction with NO(2)(*) at pH > 8. The thiyl radical from GSH reacted with urate with a rate constant of approximately 3 x 10(7) M(-1) s(-1). The implications of these values are: (i) the lifetime of NO(2)(*) in cytosol is < 10 micros; (ii) thiols are the dominant 'sink' for NO(2)(*) in cells/tissue, whereas urate is also a major scavenger in plasma; (iii) the diffusion distance of NO(2)(*) is approximately 0.2 microm in the cytoplasm and < 0.8 microm in plasma; (iv) urate protects GSH against depletion on oxidative challenge from NO(2)(*); and (v) reactions between NO(2)(*) and thiols/urate severely limit the likelihood of reaction of NO(2)(*) with NO* to form N(2)O(3) in the cytoplasm.

Oxidation of tetrahydrobiopterin by biological radicals and scavenging of the trihydrobiopterin radical by ascorbate

One-electron oxidation of (6R)-5,6,7,8-tetrahydrobiopterin (H(4)B) by the azide radical generates the radical cation (H(4)B(*)(+)) which rapidly deprotonates at physiological pH to give the neutral trihydrobiopterin radical (H(3)B(*)); pK(a) (H(4)B(*)(+) <==> H(3)B(*) + H(+)) = (5.2 +/- 0.1). In the absence of ascorbate both the H(4)B(*)(+) and H(3)B(*) radicals undergo disproportionation to form quinonoid dihydrobiopterin (qH(2)B) and the parent H(4)B with rate constants k(H(4)B(*)(+) + H(4)B(*)(+)) = 6.5 x 10(3) M(-1) s(-1) and k(H(3)B(*) + H(3)B(*)) = 9.3 x 10(4) M(-1) s(-1), respectively. The H(3)B(*) radical is scavenged by ascorbate (AscH(-)) with an estimated rate constant of k(H(3)B(*) + AscH(-)) similar 1.7 x 10(5) M(-1) s(-1). At physiological pH the pterin rapidly scavenges a range of biological oxidants often associated with cellular oxidative stress and nitric oxide synthase (NOS) dysfunction including hydroxyl ((*)OH), nitrogen dioxide (NO(2)(*)), glutathione thiyl (GS(*)), and carbonate (CO(3)(*-)) radicals. Without exception these radicals react appreciably faster with H(4)B than with AscH(-) with k(*OH + H(4)B) = 8.8 x 10(9) M(-1) s(-1), k(NO(2)(*) + H(4)B) = 9.4 x 10(8) M(-1) s(-1), k(CO(3)(*-) + H(4)B) = 4.6 x 10(9) M(-1) s(-1), and k(GS(*) + H(4)B) = 1.1 x 10(9) M(-1) s(-1), respectively. The glutathione disulfide radical anion (GSSG(*-)) rapidly reduces the pterin to the tetrahydrobiopterin radical anion (H(4)B(*-)) with a rate constant of k(GSSG(*-) + H(4)B) similar 4.5 x 10(8) M(-1) s(-1). The results are discussed in the context of the general antioxidant properties of the pterin and the redox role played by H(4)B in NOS catalysis.

Free radical scavenging behavior of folic acid: evidence for possible antioxidant activity

The free radical scavenging properties and possible antioxidant activity of folic acid are reported. Pulse radiolysis technique is employed to study the one-electron oxidation of folic acid in homogeneous aqueous solution. The radicals used for this study are CCl(3)O(2)(*), N(3)(*), SO(4)(*-), Br(2)(*-), *OH, and O(*-). All these radicals react with folic acid under ambient condition at an almost diffusion-controlled rate producing two types of transients. The first transient absorption maximum is around 430 nm, which decays, and a simultaneous growth at around 390 nm is observed. Considering the chemical structure of folic acid, the absorption maximum at 430 nm has been assigned to a phenoxyl radical. The latter one is proposed to be a delocalized molecular radical. A permanent product has been observed in the oxidation of folic acid with CCl(3)O(2)(*) and N(3)(*) radicals, with a broad absorption band around 370-400 nm. The bimolecular rate constants for all the radical-induced oxidation reactions of folic acid have been measured. Folic acid is seen to scavenge these radicals very efficiently. In the reaction of thiyl radicals with folic acid, it has been observed that folic acid can not only scavenge thiyl radicals but can also repair these thiols at physiological pH. While carrying out the lipid peroxidation study, in spite of the fact that folic acid is considerably soluble in water, we observed a significant inhibition property in microsomal lipid peroxidation. A suitable mechanism for oxidation of folic acid and repair of thiyl radicals by folic acid has been proposed.

On the Oxidation of Cytochrome c by Hypohalous Acids

Oxidation of cytochrome c, a key protein in mitochondrial electron transport and a mediator of apoptotic cell death, by reactive halogen species (HOX, X2), i.e., metabolites of activated neutrophils, was investigated by stopped-flow. The fast initial reactions between FeIIIcytc and HOX species, with rate constants (at pH 7.6) of k > 3 x 10(6) M(-1) s(-1) for HOBr, k > 3 x 10(5) M(-1) s(-1) for HOCl, and k = (6.1+/-0.3) x 10(2) M(-1) s(-1) for HOI, are followed by slower intramolecular processes. All HOX species lead to a blue shift of the Soret absorption band and loss of the 695-nm absorption band, which is an indicator for the intact iron to Met-80 bond, and of the reducibility of FeIIIcytc. All HOX species do, in fact, persistently impair the ability of FeIIIcytc to act as electron acceptor, e.g., in reaction with ascorbate or O2*-. I2 selectively oxidizes the iron center of FeIIcytc, with a stoichiometry of 2 per I2, and with k(FeIIcytc + I2) approximately 4.6 x 10(4) M(-1) s(-1) and k(FeIIcytc + I2*-) = (2.9+/-0.4) x 10(8) M(-1) s(-1). Oxidation of FeIIcytc by HOX species is not selectively directed toward the iron center; HOBr and HOCl are considered to react primarily by N-halogenation of side chain amino groups, and HOI mainly by sulfoxidation. There is some evidence for the generation of HO* radicals upon reaction of HOCl with FeIIcytc. Chloramines (e.g., NH2Cl), bromamine (NH2Br), and cyclo-Gly2 chloramide oxidize FeIIcytc slowly and unselectively, but iodide efficiently catalyzes reactions of these N-halogens to yield fast selective oxidation of the iron center; this is due to generation of I2 by reaction of I- with the N-halogen and recycling of I- by reaction of I2 with FeIIcytc. Iodide also catalyzes methionine sulfoxidation and thiol oxidation by NH2Cl. The possible biological relevance of these findings is discussed.

Oxygen is not required for degradation of DNA by glutathione and Cu(II)

Previous studies by others have shown that thiols, such as glutathione, cause cleavage of DNA in the presence of Cu(II) ions and that the hydroxyl radical derived from molecular oxygen is the major cleaving species. In this paper, we present several lines of evidence that strongly suggest that molecular oxygen is not essential for DNA cleavage and that thiyl radicals may also be involved. Indirect evidence is presented to indicate that glutathione may substitute oxygen as an electron acceptor. In addition, DNA degradation occurs to a significant extent under anaerobic conditions and no inhibition of single-strand cleavage of supercoiled plasmid DNA is seen in the presence of superoxide dismutase and catalase. In view of the ubiquitous presence of glutathione, these results could be of interest under certain diseased conditions where copper concentrations are elevated.

First electron spin resonance evidence for the production of semiquinone and oxygen free radicals from orellanine, a mushroom nephrotoxin

Orellanine is the tetrahydroxylated and di-N-oxidized bipyridine toxin from several Cortinarius mushrooms. The mechanism responsible for its lethal nephrotoxicity was unknown until now. Our present ESR spectroscopic study of the redox properties of the toxin is an original contribution to the knowledge of its toxicity. It was achieved in particular by comparison of the properties of orellanine to that of other bipyridine compounds. After a one-electron oxidation (e.g., photochemical oxidation upon visible light), a radical form of orellanine is observed at physiological pH under aerobic or anaerobic conditions. This radical, identified as ortho-semiquinone anion radical, can also be generated by oxidation with biological oxidizing agents or enzymatic systems. Production of superoxide and hydroxyl radicals is shown by the spin-trapping method using DMPO as a spin trap. Bioreducing agents like GSH and cysteine involve in vitro the semiquinone radical and orellanine in a redox cycling process resulting in the production of glutathionyl and oxygen free radicals. This process leads in vitro to a large oxygen consumption and to a dramatic depletion of glutathione level. The formation of an apparently stable ortho-semiquinone anion radical and of reactive oxygen radical species is observed for the first time with a mushroom toxin. It is due to the catechol-like functions borne by the di-N-oxidized bipyridine structure of the toxin and may at least partly explain the toxicity of orellanine.

Generation and reactions of the disulphide radical anion derived from metallothionein: a pulse radiolytic study

OH-radicals were generated by pulse radiolysis of aqueous solutions of rabbit (Zn,Cd)-metallothionein (MT). They react with MT mainly by forming a thiyl radical with a rate constant of 1.7 x 10(12) dm3 mol-1 s-1. The thiyl radical reacts rapidly but reversibly with a thiolate function to form RSSR.-: RS + RS- reversible RSSR.-. The kinetics of the formation and decay of this radical anion have been studied pulse radiolytically by monitoring the evolution of the optical absorption of RSSR.- at 450 nm. This process is mostly intermolecular, i.e. bimolecular in MT. In the absence of O2, RSSR.- decays bimolecularly: RSSR.(-)+RS.-->RSSR + RS-. In the presence of O2, RS. may be scavenged by O2 and thus the yield of RSSR.- decreases: RS.+O2 reversible RSOO.. Under these conditions RSSR.- decays by first-order kinetics: RSSR.(-)+O2-->RSSR + O2.-. The rate constants of these reactions have been determined at room temperature: k4 = 1.8 x 10(9) dm3 mol-1 s-1, k5 = 7 x 10(4) s-1, k10 = 9.2 x 10(8) dm3 mol-1 s-1, and k18 about 3 x 10(7) dm3 mol-1 s-1. From the dependence of the maximal absorbance at 450 nm on the thiolate concentration in the absence of oxygen, epsilon (RSSR.-) = 9 x 10(3) dm3 mol-1 cm-1 and the stability constant (K4/5) of 2.3 x 10(4) dm3 ml-1 was determined. K4/5 is in good agreement with that determined kinetically, k4/k5 = 2.6 x 10(4) dm3 mol-1. The stability constant K15/16 of the thiylperoxyl radical, RSOO., was determined to be 5.5 x 10(3) dm3 mol-1.