Thiyl radicals, reaction kinetics and redox chemistry

In focusing on the free radical chemistry of thiyl radicals,two importantclasses are dealt with, namely, the amino acidalkylthiyl and the acylthiyl radicals, where pulse radiolysisand steady state γ-radiolysis are applied. Amino acid thiyl radicals (RS•), are important in manybiological systems,as intermediates in the repair reactions ofthiols towards targeted carbon-centeredradicals and ascatalytic centers in many enzymatic reactions. Initially,thereactions following one-electron oxidation of cysteine(CysSH), homocysteine(HCysSH) and glutathione (GSH) in aqueoussolutions are observed, where kineticanalysis reveals thatintramolecular hydrogen abstraction by thiyl radicals fromtheα-amino-α-carboxyl C-H bonds is general for thethree investigated. Theintramolecular hydrogen abstraction of aglutathione thiyl radical from α-amino-α-carboxylC-Hbond discloses an equilibrium constant larger than 10 4 asevidencedby ammonia formation and thiol consumption. Thus,based on reduction potentialE°(RS•,H + /RSH) being1.36 V, E°(NH 2 (C•)RCO 2 ¯, H + /NH 2 CHRCO 2¯) andE°(NH 2 (C•)RCO 2 H, H + /NH 2 CHRCO 2 H)are derived as lower than 1.12 V and0.98 V vs NHE,respectively. These are, to my knowledge, the firstexperimentalvalues relating to C-H bond strengths of aminoacids with their biologicalimplications clearly appraised. Further, spectroscopic characteristics and one-electronreduction potential ofacylthiyl, including acetylthiyl,benzoylthiyl and 4-substituted benzoylthiylradicals arestudied. The kinetics and the thermodynamics of theß-fragmentationreactions, i.e. RC(O)S•\u97\u97>R• + COS, for the CH 3 C(O)S•,PhC(O)S• and 4-X-PhC(O)S• radicals are observed,with experimental results compared andevaluated along withtheir oxygen counterparts CH 3 C(O)O•, PhC(O)O• and4-XPhC(O)O• radicals. Keywords:Thiyl, Glutathione,α-Amino-α-Carboxyl Carbon-Centered Radical,Acylthiyl,Aroylthiyl, Benzoylthiyl, 4-Substituted Benzoylthiyl Radical,PulseRadiolysis, Kinetics, One-Electron Reduction Potential,Bond Dissociation Energy, β-Fragmentation.NR 2014080