Thiyl radicals, reaction kinetics and redox chemistry

Abstract: 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•——>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.