˙OH radical induced decarboxylation of γ-glutamylmethionine and S-alkylglutathione derivatives: evidence for two different pathways involving C- and N-terminal decarboxylation

Abstract

The hydroxyl radical induced oxidation of γ-glutamylmethionine and S-alkylglutathione derivatives (alkyl = CH₃, C₂H₅, C₄H₉, C₆H₁₃, C₉H₁₉) in aqueous solution results in significantly different decarboxylation yields upon variation of the peptide concentration, pH and chain length of alkyl substituents adjacent to the sulphur. Mechanistically, the decarboxylation is considered to proceed via two different routes: (i) electron transfer between oxidized sulphur, > S˙⁺, and the C-terminal carboxyl group (pseudo-Kolbe mechanism) whenever both reactants are located within the same peptide unit, and (ii) interaction between an ˙OH adduct, > ˙S–OH, and a protonated amino group which is positioned α to a carboxyl group (N-terminal decarboxylation). The latter mechanism also occurs if both reaction centres are not located within the same peptide unit.