The Role of Sulphur Peptide Functions in Free Radical Transfer: A Pulse Radiolysis Study

Summary Cascading transfers of free radical centres, involving sulphur and aromatic protein functions, have been studied in further detail. The disulphide radical anion appears to be an important terminus of both oxidative and reductive radical transfer. In deaerated solutions of cysteine (20 mmol dm−3) the yield of Cys2/SS·− closely resembles the yield of all primary free radicals generated by water radiolysis (·OH, H· and eaq−). The alanyl Ala/Cβ·, formed by electron addition to cysteine and subsequent SH− elimination, oxidizes cysteine with a rate constant of k8 = 5·0 × 106 dm3 mol−1 s−1 at pH 6 to 7 and 3·6 × 106 dm3 mol−1 s−1 at pH9 to 10. In the case of glutathione (GSH) the eaq−-induced carbon-centred radical oxidizes the parent thiol with rate constants k(G· + GSH) of 7·0 × 106 and 1·3 × 106 dm3 mol−1 s−1 at pH 8 and pH 10, respectively; and with dithiothreitol (D(SH)2) the corresponding reaction rate is k(·DSH + D(SH)2) = 5·5 × 106 dm3 mol−1 s−1 at pH 7·0. The decarboxylated methionyl Met/C·α, formed by reaction of ·OH with methionine, is capable of electron transfer to cystine, indicating a reduction potential for decarboxylated methione more negative than −1·6 V. The ring-closed methionyl radical cation Met/SN·+, formed by reaction of ·OH with Met-Gly, oxidizes azide via equilibration, Met/SN·+ + H+ + N3− ⇌ Met + N3·, which enables an estimate to be given for the one-electron reduction potential: E°(Met/SN·+ + H+; Met) = + 1·42 ± 0·3 V (pH 6·8). Some further reactions of oxidizing dimeric Met2/SS·+ species in neutral solution have been demonstrated. The direction and nature of the transfers can be expressed by the scheme: