AN ELECTRON-SPIN-RESONANCE STUDY OF GAMMA-IRRADIATED POLYCRYSTALLINE METHIONINE-CONTAINING PEPTIDES

The radicals formed on gamma-radiolysis of several methionine containing peptides in the polycrystalline state at 77 K and their reactions on subsequent warming over the temperature range 77-293 K are reported. For all peptides the radical species present at 77 K are peptide anion radicals. For peptides of the type x-met (x = gly, ala, val, leu, ser), the radical anions decay to produce deamination radicals on warming to 120-230 K. On further warming to 293 K, the presence of decarboxylation radicals and H-abstraction radicals at the alpha-carbon of the methionine residue were detected for gly-met, ala-met, and val-met, leu-met, ser-met, respectively. For peptides of the type met-x (x = gly, ala, leu, ser, asp, glu, gly-gly) the H H-abstraction radicals at the alpha-carbon of the C-terminal amino acid residue are the only radical species present at room temperature. For val-met, leu-met, ser-met and met-x peptides the H-abstraction radicals in the side chain of methionine residue over the temperature range 180-230 K were also observed. It is proposed that the primary localization of the positive charge on the sulphur atom accounts for the radical reaction pattern in the cationic pathway.