Tl2+ and Ag2+ metal-ion-induced oxidation of methionine in aqueous solution. A pulse radiolysis study.

Oxidation of methionine by Tl2+ and Ag2+ occurs with bimolecular rate constants of 2.5 x 10(9) and 3.3 x 10(8) mol-1 ls-1, respectively, and lead to sulphur-centred radical cations as primary intermediates. The overall mechanism is in principle the same as for OH. and X2.- (X = halide) induced oxidations with however one significant difference. Thus no oxidation of the amino group by S+. including subsequent decarboxylation is observed for methionine in its zwitterionic form while this readily occurs in the OH. and X2.- initiated mechanism. The reason is suggested to be complexing of the amino function by the metal ions which effectively lowers the reduction potential of the amino group. The sulphur-centred radical cation rather decays by a mostly first-order deprotonation process to yield CH2-S-CH2 approximately and CH3-S-CH approximately radicals of methionine. They absorb with lambda max = 290 mm and epsilon = 3000 +/- 600 mol-1 l cm-1, and exhibit reducing properties.