Photosensitized oxidation of methionine derivatives. Laser flash photolysis studies

The early events in the triplet 4-carboxybenzophenone (CB)-induced oxidation of N-acetyl-methionine methyl ester (N-Ac-Met-OCH3) are investigated in aqueous solution. Upon electron transfer from the methionine residue of N-Ac-Met-OCH3 to (CB)-C-3*, the resulting sulfur radical cation undergoes further reactions: (1) back electron transfer, (2) escape of the radical ions from the solvent cage, or (3) proton transfer and escape of the radicals. The yields and paths of these reactions are shown to depend strongly on the pH of the solution, and, similar to the previously reported results for dipeptides (Met-Gly and Gly-Met), on the structural nature of the methionine substituents. In the experiments performed in this work, low quencher concentrations were used to avoid formation of intermolecular transients (e.g., dimeric sulfur-centered radical cation (Sa ' S)(+)). Under these experimental conditions, the one-electron oxidized sulfur does not seem to become stabilized in an (Sa ' N)(+) three-electron bonded intramolecular complex. The proposed mechanism is further supported by the stable products analysis. A detailed mechanism involving characterization of the transients is discussed and compared to that of methionine and methionine-containing dipeptides (Met-Gly and Gly-Met). Moreover, a newly installed transient absorption laser system is described in details.