Sensitized photooxidation of S-methylglutathione in aqueous solution: Intramolecular (S...O) and (S...N) bonded species

Nanosecond laser flash photolysis was used to generate sulfur radical cations of the thioether, S-methylglutathione (S-Me-Glu), via the one-electron oxidation of this thioether by triplet 4-carboxybenzophenone. The purpose of this investigation was to follow the neighboring group effects resulting from the interactions between the sulfur radical cationic sites and nearby lone-pair electrons on heteroatoms within the radical cation, especially the electron lone-pairs on heteroatoms in the peptide bonds. The tripeptide, S-Me-Glu, offers several possible competing neighboring group effects that are characterized in this work. Quantum yields of the various radicals and three-electron bonded (both intramolecular and intermolecular) species were determined. The pH dependence of photoinduced decarboxylation yields was used as evidence for the identification of a nine-membered ring, sulfur–nitrogen, three-electron bonded species. The mechanisms of the secondary reactions of the radicals and radical cations were characterized by resolving their overlapping transient-absorption spectra and following their kinetic behavior. In particular, sulfur–oxygen and sulfur–nitrogen three-electron bonded species were identified where the oxygen and nitrogen atoms were in the peptide bonds.