Sensitized photo-oxidation of sulfur-containing amino acids and peptides in aqueous solution

Qualitative and quantitative studies of the photo-oxidation of sulfur-containing amino acids and methionine-containing dipeptides and tripeptides in aqueous solution sensitized by 4-carboxybenzophenone (CB) are reviewed. The mechanism of the photo-oxidation reaction was investigated using the techniques of flash photolysis, steady state photolysis and pulse radiolysis. The rate constants for quenching of the CB triplet by twelve sulfur-containing amino acids and six methionine-containing peptides were determined to be in the range 108–109 M−1 s−1 for both neutral and alkaline solutions. The amino acids varied in structure, having different numbers of COCH and NH2 terminal groups and their sulfur atom at different locations relative to the terminal groups. The methionine-containing peptides were MetGly, GlyMet, MetMet MetGlyGly, GlyMetGly and GlyGlyMet. Time-resolved transient spectra accompanying the quenching events were assigned to the triplet states of CB, ketyl radicals of CB, radical anions of CB and radical anions of CB and radical cations derived from the amino acids and peptides. The radical cations identified were intermolecularly (S.·.S)+-bonded cations, intramolecularly (S.·.N)+-bonded cations and an intramolecularly (S.·.S)+-bonded radical cation that was observed in experiments with MetMet. The quantum yields of the transients and their kinetics of formation and decay were measured by flash photolysis. The quantum yields of CO2 formation were determined by steady state photolysis. Electron transfer from the sulfur atom to the triplet state of the ketone was found to be a primary photochemical step. A detailed mechanism of the CB-sensitized photo-oxidation of sulfur-containing amino acids and methionine-containing peptides, including primary and secondary photoreactions, is proposed and discussed. Within the mechanism, contrasting behavior between the peptides and amino acids as quenchers is emphasized.