SENSITIZED PHOTOOXIDATION 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 tripep tides in aqueous solution sensitized by 4-carboxybenzophenone (CB) are reviewed. The mechanism of the photo-oxidation reaction was investiga ted using the techniques of flash photolysis, steady state photolysis and pulse radiolysis. The rate constants for quenching of the CB tripl et by twelve sulfur-containing amino acids and six methionine-containi ng peptides were determined to be in the range 10(8)-10(9) M(-1) s(-1) for both neutral and alkaline solutions. The amino acids varied in st ructure, having different numbers of COOH and NH2 terminal groups and their sulfur atom at different locations relative to the terminal grou ps. The methionine-containing peptides were Met-Gly, Gly-Met, Met-Met, Met-Gly-Gly, Gly-Met-Gly and Gly-Gly-Met. Time-resolved transient spe ctra accompanying the quenching events were assigned to the triplet st ates of CB, ketyl radicals of CB, radical anions of CB and radical cat ions derived from the amino acids and peptides. The radical cations id entified were intermolecularly (S therefore S)(+)-bonded cations, intr amolecularly (S therefore N)(+)-bonded cations and an intramolecularly (S therefore S)(+)-bonded radical cation that was observed in experim ents with Met-Met. The quantum yields of the transients and their kine tics of formation and decay were measured by hash photolysis. The quan tum yields of CO2 formation were determined by steady state photolysis . Electron transfer from the sulfur atom to the triplet state of the k etone was found to be a primary photochemical step. A detailed mechani sm of the CB-sensitized photo-oxidation of sulfur-containing amino aci ds and methionine-containing peptides, including primary and secondary photoreactions, is proposed and discussed. Within the mechanism, cont rasting behavior between the peptides and amino acids as quenchers is emphasized.