Photo-oxidation of methionine-containing peptides by the 4-carboxybenzophenone triplet state in aqueous solution. Competition between intramolecular two-centered three-electron bonded (S therefore S)(+) and (S therefore N)(+) formation

Quantum yields for the formation of transients were measured following the quenching of triplet 4-carboxy-benzophenone ((CB)-C-3*) by methionine-conta ining peptides in aqueous solutions. Ketyl radicals (CBH.), ketyl radical a nions (CB.-) and various sulfur radical cations were identified following t he triplet-quenching events, The presence of these intermediates indicated that the triplet-quenching mechanism can be characterized as mainly electro n-transfer in nature. The quenching rate constants were of the order of 2 x 10(9) M-1 s(-1). There were small, but significant, differences in the tri plet-quenching rate constants, and these trends indicate the existence of m ultiple sulfur targets in the quenchers, The absorption of the transient pr oducts was followed in detail by using spectral-resolution analysis. From t he absorption data, quantum yields were estimated for the formation of the various transients. There were differences found in the yields of the trans ient products between the experiments, where the quenchers were the "mixed" stereoisomers of methionylmethionine (L,D and D,L) and experiments where t he quenchers were L,L and D,D stereoisomers, Triplet-quenching data from se veral other methionine-containing small oligopeptides were analyzed in an a nalogous manner. Systematic variations were observed, and these patterns we re discussed in terms of competitive donation of protons to the CB.- within the charge-transfer complex. The competition was between protons on carbon s adjacent to the sulfur-radical center and protons on the protonated amino groups of the radical cation, In addition, there was a competition between the two intramolecular two-centered, three-electron bonded species (S ther efore S)(+) and (S therefore N)(+) that play roles in the secondary kinetic s.