Photooxidation of methionine derivatives by the 4-carboxybenzophenone triplet state in aqueous solution. Intracomplex proton transfer involving the amino group

Oxidation of the triplet state of 4-carboxybenzophenone (CB) by a series of five substituted methionines and three methionine-containing dipeptides wa s monitored under laser flash photolysis conditions in aqueous solution. Sp ectral resolution techniques were employed to follow the concentration prof iles of the intermediates formed from the quenching events. From these conc entration profiles, quantum yields for the intermediates were determined. B ranching ratios were evaluated for the decay of the charge-transfer complex by the competing processes of back electron transfer, proton transfer and escape of radical ions. The relative prominence of these processes was disc ussed in terms of the proton-transfer tendencies of the nominal sulfur-radi cal-cationic species. A systematic decrease was observed in the quantum yie lds for the escape of radical ions along with a correlated increase in the proton-transfer yields. The enhanced propensity of the sulfur radical catio ns to deprotonate is due to deprotonation at the carbons adjacent to the su lfur-cationic site and at the unsubstituted amino groups when present, This scheme was supported by an observed decrease in the yields of dimeric sulf ur radical cations with an increase in the electron-withdrawing abilities o f the substituents, making the radical-cationic species stronger acids. The involvement of protons on the amino groups was implicated by the correlati on of the quantum yields of ketyl radical formation in the photochemistry e xperiments with the rate constants for the reaction of the CB radical anion with the sulfur-containing substrates in pulse radiolysis experiments.