pH effects on the photooxidation of methionine derivatives by the 4-carboxybenzophenone triplet state

Reduction of the triplet state of 4-carboxybenzophenone (CB) by a series of five substituted methionines was monitored under laser flash photolysis co nditions in aqueous solution. Spectral resolution techniques were employed to determine initial quantum yields of the photoproducts. Branching ratios were evaluated for the decay of the charge-transfer complex by the competin g processes of back electron transfer, proton transfer, and escape of radic al ions. The relative prominence of these processes was discussed in terms of the proton-transfer tendencies of the nominal sulfur-radical-cationic sp ecies. A scheme was proposed to account for the pH effects observed. The ki netic equations were solved with a minimum of assumptions, and simulations were performed that qualitatively support the proposed scheme.