4-Carboxybenzophenone-sensitized photooxidation of sulfur-containing amino acids. Nanosecond laser flash photolysis and pulse radiolysis studies

Article abstract of DOI:10.1021/ja00052a025

Sulfur-containing amino acids were oxidized via photosensitization by 4-carboxybenzophenone (CB) in neutral aqueous solutions. The mechanism of this reaction was investigated with flash photolysis and pulse radiolysis techniques. The rate constants were determined for the quenching of the CB triplet state by 12 amino acids (with variable relative location and number of terminal functions COOH and NH₂ with respect to the sulfur atom) and were found to be 1.8 × 10⁸-2.9 × 10⁹ M^-1s^-1. Time-resolved transient spectra accompanying the quenching events were assigned to the triplet state, the ketyl radical of CB, the radical anion of CB, and the (S∴S)⁺ radical cations of some of the amino acids. The presence of the radical ions showed the nature of the quenching process to be electron-transfer in character. Two temporally distinct processes were observed for ketyl radical formation. A fast component occurred on a nanosecond time scale. It is ascribed to electron transfer from the sulfur atom to the triplet state of CB followed by (1) the diffusion apart of the charge-transfer (CT) complex and (2) the intramolecular proton transfer within CT complex. The first process was the more efficient one and led to the formation of sulfur-centered radical cations and ketyl radical anions which undergo fast protonation. A slower formation of ketyl radicals occurred on a microsecond time scale and is characterized by pseudo-first-order rate constants, which depend linearly on the CB ground-state concentration (k ～ 10⁹ M^-1 s^-1). This dark reaction is assigned to the one-electron reduction of CB by the α-aminoalkyl radicals produced from the free-radical cation of the amino acids as a result of intramolecular electron transfer from the carboxyl group to the sulfur-centered radical cation followed by decarboxylation. Yields were determined for some of the processes: the limiting overall quantum yields of ketyl radical (0.4-1.4), the quantum yields of ketyl radical formed in the primary photochemical process and in the dark reaction, and quantum yields of ketyl radical anion. A detailed mechanism for the CB-sensitized photooxidation of sulfur-containing amino acids is proposed and discussed.