Mechanism of Dye-sensitized Photooxidation of Tryptophan, Tryptamine, and Their Derivatives. Singlet Oxygen Process in Competition with Type I Process

Article abstract of DOI:10.1246/bcsj.55.2959

Photosensitized oxidation of tryptophan (Trp), tryptamine, and their derivatives were inestigated with a variety of sensitizers.Rose Bengal sensitized photooxidations proceed exclusively via Type II mechanism (singlet oxygen mechanism), whereas for thionine-sensitized photooxidations both Type I (nonsinglet oxygen mechanisms) and Type II mechanisms are operative under a given set of conditions.Rate constants of reaction, in units of 10⁶ M^-1s^-1, were determined to be 3 for Trp, 4 for tryptophan methyl ester, 4 for N_b-acetyltryptophan methyl ester, 9 for tryptamine, and 7 for N_b-methoxycarbonyltryptamine in the Rose Bengal-sensitized photooxidations.These values are in good agreement with those obtained from reactions with chemically generated singlet oxygen.Quenching rate constants are somewhat larger than reaction rates constants, indicating that the quenching process is the major path for the interaction of singlet oxygen with these indolic substrates.Oxidation products in chemically generated singlet-oxygen reaction of Trp are compared with those obtained from dye-sensitized photooxidations.