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

Abstract

Photosensitized oxidations of tryptophan (Trp), tryptamine, and their derivatives were investigated 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−1 s−1, were determined to be 3 for Trp, 4 for tryptophan methyl ester, 4 for Nb-acetyltryptophan methyl ester, 9 for tryptamine, and 7 for Nb-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.