Kinetic aspects of the rose bengal-sensitized photo-oxygenation of tryptophan alkyl esters. Ground state and photopromoted dye-tryptophan derivative interactions

The kinetics of the rose bengal (RB)-promoted photodynamic action on tryptophan (trp) and a series of tryptophan alkyl esters (methyl, butyl and octyl derivatives) was studied. The employment of auxiliary specific quenchers for superoxide ion and singlet molecular oxygen (O-2((1) Delta(g))) indicates that the O-2((1) Delta(g))-mediated process constitutes the major path in the photo-oxygenation process (with a contribution of about 70% to the total oxygen consumption upon RE-sensitized irradiation of the tryptophan derivatives). Blocking of the carboxylic group (esterification) greatly increases the O-2((1) Delta(g)) physical quenching ability of the amino acid derivatives. The effect is more pronounced for those compounds possessing longer hydrocarbon chains. Besides, the pathway for chemical (reactive) quenching is slightly decreased in a general manner, this behaviour being independent of the blocking group. Results were corroborated by employing methylene blue as a dye sensitizer. Additionally, the interactions of the amino acid derivatives with RE excited and ground states were also studied. Laser flash photolysis determinations indicate that the tryptophan derivatives interact with the excited triplet state of the dye with rate constants of the order of 10(7) M(-1) s(-1). A ground state complexation between RE and the amino acid derivatives took place in the dark. Association constants, determined by static fluorescence quenching of the dye, increased with the length of the hydrocarbon chain of the ester. They ranged from 40 M(-1) for trp to 18 000 M(-1) for trp-octyl ester. The interaction was governed by an additive effect of charge transfer and hydrophobic bond formation.