KINETIC ASPECTS OF THE ROSE BENGAL-SENSITIZED PHOTOOXYGENATION OF TRYPTOPHAN ALKYL ESTERS - GROUND-STATE AND PHOTOPROMOTED DYE-TRYPTOPHAN DERIVATIVE INTERACTIONS

The kinetics of the rose bengal (RB)-promoted photodynamic action on t ryptophan (trp) and a series of tryptophan alkyl esters (methyl, butyl and octyl derivatives) was studied. The employment of auxiliary speci fic quenchers for superoxide ion and singlet molecular oxygen (O-2((1) Delta(g))) indicates that the O-2((1) Delta(g))-mediated process cons titutes the major path in the photo-oxygenation process (with a contri bution of about 70% to the total oxygen consumption upon RE-sensitized irradiation of the tryptophan derivatives). Blocking of the carboxyli c group (esterification) greatly increases the O-2((1) Delta(g)) physi cal quenching ability of the amino acid derivatives. The effect is mor e pronounced for those compounds possessing longer hydrocarbon chains. Besides, the pathway for chemical (reactive) quenching is slightly de creased in a general manner, this behaviour being independent of the b locking group. Results were corroborated by employing methylene blue a s a dye sensitizer. Additionally, the interactions of the amino acid d erivatives with RE excited and ground states were also studied. Laser flash photolysis determinations indicate that the tryptophan derivativ es interact with the excited triplet state of the dye with rate consta nts of the order of 10(7) M(-1) s(-1). A ground state complexation bet ween RE and the amino acid derivatives took place in the dark. Associa tion constants, determined by static fluorescence quenching of the dye , increased with the length of the hydrocarbon chain of the ester. The y ranged from 40 M(-1) for trp to 18 000 M(-1) for trp-octyl ester. Th e interaction was governed by an additive effect of charge transfer an d hydrophobic bond formation.