CHLOROPHYLL-QUINONE PHOTOCHEMISTRY IN LIPOSOMES: MECHANISMS OF RADICAL FORMATION AND DECAY*

Abstract— Laser flash photolysis has been used to investigate the mechanism of formation and decay of the radical species generated by light-induced electron transfer from chlorophyll a (Chi) triplet to various quinones in egg phosphatidyl choline bilayer vesicles. Chlorophyll triplet quenching by quinone is controlled by diffusion occurring within the bilayer membrane (kq～ 10⁶M^?1 s^?1. as compared to ～ 10⁹ M^?1 s^?1 in ethanol) and reflects bilayer viscosity. Radical formation via separation of the intermediate ion pair is also inhibited by increased bilayer viscosity. Cooperativity is observed in the radical formation process due to an enhancement of radical separation by electron transfer from semiquinone anion radical to a neighboring quinone molecule. Two modes of radical decay are observed, a rapid (t_1/2= 150μ) recombination between Chi and quinone radicals occurring within the bilayer and a much slower (t_1/2= 1–100 ms) recombination occurring across the bilayer-water interface. The latter is also cooperative, which accounts for a t_1/2 which is dependent upon quinone concentration. The slow decay is only observed with quinones which are not tightly anchored into the bilayer, and is probably the result of electron transfer from semiquinone anion radical formed within the bilayer to a quinone molecule residing at the bilayer-water interface. Direct evidence for such a process has been obtained from experiments in which both ubiquinone and benzoquinone are present simultaneously. With benzo-quinone, approx. 60% of the radical decay occurs via the slow mode. Triplet to radical conversion efficiencies in the bilayer systems are comparable to those obtained in fluid solution (～ 60%). However, radical recombination, at least for the slow decay mechanism, is considerably retarded.