DIRECT OBSERVATION OF ELECTRON TRANSFER ACROSS A LIPID BILAYER: PULSED LASER PHOTOLYSIS OF AN ASYMMETRIC VESICLE SYSTEM CONTAINING CHLOROPHYLL,METHYL VIOLOGEN AND EDTA*

Abstract— Suspensions of vesicles composed of chlorophyll a (Chi) and phospholipid that were asymmetric with respect to aqueous solutions of methyl viologen (MV₂₊), an electron acceptor, and EDTA, an electron donor, were investigated using both flash and steady-state photolysis techniques. It was shown that Chl-photosensitized electron transfer occurred across the walls of the vesicles from EDTA to MV₂₊. Flash photolysis indicated that MV₂₊ dissolved in the interior aqueous compartments of the vesicles oxidized only those triplet excited state Chi molecules that were dissolved in the inner monolayers of the vesicle walls. The resultant radical products, Chi₊ and MV₊, recombined with a halftime of the order of 10_-4s. EDTA, added externally to the vesicles, competed effectively with MV₊ as a reducing agent for Chl₊. This places a lower limit of 10₄ s_-1 on the rate constant for transmembrane electron transfer. Compartmentalization by the vesicle wall of the competing pathways for the reduction of Chi₊ resulted in a nonlinear dependence of the rate constant of Chl₊ decay on EDTA concentration. The magnitude of the rate constant of electron transfer through the membrane and the way that the kinetics of Chl₊ decay depended on the concentration of Chi in the membrane strongly suggest that the electron transfer occurred by electron exchange between Chi and Chl₊.