CHLOROPHYLL PHOTOSENSITIZED ELECTRON TRANSFER REACTIONS IN LIPID BILAYER VESICLES: GENERATION OF PROTON GRADIENTS ACROSS THE BILAYER COUPLED TO QUINONE REDUCTION AND HYDROQUINONE OXIDATION

Abstract— A chlorophyll-containing small unilamellar lipid bilayer vesicle system with a sulfonated quinone molecule (MQS) in one aqueous compartment and a sulfonated hydroquinone molecule (H₂QS) in the other has been investigated, using laser flash photolysis and steady-state irradiation, as a means of storing light energy in the form of a proton gradient across the lipid bilayer. Under optimal conditions, an efficiency of 39% based on the chlorophyll triplet state quenched has been achieved for vectorial electron transfer across the bilayer; this corresponds to a quantum yield of 23% based on absorbed photons. As a consequence of irradiation by a single laser flash, 0.2 μ M of protons were taken up by quinone reduction (MQS → H₂MQS) in the outer compartment. The same number of protons were released in the inner compartment by hydroquinone oxidation (H₂QS → QS). Since the volume occupied by the vesicles was only 1/1000 of the total volume of the sample, the local concentration of protons in the inner compartment was 1000 times larger ( i.e. ? 200 μ M ), resulting in the generation of an appreciable proton gradient across the bilayer.