CHLOROPHYLL PHOTOSENSITIZED ELECTRON TRANSFER IN PHOSPHOLIPID VESICLE SYSTEMS: COMPARISON OF INSIDE‐OUTSIDE ASYMMETRY BETWEEN LARGE AND SMALL UNILAMELLAR VESICLES*

Abstract

Abstract— We have determined the chlorophyll triplet quenching efficiencies, the chlorophyll cation radical yields and the conversion efficiencies of chlorophyll triplet to radical in large and small unilamellar phosphatidylcholine vesicles (LUV and SUV, respectively) in the presence of electrically‐charged electron acceptors (ferricyanide and oxidized cytochrome c) located in either the inner or outer aqueous compartments of the vesicles. Both types of vesicles displayed inside‐outside asymmetry, although the properties were reversed. Triplet quenching in SUV was more efficient when ferricyanide was located within the vesicle interior, whereas the reverse was true in LUV. When ferricyanide was located on the outside of the vesicles, the extent of triplet quenching in LUV was about two times that in SUV and the amount of cation radical formed in LUV was about two times that in SUV. Under these conditions, the conversion efficiencies of chlorophyll triplet to radical were 12.2% for LUV and 8.5% for SUV. With cytochrome c as an electron acceptor in negatively charged vesicles (25 mol per cent dixhexadecylphosphate incorporated) similar results were obtained. Again, the triplet quenching and radical yield inside‐outside asymmetry properties were reversed between the two types of vesicles, and radical formation efficiencies when cyt c was located outside the vesicles were higher in LUV (11.7%) than in SUV (4.2%). We conclude that the inside‐outside asymmetric photochemical behavior of unilamellar phosphatidylcholine vesicles is influenced by factors in addition to the difference in radius of curvature between the inside and outside surfaces. It is suggested that transmembrane electrostatic potentials may be involved. Furthermore, in the present system the properties of LUV were more favorable to photochemical electron transfer product formation than those of SUV.