CHLOROPHYLL PHOTOSENSITIZED ELECTRON TRANSFER IN PHOSPHOLIPID VESICLE BILAYERS: INSIDEVSOUTSIDE ASYMMETRY*

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Abstract
Abstract—We have determined triplet quenching efficiencies. radical yields and radical recombination kinetics in mixed chlorophyll (Chl)‐egg phosphatidylcholine vesicle suspensions in the presence of electrically‐charged electron acceptors located either in the external. continuous aqueous phase or within the internal aqueous volume of the vesicles. There was a marked asymmetry between these processes as to whether they occurred at the outer or inner bilayer‐water interfaces.With methyl viologen (MV2+) as acceptor, 52 ± 4% of the total Chl triplet could be quenched from the inside. whereas only 16 ± 2% was quenchable from the outside. Approximately 35% of the triplet population was inacccssible to quenching by MV2+from either inside or outside. Ouenching rate constants were higher from the outside than from the inside (2 × 106M−lS‐Ivs1 × 106M−Is−1). A similar pattern was obtained when anthraquinone disulfonate or ferricyanide were used as acceptors.Radical yields and recombination kinetics also displayed asymmetric behaviour. From the inside. only 4 ± 2% of the quenched triplets gave rise to separated radicals using MV2+as acceptor, whereas from the outside the conversion yield was 32 ± 2%. The halftime for the Chl+MV+reaction was approximately 100 times longer at the outer surface than at the inner surface.We conclude the following: (a) Chl is distributed asymmetrically within the bilayer such that more triplet Chl is located within quenching distance of the interface at the inner surface than at the outer surface. Furthermore, an appreciable fraction of the triplet Chl is located sufficiently far from either interface so that quenching is not possible. (b) The mobility of Chl and quencher molecules is greater at the outer surface of the vesicles than at the inner surface.

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