PHOTOCHEMICALLY CONTROLLED ION PERMEABILITY OF LIPOSOMAL MEMBRANES CONTAINING AMPHIPHILIC AZOBENZENE

We have succeeded in reversible control by the photochemical method of the K+ permeability across a membrane of liposomes consisting of L-alpha-dimyristoyl phosphatidylcholine (DMPC), dicetyl phosphate (DCP), and 4-octyl-4'-(5-carboxypentamethyleneoxy)azobenzene (8A5) (molar ratio, [DMPC]:[DCP]:[8A5] = 10:1:0-1). In the dark at 288 K, permeability coefficient, P, was 10(-12)-10(-13) cm/s. When liposome dispersion was irradiated with UV light, trans-to-cis photoisomerization of 8A5 occurred and two regions distinguished by variation of P with time were observed, the "main region" and "following region". In the former region, P increased rapidly to saturated value immediately after the onset of UV light and remained constant for > 30 min. In the latter region, P slowly increased again. In the "main region", 5000 times enhancement of P was observed, which resulted in P of 10(-8) cm/s. When the liposome dispersion was irradiated with visible light after UV irradiation, P decreased again to the original value. The exponential relation between P and molar ratio of [cis-8A5] to [DMPC] has been established. It has been found that permeability of the bulky ion having negative charges, [Fe(CN)6]3-, is also increased by UV irradiation. Steady increase of P in the "following region", which occurred after prolonged UV irradiation at 288 K, was attributed to deformation and mutual fusion of liposomes.