pH-dependence of phase transition of the lipid bilayer of liposomes of stratum corneum lipids

Phase transition of the lipid bilayer of liposomes of stratum corneum lipids, consisting of ceramide, cholesterol, cholesteryl sulfate and palmitic acid, was examined at different pH values by monitoring the fluorescence anisotropy of diphenylhexatriene (DPH). The phase transition temperature decreased about 15 degrees C with decrease of the pH from 6.2 to 4.0. The decrease depended on the content of palmitic acid in liposomes and corresponded with the protonation of the anionic form of fatty acid judging from the fluorescence intensity and fluorescence anisotropy of the cationic trimethylammonium derivative of DPH (TMA-DPH). These results and pH-dependent change in fluorescence anisotropy of 2-(9-anthroyloxy)palmitic acid (2-AP) indicate that perturbation of lamellar lipids is induced by the undissociated form of fatty acids. Stimulation of transdermal penetration of phenol at acidic pH was observed which was consistent with the results on liposomes. A pH-dependent decrease of the phase transition temperature of liposomes was also observed on replacement of palmitic acid by other saturated fatty acids, the decreases being in the order lauric, myristic > palmitic > stearic acid. In the presence of lauric acid at pH 4.0, the fluorescence anisotropy of DPH also decreased at physiological temperatures, this effect being similar to those of long-chain cis-unsaturated fatty acids.