COMPARATIVE-STUDY OF FLUORESCENCE ANISOTROPY IN SURFACE MONOLAYERS OF EMULSIONS AND BILAYERS OF VESICLES

Surface rigidities of emulsion particles composed of triglyceride (TG) and phosphatidylcholine (PC), and PC vesicles were investigated using the fluorescent probe 1-[4-(trimethylamino)phenyl]phenylhexa-1,3,5-triene (TMA-DPH), which was anchored at the phospholipid-water interface. Steady-state fluorescence anisotropy of TMA-DPH in surface monolayers of emulsion particles was higher than that in bilayers of vesicles. A longer fluorescence lifetime of TMA-DPH in emulsion surface monolayers was observed compared to bilayers. These results indicated that the surface monolayers of emulsion particles were more rigid than bilayers, consistent with the finding that the PC molecules formed a condensed monolayer at the TG-saline interface (Handa, T.; Saito, H.; Miyajima, K. Biochemistry 1990, 29, 2884-2890). Distribution of cholesterol (Chol) between the emulsion surface and core was evaluated on the basis of interfacial tension measurements. The anisotropy value in the emulsion particles did not change up to 20 mol % and began to increase at about 30 mol % or greater of surface Chol, in contrast to continuous increases in the fluorescence anisotropy in bilayer vesicles. When an approximately 20 mol % of Chol was present in surface layers, the fluorescence lifetimes of TMA-DPH increased for bilayer vesicles but did not change for emulsion particles. Fluorescence lifetimes increased for both emulsion particles and bilayer vesicles with 40 mol % of surface Chol. These effects of Chol on the fluorescence properties of emulsion particles and bilayers suggested that Chol was accommodated in a different way for the surface monolayers of emulsion particles and bilayers of vesicles.