COMPARATIVE-STUDY OF FLUORESCENCE ANISOTROPY IN SURFACE MONOLAYERS OFEMULSIONS 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 -[4-(trimethylamino)phenyl]phenylhexa-1,3,5-tri ene (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 monolayer s 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 (Ch ol) between the emulsion surface and core was evaluated on the basis o f interfacial tension measurements. The anisotropy value in the emulsi on particles did not change up to 20 mol % and began to increase at ab out 30 mol % or greater of surface Chol, in contrast to continuous inc reases in the fluorescence anisotropy in bilayer vesicles. When an app roximately 20 mol % of Chol was present in surface layers, the fluores cence lifetimes of TMA-DPH increased for bilayer vesicles but did not change for emulsion particles. Fluorescence lifetimes increased for bo th emulsion particles and bilayer vesicles with 40 mol % of surface Ch ol. These effects of Chol on the fluorescence properties of emulsion p articles and bilayers suggested that Chol was accommodated in a differ ent way for the surface monolayers of emulsion particles and bilayers of vesicles.