Bilayer-to-micelle transitions of phospholipid vesicles induced by surfactant interactions

Abstract

Interactions between two different supramolecular self-assemblies, phospholipid bilayer vesicles and surfactant micelles, and thermotropic and lyotropic phase behaviors in aqueous mixtrues of them were systematically investigated by methods of turbidity titration, dynamic laser light scattering, fluorometric assay detecting the release of fluoropore, and differential scanning calorimetry. The effects of cholesterol and cetyl alcohol on the stability of bilayer vesicles against various surfactants were also studied. The bilayer vesicle despersions were made of egg yolk lecithin(EYL), egg yolk phosphatidylcholine (EPC), dipalmitoyl phosphatidylcholine (DPPC), and dioleyl phosphatidylethanloamine(DOPE) vesicles incorporated with immunoglobulin G(IgG), respectively. As the water-soluble and micelle forming amphiphiles, monosialoganglioside, poly(oxyethylene) fatty alcohols as diblock copolymer which have various lengths of tails and headgroups, and conjugated and unconjugated bile salts wewr used to analyze the effect of various surfactant structures, effects of the values of hydrophile-lipophile balance graded by either size of hydrophobic tail or head group, and effects of configurational and/or conjugational states of bile salts. The turbidities of the mixed dispersions of sonicated phospholipid vesicles and nonionic-, diblock copolymer surfactants, POE(n) fatty alcohols, were systematically measured as a function of the surfactant added for a wide range of lipid concentrations (from 0.51 to 6.35 mM EPC). The results allowed the phase diagrams of EPC-POE(n) fatty alcohols-excess water systems as well as the quantitative determinations of solubilization parameters below. Considering the three-stage model for bilayer-to-micelle phase transitions of phospholipid vesicles induced by surfactants, namely solubilization, the effective molar ratios of surfactant to phosphilipid in the surfactant-saturated liposomes, $R^sat$, and in the mixed micelle state, $R^sol$, were...