Action of octylglucoside on non-ionic monoalkyl amphiphile-cholesterol vesicles: study of the solubilization mechanism

Abstract

Non-ionic surfactant vesicles (NSV) were prepared at room temperature, from a mixture of diglycerol hexadecylether (C 16G 2) and cholesterol (CHOL) with a small amount of dicetylphosphate (DCP) (47.5, 47.5, 5 wt%) by either sonication or detergent dialysis of octylglucoside (OG)-lipids mixed micelles. NSV were characterized by quasielastic light scattering (QLS) and high performance liquid chromatography (HPLC) on gel exclusion column. Respective mean diameters of 72 nm for sonicated NSV and of 287 and 322 nm for NSV prepared by detergent dialysis were found. The continuous dissolution of both small vesicles (SV) and large ones (LV) by a 100-mM or 400-mM OG micellar solution was systematically examined by monitoring turbidity at 350 nm. The molecular composition of aggregates [OG/lip] agg as well as the OG concentration in the continuous phase [OG] bulk were determined at break points observed on the solubilization curves. Initial vesicles and mixed aggregates at each break point were also characterized by plots of optical density (OD) versus total lipid concentration [lip] tot. The solubilization curves of SV and LV exhibit different shapes until [OG] bulk reaches about the critical micellar concentration (cmc) of pure OG; thereafter one single dissolution process occurs involving the same intermediate aggregates for SV and LV. At any stage of the solubilization, [OG/lip] agg and [OG] bulk remained independent of the size of the corresponding aggregates, suggesting that NSV solubilization is governed by molecular processes. By comparison with the OG-egg phosphatidylcholine (EPC) system previously studied, it was found that the mechanism of NSV dissolution by OG is very similar to that of EPC small unilamellar vesicles (EPC SUV) solubilization by the same detergent. However, it has been shown that (i) NSV are impermeable to OG until [OG] tot = 13 mM, (ii) their transformation into mixed micelles is kinetically dependent on the lateral diffusion of the detergent molecules in lipid bilayers. The rather efficient barrier of the NSV membrane to OG suggests a strong cohesion of C 16G 2 and CHOL.