Kinetic aspects of the solubilization of non-ionic monoalkyl amphiphile-cholesterol vesicles by octylglucoside

Abstract

Sonicated non-ionic surfactant vesicles (NSV) ranging from 66–72 nm in mean diameter (MD) and based on a mixture of diglycerol hexadecylether (C 16G 2), cholesterol (CHOL) and dicetylphosphate (DCP) (47.5, 47.5, 5 wt%) were analyzed by quasielastic light scattering (QELS) and high performance liquid chromatography (HPLC) on gel exclusion column. Sonicated NSV evidenced stable average size and size distribution for 2 weeks at room temperature whereas, after this time, QELS MD values were slightly increasing, mainly due to vesicle aggregation. The continuous dissolution of non-aggregated vesicles by a 100 mM octylglucoside (OG) micellar solution added at different rates (ROG) ranging from 1.39 × 10 −2 to 6.94 × 10 −5 mmol/min was followed at 25°C by monitoring turbidity at 350 nm as a function of time. Lipids-OG mixed aggregates were also prepared by rapidly adding detergent to vesicles and were allowed to equilibrate before their characterization by QELS and turbidity at 350 nm. The results suggested that the solubilization process of sonicated NSV by OG is composed of two main steps depending on the detergent concentration in the continuous phase: (i) the size increase of the vesicles related to a complex mechanism of swelling and fusion, (ii) the rearrangement of detergent and lipid molecules in the aggregates which leads first to the decrease of the apparent particle size, then followed by a macroscopic phase separation and which finally yields pure lipids-OG mixed micelles. The influence of ROG on the detergent composition of both lipid aggregates and aqueous medium showed that these steps are kinetically limited by the insertion of detergent molecules in the vesicle membrane and their further diffusion in the lipid bilayer.