Systematic study on the solubilization of phospholipid vesicles by various surfactants

Abstract

The solubilization of phospholipid vesicles by surfactants were investigated for systematically selected lipid—surfactant combinations. The solubilization process was followed by the change in 90° scattered-light intensity resulting from the stepwise addition of surfactant solution to the vesicle preparation. Two break points appeared in the turbidity curve, which allowed a determination of the surfactant concentrations corresponding to the onset and completion of the solubilization. From the lipid concentration dependence of these critical surfactant concentrations, the effective surfactant to lipid molar ratios in the surfactant-saturated lipid bilayer, R sat, and in the lipid-saturated mixed micelle, R sol, were estimated according to the “three-stage” model proposed for the vesicle solubilization. For phosphatidylcholine vesicles, both R sol and R sat exhibited no significant dependence on the alkyl chain length and head group type of the surfactants. When the acyl chain length of the lipids was shortened, the R sol value decreased, while R sat remained unaltered. For negatively charged phosphatidic acid vesicles, R sol depended strongly on the head group charge of the surfactants. The evaluation of the solubilizing power of surfactants in terms of R sol values leads to the conclusion that the solubilizing power for electrically neutral phosphatidylcholine vesicles is primarily determined by the lipid acyl chain length rather than the alkyl chain length and head group type of the surfactants. The viscosity measurements demonstrated that the rod-like or disc-like mixed micelles initially formed by the lipid solubilization transform to spherical micelles at higher concentration of the surfactants.