The vesicle-to-micelle transformation of phospholipid-cholate mixed aggregates: a state of the art analysis including membrane curvature effects

Abstract

We revisited the vesicle-to-micelle transformation in phosphatidylcholine-cholate mixtures paying special attention to the lipid bilayer curvature effects. For this purpose, we prepared unilamellar vesicles with different starting sizes (2r(v)=45-120nm). We then studied mixtures of the unilamellar vesicles (1-8mmol kg(-1)) and sodium cholate (0-11.75mmolkg(-1)) by static and dynamic light scattering. The transformation generally comprises at least two, largely parallel phenomena; one increases and the other decreases the average mixed aggregate size. In our view, cholate first induces bilayer fluctuations that lead to vesicle asphericity, and then to lipid bilayer poration followed by sealing/reformation (or fusion). The cholate-containing mixed bilayers, whether in vesicular or open form, project thread-like protrusions with surfactant enriched ends even before complete bilayer solubilisation. Increasing cholate concentration promotes detachment of such protrusions (i.e. mixed micelles formation), in parallel to further softening/destabilising of mixed amphipat bilayers over a broad range of concentrations. Vesicles ultimately fragment into mixed thread-like micelles. Higher cholate relative concentrations yield shorter thread-like mixed micelles. Most noteworthy, the cholate-induced bilayer fluctuations, the propensity for large aggregate formation, the transformation kinetics, and the cholate concentration ensuring complete lipid solubilisation all depend on the starting mean vesicle size. The smallest tested vesicles (2r(v)=45nm), with the highest bilayer curvature, require ~30% less cholate for complete solubilisation than the largest tested vesicles (2r(v)=120nm).