Lipid redistribution in phosphatidylserine-containing vesicles adsorbing on titania

Lipid vesicles (liposomes) exhibit a wide range of behavior at inorganic (oxide) surfaces. A complete understanding of the vesicle-surface interactions, and of the ensuing transformations surface adsorbed liposomes undergo, has proven elusive. This is at least in part due to the large number of degrees of freedom of the system comprising vesicles with their molecular constituents, substrate surface, and electrolyte solution. The least investigated among these degrees of freedom are those intrinsic to the vesicles themselves, involving rearrangements of lipid molecules. In this study, the adsorption of two-component vesicles (phosphatidylcholine:phosphatidylserine) on titanium dioxide was investigated by dual polarization interferometry. Mixtures of these two lipids containing more than 20% of phosphatidylserine form supported bilayers on titania, with phosphatidylserine predominantly facing the surface of the oxide. The purpose of this investigation is to ascertain whether redistribution of phosphatidylserine occurs already in the adsorbing vesicles. Indeed, this was found to be the case. A possible mechanism of this process is discussed.