Measuring how lipid membranes respond to physical perturbations: How charge alters lipid miscibility, how shear disrupts interleaflet coupling, and how prebiotic compounds affect membrane stability

Abstract

The physical and chemical properties of lipid bilayers determine their biological function as the main structural component of cell membranes. The response of lipid membranes to various perturbations is, in general, difficult to predict. This dissertation will discuss four sets of experiments to probe the behavior of lipid membranes in different conditions. In the first set of experiments, we show that the presence of charged lipids has a much smaller effect on the miscibility phase separation of lipid bilayers than predicted by simple theories. This conclusion is supported by the finding that the transition temperature of systems containing charged lipids are similar to the transition temperatures of systems without charged lipids, and by the finding that the addition of monovalent salt, which screens charge interactions, has a small effect on the transition temperatures of bilayers containing charged lipids. The second set of experiments examines the effect of high shear on phase-separated membranes. We find that an external shear is sufficient to move domains in each leaflet out of registration. By quantifying these results, we obtain a value for the free energetic cost per unit area of misregistration. In the third set of experiments, we show that it is possible to create bilayers containing high fractions of charge and in highly salty solutions. We achieve this using the method cDICE, where vesicles are created by centrifuging aqueous droplets through a layer of lipids dissolved in oil. We further show that in this scheme, no detectable amount of cholesterol is incorporated into bilayers. Finally, we examine interactions between bilayers of fatty acids and the building blocks of RNA, nucleobases and sugars. We show that the nucleobases and sugar that are found in RNA bind to fatty acid aggregates, and do so more strongly than chemically similar molecules. We also find that nucleobases and sugar stabilize fatty acid vesicles against flocculation in salty solutions. In contrast, under control conditions, salt induces the aggregation of vesicles into dense structures. These two effects suggest a role for fatty acid vesicles in the origin of life as a stable membrane in a salty ocean that could select and concentrate the components required to make RNA.