Temperature Behavior of Nanometer-Size Lipid Domains in DPPC:DLPC Model Membranes Studied by Small Angle Neutron Scattering

Abstract

Cellular membranes are no longer viewed as a heterogeneous mix of lipids and proteins, but rather as having distinct lipid domains which are key for many biological processes. Much work devoted to understanding the mechanisms that drives lateral organization in cell membranes has been done on model membrane systems. The study of lipid-lipid phase separation contributes to our understanding of the structure-function relation in the cell membrane. Phase behavior of lipid mixtures has been studied extensively in model lipid membranes using microscopy and spectroscopy techniques. By microscopy, the domains are found to be microns in size. A hypothesis to explain small domains in real cell membranes is that the cytoskeleton generates boundaries, generating small membrane regions with access to only a small amount of lipids and other components. Therefore, to be able to correlate studies of model membranes to the actual plasma membrane, there is a need to characterize lipids domains in a system where they cannot grow more than few nanometers in size. To achieve such a goal, we use Small Unilamellar Vesicles made of a 1:1 ratio of deuterated DPPC and DLPC for which phase separation in large vesicles has been observed. Small Angle Neutron Scattering was used to characterize the size and composition of the domains, which appeared as the temperature was lowered below the Tm of the system (which lies between the two Tm values of each lipid depending on the composition). The scattering was fitted using an ab initio method developed by Svergun and colleagues to analyze scattering data from biological macromolecules. The results show that domains in these systems do not coalesce to form a single stable domain but rather break-up into smaller domains as the temperature lowers.