Understanding the Behavior of Nanometer-Size Lipid Domains in Model Membranes: A Small Angle Neutron Scattering Study

Abstract

Lipid-lipid phase separation is important in understanding the behavior of the biological membrane. Such phenomenon has been studied extensively in model lipid membranes using microscopy techniques where domains are found to be microns in size. In actual biological membranes, however, domains are smaller, and microscopy techniques are unable to detect them. A hypothesis to explain these small domains is that the cytoskeleton generates boundaries to compartmentalize the membrane into small sub-membrane regions with access to only small amounts of lipids in the lifespan of lipid domains. Therefore, to be able to correlate studies of model membranes to the actual plasma membrane, there is a need to characterize lipid domains in a system where they cannot grow more than few nanometers in size. To achieve such a goal, we use small Unilemellar Vesicles (ULVs) made of 1:1 and 3:7 ratios of DPPC (deuterated-DPPC) and DLPC respectively for which phase separation in large vesicles has been observed. Using small vesicles with varying sizes (diameters from 30nm to 400nm) not only provides a means to control curvature, but also limits the amount of available lipids for domain growth. Small Angle Neutron Scattering was used to characterize the size, density and average composition of the domains, which appeared as the temperature was lowered below Tm, the melting temperature of the system. The scattering curves were fitted using a pair-correlation method in order to extract the “local structure” of the vesicles. The results interestingly suggest that the nanometer domains in these systems do not coalesce to form a single stable domain as observed in giant vesicles. Overall, thiswork provides insight into the behavior of nano-meter size lipid-lipid phase separation as a function of composition, temperature, vesicle-size and curvature.