Understanding The Nucleation and Growth of Lipid-Lipid Phase Separation at Nanometer-Length-Scale: A Small Angle Neutron Scattering Study

Abstract

Although lipid-lipid phase separation has been studied extensively in model lipid membranes, most of the available information is limited to micron-size domains. Lack of information on the nanometer-length-scale is a big hurdle in understanding the fundamental basis for lipid domain nucleation, in the first place, and growth. How do small unstable domains grow? What is the effect of temperature and membrane curvature? To answer these questions, this work presents Small Angle Neutron Scattering (SANS) studies on small Unilemellar Vesicles (ULVs) (diameter varying from 30nm to 400nm) made of 1:1 and 3:7 ratios of deuterated DPPC (dDPPC) and hydrogenated DLPC respectively. Small vesicles with varying sizes not only provide a means to control the curvature, but also limit the amount of available lipids for domain growth. Experiments were performed in contrast matched conditions, such that the scattering length density of the solvent and that of the homogeneous lipid vesicle (above the melting transition temperature, Tm, of the mixture) were exactly equal. As the sample temperature was lowered below Tm, the lipids started to phase separate and an excess scattering characteristic of lipid segregation into nano-size domains was observed. Notable trends were observed in the scattering curves as the temperature was lowered below the Tm of the mixtures. Interestingly, these results show that the phase separation behavior varies significantly between the small and large size vesicles. Insight in to the basic mechanism for the formation of lipid-lipid phase separation as a function of temperature and curvature will be discussed based on the analysis of the scattering intensities and Differential Scanning Calorimetry experiments.