In Situ Determination of Structure and Fluctuations of Coexisting Fluid Membrane Domains

Biophysical understanding of membrane domains requires accurate knowledge of their structural details and elasticity. We report on a global small angle x-ray scattering data analysis technique for coexisting liquid-ordered (L(o)) and liquid-disordered (L(d)) domains in fully hydrated multilamellar vesicles. This enabled their detailed analysis for differences in membrane thickness, area per lipid, hydrocarbon chain length, and bending fluctuation as demonstrated for two ternary mixtures (DOPC/DSPC/CHOL and DOPC/DPPC/CHOL) at different cholesterol concentrations. L(o) domains were found to be ∼10 Å thicker, and laterally up to 20 Å(2)/lipid more condensed than L(d) domains. Their bending fluctuations were also reduced by ∼65%. Increase of cholesterol concentration caused significant changes in structural properties of L(d), while its influence on L(o) properties was marginal. We further observed that temperature-induced melting of L(o) domains is associated with a diffusion of cholesterol to L(d) domains and controlled by L(o)/L(d) thickness differences.