X-ray Studies of Free Floating and Supported Bilayers Formed from Cholesterol Phospholipid Mixtures

Phospholipid bilayers form the structural scaffolding around which the plasma membranes of eukaryotic cell membranes are built. As a consequence, the structural and mechanical properties of these bilayers provide important input for understanding functions of the membrane such as transport, trafficking and protein-membrane interactions. We have used small angle x-ray scattering (SAXS) and x-ray reflectivity (XRF) to study the structure of bilayers comprised of varying mixtures of DPPC and Cholesterol in both the supported geometry (on top of single crystal Si-111 substrates) and as free floating liposomes. Analysis of the scattering provides information on head group spacing, roughness, the liquid-gel transition, leaflet asymmetries and the differences between supported and free bilayers. These results may provide insight into how cholesterol content is related to membrane function, and why certain membranes, such as the mammalian eye-lens membranes have extraordinarily high cholesterol content.