NSLS-II Seminar

The model phospholipid membranes are of considerable scientific and practical interest because they provide a window into understanding the thermodynamics for phase separation of membrane components in a living cell. Within this framework, intense debate currently surrounds the understanding of the fundamental basis for the formation and dynamics of the so called raft micro-domains, which have been broadly implicated in many membrane functions. Here we report an unusual consequence of a physical property of the membrane-substrate interface in influencing phase separation in supported bilayers. The planarity of the interface constrains the head-groups of the lower leaflet to organize in a single topologically-defined plane. As a consequence, we find that the topology of the outer leaflet becomes strongly corrugated in multicomponent bilayers. Influences of such imposed physical topology on phase separation are also discussed. The effect of the membrane-substrate interaction is further demonstrated in a multi-stack bilayer system. Off-specular diffuse scattering shows a kink which corresponds to the cutoff wave vector of surface capillary waves. In order to circumvent the substrate proximity effect and fully lift the dynamics of lipid membranes, we studied lipid membrane multilayers in humidity. Both x-ray and neutron scattering techniques are applied to study their phase transition. Below the phase transition temperature, a new phase which is related to the raft formation appears with distinct bilayer spacing and pops off from the lipid mixture. We also observed for the first time the dynamics of lipid membrane multilayers in humidity with X-ray Photon Correlation Spectroscopy (XPCS).