Structural Characterization on Asymmetric Lipid Vesicles at Subnanometer Resolution

Abstract

Mammalian plasma membranes consist of an asymmetric lipid distribution along the two leaflets, in other word the inner leaflet is compositional different from the outer. Bilayer asymmetry is hypothesized to affect various membrane properties, such as membrane potential, surface charge, permeability, and stability. Bilayer asymmetry is also expected to affect structural properties of the membrane, like bilayer thickness and thickness of the single leaflets for example. However, due to the difficulty of preparing asymmetric vesicles the majority of model membrane studies have been performed on symmetric bilayers, where inner and outer membrane leaflets are identical in composition. Of recent, we developed new protocols for the construction and characterization of asymmetric vesicles amiable for scattering and NMR experiments with a well-defined inner and outer leaflet composition. Quantification of bilayer composition and degree of asymmetry enables the determination of transverse structural parameters, such as, the bilayer thicknesses and area per lipid of the various phases in each leaflet. We are able to ascertain these structural parameters through a joint analysis of small angle neutron scattering (SANS) data exploiting D/H contrast variation and small angle X-ray scattering (SAXS). Here we report on the first probe-free analysis yielding insights into a transbilayer coupling mechanisms. First results have shown a decrease in lipid packing density at room temperature of the DPPC-rich phase (outer leaflet) compared to typical gel phase packing, indicating a disordering effect from coupling to the fluid inner leaflet.This work is supported by the Austrian Science Fund FWF, Project No.P27083-B20 (to G.P.).