Investigation of Transbilayer Coupling in Gel-Fluid Asymmetric Lipid Vesicles

Biological membranes display an asymmetric lipid distribution along the bilayer normal. Membrane asymmetry is also thought to affect the bilayer and the individual bilayer leaflet thicknesses. Recently, protocols enabling the preparation and characterization of stress-free, free-floating asymmetric vesicles (aLUVs) via a cyclodextrin mediated exchange were developed [1]. Here, we report of aLUVs studies with gel and fluid phase bilayer leaflets using small- and wide-angle x-ray scattering (SWAXS), and small-angle neutron scattering (SANS). Together, these techniques allow for obtaining the structural details of the two membrane leaflets. No hydrocarbon chain correlation peak was observed in the wide-angle regime of DPPC/POPC aLUVs at room temperature, consistent with an earlier observation of the partial fluidization of gel DPPC through coupling to the inner, fluid POPC leaflet [1]. On the other hand, POPE/POPC aLUVs, below the melting temperature of pure POPE, showed a WAXS-peak typical of hydrocarbons packed in a 2D hexagonal lattice. This coupling between the two bilayer leaflets manifested itself as through a small increase in area per lipid. However, our studies also showed that the underlying energetic contributions are insufficient to break up the hydrogen bond-stabilizing network formed by PE headgroups.This work is supported by the Austrian Science Fund FWF, Project No.P27083-B20 (to G.P.).[1] Heberle, F. A., Marquardt, D., Doktorova, M., Geier, B., Standaert, R. F., Heftberger, P., Kollmitzer, B., Nickels, J. D., Dick, R. A., Feigenson, G. W. et al. (2016). Langmuir, 32(20), 5195(5200).