Asymmetric distribution of cone-shaped lipids in a highly curved bilayer revealed by asmall angle neutron scattering technique

Abstract

We have investigated the lipid sorting in a binary small unilamellar vesicle (SUV)composed of cone-shaped (1,2-dihexanoyl-sn-glycero-3-phosphocholine: DHPC) andcylinder-shaped (1,2-dipalmitoyl-sn-glycero-3-phosphocholine: DPPC) lipids. In order toreveal the lipid sorting we adopted a contrast matching technique of small angle neutronscattering (SANS), which extracts the distribution of deuterated lipids in the bilayerquantitatively without steric modification of lipids as in fluorescence probe techniques.First the SANS profile of protonated SUVs at a film contrast condition showed that SUVshave a spherical shape with an inner radius of 190 Å and a bilayer thickness of 40 Å. The SANSprofile of deuterated SUVs at a contrast matching condition showed a characteristicscattering profile, indicating an asymmetric distribution of cone-shaped lipids in thebilayer. The characteristic profile was described well by a spherical bilayer model.The fitting revealed that most DHPC molecules are localized in the outer leaflet.Thus the shape of the lipid is strongly coupled with the membrane curvature.We compared the obtained asymmetric distribution of the cone-shaped lipids inthe bilayer with the theoretical prediction based on the curvature energy model.