The complexity of cellular membranes is not only reflected by their wide molecular composition but also in the wide range of morphologies with coexisting high and low curvature regions. Very little is known about the nanostructure of curved membranes, due to limitations in experimental and simulation approaches. Even though curvature-induced microscopic phase separation has been demonstrated experimentally using a range of microscopy-based techniques, structural studies are basically non-existent mainly due to a lack of suitable methods to study such membrane. Recently, we have overcome such limitations by using “diffracting scaffolds” formed by SiO₂ nanoparticles (NPs) ranging from 200 to 50 nm in diameter forming a dense coating on solid surfaces.

We deposited supported lipid bilayer (SLB) using the 1-palmitoyl, 2-oleoyl phosphatidylcholine (POPC) vesicle fusion on both the SiO₂ NPs and the flat regions in between these NPs. We characterised the bilayer structure using neutron reflection (NR) and the coating of the NPs was visible in grazing-incidence small-angle scattering (GISANS).

We aim to build on this knowledge to probe the curvature-induced phase separation of binary lipid bilayers as a function of curvature. We have shown that POPC lipid bilayers could form well on NPs having 200 nm in diameter, but the SLB coverage was worse when going down to NP having 50 nm in diameter. This could be due to a limitation on the intrinsic curvature of POPC which has a critical packing parameter of 1. By expanding to lipid mixtures that include lipids with high and low packing parameter such as cardiolipin and glycolipids, curvature-induced phase separation in terms of the structure of the bilayer across and in the plane will be studied. NR and GISANS using contrast matching and lipid deuteration are the ONLY techniques that can provide direct evidence on the structure of coexisting membranes sustaining high and low curvature.

This work resulted from collaborative efforts between the Department of Biomedical Sciences at Malmö University, Uppsala University, the National deuteration facility at Australian Nuclear Science and Technology Organization, the Institute Laue Langevin, ISIS neutron and muon source, Nanyang Technological University. This project is funded by the Swedish Research Council, Nordforsk − Nordic Neutron Science Program and Wennergren Foundation. Experiments were performed at ILL and the ISIS neutron and muon source.

Contact:

Prof. Marité Cárdenas
Malmö University
marite.cardenas@mau.se