Small-angle scattering from flat bilayers containing correlated scattering length density inhomogeneities

Model lipid bilayers have been widely employed as a minimal system to investigate the structural properties of biological membranes by small-angle X-ray (SAXS) and neutron scattering (SANS) techniques. These have nanometre resolution and can give information regarding membrane thickness and scattering length densities (SLDs) of polar and apolar regions. However, biological membranes are complex systems containing different lipids and protein species, in which lipid domains can be dynamically assembled and disassembled. Therefore, SLD variations can occur within the biomembrane. In this work, a novel method has been developed to simulate SAXS and SANS profiles obtained from large unilamellar vesicles containing SLD inhomogeneities that are spatially correlated over the membrane surface. Such inhomogeneities are represented by cylindrical entities with equivalent SLDs. Stacking of bilayers is also included in the model, with no correlation between horizontal and vertical order. The model is applied to a lipid bilayer containing SLD inhomogeneities representing pores, lipid domains, and transmembrane, partially immersed and anchored proteins. It is demonstrated that all the structural information from the host lipid bilayer and from the SLD inhomogeneity can be consistently retrieved by a combined analysis of experimental SAXS and SANS data through the methodology proposed here.