Reconstruction and interpretation of lipid bilayer structure from X-ray scattering often rely on assumptions regarding the molecular distributions across the bilayer. It is usually assumed that changes in head-head spacings across the bilayer, as measured from electron density profiles, equal the variations in hydrocarbon thicknesses. One can then determine the structure of a bilayer by comparison to the known structure of a lipid with the same headgroup. Here we examine this procedure using simulated electron density profiles for the benchmark lipids DMPC and DPPC. We compare simulation and experiment in both real and Fourier space to address two main aspects: (i) the measurement of head-head spacings from relative electron density profiles, and (ii) the determination of the absolute scale for these profiles. We find supporting evidence for the experimental procedure, thus explaining the robustness and consistency of experimental structural results derived from electron density profiles. However, we also expose potential pitfalls in the Fourier reconstruction that are due to the limited number of scattering peaks. Volumetric analysis of simulated bilayers allows us to propose an improved, yet simple method for scale determination. In this way we are able to remove some of the restrictions imposed by limited scattering data in constructing reliable electron density profiles.
Bibliographical noteFunding Information:
We thank Dr. John F. Nagle for valuable discussions and comments on the manuscript, and Dr. Thomas Huber for discussions of continuous transforms and of his simulation results. J.N.S. thanks the Whitaker Foundation for Biomedical Engineering for graduate fellowship support.
Copyright 2017 Elsevier B.V., All rights reserved.
- Bilayer thickness
- Electron density profile
- Form factors
- Fourier reconstruction
- Lipid headgroup
- Lipid volume