Determining the Bending Moduli of Asymmetric Bilayers by Simulation

Abstract

Biological membranes are almost always asymmetric in terms of lipid composition. Such asymmetry is thought to play an important part in biological functions via alteration of the membrane's elastic properties, including its bending modulus. Recently experimental techniques have been developed which enable the preparation of asymmetric GUVs and therefore allow the measurement and direct comparison of bending moduli for pure bilayers, mixed symmetric bilayers and asymmetric bilayers composed of similar lipid species. These experiments show that asymmetry does indeed have a marked effect on bilayer stiffness. In order to reproduce these data and develop a molecular level understanding of them, we have taken a computational approach using both atomistic and coarse grained forcefields for a variety of system sizes and lipid species. We have performed analysis of our trajectories using a range of theoretical models based on a variety of different analysis techniques including those based directly on membrane area compressibility; those based on spectral analysis involving 2D discrete Fourier transforms; and those based on the distribution of angles of splay. Our results show that the different models employed provide consistent results across a range of system sizes both with each other and the experimental data, as well as suggesting promising new directions for experimental investigation.