The Interplay of Structure and Dynamics in Mixed Lipid Bilayers

Abstract

The implication of lipids in membrane mediated processes has motivated significant theoretical and experimental research interest to understand the functional significance of lipid chemical structure variations on the biomembrane structure and dynamics. In particular, hydrophobic mismatch between lipids with different acyl chain lengths is expected to play an important role in determining the local membrane structure and collective membrane dynamics. Herein we investigated these phenomena in large unilamellar vesicles composed of binary mixtures of lipids with a 4 carbon mismatch in tail length, dimyristoylphosphatidylcholine (DMPC, 14:0 C) and distearoylphosphatidycholine (DSPC, 18:0 C), using neutron scattering techniques. Structural studies using small angle neutron scattering (SANS) revealed that the mixed lipid bilayers were thinner than expected for a simple composition-weighted average of the pure component bilayers. The structural differences were accompanied by dramatic differences in the mixed lipid membrane dynamics. Neutron spin echo spectroscopy (NSE) experiments showed that the mixed lipid bilayers were more dynamic than their single component analogs, with the mixed bilayers having a lower bending modulus and increased thickness fluctuation amplitude than the pure DMPC or DSPC bilayers. Both of these enhanced dynamics were consistent with a decrease in the area expansion modulus of the mixed systems. Together our results demonstrate the influence of lipid composition on the bilayer biophysical properties and highlight the complex interplay between structure and dynamics in lipid membranes.