Free volume properties of sphingomyelin, DMPC, DPPC, and PLPC bilayers

Free volume pockets or voids are crucial for a variety of dynamic processes in lipid membranes. Voids facilitate the diffusion of lipid molecules in the plane of the membrane and are highly relevant for the permeation of small solutes across the membrane. We employ atomic-scale molecular dynamics simulations to study the free volume and packing properties of different lipid membrane systems, focusing on lipids commonly found in lipid rafts. We find that the free volume properties of membranes comprised of saturated (DMPC, DPPC) and diunsaturated (PLPC) phosphaticlylcholine (PC) molecules have many common features, while bilayers consisting of pal mitoylsphingomyelin (PSM) are distinctly different. PSM has a significantly smaller average close-packed cross-sectional area than the PCs. The free volume fraction is significantly larger in the center of a PSM bilayer than in the center of a DPPC bilayer. The opposite is true for the acyl chain and head group regions: here DPPC has a higher free volume fraction. A detailed analysis of the size, shape and orientation of voids in DPPC and PSM shows that the properties of voids are quite different in bilayers consisting of DPPC and PSM. Compared to DPPC, the number density of voids of all sizes is reduced in the head group and acyl chain regions of PSM. In the bilayer center the situation is reversed. Also the shapes and orientations of voids differ, especially in the acyl chain region. Together with recent work on DPPC/cholesterol mixtures, 15,20 this article summarizes the central role of free volume in comprehending the structural properties of membrane domains rich in cholesterol and sphingomyelin.