Atomistic simulations of a multicomponent asymmetric lipid bilayer.

The cell membrane is inherently asymmetric and heterogeneous in its composition, a feature that is crucial for its function. Using atomistic molecular dynamics simulations, the physical properties of a 3-component asymmetric mixed lipid bilayer system comprising an unsaturated POPC (palmitoyloleoylphosphatidylcholine), a saturated PSM (palmitoylsphingomyelin), and cholesterol are investigated. Our simulations explore both the dynamics of coarsening following a quench from the mixed phase and the final phase-segregated regime obtained by equilibrating a fully segregated configuration. Following a quench, the membrane quickly enters a coarsening regime, where the initial stages of liquid ordered, l(o), domain formation are observed. These growing domains are found to be highly enriched in cholesterol and PSM. Consistent with this, the final phase-segregated regime contains large l(o) domains at equilibrium, enriched in cholesterol and PSM. Our simulations suggest that the cholesterol molecules may partition into these PSM-dominated regions in the ratio of 3:1 when compared to POPC-dominated regions. PSM molecules exhibit a measurable tilt and long-range tilt correlations within the l(o) domain as a consequence of the asymmetry of the bilayer, with implications to local membrane deformation and budding. Tagged particle diffusion for PSM and cholesterol molecules, which reflects spatial variations in the physical environment encountered by the tagged particle, is computed and compared with recent experimental results obtained from high-resolution microscopy.