Combined Monte Carlo and molecular dynamics simulation of hydrated lipid-cholesterol lipid bilayers at low cholesterol concentration

We have applied a hybrid equilibration and sampling procedure for the atomi c level simulation of a hydrated lipid bilayer to systems consisting of dip almitoyl phosphatidylcholine (DPPC) and cholesterol, and palmitoyl-oleyl ph osphatidylcholine (POPC) at low (similar to6%) cholesterol concentration. T he procedure is applied to bilayers of 94 molecules of DPPC, 6 molecules of cholesterol, and 3205 water molecules, and to bilayers of 120 molecules of POPC, 8 molecules of cholesterol, and 4268 water molecules, at a temperatu re of 325 K. After equilibration, three separate 400-ps continuous molecula r dynamics runs, separated by 10,000 configurational bias Monte Carlo steps , were carried out for each system. Properties of the systems were calculat ed and averaged over the three separate runs. Results of the simulations ar e presented and compared with experimental data and with other recent simul ations of DPPC and cholesterol, and of pure DPPC, and pure POPC. Certain pr operties of the bilayers are indistinguishable from cholesterol-free bilaye rs, including lateral diffusion and electron density. Other properties, mos t notably the order parameter profile, show the effect of cholesterol even at low concentrations.