BEHAVIOR OF CHOLESTEROL AND ITS EFFECT ON HEAD GROUP AND CHAIN CONFORMATIONS IN LIPID BILAYERS - A MOLECULAR-DYNAMICS STUDY

Cholesterol molecules were put into a computer-modeled hydrated bilaye r of dimyristoyl phosphatidyl choline molecules, and molecular dynamic s simulations were run to characterize the effect of this important mo lecule on membrane structure and dynamics. The effect was judged by ob serving differences in order parameters, tilt angles, and the fraction of gauche bonds along the hydrocarbon chains between lipids adjacent to cholesterol molecules and comparing them with those further away. I t was observed that cholesterol causes an increase in the fraction of trans dihedrals and motional ordering of chains close to the rigid ste roid ring system with a decrease in the kink population. The hydrogen- bonding interactions between cholesterol and lipid molecules were dete rmined from radial distribution calculations and showed the cholestero l hydroxyl groups either solvated by water, or forming hydrogen bond c ontacts with the oxygens of lipid carbonyl and phosphate groups. The d ynamics and conformation of the cholesterol molecules were investigate d and it was seen that they had a smaller tilt with respect to the bil ayer normal than the lipid chains and furthermore that the hydrocarbon tail of the cholesterol was conformationally flexible.