Saturation with cholesterol increases vertical order and smoothes the surface of the phosphatidylcholine bilayer: a molecular simulation study.

Molecular dynamics (MD) simulations of a mono-cis-unsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayer and a POPC bilayer containing 50mol% cholesterol (POPC-Chol50) were carried out for 200ns to compare the spatial organizations of the pure POPC bilayer and the POPC bilayer saturated with Chol. The results presented here indicate that saturation with Chol significantly narrows the distribution of vertical positions of the center-of-mass of POPC molecules and POPC atoms in the bilayer. In the POPC-Chol50 bilayer, the same moieties of the lipid molecules are better aligned at a given bilayer depth, forming the following clearly separated membrane regions: the polar headgroup, the rigid core consisting of steroid rings and upper fragments of the acyl chains, and the fluid hydrocarbon core consisting of Chol chains and the lower fragments of POPC chains. The membrane surface of the POPC-Chol50 bilayer is smooth. The results have biological significance because the POPC-Chol50 bilayer models the bulk phospholipid portion of the fiber-cell membrane in the eye lens. It is hypothesized that in the eye lens cholesterol-induced smoothing of the membrane surface decreases light-scattering and helps to maintain lens transparency.