A Mechanistic Perspective on the Effect of Cholesterol in Phospholipid Bilayers

Cholesterol (CHOL) is one of the most important components of plasma membranes of higher cells and one of the main factors for the formation of (nano)domains. In this work, molecular dynamics (MD) simulations of mixtures of CHOL with either DPPC (saturated lipid) or DLiPC (unsaturated lipid) as standard phospholipids (PLs) are presented in a wide range of CHOL concentrations. Information is extracted from the simulations with the ultimate goal to formulate a lattice model for PL/CHOL mixtures where each molecule is characterized by its location and additionally each PL by its acyl chain order parameter. All ingredients, namely the nearest-neighbor interactions, the acyl-chain entropy as well as its topology are taken from the simulations. Since the enthalpic contributions can be expressed in terms of the local CHOL arrangement, they are applicable for the whole range of CHOL concentrations. Simulations of the lattice model with standard Monte Carlo techniques allows one to reproduce the dependence of the average PL chain order on CHOL concentrations. Furthermore, comparison of the ingredients for DPPC/CHOL and DLiPC/CHOL allows one to identify why both systems display, e.g., largely different order parameters. Finally, it is verified that the interaction ingredients hardly change when comparing binary and ternary DPPC/DLiPC/CHOL mixtures, suggesting applicability of the concepts to more complex mixtures.