  The connection between membrane inhomogeneity and the structural basis of
lipid rafts has sparked interest in the lateral organization of model lipid
bilayers of two and three components. In an effort to investigate anisotropic
lipid distribution in mixed bilayers, a self-consistent mean-field theoretical
model is applied to palmitoyloleoylphosphatidylcholine (POPC) - palmitoyl
sphingomyelin (PSM) - Cholesterol mixtures. The compositional dependence of
lateral organization in these mixtures is mapped onto a ternary plot. The model
utilizes molecular dynamics simulations to estimate interaction parameters and
to construct chain conformation libraries. We find that at some concentration
ratios the bilayers separate spatially into regions of higher and lower chain
order coinciding with areas enriched with PSM and POPC respectively. To examine
the effect of the asymmetric chain structure of POPC on bilayer lateral
inhomogeneity, we consider POPC-POPC interactions with and without angular
dependence. Results are compared with experimental data and with results from a
similar model for mixtures of dioleoylphosphatidylcholine (DOPC), steroyl
sphingomyelin, and Cholesterol.
