Configurations of fatty acyl chains in egg phosphatidylcholine-cholesterol mixed bilayers

Based on the structural properties of cholesterol and egg phosphatidylcholine, and on the assumption that the van der Waals' type attactive interaction between the steroid nucleus and the fatty acyl chains provides a stabilizing force for the cholesterol-egg phosphatidylcholine complex, some specific orientation and configurations of the fatty acyl chains around the steroid nucleus in the interacting system are proposed in terms of an optimal packing. The proposed model suggests thathe saturated chains are largely facing the flattened (alpha) surface of the steroid nucleus of cholesterol, while the unsaturated chains can interact with both the alpha and beta surfaces of the steroid nucleus. It is also suggested that the angular methyl groups on the beta surface of the steroid nucleus lock the unsaturated fatty acyl chain in a relatively immobile configuration. Experimental evidence which provides support for the proposed stereochemical model is presented.