Cholesterol-phospholipid interaction in membranes. 2. Stoichiometry and molecular packing of cholesterol-rich domains

A model for the molecular interaction between cholesterol and phospholipid in bilayer membranes is presented. We propose that cholesterol forms associations with phospholipids with stoichiometries of both 1:1 and 1:2. A hydrogen bond between the beta-OH of cholesterol and the glycerol ester oxygen of a phospholipid is suggested as a likely mechanism for tight binding in a 1:1 complex. A second phospholipid molecule is loosely associated with the complex to form domains of 1:2 stoichiometry, which may coexist with pure phospholipid domains. Interfacial boundary phospholipid separates these two domains. Under conditions in which interfacial phospholipid is maximal, the perturbed phospholipid assumes a composition of 20 mol % cholesterol. To account for the phase behavior and surface properties of cholesterol-lipid membranes, we propose a molecular packing model for linear arrays within the cholesterol-rich domains. In this arrangement, two rows of 1:1 complex run antiparallel with loosely associated phospholipid intercalated between them. The loosely associated phospholipid can pack in the nearly hexagonal manner in which pure crystalline phospholipid is known to pack. The model provides maximal van der Waals contact in the hydrocarbon region of the bilayer and can maintain phospholipids as cholesterol's nearest neighbors at all concentrations up to 50 mol % cholesterol. The model is compatible with the diverse experimental observations compiled by many investigators over the past decade.