Effects of cholesterol on the properties of equimolar mixtures of synthetic phosphatidylethanolamine and phosphatidylcholine. A 31P NMR and differential scanning calorimetry study.

1. The interaction of cholesterol with equimolar mixtures of 18 : l e /18 : I c phosphatidylethanolamine and 16 : 0/16 : 0 phosphatidylcholine has been investigated employing differential scanning calorimetry and 3,p NMR. At temperatures where lateral phase separation occurs cholesterol interacts preferentially with the (gel state) 1 6 : 0 / 1 6 : 0 phosphatidylcholine component , in agreement with previous studies (van Dijck et al. {1976) Biochim. Biophys. Acta 455 ,576-587) . Bilayer structure is maintained in such situations, and the effects do not therefore arise due to a preference of cholesterol for phospholipids in bilayer or non-bilayer phases. Further, the 3,p NMR results indicate that when both species are in the liquid crystalline state the preferential interaction of cholesterol with the phosphatidylcholine component is maintained. 2. The addition of cholesterol to {liquid crystalline) equimolar mixtures of 18 : l c / 1 8 : lc phosphatidylethanolamine and 16 : 0/16 : 0 phosphatidylcholine stabilizes the bilayer phase. Alternatively, cholesterol destabilizes the bilayer phase in equimolar mixtures of 18 : l c / 1 8 : lc phosphatidylethanolamine and 1 8 : l c / 1 8 : 1 c phosphatidylcholine, reducing the temperature at which non-bilayer phases are formed. Intermediate cholesterol concentrations encourage formation of a phase which possibly has cubic structure, whereas higher cholesterol contents (and higher temperatures) encourage formation of the hexagonal (HI l ) phase. 3. The temperature-dependent polymorphic phase behaviour of equimolar mixtures of 18 : l c / 1 8 : lc phosphatidylethanolamine and 18 : l c /18 : lc