Polymorphic phase behaviour of lipid mixtures as detected by 31P NMR. Evidence that cholesterol may destabilize bilayer structure in membrane systems containing phosphatidylethanolamine.

1. 31p NMR is a useful analytical technique for the study of the polymorphic phase behaviour of hydrated phospholipids in excess water. Such possibilities arise due to lateral diffusion of phospholipids which, in non-bilayer phases, produce additional motional averaging mechanisms. This results in distinctive 31p NMR spectra for lipids in the bilayer phase, hexagonal (Hll) phase, or phases such as the inverted micellar, cubic or rhombic. 2. The polymorphic phase behaviour of soya phosphatidylethanolamine and mixtures of soya phosphatidylethanolamine with saturated and unsaturated phosphatidylcholines has been investigated. As the temperature is decreased below --10°C soya phosphatidylethanolamine tends to enter the bilayer phase. At higher temperatures (0--50°C) the hexagonal (H~) phase is observed. The addition of 50 mol% egg yolk or (liquid~rystalline) 16 : 0/16 : 0 phosphatidylcholine is sufficient to stabilize the bilayer phase. 3. The addition of equimolar cholesterol to 18 : lc /18 : le phosphatidylethanolamine results in the bilayer-hexagonal (Hll) transition moving to lower temperatures. Similarly, the addition of equimolar cholesterol to soya phosphat idylethanolamine/35 mol% egg yolk phosphatidylcholine mixtures destabilizes the component in the bilayer phase. Alternatively, the addition of cholesterol to mixtures containing 30 mol% 16 : 0/16 : 0 phosphatidylcholine stabilizes the bilayer configuration. These results are discussed with regard to the observation that biological membranes containing high concentrations of phosphatidylethanolamine contain little or no cholesterol. It