Bilayer to non-bilayer transition in mixtures of phosphatidylethanolamine and phosphatidylcholine: Implications for membrane properties

Abstract

The structural phases in various mixtures of soybean phosphatidylethanolamine and egg phosphatidylcholine were studied by X-ray diffraction, freeze fracture electron microscopy, and 31P NMR. An intermediate state between bilayer and hexagonal structures was found at a composition of 10–25 mol% of phosphatidylcholine. The intermediate state consists of closely packed multilayers, together with arrays of lipidic intramembranous particles. The arrays of lipidic intramembranous particles, possibly membrane invaginations, give rise to an anisotropic 31P NMR spectrum commonly accredited to a hexagonal structure. A phase diagram of this mixed system is proposed. The compositional range at which the intermediate state exists coincides with the range of maximal mitochondrial ATPase activity when these lipids are used in reconstitution experiments.