Physical properties and surface interactions of bilayer membranes containing N-methylated phosphatidylethanolamines

Abstract

The structure and physical properties of aqueous dispersions of 1,2-diacyl-sn-glycero-3-phosphoethanolamines (PE) and their N-methylated analogs were studied by scanning calorimetry, 31P NMR, and freeze-fracture EM. While successive N-methylations of a diacylphosphatidylethanolamine cause only modest decreases in its gel to liquid-crystalline phase transition temperature, the introduction of even a single N-methyl group sharply increases the temperature at which the lipid forms a hexagonal II phase. 31P NMR and EM show that unlike pure PE species, N-methylated PE can form a variety of irregular nonlamellar structures at temperatures well below that at which a well-defined hexagonal II phase is formed. The rate of Ca-induced leakage of encapsulated carboxyfluorescein from large unilamellar vesicles composed of dioleoyl or dielaidoylphosphatidylserine and the corresponding PE is strongly reduced when PE is replaced by N-methylated derivatives. The rate of Ca-induced intermixing of lipids of PE/phosphatidylserine (PS) vesicles steadily decreases as the PE component is successively replaced by its mono-, di and tri-N-methylated (phosphatidylcholine) derivatives. By correlating calorimetrically obtained phase diagrams with measurements of vesicle lipid intermixing, dielaidoyl-N-methylphosphatidylethanolamine, like PE, can support direct interactions between the surfaces of PS/N-methyl-PE vesicles without lateral separation of a PS (Ca2+)-rich phase, while dielaidoyl-N,N-dimethyl-PE (and phosphatidylcholine) cannot. When dioleoyl lipids are examined, both the monomethyl- and dimethyl-PE species can support lipid intermixing between the membranes of vesicles containing even low amounts of PS. The ability of PE to support PS/Ca2+-initiated interactions between vesicle surfaces in invariably superior to that of its N-methylated derivatives, a fact that appears to reflect the higher H bond donating ability of unsubstituted PE more than the relatively small size of its head group.