Physical Properties and Surface Interactions of Bilayer Membranes Containing N-Methylated Phosphatidylethanolamines

The structure and physical properties of aqueous dispersions of l,2-diacyl-sn-glycero-3-phosphoethanolamines (PE's) and their N-methylated analogues have been studied by scanning calorimetry, 31P nuclear magnetic resonance, and freeze-fracture electron microscopy. 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. However, 31P nuclear magnetic resonance and electron microscopy show that unlike pure PE species, N-methylated PE's 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 calcium-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 calcium-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, we conclude that 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, however, 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 is invariably superior to that of its N-methylated derivatives, a fact that appears to reflect the higher hydrogen bond donating ability of unsubstituted PE more than the relatively small size of its head group.