Physical properties and surface interactions of bilayer membranes containing N-methylated phosphatidylethanolamines.
The structure and physical properties of aqueous dispersions of 1,2-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.(ABSTRACT TRUNCATED AT 250 WORDS)
D002152:Calorimetry, Differential Scanning; D005614:Freeze Fracturing; D008051:Lipid Bilayers; D009682:Magnetic Resonance Spectroscopy; D008745:Methylation; D008854:Microscopy, Electron; D008968:Molecular Conformation; D010714:Phosphatidylethanolamines / Q000138:chemical synthesis; D013329:Structure-Activity Relationship
D008051:Lipid Bilayers; D010714:Phosphatidylethanolamines