Tracking phospholipid populations in polymorphism by sideband analyses of 31P magic angle spinning NMR.

A method was developed to track the distributional preferences of phospholipids in polymorphism based on sideband analyses of the 31P magic angle spinning nuclear magnetic resonance spectra. The method was applied to lipid mixtures containing phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn) and either cholesterol (Chol) or tetradecane, as well as mixtures containing the anionic phosphatidylmethanol, phosphatidylethanolamine, and diolein. The phospholipid composition of coexisting lamellar (Lalpha) and inverted hexagonal (HII) phases remained constant throughout the Lalpha --> HII transition in all mixtures, except those that contained saturated PtdCho and unsaturated PtdEtn in the presence of cholesterol-mixtures that are known to be microimmiscible because of favored associations between Chol and saturated acyl chains. In the latter mixture, saturated PtdCho was enriched in the planar bilayer structure, and unsaturated PtdEtn was enriched in the highly curved HII structure. This enrichment was coincident with an increase in the transition width. When compositional heterogeneity among coexisting phases was observed, it appeared that preexisting lateral microheterogeneities led to compositionally distinct transitional clusters, such that the distributional preferences that resulted were not those of the individual phospholipids.