TRACKING PHOSPHOLIPID POPULATIONS IN POLYMORPHISM BY SIDE-BAND ANALYSES OF P-31 MAGIC-ANGLE-SPINNING NMR

A method was developed to track the distributional preferences of phos pholipids in polymorphism based on sideband analyses of the P-31 magic angle spinning nuclear magnetic resonance spectra, The method was app lied to lipid mixtures containing phosphatidylcholine (PtdCho), phosph atidylethanolamine (PtdEtn) and either cholesterol (Chol) or tetradeca ne, as well as mixtures containing the anionic phosphatidylmethanol, p hosphatidylethanolamine, and diolein. The phospholipid composition of coexisting lamellar (L-alpha) and inverted hexagonal (H-II) phases rem ained constant throughout the L-alpha --> H-II transition in all mixtu res, except those that contained saturated PtdCho and unsaturated PtdE tn in the presence of cholesterol-mixtures that are known to be microi mmiscible because of favored associations between Chol and saturated a cyl chains. In the latter mixture, saturated PtdCho was enriched in th e planar bilayer structure, and unsaturated PtdEtn was enriched in the highly curved H-II structure. This enrichment was coincident with an increase in the transition width. When compositional heterogeneity amo ng coexisting phases was observed, it appeared that preexisting latera l microheterogeneities led to compositionally distinct transitional cl usters, such that the distributional preferences that resulted were no t those of the individual phospholipids.