P-31 nuclear magnetic resonance studies of the appearance of an isotropic component in dielaidoylphosphatidylethanolamine.

We have utilized phosphorus nuclear magnetic resonance, which provides an excellent means of characterizing the physical state of lipids, to investigate the polymorphic phase behavior of pure dielaidoylphosphatidylethanolamine (DEPE). We have observed a sharp isotropic component in the typical bilayer and inverted hexagonal P-31 NMR spectra. This component appears in the spectra of both the bilayer and inverted hexagonal lipid phases after several cycles through the bilayer-to-hexagonal phase transition. The magnitude of the isotropic component increased as a function of the number of cycles through the transition. The appearance of this component was not a function of time at constant temperature, but only a function of the number of cycles through the transition. The isotropic component is stable at all temperatures above the gel-to-liquid crystal transition, but it abruptly disappears when the lipid is cooled below the gel-to-liquid crystal phase transition. It is suggested that this isotropic phase is similar to the isotropic phase observed in dioleoylphosphatidylethanolamine (DOPE) by x-ray diffraction and identified as a cubic phase (Shyamsunder, E., S. M. Gruner, M. W. Tate, D. C. Turner, P. T. C. So, and C. P. S. Tilcock. 1988. Biochemistry. 27:2332-2336).