Gel-to-inverted hexagonal (L beta-HII) phase transitions in phosphatidylethanolamines and fatty acid-phosphatidylcholine mixtures, demonstrated by 31P-NMR spectroscopy and x-ray diffraction.

The phase behaviour of distearoyl- and dihexadecylphosphatidylethanolamine at high salt concentration and of dipalmitoylphosphatidylcholine + 66 mol% palmitic acid at pH 4.0 have been studied by high-field 31P-NMR spectroscopy and X-ray diffraction. In saturated NaCl, dihexadecylphosphatidylethanolamine undergoes a reversible transition at 74 degrees C directly from the lamellar gel phase L beta to the inverted hexagonal HII phase. A similar transition is observed at 78.4 degrees C for distearoylphosphatidylethanolamine in saturated NaCl (cooling scan). The single, sharp calorimetric transition at 61 degrees C observed in dipalmitoylphosphatidylcholine + 66 mol% palmitic acid at pH 4.0 also corresponds to a transformation directly from the lamellar gel to the inverted hexagonal phase, without an intervening fluid lamellar phase.