Influence of calcium on phosphatidylethanolamine. An investigation of the structure at high pH

The influence of calcium on the structure of 1,2-dihexadecyl-sn-glycero-3-phosphoethanolamine was investiged at pH 8 and 12 by differential scanning calorimetry and by X-ray diffraction. At pH 12, where the amino group of phosphatidylethanolamine is partly deprotonated, two separate lamellar phases are observed at 60 degrees C in the presence of CaCl2; one phase is metastable and the other is present in a crystalline structure. At high temperature (90 degrees C), the small-angle diffraction lines indicate the existence of a hexagonal phase at pH 8. At pH 12 a hexagonal phase is detected only in the presence of calcium.