Partial dehydration of phosphatidylethanolamine phosphate groups during hexagonal phase formation, as seen by i.r. spectroscopy.

The gel-to-fluid and lamellar-to-HII-hexagonal thermotropic phase transitions of egg-yolk phosphatidylethanolamine have been examined by Fourier-transform infrared spectroscopy under a variety of conditions, namely excess water at pH 5.0, excess water at pH 9.5 and low hydration. The various lamellar and hexagonal phases have been characterized by X-ray diffraction. At pH 5.0, gel-fluid and lamellar-hexagonal transitions were detected at 10 and 32 degrees C respectively, in accordance with previous data. At pH 9.5, only the first of these two transitions was detected. In the partially hydrated sample a single phenomenon was observed, probably encompassing both transitions, so that, in practice, a gel-HII-hexagonal transition appears to occur. The region of the i.r. spectrum corresponding to the phospholipid phosphate group reveals that the lamellar-hexagonal, but not the gel-fluid, transition is accompanied by a weakening in the shell of hydrogen-bonded water, thus providing direct evidence that, in a pure lipid/water system, hexagonal phase formation requires partial dehydration of the phospholipid phosphate group. X-ray diffraction data support this conclusion, since, at least in the low-hydration system, the average surface area per lipid polar group decreases with the thermotropic lamellar-hexagonal transition.