title: Partial dehydration of phosphatidylethanolamine phosphate groups during hexagonal phase formation, as seen by i.r. spectroscopy.
abstract: 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.
authors.affiliation: Department of Biochemistry, University of the Basque Country, Bilbao, Spain.
authors.forename: J
authors.initials: J
authors.lastname: Castresana
authors.forename: J L
authors.initials: JL
authors.lastname: Nieva
authors.forename: E
authors.initials: E
authors.lastname: Rivas
authors.forename: A
authors.initials: A
authors.lastname: Alonso
mesh_terms: D005583:Fourier Analysis; D009943:Organophosphorus Compounds / Q000737:chemistry*; D010714:Phosphatidylethanolamines / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013696:Temperature; D014867:Water / Q000737:chemistry*
chemical_list: D009943:Organophosphorus Compounds; D010714:Phosphatidylethanolamines; D014867:Water