Fourier-transform infrared spectroscopic evidence for a novel lyotropic phase transition occurring in dioleoylphosphatidylethanolamine

The hydration of 1,2-dioleoyl-sn-glycerophosphoethanolamine (DOPE)* has been studied by Fourier-transform infrared spectroscopy applied to macroscopically oriented films in comparison to related phospholipids (DPPE and DPPC). DOPE differs from the other lipids mainly in one respect, since it displays a number of relatively drastic, correlated spectroscopic changes within a distinct very narrow range of water activities at an ambient relative humidity of similar to 50%. These striking alterations can be observed especially for a set of infrared bands due to the headgroup (phosphoethanolamine) moieties whose parameters are simultaneously changed at a certain degree of hydration estimated to be less than one in the water-per-DOPE molecular ratio. This unique spectral behaviour being much more dramatic than, for instance, that observed for the main transition of lipids seems to reflect the occurrence of considerable structural reorientations within the polar part of DOPE molecules and may be explained as indicating the existence of a lyotropic phase transition. Discussion of the peculiar nature of this transformation results in a tentative assignment of the participating phases as belonging to the non-lamellar aggregations enclosing most probably the inverse hexagonal phase (at higher hydration) and the inverse fluid ribbon phase (at lower hydration).