X-Ray Diffraction Structures of Some Phosphatidylethanolamine Lamellar and Inverted Hexagonal Phases

X-ray diffraction is used to solve the low-resolution structures of fully hydrated aqueous dispersions of seven different diacyl phosphatidylethanolamines (PEs) whose hydrocarbon chains have the same effective chain length but whose structures vary widely. Both the lower-temperature, liquid-crystalline lamellar (L α ) and the higher-temperature, inverted hexagonal (H II) phase structures are solved, and the resultant internal dimensions (d-spacing, water layer thickness, average lipid length, and headgroup area at the lipid–water interface) of each phase are determined as a function of temperature. The magnitude of the L α and H II phase d-spacings on either side of the L α /H II phase transition temperature ( T h) depends significantly on the structure of the PE hydrocarbon chains. The L α phase d-spacings range from 51.2 to 56.4 Å, whereas those of the H II phase range from 74.9 to 82.7 Å. These new results differ from our earlier measurements of these PEs (Lewis et al., Biochemistry, 28:541–548, 1989), which found near constant d-spacings of 52.5 and 77.0–78.0 Å for the L α and H II phases, respectively. In both phases, the d-spacings decrease with increasing temperature independent of chain structure, but, in both phases, the rate of decrease in the L α phase is smaller than that in the H II phase. A detailed molecular description of the L α/H II phase transition in these PEs is also presented.