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-alpha) 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-alpha and H-II, phase d-spacings on either side of the L-alpha/H-II phase transition temperature (T-h) depends significantly on the structure of the PE hydrocarbon chains. The L. phase cl-spacings; range from 51.2 to 56.4 A, whereas those of the H-II phase range from 74.9 to 82.7 Angstrom. 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 Angstrom for the L-alpha 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-alpha phase is smaller than that in the H, phase. A detailed molecular description of the L-alpha/H-II phase transition in these PEs is also presented.