X-ray diffraction reconstruction of the inverted hexagonal (HII) phase in lipid-water systems

The structure of the inverted hexagonal (HII) phase in biological lipid-water systems is studied to examine the physical interactions which drive the polymorphic phase behavior and which are also thought to play a relevant role in biological membrane function. A method is derived which yields the complex phase factors of the HII phase diffraction amplitudes from examination of a single sample. This method is applied to a low-resolution Fourier reconstruction of the HII phase in dioleoylphosphatidylethanolamine (DOPE) + water, specifically to examine deviations from the presumed circular model of the HII phase. It is found that the average radius of the water core, Rw, as determined from a Fourier reconstruction, is in good agreement with previously measured values of Rw obtained from more time-consuming traditional methods [Tate, M. W., & Gruner, S. M. (1989) Biochemistry 28, 4245]. In addition to the average value of Rw, the Fourier reconstruction also can be used to determine the true shape of the water core. It is found that the water core is circular to within 5% of Rw when the unit cell size is less than approximately 75 A. Above 75 A, however, a definite shape deformation becomes apparent, with radial noncircularities of 5-10%, probably in response to the increased entropic cost of packing the hydrocarbon chains into the anisotropic environment of the HII unit cell [Kirk, G. L., Gruner, S. M., & Stein D. E. (1984) Biochemistry 23, 1093]. As a more direct probe of the packing anisotropy, Fourier reconstructions of DOPE + dodecane and DOPE + squalene systems were compared with the reconstruction of DOPE. These oils are known to promote the low temperature occurrence of the HII phase, presumably by a reduction in the hydrocarbon packing stress. In support of this hypothesis, the alkanes were observed to relax the water core to a circular shape for even large lattices. In addition, anisotropy of the electron density near the end of the lipid chains is reduced when alkane is added, implying a more uniform hydrocarbon packing environment, consistent with the results of neutron diffraction upon the addition of deuterated decane [Turner, D. C., Gruner, S. M., & Huang, J. (1992) Biochemistry (following paper in this issue)].