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

Abstract

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)].

Stalk structures in lipid bilayer fusion studied by x-ray diffraction

“…Also in case of all studies of nonlamellar phospholipid mesophases we are aware of, be it inverted hexagonal, ripple, cubic (e.g. [111,112,113]) or the rhombohedral phase, the assumption of centrosymmetry has been made and yielded reasonable structural results. It has been attempted to motivate this by the fact that nonlamellar mesophases are obtained by phase transitions from symmetric lipid bilayers, and centrosymmetry should somehow be conserved [114].…”

Effects of oleic acid and its congeners, elaidic and stearic acids, on the structural properties of phosphatidylethanolamine membranes

“…PE is mainly organized into lamellar structures that define the cell, constituting a physical boundary and support for the membrane proteins. PEs are hexagonal (H II )-prone lipids at high temperatures because of their "molecular shape," which resembles a truncated cone and confers a negative curvature strain to model (17,18) and biological membranes (19).…”

Effects of unsaturated fatty acids and triacylglycerols on phosphatidylethanolamine membrane structure

“…However, PE PLs are also prone to form nonlamellar structures, such as the inverted hexagonal H II phase (25)(26)(27)(28)(29). Some special features, such as the control of functions of membrane proteins and the structural organization inside cells, were attributed to H II -prone PLs (30)(31)(32)(33).…”