Structural Investigations of the Inverted Hexagonal and Inverted Cubic Phases in Lipid-Water Systems.

The structure of the inverted hexagonal (H _{rm II}) and inverted cubic liquid crystalline phases in biological lipid-water systems is studied to examine the physical interactions which drive the polymorphic phase behavior, though also to play a relevant role in biological membrane function. A method is derived which yields the complex phase factors of the H _{rm II} phase diffraction amplitudes from examination of a single sample. This method is applied to a low resolution Fourier reconstruction of the H_{rm II} phase in dioleoylphosphatidylethanolamine (DOPE) + water. The shape of the H_{rm II} water core is found to be circular to within 5% of the water core radius, R_{omega} , when the unit cell size is less than ~75A. Above 75A, however, a definite shape deformation becomes apparent, with radial non-circularities of 5 to 10%, probably in response to the increased entropic cost of packing the hydrocarbon chains into the anisotropic environment of the H_{rm II} unit cell (Kirk, Gruner and Stein 1984, Biochem. 23, 1093). As a more direct probe of the packing anisotropy, Fourier reconstructions of DOPE + alkane systems were compared with the reconstruction of DOPE. Alkanes are known to promote the H_{rm II} 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, anisotopy of the electron density near the end of the lipid chains is also reduced when alkane is added, implying a more uniform hydrocarbon packing environment. Neutron diffraction results agree with this conclusion, showing that the alkane preferentially packs into the interstitial regions of the unit cell, where the packing stress is though to be largest. In another set of experiments the temperature -composition phase diagram of a 12 carbon glucolipid is presented. Assuming that the structure of two observed cubic phases (Ia3d and Pn3m) are based on infinitely periodic minimal surfaces (IPMS) the interfacial curvature can be calculated. A simple curvature free energy model is presented which qualitatively describes the phase transtion. One of the important results of this model is a determination of the Gaussian bending modulus, the first such measurement for a dual chain lipid.