Directly measured deformation energy of phospholipid HII hexagonal phases

Abstract

Osmotic stress combined with X-ray diffraction has been used to measure the energies of inverted hexagonal (H_II) phases containing dioleylphosphatidyl-choline (DOPC) and -ethanolamine (DOPE). To distinguish chain packing energies from the action of other factors governing stability, energies with and without added tetradecane have been compared. Little hydrocarbon strain energy is stored in the H_II lattice, but alkane promotes the transition from a lamellar to an H_II structure. In the presence of excess water, various H_II lattices seek spontaneous radii of curvature, r₀, of the aqueous–polar interface. Under low osmotic stress, the work of deforming from this spontaneous radius appears to be quadratic in curvature, can be well described by a bending modulus that is independent of the initial r₀, and is related to bending moduli measured on planar bilyaers. At high stress, the work of water removal and lattice deformation seems to be that of removing water from a polar ‘mash’ composed of lipid headgroups and water, dependent only weakly on the DOPE/DOPC ratio.