Lyotropic effects of alkanes and headgroup composition on the lα -Hii lipid liquid crystal phase transition : hydrocarbon packing versus intrinsic curvature

Abstract

The effects of mixed phospholipid headgroup composition and the addition of small amounts of alkane were examined with respect to the lamellar (L α) to inverse hexagonal (HII) phase transition in phospholipid-water liquid crystals. X-ray diffraction was used to probe the lattices and determine the phases. It has been postulated that competition between hydration repulsion, lipid monolayer curvature elasticity, and hydrocarbon packing determine the Lα-HIItransition, [Kirk, Gruner and Stein, Biochem. 23(1984) 1093]. Here, experiments which explored the effects of curvature and packing are described. It is shown that the radius of curvature of the lipid tubes of the HII phase could be adjusted by mixing high and low curvature lipids. However, large curvatures could not be expressed unless lipid hydrocarbon packing constraints were relieved, for instance, by the addition of alkane. Without alkane, the Lα phase extended to high temperatures; adding just 5 % alkane significantly reduced the temperature span of the Lα phase, by lowering the Lα to HII transition temperature, and dramatically expanded the HII lattice. Measurements of the internal dimensions of the HII lattice showed the lipid layer thickness to be nearly constant at a given temperature and that almost all of the lattice expansion was due to an increase in the radius of the water cores. Biological implications are discussed