Lipid polymorphism of mixtures of dioleoylphosphatidylethanolamine and saturated and monounsaturated phosphatidylcholines of various chain lengths.

The L alpha-HII phase transition behavior of many lipid-water liquid crystals is dominated by the competition between the tendency to curl the lipid layers to an intrinsic radius of curvature and opposing hydrocarbon packing constraints. In particular, packing constraints can increase the free energy of the inverted hexagonal (HII) phase as compared to that of the lamellar (L alpha) phase. This is especially true where the lipid molecule is not long enough to reach into the corners of the lattice in large hexagonal structures necessitated by a large intrinsic radius of curvature. In this paper it is shown that the addition of a minor fraction long-chain lipid to a system of otherwise uniform chain composition can also relax packing constraints, thereby lowering the lamellar to hexagonal transition temperature. For the specific systems used, dioleoylphosphatidylethanolamine (di-18:1c-PE) with minor fractions of 1,2-diacyl-sn-glycero-3-phosphocholines [di-n:1c-PC (n = 14, 18, 22, and 24)], the observed HII lattices systematically increased in size with increasing chain length, suggesting that the chain length also may affect the intrinsic curvature of the mixture. These experiments demonstrate that the lipid "shape concept", which is a qualitative expression of the concept quantitatively described by the intrinsic radius of curvature, is insufficient to understand the L alpha-HII transition. It is necessary to, at least, consider the competition between curvature and packing.