Reversed hexagonal phase formation in lecithin-alkane-water systems with different acyl chain unsaturation and alkane length

Abstract

Investigations of lipid-alkane systems are important for an understanding of the interactions between lipids and hydrophobic/amphiphilic peptides or other hydrophobic biological molecules. A study of the formation of nonlamellar phases in several phosphatidylcholine (PC)-alkane-2H2O systems has been performed. The PC molecules chosen in this work are dipalmitoyl-PC (DPPC), 1-palmitoyl-2-oleoyl-PC (POPC), dioleoyl-PC (DOPC), and dilinoleoyl-PC(DLiPC), lipids that in excess water form just a lamellar liquid-crystalline phase up to at least 90.degree.C. The addition of n-alkanes (C8-C20) to these PC-2H2O systems induces the formation of reversed hexagonal (HII) and isotrophic phases. The water and dodecane concentrations required to form these phases depend on the degree of acyl chain unsaturation of the PC molecules and increase in the order DLiPC .apprxeq. DOPC < POPC <DPPC. The most likely explanation to this result is that the diameter of the lipid-water cylinders in the HII phase grows gradually larger with increased acyl chain saturation and more water and dodecane are consequently needed to fill the water cylinders and the void volumes between the cylinders, respectively. The ability of the alkanes to promote the formation of an HII phase is strongly chain length dependent. Although the number of alkane carbon atoms added per DOPC molecule in the DOPC-n-alkane-2H2O mixtures was kept constant, this ability decreased on going from octane to eicosane. The thermal history of a DPPC-n-dodecane-2H2O samples was important for its phase behavior. A large fraction of the sample formed an isotropic phase at 65.degree.C when heated at 10.degree.C/24 h while an HII phase dominated at this temperature when the sample was heated at 10.degree.C/h. The results obtained are consistent with the theoreis presented by Gruner [Gruner, S. M. (1985) Proc. Natl. Aca. Sci. U.S.A. 82, 3655-3669] and Siegel [Siegel, D. P. (1986) Chem. Phys. Lipids 42, 279-301].