Phase properties of binary mixtures of monogalactosyldiacylglycerols differing in hydrocarbon chain substituents dispersed in aqueous systems

Abstract

Monogalactosyldiacylglycerols isolated from spinach leaves contain a high proportion of polyunsaturated fatty acyl substituents and form hexagonal-II structures when dispersed in excess water. Catalytic hydrogenation of the lipid in the presence of Adam's catalyst completely saturates the hydrocarbon chains and the lipid forms typical open sheet bilayer structures in water at 20°C. Binary mixtures of the native and hydrogenated lipid tend to phase separate at 20°C. Freeze-fracture electron microscopy reveals lamellar phase lipid indispersed with regions of hexagonal-II structure and the proportions of each reflect the composition of the mixture. X-ray diffraction in both wide- and low-angle regions show that the saturated lipid forms the typical stable gel-phase structure in mixtures that are allowed to equilibrate over three days at 20°C. The phase transition behaviour of binary mixtures of the two galactolipids was investigated by differential scanning calorimetry and fluorescence probe methods. Thermal studies indicate that the phase-separated gel structure undergoes an anomalous transition compared with the saturated pure lipid in that the transition temperature is reduced from about 57°C to 41°C and the enthalpy of the transition is also somewhat reduced. Furthermore, the transition appears to involve the conversion of the completely phase-separated system into bilayer coexisting with phases intermediate between bilayer and hexagonal-II. A homogeneous hexagonal-II phase is presumably formed at higher temperatures. The thermal and structural studies were consistent with fluorescence polarization measurements of 1,6-diphenyl-1,3,5-hexatriene interpolated into the hydrocarbon domain of the structure.