The structure and thermotropic properties of pure 1,2-diacylgalactosylglycerols in aqueous systems.

Pure 3-sn-monogalactosyldilinolenoylglycerol and 3-sn-digalactosyldilinolenoylglycerol have been isolated from bean leaves. Distearoyl derivatives have been prepared by catalytic hydrogenation of the unsaturated galactolipids. The unsaturated lipids form stable monomolecular films at the air/water interface which are similar to liquid-expanded phospholipid monolayers. The limiting areas were about 0.57 nm2 and 0.62 nm2 for the mono- and digalactosyldiacylglycerols, respectively. The saturated galactolipids formed condensed monolayers that were relatively unstable. The surface pressure-area isotherm of the digalactosyl derivative was more expanded than that of the monogalactosyldiacylglycerol especially at low surface pressures. Low-angle X-ray diffraction and freeze-fracture electron microscopy studies of the monogalactosyldiacylglycerols showed that an hexagonal-type structure was formed by the unsaturated lipid in aqueous systems, whilst the saturated lipid was arranged in a lamellar configuration. Both digalactosyldiacylglycerols form lamellar structures in water. A gel-to-liquid-crystalline phase transition of distearoyldigalactosylglycerol was observed at about 51 degrees C by fluorescence depolarization measurements, using 1,6-diphenylhexatriene, and by differential scanning calorimetry. The saturated monogalactosyldiacylglycerol did not form dispersions suitable for fluorescence probe studies of a phase transition. A complex pattern of endotherms was observed for this lipid by differential scanning calorimetry.