Lamellar-non-lamellar phase transitions in synthetic glycoglycerolipids studied by time-resolved X-ray diffraction

The phase sequences of eight fully hydrated synthetic, stereochemically pure glycoglycerolipids with saturated alkyl chains 12-18 carbon atoms long and a glucose, galactose or mannose head group are followed in real time during heating and cooling scans using synchrotron X-ray diffraction. One of them, 1,2-di-O-hexadecyl-3-O-β-D-glucosyl-sn-glycerol, has been characterized by X-ray diffraction for the first time. A summary of the lamellar-non-lamellar transition sequences and reversibility for all eight glycoglycerolipids studied is provided. It includes also observations of intermediate phases, previously not detected. Lattice parameters of the various phases have been determined as functions of chain length in monoglucosides. While the repeat periods of the lamellar phases increase linearly with chain length, an anomalously high lattice spacing of the inverted hexagonal phase is observed at a chain length of 14 carbon atoms. This maximum coincides with the disappearance of the cubic phases from the phase sequence upon chain elongation from 12 to 14 carbon atoms. It thus appears that the expanded H_II phase in 14-Glc retains structural characteristics of the anticipated cubic phases. Upon heating to high temperatures, its high lattice spacing gradually approaches that of the 'normal' hexagonal phase. A direct transition from lamellar subgel to inverted hexagonal phase has been observed to proceed without intermediate structures, but with an extended phase coexistence region, in 1,2-di-O-tetradecyl-3-O-β-D-galactosyl-sn-glycerol and 1,2-di-O-octadecyl-3-O-β-D-galactosyl-sn-glycerol. This transition is not reversible on cooling when lamellar phases skipped in the heating scan intervene. By contrast, the direct lamellar gel-inverted hexagonal phase transitions are fully reversible with minor or absent temperature hysteresis.