LAMELLAR NON-LAMELLAR PHASE-TRANSITIONS IN SYNTHETIC GLYCOGLYCEROLIPIDS STUDIED BY TIME-RESOLVED X-RAY-DIFFRACTION

The phase sequences of eight fully hydrated synthetic, stereochemicall y pure glycoglycerolipids with saturated alkyl chains 12-18 carbon ato ms long and a glucose, galactose or mannose head group are followed in real time during heating and cooling scans using synchrotron X-ray di ffraction. One of them, ,2-di-O-hexadecyl-3-O-beta-D-glucosyl-sn-glyce rol, has been characterized by X-ray diffraction for the first time. A summary of the lamellar-non-lamellar transition sequences and reversi bility for all eight glycoglycerolipids studied is provided. It includ es also observations of intermediate phases, previously not detected. Lattice parameters of the various phases have been determined as funct ions of chain length in monoglucosides. While the repeat periods of th e 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 disap pearance of the cubic phases from the phase sequence upon chain elonga tion from 12 to 14 carbon atoms. It thus appears that the expanded H-I I phase in 14-Glc retains structural characteristics of the anticipate d cubic phases. Upon heating to high temperatures, its high lattice sp acing gradually approaches that of the 'normal' hexagonal phase. A dir ect transition from lamellar subgel to inverted hexagonal phase has be en observed to proceed without intermediate structures, but with an ex tended phase coexistence region, in -di-O-tetradecyl-3-O-beta-D-galact osyl-sn-glycerol and -di-O-octadecyl-3-O-beta-D-galactosyl-sn-glycerol . This transition is not reversible on cooling when lamellar phases sk ipped in the heating scan intervene. By contrast, the direct lamellar gel-inverted hexagonal phase transitions are fully reversible with min or or absent temperature hysteresis.