Different phase behaviour of the sn-1 and sn-3 stereoisomers of the glycolipid di-tetradecyl-beta-D-galactosylglycerol.

The phase behaviour of a synthetic, stereochemically pure glycolipid 2,3-di-O-tetradecyl-1-O-beta-D-galactosyl-sn-glycerol (14-2,3-Gal) in excess water has been characterized by differential scanning calorimetry and time-resolved X-ray diffraction and compared with that of the previously studied sn-3 stereoisomer, 1,2-di-O-tetradecyl-3-O-beta-D-galactosyl-sn-glycerol (14-1,2-Gal), and 1,2-di-O-tetradecyl-3-O-beta-D-glucosyl-sn-glycerol (14-1,2-Glc). The properties of 14-1,2-Gal and 14-2,3-Gal are completely different with respect to phase sequences, metastable behaviour, transition temperatures and enthalpies, but there is a rather close similarity between the phase patterns of 14-2,3-Gal and 14-1,2-Glc. The sn-3 stereoisomer, 14-1,2-Gal, exhibits a direct lamellar crystalline to inverted hexagonal phase transition (Lc-->HII) on heating and a HII-->L alpha (metastable)-->L beta(metastable)-->Lc phase sequence in subsequent cooling, while both 14-2,3-Gal and 14-1,2-Glc are characterized by an Lc-->L alpha-->HII sequence in first heating, and reversible L beta<-->L alpha<-->HII phase sequences in subsequent heating and cooling scans. The peak areas and temperatures of the L beta-->L alpha transitions are practically identical, while the Lc-->L alpha and L alpha-->HII temperatures for 14-2,3-Gal are about 5 degrees C higher than the corresponding temperatures for 14-1,2-Glc. These data are interpreted in terms of a significantly greater stability and faster formation kinetics of the lamellar crystalline Lc phase of 14-1,2-Gal.(ABSTRACT TRUNCATED AT 250 WORDS)