Different phase behaviour of the sn-1 and sn-3 stereoisomers of the glycolipid di-tetradecyl-β- d-galactosylglycerol

Abstract

The phase behaviour of a synthetic, stereochemically pure glycolipid 2,3- di-O- tetradecyl-l-O-β- 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-β- d- galactosyl-sn- glycerol (14-1,2-Gal), and 1,2- di-O- tetradecyl-3-O-β- 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 (L c → H II ) on heating and a H II → L α (metastable) → L β(metastable) → L c phase sequence in subsequent cooling, while both 14-2,3-Gal and 14-1,2-Glc are characterized by an L c → L α → H II sequence in first heating, and reversible L β ↔ L α ↔ H II phase sequences in subsequent heating and cooling scans. The peak areas and temperatures of the L β → L α transitions are practically identical, while the L c → L α and L α → H II temperatures for 14-2,3-Gal are about 5°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 L c phase of 14-1,2-Gal. They show also that the property of galactose head groups of imparting higher stability on glycolipid membranes is strongly dependent on the glycerol stereoconfiguration. Changing it from sn-3 to sn-1 results in a large stability loss of the L c phase and convergence of the galactolipid phase pattern to that of the corresponding glucolipid 14-1,2-Glc.