Differential scanning calorimetry and X-ray diffraction studies of the thermotropic phase behavior of the diastereomeric di-tetradecyl-beta-D-galactosyl glycerols and their mixture

Abstract

We have investigated the thermotropic phase behavior of aqueous dispersions of the 1,2- and 2,3-di-O-tetradecyl-1(3)-O-(beta-D-galactopyranosyl)-sn- glycerols and their diastereomeric mixture using differential scanning calorimetry and low-angle and wide-angle x-ray diffraction. Upon heating, unannealed aqueous dispersions of these compounds all exhibit a lower temperature, moderately energetic phase transition at approximately 52 degrees C and a higher temperature, weakly energetic phase transition at approximately 63 degrees C, both of which are reversible on cooling. X-ray diffraction measurements identify these events as the L beta (or L' beta)/L alpha and L alpha/HII phase transitions, respectively. The structures of the L beta, L alpha, and HII phases of these lipids, as determined by x-ray diffraction measurements, are identical within the error bars for all of these lipids. On annealing below the L beta/L alpha phase transition temperature, the L beta phase converts to an Lc phase at a rate which is strongly dependent on the chirality of the glycerol backbone (1,2-sn > 1,2-rac > 2,3-sn). The temperature of the phase transition from the Lc phase seen on reheating is also dependent on the glycerol chirality. In addition, the nature of the Lc phase changes on subsequent heating in the 1,2-sn and 1,2-rac lipids, but we have not been able to detect this Lc1/Lc2 phase transition by calorimetry. However, wide-angle x-ray diffraction measurements indicate that these Lc phases differ mostly in their hydrocarbon chain packing modes. The Lc2 phase does not appear to be present in the 2,3-sn compound, suggesting that its formation is not favored in this diastereomeric isomer. These observations are discussed in relation to the effect of glycerol chirality on the molecular packing of these glycolipids, particularly on hydrogen bonding and hydration in the interfacial region of the bilayer.