EFFECT OF THE CHIRALITY OF THE GLYCEROL BACKBONE ON THE BILAYER AND NONBILAYER PHASE-TRANSITIONS IN THE DIASTEREOMERS OF DI-DODECYL-BETA-D-GLUCOPYRANOSYL GLYCEROL

We have studied the physical properties of aqueous dispersions ot 1,2-sn- and 2,3-sn-didodecyl-beta-D-glucopyranosyl glycerols, as well as their diastereomeric mixture, using differential scanning calorimetry and low angle x-ray diffraction. Upon heating, both the chiral lipids and the diastereomeric mixture exhibit characteristically energetic L(beta)/L(alpha) phase transitions at 31.7-32.8-degrees-C and two or three weakly energetic thermal events between 49-degrees-C and 89-degrees-C. In the diastereomeric mixture and the 1,2-sn glycerol derivative, these higher temperature endotherms correspond to the formation of, and interconversions between, several nonlamellar structures and have been assigned to L(alpha)/Q(parallel-to)a, Q(parallel-to)a/Q(parallel-to)b, and Q(parallel-to)b/H(parallel-to) phase transitions, respectively The cubic phases Q(parallel-to)a and Q(parallel-to)b, whose cell lattice parameters are strongly temperature dependent, can be identified as belonging to space groups 1a3d and Pn3m/Pn3, respectively. In the equivalent 2,3-sn glucolipid, the Q(parallel-to)a, phase is not observed and only two transitions are seen at 49-degrees-C and 77-degrees-C, which are identified as L(alpha)/Q(parallel-to)b Q(parallel-to)b/H(parallel-to) phase transitions, respectively. These phase transitions temperatures are some 10-degrees-C lower than those of the corresponding phase transitions observed in the diastereomeric mixture and the 1,2-sn glycerol derivative. On cooling, ail three lipids exhibit a minor higher temperature exothermic event, which can be assigned to a H(parallel-to)/Q(parallel-to)b phase transition. An exothermic L(alpha)/L(beta) phase transition is observed at 30-31-degrees-C. A shoulder is sometimes discernible on the high temperature side of the L(alpha)/L(beta) event, which may originate from a Q(parallel-to)b/L(alpha) phase transition prior to the freezing of the hydrocarbon chains. None of the lipids show evidence of a Q(parallel-to)a, phase on cooling. No additional exothermic transitions are observed on further cooling to -3-degrees-C. However, after nucleation at 0-degrees-C followed by a short period of annealing at 22-degrees-C, the 1,2-sn glucolipid forms an L(c) phase that converts to an L(alpha) phase at 39.5-degrees-C on heating. Neither the diastereomeric mixture nor the 2,3-sn glycerol derivative shows such behavior even after extended Our results suggest that the differences in the phase behavior of these glycolipid isomers may not be attributable to headgroup size per se, but rather to differences in the stereochemistry of the lipid polar/apolar interfacial region, which consequently effects hydrogen-bonding, hydration, and the hydrophilic/hydrophobic balance.