THERMOTROPIC PHASE PROPERTIES OF 1,2-DI-O-TETRADECYL-3-O-(3-O-METHYL-BETA-D-GLUCOPYRANOSYL)-SN-GLYCEROL

The hydration properties and the phase structure of 1,2-di-O-tetradecyl-3-O-(3-O-methyl-beta-D-glucopyranosyl)-sn-glycerol (3-O-Me-beta-D-GlcDAIG) in water have been studied via differential scanning calorimetry, H-1-NMR and H-2-NMR spectroscopy, and x-ray diffraction. Results indicate that this lipid forms a crystalline (L(c)) phase up to temperatures of 60-70 degrees C, where a transition through a metastable reversed hexagonal (H-II) phase to a reversed micellar solution (L(2)) phase occurs. Experiments were carried out at water concentrations in a range from 0 to 35 wt %, which indicate that all phases are poorly hydrated, taking up <5 mol water/mol lipid. The absence of a lamellar liquid crystalline (L(alpha)) phase and the low levels of hydration measured in the discernible phases suggest that the methylation of the saccharide moiety alters the hydrogen bonding properties of the headgroup in such a way that the 3-O-Me-beta-D-GlcDAIG headgroup cannot achieve the same level of hydration as the unmethylated form. Thus, in spite of the small increase in steric bulk resulting from methylation, there is an increase in the tendency of 3-O-Me-beta-D-GlcDAIG to form nonlamellar structures. A similar phase behavior has previously been observed for the Acholeplasma laidlawii A membrane lipid 1,2-diacyl-3-O-(6-O-acyl-alpha-D-glucopyranosyl)-sn-glycerol in water (Lindblom et al. 1993. J. Biol. Chem. 268:16198-16207). The phase behavior of the two lipids suggests that hydrophobic substitution of a hydroxyl group in the sugar ring of the glucopyranosylglycerols has a very strong effect on their physicochemical properties, i.e., headgroup hydration and the formation of different lipid aggregate structures.