The dependence of glyceroglycolipid orientation and dynamics on head-group structure

Abstract

The head-group orientations and molecular dynamics of three glyceroglycolipids in aqueous dispersions, as determined by 2H-NMR, are compared. 1,2- Di-O- tetradecyl-3-O-(α- d - glucopyranosyl)-sn- glycerol (α-DTGL) and 1,2- di-O- tetradecyl-3-O-(α- d - mannopyranosyl)-sn- glycerol (α-DTML), selectively 2H-labelled on the pyranose ring, at the exocyclic hydroxymethyl group, and at C3 of glycerol, have been studied by 2H-NMR and the results compared with those reported earlier for 1,2- di-O- tetradecyl-3-O-(β- d - glucopyranosyl)-sn- glycerol (β-DTGL). The α-glucolipid exhibits a gel-to-liquid crystal phase transition and a lamellar to hexagonal mesophase transition at temperatures which are similar to those of the β-anomer, β-DTGL. However, α-DTGL exhibits head group orientations and molecular ordering in the lamellar and hexagonal phases which differ strikingly with those reported for the corresponding β-glucolipid. Whereas the head group of β-DTGL is extended away from the bilayer surface into the aqueous phase, that of α-DTGL is almost parallel to the bilayer surface. α-DTGL exhibits a molecular order parameter of 0.56 which is substantially greater than that of its anomer, β-DTGL, 0.45. The latter indicates that the head group region of the α-glyceroglucolipid is characterized by smaller angular fluctuations than that of β-DTGL. On entering the hexagonal mesophase the pyranose ring of the β-glucolipid undergoes a large reorientation relative to the motional axis of the head group, whereas the α-anomer exhibits only a small orientational change. 1,2- Di-O- tetradecyl-3-O-(α- d - mannopyranosyl)-sn- glycerol (α-DTML) undergoes a phase transition at 47°C, attributed to the unusual lamellar gel to hexagonal phase transition. The pyranose ring of α-DTML, in a mixture with dimyristoylphosphatidylcholine (1:9 mol ratio) to give a lamellar liquid crystalline phase, is oriented away from the bilayer surface into the aqueous environment and has an S mol of 0.75. The results for α-DTML, 2H-labelled at the C3 position of glycerol, suggest that this segment also has high molecular ordering. α-DTML in a lamellar environment has the least flexible membrane surface of the glyceroglycolipids investigated to date. 2H-NMR spin lattice relaxation times have been used to probe the head group motions of the glycolipids. The results indicate that the rate of head group motion increases in the order α-DTML < α-DTGL < β-DTGL. The glycolipid head group has lower motional rates than the corresponding motion of phospholipids, with the possible exception of cardiolipin.