Structure and dynamics of a glyceroglycolipid: A 2H NMR study of head group orientation, ordering, and effect on lipid aggregate structure

The head group orientation and the motional characteristics of 1,2-di-O-tetradecyl-3-O-β-D-glucopyranosylglycerol selectively 2H-labeled on the glucose moiety have been investigated by differential scanning calorimetry and 2H NMR. The glycolipid undergoes a major endothermic transition at 520C, which is attributed to the gel to liquid-crystal phase transition. The nature of a less energetic endothermic transition at 580C, determined to be a lamellar to hexagonal mesophase transition by 2H NMR, is confirmed by x-ray diffraction. In the Lamellar phase, the glycolipid head group undergoes axially symmetric motion and has an orientational order parameter S/sub mol/ of 0.45. The head group is extended away from the bilayer surface. On entering the hexagonal mesophase, the orientational order parameter for the sugar ring is reduced slightly to 0.38, but the local rotation axis undergoes a large reorientation with respect to the carbohydrate ring. In a phospholipid matrix, the orientation of the carbohydrate head group of the glycolipid is affected by the greater extension of the surface residues of the host lipid