Determination of conformational properties of glycolipid head groups by 2H NMR of oriented multibilayers

Conformations and orientations of the glucose and glycerol moieties of a monoglucosyl lipid in hydrated bilayers were determined in detail by deuterium nuclear magnetic resonance (2H NMR). Multibilayer membranes of 1,2-di-O-tetradecyl-3-O-(β-D-glucopyranosyl)glycerol (DTGL), of dimyristoylphosphatidylcholine (DMPC), and a mixture of DTGL and DMPC were oriented between glass plates. The glucolipid was selectively labeled with deuterium on the pyranose ring and at C3 of glycerol, whereas DMPC was labeled at the C4 position of the sn-2 chain. Quadrupolar splittings were measured as a function of the angle between the bilayer normal and the magnetic field direction. The results establish that the director of motional averaging, the direction about which motion and order are axially symmetric, is the bilayer normal for all the head group, the glycerol backbone, and the hydrophobic core. Segmental order parameters were determined to be 0.45, 0.65, and 0.40, respectively, for the various regions of DTGL in the membranes. The latter results indicate that there is some motion on the time scale of 105 s-1 about the Cl'(glucose)-O-C3(glycerol) glycosidic bond but that its amplitude is very restricted. Comparison of 1H-decoupled and 1H-coupled 2H NMR spectra of the C3-labeled glycolipid gave estimates of the 2H-2H dipolar coupling between the deuterons at this position. The orientation of the glycerol C3 hydroxymethylene subunit was calculated relative to the bilayer normal and the C2-C3 bond was found to be tilted away from the bilayer normal by 3 +/- 10. An estimate of the 1H-2H dipolar coupling between the hydrogen at C2 and the deuterons at C3 of glycerol of DTGL was obtained. The conformation about the glycosidic bond of the glycolipid has the sugar ring fully extended away from the bilayer surface. This conformation differs considerably from that associated with simple glycosides