Disaccharide Headgroup Research Articles

Glycolipid membrane surface structure: orientation, conformation, and motion of a disaccharide headgroup.

The orientation of the disaccharide headgroup of a lactose-containing lipid, 3-O-(4-O-beta-D-galactopyranosyl-beta-D-glucopyranosyl)-1,2-di-O-tetrade cyl-sn- glycerol (DTLL), relative to the surface of bilayer membranes has been determined via 2H NMR. The lactosyl headgroup is extended away from the membrane surface into the aqueous phase. The headgroup motion has axial symmetry as evidenced by the spectral line shape and order parameter tensor. 2H NMR of oriented multibilayers of DTLL confirms that the director of motional averaging is the bilayer normal. The two sugar residues have segmental order parameters S (glucose, 0.53; galactose, 0.51) which indicate that the headgroup fluctuates about the bilayer normal as a rigid unit. 2H spin-lattice relaxation times T1z for deuterons on each of the two sugar rings are similar, indicating further that there is no substantial motion about the disaccharide linkage within the headgroup. The magnitude of the relaxation times (4 ms) suggests that the rigid body motions of the headgroup are approaching the Larmor frequency; however, they increase with increasing temperature, indicating that the motions are rapid enough to be in the fast motional regime (omega o2 tau c2 less than 1). The conformation about the galactose-glucose intersaccharide linkage, calculated from the 2H NMR data, is shown to differ substantially from those found in X-ray diffraction studies of crystalline lactose and high-resolution NMR studies of methyl lactoside in nonviscous solution. The orientations of the hydroxymethyl groups in the headgroup have been calculated from the 2H NMR data. For the galactosyl residue the data are consistent with the presence of more than one rotamer about the C5"-C6" bond which are in fast exchange on the 2H NMR time scale. The hydroxymethyl group of the glucose residue exists in two rotameric forms about the C5'-C6' bond which have relative populations of ca. 2:1 and which are in slow exchange on the 2H NMR time scale (10(-5) s). The two rotamers differ from those deduced from X-ray crystallography of crystalline lactose and 13C NMR studies of methyl lactoside in solution.

Galactosyl ceramides of the myelin sheath: Thermal studies

The thermal properties of several hydrated glycosyl ceramides have been investigated by differential scanning calorimetry in relation to the role of galactosyl ceramides in the myelin sheath of central nerve tissue. Myelin galactosyl ceramides are characterised by the presence of a large proportion of α-hydroxy fatty acids which lower the phase transition of this lipid class in comparison to galactosyl ceramides with normal fatty acids of similar chain length. Alteration of the sugar headgroup from galactose to glucose produced no significant change in the transition temperature of the lipid. The presence of a disaccharide headgroup lowered the transition temperature and altered the structure of the endotherm. Increasing concentrations of cholesterol in aqueous dispersions of galactosyl ceramide progressively decreased the enthalpy of the phase transition until, in the presence of 34 mole % cholesterol, it was no longer detectable. Examination of the effects of a range of phospholipids on the phase transition of galactosyl ceramides suggest that the phase behaviour of phospholipid-galactosyl ceramide mixtures is complex. However, only a single endotherm was observed in all the phospholipid-galactosyl ceramide systems examined. This tends to indicate that segregated phases enriched in galactosyl ceramides do not occur in the intact myelin membrane.