Conformational Studies of Monosaccharides and Oligosaccharides by Nmr Spectroscopy and X-Ray Crystallography

1H NMR spectra of aldohexopyranosyl rings containing 13C-enrichment reveal the presence of long-range 4JC1,H6R/S and 4JC3,H6R/S whose magnitudes depend mainly on the O5-C5-C6-O6 torsion angle. Using theoretical calculations and conformationally constrained experimental model compounds, the magnitudes and signs of 4JC1,H6R/S and 4JC3,H6R/S have been established, and their dependencies on the geometry of the C1-O5-C5-C6-H6R/S and C3-C4-C5-C6-H6R/S coupling pathways, respectively, were determined. A novel method involving the use of trans-glycoside J-coupling constants is applied to evaluate the conformational properties of structurally-related glycosidic linkages in oligosaccharides. By incorporating dual 13C-labeling at the carbons participating directly in the linkage and measuring the ensemble of trans-glycoside J-couplings across the linkage, firm qualitative and semi-quantitative information can be obtained rapidly about whether linkage conformations are similar or different. A simplified cyanohydrin route was developed to synthesize isotopically labeled 2-amino-2-deoxy-D-aldohexoses and their methyl glycosides by Yuping Zhu and other members of the Serianni laboratory. The hydrolysis of the starting materials for this synthesis, the aldopentopyranosylamines, was investigated by NMR to determine optimal reaction conditions for the cyanide addition reaction. Multiple 13C isotopomers of di- and trisaccharides containing N-acetyl-D-glucosamine (GlcNAc) and D-mannose (Man) representing nested fragments of biologically important N-glycans were synthesized. NMR studies yielded trans-glycoside JCC and JCH within related structures, providing new information on conformational information of constituent linkages and the effect of molecular context in dictating preferred linkage conformation in solution. Conformations of four glycosidic linkages in the pentasaccharide core were determined by semi-quantitative fitting of the coupling constants to Karplus equations, followed by filtering with NOE data. The effect of structural context was examined by comparing the corresponding trans-glycoside J-couplings observed in the trisaccharide (TriMan) unit to those in the individual disaccharide components.Two disaccharide crystals were studied by X-ray crystallography. Methyl alpha-lactoside, C13H24O11, has glycosidic torsion angles phi (O5gal-C1gal-O1gal-C4glc) and psi (C1gal-O1gal-C4glc-C5glc) of -93.52 (13) -144.83 (11)åÁ, respectively. Methyl beta-L-lactoside, C13H24O11, has glycosidic torsion angles phi and psi of 93.89 (13) and -127.43 (13)å_, respectively.