Investigation of Carbohydrate Conformation in Solution and in Powders by Double-Quantum NMR

Abstract

Double-quantum heteronuclear local field NMR was applied to two 13 C2-labeled carbohydrate samples, (1,2- 13 C2)-glucose and methyl-R-D-(1,3- 13 C2)-glucose. The geometry of the H- 13 C- 13 C-H moeity was estimated using the evolution of double-quantum coherences under correlated heteronuclear dipolar interactions. For (1,2- 13 C2)-glucose, double-quantum techniques were used both in solution and solid phases. The measured H-C1-C2-H torsion angles in crystalline glucose were 170° ( 5° for the ‚-anomer and 40° ( 15° for the R-anomer, in good agreement with reported crystal structures. In the solution phase we give a full analysis of an experiment in which the cross-correlation effects are isolated by the use of a heteronuclear multiple-quantum filter. We consider the influence of anisotropic rotational diffusion, chemical shift anisotropy, and proton - proton spin diffusion on the torsion angle estimate. We show that it is possible to determine the torsion angle and the rotational correlation time independently. The measured H-C1-C2-H torsion angles in solution differ slightly from the solid-state results: 159 ° ( 10° for the ‚-anomer and 57° ( 7° for the R-anomer. For methyl-R-D-(1,3- 13 C2)-glucose, the solid phase double-quantum heteronuclear local field experiment was applied for the first time to a HCCH moiety in which the carbons are not directly bonded. These techniques may be applied to other structural problems such as the determination of glycosidic linkage conformations and the conformation of sugar rings in nucleotides.