Modified GOESY in the Analysis of Disaccharide Conformation

One-dimensional nuclear magnetic resonance techniques were applied to the conformational investigation of a disaccharide. More specifically, nuclear Overhauser enhancements (NOEs) of protons on either side of the glycosidic bond have been used to determine the conformation of the disaccharide α-l-Rhap-(1 → 2)-α-l-Rhap-OMe. A modified GOESY sequence, incorporating selective excitation and pulsed field gradient enhancement, was developed and used to accurately measure small NOE signals of interest. These experiments were named M-GOESY, for modified GOESY, and the data they provided were used to calculate internuclear distances in the disaccharide molecule. The accuracy of the M-GOESY measurements was enhanced by elimination of indirect effects, or spin diffusion, by selective inversion(s) of either the intermediate magnetization or the source and target magnetization during the mixing time. Results of this study indicate that the α-l-Rhap-(1 → 2)-α-l-Rhap-OMe disaccharide molecule exists primarily in one conformation, with the glycosidic torsion angle ψ ≈ −30° based on past molecular dynamics simulations.