NMR solution geometry of saccharides containing the 6-O-(α-D-glucopyranosyl)-α/β-D-glucopyranose (isomaltose) or 6-O-(α-D-galactopyranosyl)-α/β-D-glucopyranose (melibiose) core

Abstract

The solution geometries of D-Glcp, Me-D-Glcp, 6-O-Me-D-Glcp, Me-6-O-Me-D-Glcp, D-Glcp-(α-1,6)-D-Glcp (isomaltose), D-Glcp-(α-1,6)-D-Glcp-(α-1,6)-D-Glcp (isomaltotriose), D-Galp-(α-1,6)-D-Glcp (melibiose), D-Galp-(α-1,6)-D-Glcp-(α-1,2)-D-Fruf (raffinose), and D-Galp-(α-1,6)-D-Galp-(α-1,6)-D-Glcp-(α-1,2)-D-Fruf (stachyose) in water are described by NMR spectroscopy, molecular dynamic simulations and quantum mechanical calculations. Overall, a change in anomeric configuration at the reducing end and/or anomeric substitution (methylation) changed the conformational space of the terminal CH2OH group significantly. Conformational analysis of the free monosaccharides matched literature results very well. Dihedral angle histograms weighted against published Karplus equations yielded excellent matches of experimental J-values in some cases but significant deviations in other. The anomeric hemiacetal configuration appeared to have a significant remote influence on the conformational space of the α-1,6-glycosidic linkage. Rigid glycosidic φ-conformations (g+) combined with mostly st-conformations for glycosidic ψ-angles from computations matched experimental nuclear Overhauser enhancements in all cases. While the investigated Glcp-α-1,6-Glcp linkages were nearly identical in φ/ψ-conformation, differences were apparent in the Galp-α-1,6-Galp linkage of stachyose. Of twenty-one crystal structures, a total of fourteen had ligand conformations corresponding to the most abundant or second-most abundant solution geometry determined in this study.