Conformational Preferences at the Glycosidic Linkage of Saccharides in Solution as Deduced from NMR Experiments and MD Simulations: Comparison to Crystal Structures

Abstract

Glycans are central to information content and regulation in biological systems. These carbohydrate molecules are active either as oligo‐ or polysaccharides, often as glycoconjugates. The conformational preferences and population distributions at the glycosidic torsion angles ϕ and ψ have been investigated for O‐methyl glycosides of three disaccharides where the substitution takes place at a secondary alcohol. Stereochemical differences at or adjacent to the glycosidic linkage were explored by solution state NMR spectroscopy using one‐dimensional 1H,1H‐NOESY NMR experiments to obtain transglycosidic proton‐proton distances and one‐ and two‐dimensional heteronuclear NMR experiments to obtain 3JCH transglycosidic coupling constants related to torsion angles ϕ and ψ. Computed effective proton‐proton distances from molecular dynamics (MD) simulations showed excellent agreement to experimentally derived distances for the α‐(1→3)‐linked disaccharides and revealed that for the bimodal distribution at the ψ torsion angle for the α‐(1→4)‐linked disaccharide experiment and simulation were at variance with each other, calling for further force field developments. The MD simulations disclosed a highly intricate inter‐residue hydrogen bonding pattern for the α‐(1→4)‐linked disaccharide, including a nonconventional hydrogen bond between H5' in the glucosyl residue and O3 in the galactosyl residue, supported by a large downfield 1H NMR chemical shift displacement compared to α‐d‐Glcp‐OMe.