Conformational flexibility of the disaccharide β--Fuc-(1→4)-α--Glc-OMe as deduced from NMR spectroscopy experiments and computer simulations

Carbohydrates in biological systems are referred to as glycans and modification of their structures is a hallmark indicator of disease. Analysis of the three-dimensional structure forms the basis for further insight into how they function and comparison of crystal structure with solution-state conformation(s) is particularly relevant, which has been performed for the disaccharide β- l -Fuc p -(1→4)-α- d -Glc p -OMe. In water solution the conformational space at the glycosidic linkage between the two sugar residues is identified from molecular dynamics (MD) simulations as having a low-energy exo-syn conformation, deviating somewhat from the solid-state conformation, and two anti -conformational states, i.e. , anti- and anti-ψ , indicating flexibility at the glycosidic linkage. NMR data were obtained from 1D 1 H, 1 H-NOESY and STEP-NOESY experiments, measurement of transglycosidic 3 J CH coupling constants and NMR spin-simulation. The free energy profile of the ω torsion angle computed from MD simulation was in excellent agreement with the rotamer distribution from NMR experiment being for gt : gg : tg 38 : 53 : 9, respectively, with a proposed inter-residue O5′ HO6 hydrogen bond being predominant in the gg rotamer. Quantum mechanics methodology was used to calculate transglycosidic NMR 3 J CH coupling constants, averaged over a conformational ensemble of structures representing various rotamers of exocyclic groups, in good to excellent agreement with Karplus-type relationships previously developed. Furthermore, 1 H and 13 C NMR chemical shifts were calculated using the same methodology and were found to be in excellent agreement with experimental data. Quantum mechanics methodology was used to calculate solution-state NMR observables such as transglycosidic 3 J CH coupling constants, and 1 H and 13 C NMR chemical shifts in excellent agreement with experimental data for a flexible disaccharide.