Conformational flexibility of the disaccharide & beta;-l-Fucp-(1 & RARR;4)-& alpha;-d-Glcp-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 & beta;-l-Fucp-(1 & RARR;4)-& alpha;-d-Glcp-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-& phi; and anti-& psi;, indicating flexibility at the glycosidic linkage. NMR data were obtained from 1D H-1,H-1-NOESY and STEP-NOESY experiments, measurement of transglycosidic (3)J(CH) coupling constants and NMR spin-simulation. The free energy profile of the & omega; 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 & PRIME;MIDLINE HORIZONTAL ELLIPSISHO6 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, H-1 and C-13 NMR chemical shifts were calculated using the same methodology and were found to be in excellent agreement with experimental data.