Combined NMR and molecular modeling study of an iduronic acid-containing trisaccharide related to antithrombotic heparin fragments Original Research Article

An iduronic acid-containing trisaccharide, methyl-O-(4-O-methyl-2,3,6-tri-O-sulfo-α-d-glucopyranosyl-(1→4)-O-(2-O-sulfo-α-l-idopyranosyluronic acid)-(1→4)-O-2, 6-di-O-sulfo-α-d-glucopyranoside, related to antithrombotic heparin fragments has been subjected to a combined NMR and molecular modeling investigation. The conformational behavior of the two constituting disaccharide segments was investigated using a systematic grid search approach with the MM3 force field along with the proper parameters for the sulfate ester group. The exploration of the potential energy surfaces of the trisaccharide was performed through the use of the CICADA methods interfaced with the MM3 force field. In all cases, the 2-O-sulfo-α-l-iduronate moiety was given the three favored ring conformations 1C4, 4C1, and 2S0. Conformations were clustered into families, four of which are likely to exhibit significant occupancy in solution. The different low-energy conformational families display different orientations at the glycosidic linkages and/or different ring shapes for the iduronate ring. The 2S0 conformation is the major one for the 2-O-sulfo-α-l-iduronate but is still in equilibrium with the 1C4 ring shape. The occurrence of such a conformational equilibrium in solution was probed via high-resolution NMR spectroscopy through measurements of coupling constants and NOE build-up. These results are in keeping with the observation that 2-O-sulfated pentasaccharides display a similar affinity for antithrombin III as their 2-N-sulfated counterparts.