COMBINED NMR AND MOLECULAR MODELING STUDY OF AN IDURONIC ACID-CONTAINING TRISACCHARIDE RELATED TO ANTITHROMBOTIC HEPARIN FRAGMENTS

An iduronic acid-containing trisaccharide, yl-(1-->4)-O-(2-O-sulfo-alp ha-L-idopyranosyluronic -(1-->4)-O-2,6-di-O-sulfo-alpha-D-glucopyranos ide, related to antithrombotic heparin fragments has been subjected to a combined NMR and molecular modeling investigation. The conformation al behavior of the two constituting disaccharide segments was investig ated using a systematic grid search approach with the MM3 force field along with the proper parameters for the sulfate ester group. The expl oration of the potential energy surfaces of the trisaccharide was perf ormed through the use of the CICADA methods interfaced with the MM3 fo rce field. In all cases, the 2-O-sulfo-alpha-L-iduronate moiety was gi ven the three favored ring conformations C-1(4), C-4(1), and S-2(0). C onformations were clustered into families, four of which are likely to exhibit significant occupancy in solution. The different low-energy c onformational families display different orientations at the glycosidi c linkages and/or different ring shapes for the iduronate ring. The S- 2(0) conformation is the major one for the 2-O-sulfo-alpha-L-iduronate but is still in equilibrium with the C-1(4) ring shape. The occurrenc e of such a conformational equilibrium in solution was probed via high -resolution NMR spectroscopy through measurements of coupling constant s and NOE build-up. These results are in keeping with the observation that 2-O-sulfated pentasaccharides display a similar affinity for anti thrombin III as their 2-N-sulfated counterparts. (C) 1997 Elsevier Sci ence Ltd.