Hydroxyl-Proton Hydrogen Bonding in the Heparin Oligosaccharide Arixtra in Aqueous Solution

Heparin is best known for its anticoagulant activity, which is mediated by the binding of a specific pentasaccharide sequence to the protease inhibitor antithrombin-III (AT-III). Although heparin oligosaccharides are thought to be flexible in aqueous solution, the recent discovery of a hydrogen bond between the sulfamate (NHSO₃^–) proton and the adjacent 3-<i>O</i>-sulfo group of the 3,6-<i>O</i>-sulfated <i>N</i>-sulfoglucosamine residue of the Arixtra (fondaparinux sodium) pentasaccharide demonstrates that definable elements of local structure are accessed. Molecular dynamics simulations of Arixtra suggest the presence of additional hydrogen bonds involving the C3-OH groups of the glucuronic acid and 2-<i>O</i>-sulfo-iduronic acid residues. NMR measurements of temperature coefficients, chemical shift differences, and solvent exchange rate constants provide experimental confirmation of these hydrogen bonds. We note that the extraction of rate constants from cross-peak buildup curves in 2D exchange spectroscopy is complicated by the presence of radiation damping in aqueous solution. A straightforward model is presented that explicitly takes into account the effects of radiation damping on the water proton relaxation and is sufficiently robust to provide an accurate measure of the proton exchange rate between the analyte hydroxyl protons and water