N.m.r. spectroscopic observations related to the function of sulfate groups in heparin. Calcium binding vs. biological activity.

Chemically-modified heparins containing different combinations of N- and O-sulfate groups were prepared. Characterized by high field 1H- and 13C-n.m.r. spectroscopy, the polymers exhibited chemical shift variations in general accord with shielding differences expected on removal of sulfate substituents, and additional variations that probably arose from conformational changes in the polymers. Whereas the anticoagulant activity of heparins, as measured by USP, anti-Xa, and thrombin-time assays, was invariably reduced by the chemical transformations effected, the ability of heparin to bind calcium ions was found to be dependent on retention of the 2-sulfamino group, whether or not O-sulfate groups were present. The results suggest that the 2-sulfamino group is essential for maintaining a molecular conformation consistent with the ability for the L-iduronic acid residues to complex with calcium ions. Also, they show that although the anticoagulant and calcium-binding properties of heparin may be interdependent, they are not determined by the same structural entities in the polymer.