MULTINUCLEAR MAGNETIC-RESONANCE STUDIES OF THE INTERACTION OF INORGANIC CATIONS WITH HEPARIN

The interaction of Na+, Ca2+, Mg2+, Zn2+ and La3+ with heparin, a high ly negatively charged glycosaminoglycan, was studied by H-1 and Na-23 nuclear magnetic resonance spectroscopy. H-1 chemical shift and nuclea r Overhauser effect (NOE) data indicate that the counter ions Na+, Ca2 + and Mg2+ interact with the low pH, carboxylic acid form of heparin b y delocalized, long-range electrostatic interactions. At higher pH, H- 1 chemical shift and NOE data indicate that Na+ and Mg2+ continue to i nteract with heparin in the same manner, even upon deprotonation of th e carboxylic acid group; however, then is a site-specific contribution to the binding of Ca2+, Zn2+ and La3+ under these conditions. Acid di ssociation constants for heparin carboxylic acid groups and heparin-me tal binding constants were determined from the pH dependence of H-1 ch emical shifts and Na-23 spin-lattice (T-1) relaxation times. Equilibri um constants for exchange of M(2+) for heparin-bound Na+ were obtained from Na-23 T-1 data. The acid dissociation constants show a strong de pendence on Na+ concentration due to the polyelectrolyte character of heparin.