A relationship between 13C-chemical-shift displacements and counterion-condensation theory, in the binding of calcium ion by heparin.

Characteristics of the interaction between heparin and calcium ion in the presence of sodium ion have been examined by monitoring the 13C-chemical shift changes as a function of the calcium ion concentration and the total ionic strength. The results indicated that the association between the polyanion and the divalent cation is a delocalized process, as opposed to one involving specific binding. The correspondence found between chemical shift and the number of Ca2+ ions bound per charged group, as derived from the Manning counterion-condensation model, showed that the stoichiometry is not a constant quantity but, rather, varies throughout the titration, and approaches a limiting value of 2 at high dilution. Additional measurements of T1 and line-width were consistent with an intramolecular order-disorder conformational process induced by the binding of calcium ion. Moreover, binding does not occur or is relatively weak with N-desulfated heparin, or chondroitin 4-sulfate and 6-sulfate, each of which possesses fewer sulfate groups than heparin. These differences serve to emphasize the importance of the charge-density parameter in the control of counterion condensation according to the Manning model, and suggest that the spacing between the negatively charged groups is an associated factor.