title: A relationship between 13C-chemical-shift displacements and counterion-condensation theory, in the binding of calcium ion by heparin.
abstract: 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.
mesh_terms: D000818:Animals; D002118:Calcium; D002247:Carbon Isotopes; D002417:Cattle; D055598:Chemical Phenomena; D002621:Chemistry; D002809:Chondroitin Sulfates; D006493:Heparin; D008168:Lung; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D013055:Spectrophotometry, Infrared
chemical_list: D002247:Carbon Isotopes; D006493:Heparin; D002809:Chondroitin Sulfates; D002118:Calcium