An investigation of calcium ion binding to heparins

Using the line charge model of Manning and, where appropriate, a form of the empirical corrections to his limiting laws proposed by Wells, we have investigated the ion condensation phenomenon for four pure calcium heparin preparations and similar Ca-heparin/CaCl2 mixtures in aqueous solution. This has been expressed in terms of the single ion activity coefficient of the total calcium counter ion present. The results found for the single counter ion activity coefficient of pure Ca-heparin agree moderately well with the limiting law of Manning for a pure divalent counter ion polyanion salt in dilute solution. The results for Ca-heparin/CaCl2 mixtures when corrected for small ion-small ion interactions give a surprisingly good fit to the theoretical counter ion activity coefficient for a divalent counter ion/divalent cation simple electrolyte system, for values of X (X = the polyanion/co-ion ratio) up to 4, with slight deviations for values of X > 4. In the mixtures with the largest excess of polyelectrolyte, the single ion activity coefficients of the Ca2+ ion are in excess of the value predicted by Manning's theory. Ion exchange of sodium heparins to give the Ca-heparin samples used in the work did not appear to cause regular changes in the anticoagulant activity.