A new radiochemical method to investigate ion binding with polyelectrolytes

A new method for investigating the binding of ions with polyelectrolytes has been developed. This method, based on Donnan equilibrium and an isotope exchange between the electrolyte and polyelectrolyte, can distinguish territorial from specific binding of ions and can determine fractions of ions bound with the polyion. This method can determine ion binding with polyelectrolytes in a wide range of polyelectrolyte concentrations in multicomponent solutions. The method was tested with radioactive tracers 22Na+, 36Cl- and heparin sodium salt. The influence of the ionic strength on the Na+ binding with heparin was investigated at 310 K. In the limit of zero ionic strength, all Na+ ions are bound to heparin, but only 45% of them are exchangeable. Thus Na+ ions can be bound both territorially and specifically. The fraction of bound ions decreases rapidly with increasing ionic strength. The fraction of the specifically bound ions becomes negligible when the ionic strength exceeds 0.01 M, whereas the fraction of territorially bound ions can be neglected at ionic strengths higher than 0.45 M.