A 13C nuclear magnetic resonance study of selectivity in the binding of univalent counterions by heparin

The binding selectivities of univalent counterions for heparin in the systems Ca2+/M+/HEP, where M+ = Li+, Na+, K+, Cs+, TMA+, have been studied by 13C nuclear magnetic resonance spectroscopy. Chemical shift displacements of the various 13C nuclei of heparin in these systems clearly indicate that the binding selectivity of the polymer for univalent counterions increases with an increase in the radius of the hydrated cation form, namely Li+ > Na+ > K+ > Cs+ > TMA+. An explanation for this binding preference is offered on the basis of electrostatic and entropic terms, whereby the latter is reflected in changes in the hydration volumes of the univalent cations. Differences in the magnitude of these hydration volumes, in relation to the free energies of hydration, may account for a binding selectivity that favours the more highly hydrated ions.