Does It Bind? A Method to Determine the Affinity of Calcium and Magnesium Ions for Polymers Using H NMR Spectroscopy.

The binding of calcium and magnesium ions (M) by polymers and other macromolecules in aqueous solution is ubiquitous across chemistry and biology. At present, it is difficult to assess the binding affinity of macromolecules for M without recourse to potentiometric titrations and/or isothermal titration calorimetry. Both of these techniques require specialized equipment, and the measurements can be difficult to perform and interpret. Here, we present a new method based on H NMR chemical shift imaging (CSI) that enables the binding affinity of polymers to be assessed in a single experiment on standard high-field NMR equipment. In our method, M acetate salt is weighed into a standard 5 mm NMR tube and a solution of polymer layered on top. Dissolution and diffusion of the salt carry the M and acetate ions up through the solution. The concentrations of acetate, [Ac], and free (unbound) M, [M], are measured at different positions along the sample by CSI. Binding of M to the polymer reduces [M] and hinders the upward diffusion of M. A discrepancy is thus observed between [Ac] and [M] from which the binding affinity of the polymer can be assessed. For systems which form insoluble complexes with M, such as sodium polyacrylate or carboxylate-functionalized nanocellulose (CNC), we can determine the concentration of M at which the polymer will precipitate. We can also predict [M] when a solution of polymer is mixed homogeneously with M salt. We assess the binding properties of sodium polyacrylate, alginate, polystyrene sulfonate, CNC, polyethyleneimine, ethylenediamenetetraacetic acid, and maleate.