Does It Bind? A Method to Determine the Affinity of Calcium and Magnesium Ions for Polymers Using 1 H NMR Spectroscopy.

The binding of calcium and magnesium ions (M 2+ ) 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 2+ 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 1 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 2+ 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 2+ and acetate ions up through the solution. The concentrations of acetate, [Ac], and free (unbound) M 2+ , [M 2+ ] f , are measured at different positions along the sample by CSI. Binding of M 2+ to the polymer reduces [M 2+ ] f and hinders the upward diffusion of M 2+ . A discrepancy is thus observed between [Ac] and [M 2+ ] f from which the binding affinity of the polymer can be assessed. For systems which form insoluble complexes with M 2+ , such as sodium polyacrylate or carboxylate-functionalized nanocellulose (CNC), we can determine the concentration of M 2+ at which the polymer will precipitate. We can also predict [M 2+ ] f when a solution of polymer is mixed homogeneously with M 2+ salt. We assess the binding properties of sodium polyacrylate, alginate, polystyrene sulfonate, CNC, polyethyleneimine, ethylenediamenetetraacetic acid, and maleate.