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

The binding of calcium and magnesium ions (M²⁺) 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 M²⁺ 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 ¹H 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, M²⁺ 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 M²⁺ and acetate ions up through the solution. The concentrations of acetate, [Ac], and free (unbound) M²⁺, [M²⁺]_f, are measured at different positions along the sample by CSI. Binding of M²⁺ to the polymer reduces [M²⁺]_f and hinders the upward diffusion of M²⁺. A discrepancy is thus observed between [Ac] and [M²⁺]_f from which the binding affinity of the polymer can be assessed. For systems which form insoluble complexes with M²⁺, such as sodium polyacrylate or carboxylate-functionalized nanocellulose (CNC), we can determine the concentration of M²⁺ at which the polymer will precipitate. We can also predict [M²⁺]_f when a solution of polymer is mixed homogeneously with M²⁺ salt. We assess the binding properties of sodium polyacrylate, alginate, polystyrene sulfonate, CNC, polyethyleneimine, ethylenediamenetetraacetic acid, and maleate.