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

Abstract

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.

Tuning Phosphine Oxide-Substituted ortho-Carboranes for Improved Biphasic Electrochemical UO22+ Capture and Release

“…The relative binding strength of each carborane to UO 2 2+ can then readily be represented by p K eq values (eq , Table ). Bis(triphenylphosphine)iminium hexafluorophosphate ([PPN][PF 6 ]) was added to each solution as an internal standard, and its concentration was kept constant for quantitative determination of each equilibrium species by 31 P{ 1 H} NMR integration (e.g., eq for [ PO Cb ]). , C P O b + U O 2 ( N O 3 ) 2 ( T P P O ) 2 ⇄ U O 2 ( N O 3 ) 2 C P O b + 2 T P P O K = false[ U O 2 false( normalN normalO 3 false) 2 normalC normalP normalO normalb false] false[ normalT …”

“…Bis-(triphenylphosphine)iminium hexafluorophosphate ([PPN]-[PF 6 ]) was added to each solution as an internal standard, and its concentration was kept constant for quantitative determination of each equilibrium species by 31 P{ 1 H} NMR integration (e.g., eq 5 for [ PO Cb]). 27,28 V Cb UO (NO ) (TPPO)…”

Imaging Saturation Transfer Difference (STD) NMR: Affinity and Specificity of Protein–Ligand Interactions from a Single NMR Sample

“…The application of CSI NMR to a sample containing a concentration gradient has been proposed as a way to condense, in a single-tube, experiments traditionally requiring titrations, i.e., preparation of separate samples at variable concentrations of a given component. This approach has already been applied to determine the p K a and p K b of small molecules in aqueous and non-aqueous solvents, the study of weak molecular interactions through concentration gradients in agar gels, reaction monitoring, and binding affinity of macromolecules to Ca 2+ and Mg 2+ …”

“…This approach has already been applied to determine the pK a and pK b of small molecules in aqueous and non-aqueous solvents, 7 weak molecular interactions through concentration gradients in agar gels, 8 reaction monitoring, 9 and binding affinity of macromolecules to Ca 2+ and Mg 2+ . 10 STD NMR relies on the homogeneous saturation of the receptor (generally a protein), which allows magnetization transfer to any bound ligand. For each signal, the transferred saturation, reflected by the STD factor [η STD , defined as (I 0 − I sat )/I 0 ], is correlated to the proximity of the given set of protons to the protein surface, which translates into the binding epitope map of the ligand in the complex.…”

Open Linear Polymer Host–Guest Interactions Sensed by Luminescent Silver Nanodots

“…One challenge might be the lack of a sensitive method to reveal the binding. Various analytical methods, such as nuclear magnetic resonance (NMR), microbalance, calorimetry, UV–vis spectrophotometry, potentiometric titrations, isothermal titration, and atomic absorption spectroscopy have been used to monitor the host–guest interactions. − Plasmonic absorption of gold nanoparticles has also been used to indicate a cation-induced aggregation of gold nanoparticles . Otherwise, photoluminescent signals can be used to monitor the binding of cations to the conjugated polymer chain.…”