Weak Anion Binding to Poly(N-isopropylacrylamide) Detected by Electrophoretic NMR

Abstract

Ion specific effects are ubiquitous in solutions and govern a large number of colloidal phenomena. To date, a substantial and sustained effort has been directed at understanding the underlying molecular interactions. As a new approach, we address this issue by sensitive 1 H NMR methods that measure the electrophoretic mobility and the self-diffusion coefficient of poly( N -isopropylacrylamide) (PNIPAM) chains in bulk aqueous solution in the presence of salts with the anion component varied from kosmotropes to chaotropes along the Hofmeister series. The accuracy of the applied electrophoretic NMR experiments is exceptionally high, on the order of 10 –10 m 2 /(V s), corresponding to roughly 10 –4 elementary charges per monomer effectively associated with the neutral polymer. We find that chaotropic anions associate to PNIPAM with an apparent Langmuir-type saturation behavior. The polymer chains remain extended upon ion association, and momentum transfer from anion to polymer is only partial which indicates weak attractive short-range forces between anion and polymer and, thereby and in contrast to some other ion–polymer systems, the lack of well-defined binding sites.

Denaturation of proteins: electrostatic effects vs. hydration

“…Thus, Fang and Furo could show that chaotropic ions can associate to PNIPAM chains with a Langmuir-type association behavior while NaCl is only weakly adsorbed. 59 This effect measured through careful measurements of the electrophoretic mobility was found strongest for SCN − -ions. Hence, adsorption of chaotropic ions can diminish or even reverse the effective charge of unfolded proteins.…”

Relevance of the Cation in Anion Binding of a Triazole Host: An Analysis by Electrophoretic Nuclear Magnetic Resonance

“…With the aim to take into account all the factors involved in anion binding, electrophoretic NMR (eNMR) has been chosen as an advanced characterization technique. eNMR is a species-selective method, providing drift velocities of charged molecules or complexes in an electric field and thus delivering detailed information on ionic association. − The experiment is based on diffusion measurements by pulsed field gradient (PFG) NMR but includes additional electrical field pulses, which induce a collective ion drift motion evident as a shifting phase angle φ, dependent on the respective electrophoretic mobility μ. The method has been applied to ion binding in solution, as demonstrated, for example, by Giesecke et al, who investigated the binding of cations to poly­(ethylene oxide) chains, exploiting the increased charge of the polymer introduced by cations bound to the chain. , Along similar lines, eNMR has also been applied to quantify cation binding to crown ether hosts …”