An NH Moiety Is Not Required for Anion Binding to Amides in Aqueous Solution
2015
Rembert, Kelvin B. | Okur, Halil I. | Hilty, Christian | Cremer, Paul S.
Herein, we use a combination of thermodynamic and spectroscopic measurements to investigate the interactions of Hofmeister anions with a thermoresponsive polymer, poly(N,N-diethylacrylamide) (PDEA). This amide-based polymer does not contain an NH moiety in its chemical structure and, thus, can serve as a model to test if anions bind to amides in the absence of an NH site. The lower critical solution temperature (LCST) of PDEA was measured as a function of the concentration for 11 sodium salts in aqueous solutions, and followed a direct Hofmeister series for the ability of anions to precipitate the polymer. More strongly hydrated anions (CO₃²–, SO₄²–, S₂O₃²–, H₂PO₄–, F–, and Cl–) linearly decreased the LCST of the polymer with increasing the salt concentration. Weakly hydrated anions (SCN–, ClO₄–, I–, NO₃–, and Br–) increased the LCST at lower salt concentrations but salted the polymer out at higher salt concentrations. Proton nuclear magnetic resonance (NMR) was used to probe the mechanism of the salting-in effect and showed apparent binding between weakly hydrated anions (SCN– and I–) and the α protons of the polymer backbone. Additional experiments performed by attenuated total reflection Fourier transform infrared (ATR–FTIR) spectroscopy found little change in the amide I band upon the addition of salt, which is consistent with very limited, if any, interactions between the salt ions and the carbonyl moiety of the amide. These results support a molecular mechanism for ion-specific effects on proteins and model amides that does not specifically require an NH group to interact with the anions for the salting-in effect to occur.
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