title: An NH moiety is not required for anion binding to amides in aqueous solution.
abstract: 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 (CO3(2-), SO4(2-), S2O3(2-), 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.
authors.forename: Kelvin B
authors.initials: KB
authors.lastname: Rembert
authors.forename: Halil I
authors.initials: HI
authors.lastname: Okur
authors.forename: Christian
authors.initials: C
authors.lastname: Hilty
authors.forename: Paul S
authors.initials: PS
authors.lastname: Cremer
mesh_terms: D000577:Amides / Q000737:chemistry*; D000838:Anions / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry; D013696:Temperature
chemical_list: D000577:Amides; D000838:Anions; D012996:Solutions