An NH moiety is not required for anion binding to amides in aqueous solution

Kelvin B. Rembert, Halil I. Okur, Christian Hilty, Paul S. Cremer

Research output: Contribution to journalArticle

30 Citations (Scopus)

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 (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.

Original languageEnglish (US)
Pages (from-to)3459-3464
Number of pages6
JournalLangmuir
Volume31
Issue number11
DOIs
StatePublished - Mar 24 2015

Fingerprint

Amides
amides
Anions
Negative ions
Polymers
anions
aqueous solutions
Salts
salts
polymers
Nuclear magnetic resonance
Ions
proton magnetic resonance
Temperature
Fourier transform infrared spectroscopy
temperature
Protons
Precipitates
precipitates
ions

All Science Journal Classification (ASJC) codes

  • Materials Science(all)
  • Condensed Matter Physics
  • Surfaces and Interfaces
  • Spectroscopy
  • Electrochemistry

Cite this

Rembert, Kelvin B. ; Okur, Halil I. ; Hilty, Christian ; Cremer, Paul S. / An NH moiety is not required for anion binding to amides in aqueous solution. In: Langmuir. 2015 ; Vol. 31, No. 11. pp. 3459-3464.
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An NH moiety is not required for anion binding to amides in aqueous solution. / Rembert, Kelvin B.; Okur, Halil I.; Hilty, Christian; Cremer, Paul S.

In: Langmuir, Vol. 31, No. 11, 24.03.2015, p. 3459-3464.

Research output: Contribution to journalArticle

TY - JOUR

T1 - An NH moiety is not required for anion binding to amides in aqueous solution

AU - Rembert, Kelvin B.

AU - Okur, Halil I.

AU - Hilty, Christian

AU - Cremer, Paul S.

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N2 - 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.

AB - 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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