Assessing the Effect of Hofmeister Anions on the Hydrogen-Bonding Strength of Water via Nitrile Stretching Frequency Shift

The temperature dependence of the peak frequency (nu(max)) of the C equivalent to N stretching vibrational spectrum of a hydrogen-bonded C equivalent to N species is known to be a qualitative measure of its hydrogen-bonding strength. Herein, we show that within a two-state framework, this dependence can be analyzed in a more quantitative manner to yield the enthalpy and entropy changes (Delta H-HB and Delta S-HB) for the corresponding hydrogen-bonding interactions. Using this method, we examine the effect of ten common anions on the strength of the hydrogen-bond(s) formed between water and the C equivalent to N group of an unnatural amino acid, p-cyanophenylalanine (Phe(CN)). We find that based on the Delta H-HB values, these anions can be arranged in the following order: HPO42- > OAc- > F- > SO42- approximate to Cl- approximate to (H2O) approximate to ClO4- approximate to NO3- > Br- > SCN- approximate to I-, which differs from the corresponding Hofmeister series. Because PheCN has a relatively small size, the finding that anions having very different charge densities (e.g., SO42- and ClO4-) act similarly suggests that this ranking order is likely the result of specific ion effects. Since proteins contain different backbone and side-chain units, our results highlight the need to assess their individual contributions toward the overall Hofmeister effect in order to achieve a microscopic understanding of how ions affect the physical and chemical properties of such macromolecules. In addition, the analytical method described in the present study is applicable for analyzing the spectral evolution of any vibrational spectra composed of two highly overlapping bands.