Rotational dynamics of thiocyanate ions in highly concentrated aqueous solutions

Abstract

The thiocyanate (SCN⁻) anion is known as one of the best denaturants, which is also capable of breaking the hydrogen-bond network of water and destabilizing native structures of proteins. Despite prolonged efforts to understand the underlying mechanism of such Hofmeister effects, detailed dynamics of the ions in a highly concentrated solution have not been fully elucidated yet. Here, we used a dispersive IR pump–probe spectroscopic method to study the dependence of vibrational lifetimes and rotational relaxation times of thiocyanate ions on KSCN concentration in D₂O. The nitrile stretch mode is used as a vibrational probe for dispersed IR pump–probe and FTIR measurements. To avoid possible self-attenuation of the IR pump–probe signal by highly concentrated SCN⁻ ions, we added a small amount of ¹³C-isotope-labeled thiocyanate ions (S¹³CN⁻) and focused on the excited-state absorption contribution to the IR pump–probe signal of the ¹³C-isotope-labeled nitrile stretch mode. Quite unexpectedly, the vibrational lifetime of S¹³CN⁻ ions is independent of the total KSCN concentration in the range from 0.46 m (molality) to 11.8 m while the rotational relaxation time of S¹³CN⁻ ions is linearly dependent on the total KSCN concentration. By combining the present experimental findings with the fact that the dissolved ions of KSCN salt have a strong tendency to form a large ion cluster in a highly concentrated aqueous solution, we believe that the ion clusters consisting of potassium and thiocyanate ion pairs in D₂O behave like ionic liquids and the ions inside ion clusters are weakly bound by electrostatic Coulombic interactions. The ability of SCN⁻ ions to form ion clusters in aqueous protein solutions seems to be a key to understand the Hofmeister ion effect. We anticipate that the present experimental results provide a clue for further elucidating the underlying mechanism of the Hofmeister ion effects on protein stability in the future.