Rotational dynamics of thiocyanate ions in highly concentrated aqueous solutions

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 D ₂O. 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 (S ¹³CN ^-) 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 S ¹³CN ^- 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 S ¹³CN ^- 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 D ₂O 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.