Destabilization effect of imidazolium cation-Hofmeister anion salts on cytochrome c.

We have analyzed an effect of imidazolium cation-Hofmeister anion salts on stability of basic horse heart cytochrome c (cyt c) at pH4.5 (net charge +17). The effect of salts consisting of imidazolium cations, 1-ethyl-3-methylimidazolium (EMIm) and 1-butyl-3-methylimidazolium (BMIm), and five anions: chloride, bromide, iodide, nitrate, and thiocyanate on thermal and pH stability of cyt c was compared with the effect of corresponding sodium salts. Correlation between parameter of dT/d [ion] (T; thermal midpoints) with surface tension changes of solvent in the presence of both imidazolium and sodium salts implies direct interaction between ions and proteins. Surprisingly, the imidazolium salts have more pronounced destabilization effect on highly positively charged cyt c than the corresponding sodium counterparts. Our analysis suggests the direct interaction of imidazolium cations with polypeptide chain, in analogy to guanidium cation, but the destabilization effect is significantly strengthened by decreased surface tension of imidazolium salt solvents. Comparison of an effect of imidazolium and sodium salts on acidic and alkaline transitions and to thermal transition of cyt c implies a role of hydrophobic interaction between imidazolium cation and polypeptide chain.