Protein stabilization and enzyme activation in ionic liquids: specific ion effects

There are still debates on whether the hydration of ions perturbs the water structure, and what is the degree of such disturbance; therefore, the origin of the Hofmeister effect on protein stabilization continues to be questioned. For this reason, it is suggested to use the 'specific ion effect' instead of other misleading terms such as Hofmeister effect, Hofmeister series, lyotropic effect, and lyotropic series. This review first discusses the controversial aspect of inorganic ion effects on water structures, and several possible contributors to the specific ion effect of protein stability. Due to recent overwhelming attraction of ionic liquids (ILs) as benign solvents in many enzymatic reactions, this paper further evaluates the structural properties of ILs and molecular-level interactions in neat ILs and their aqueous solutions. Next, the specific ion effects of ILs on enzyme stability and activity are systematically compared and it is concluded that (a) the specificity of many enzymatic systems in diluted aqueous IL solutions is roughly in line with the traditional Hofmeister series albeit some exceptions; (b) however, the specificity follows a different track in concentrated or neat ILs because other factors (such as hydrogen-bond basicity, nucelophilicity, and hydrophobicity, etc.) are playing leading roles. In addition, some examples of biocatalytic reactions in IL systems that are guided by the empirical specificity rule are demonstrated. (c) 2015 Society of Chemical Industry