Selective Transport of Water-Soluble Proteins from Aqueous to Ionic Liquid Phase via a Temperature-Sensitive Phase Change of These Mixtures

Mixtures of some ionic liquids (ILs) and water show reversible phase change between a homogeneous mixture and phase-separated state by a small change in temperature. Some water-soluble proteins have been migrated from the aqueous to the IL phase. When tetrabutylphosphonium 2,4,6-trimethylbenzenesulfonate was used as an IL, cytochrome c (Cyt.c) was found to be extracted from the water phase to the IL phase. Conversely, both horseradish peroxidase (HRP) and azurin remained in the aqueous phase. This selective extraction was comprehended to be due to the difference in solubility of these proteins in both phases. The separated aqueous phase contained a small amount of IL, which induced the salting-out of Cyt.c. On the other hand, condensed IL phase promoted the salting-in of Cyt.c. As a result, Cyt.c was preferably dissolved in the hydrated IL phase rather than aqueous phase. In the case of HRP, there was only a salting-out profile upon increasing the concentration of IL, which induced selective dissolution of HRP in the aqueous phase. These results clearly suggest that the profile of salting-out and salting-in for proteins is the key factor to facilitate the selective extraction of proteins from aqueous to the IL phase.