Ionic liquids (ILs) have attracted attention as solvents because of their fascinating features, such as negligible vapor pressure and high thermal, chemical, and electrochemical activity. ILs have been widely studied as green, multi-use media for electrochemistry and chemisry even in bio-applications. Generally, however, most proteins are not soluble in ILs. Surface modification on the proteins, e.g. poly(ethylene oxide) (PEO) modification is one of the way to dissolve biomolecules in ILs [1]. PEO modified cytchrome c dissolved and showed the redox activity in ILs.

As another way, hydrated IL have been studied to realize the dissolution of proteins without chemical modification. Hydrated ILs maintain the basic properties of ILs, but a small amount of water was added. However, it is difficult to dissolve proteins in most hydrated ILs. Selection of component ion is essential to realize the dissolution and maintaining the higher ordered structure of proteins in hydrated IL. Hydration state of ILs which could be estimated on kosmotropicity was suggested one of the key factor to control the protein compatibility [2]. Hydrated cholinium dihydrogen phosphate ([ch][dhp]) acts as an excellent solvent to dissolve and preserve proteins for long term [3]. Furthermore, the original reactions such as enzymatic reactions and electron transfer reactions were observed in hydrated ILs, when the higher ordered structures were remained [4]. However, when the biocatalytic reaction was detected as an electrochemical response, the observed redox current was smaller than that in buffer solution. The redox reaction rate between a fixed protein and electrode was investigated to clarify the reason for the depression of the redox response of biocatalytic reactions in hydrated [ch][dhp] [5]. It was revealed that the electron transfer rate between protein and electrode was fast in hydrated [ch][dhp] as same that in buffer. Fast electron transfer reaction in hydrated ILs is expected to be more stable over a wider temperature range and longer period of time than general aqueous system.

 

References

[1] H. Ohno, C. Suzuki, K. Fukumoto, M. Yoshizawa, K. Fujita, Electron Transfer Process of Poly(ethylene oxide)-Modified Cytochrome c in Imidazolium Type Ionic Liquid, Chem. Lett., 32 (2003) 450-451.

[2] Y. Nikawa, K. Fujita, H. Ohno, Quantitative Assessment of Kosmotropicity of Hydrated Ionic Liquids by Nuclear Magnetic Resonance, Phys. Chem. Chem. Phys., 19 (2017) 8148-8151.

[3] K. Fujita, D. R. MacFarlene, M. Forsyth, Protein solubilising and stabilising ionic liquids, Chem. Commun. (2005) 4804-4806.

[4] K. Fujita, N. Nakamura, K. Igarashi, M. Samejima, H. Ohno, Biocatalytic oxidation of cellobiose in an hydrated ionic liquid, Green Chem., 11 (2009) 351-354.

[5] K. Fujita, J. Kuwahara, N. Nakamura, H. Ohno, Fast Electron Transfer Reaction of Azurin Fixed on the Modified Electrode in Hydrated Ionic Liquids, J. Electrochem. Soc., 163 (2016) G79-G81.