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 . Hydrated cholinium dihydrogen phosphate ([ch][dhp]) acts as an excellent solvent to dissolve and preserve proteins for long term . Furthermore, the original reactions such as enzymatic reactions and electron transfer reactions were observed in hydrated ILs, when the higher ordered structures were remained . 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] . 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.
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