Comparison of peroxidase activities of hemin, cytochrome c and microperoxidase-11 in molecular solvents and imidazolium-based ionic liquids

The ability of Fe(III)protoporphyrin(IX) chloride (hemin), microperoxidase-11 (MP-11), and cytochrome c (cyt-c) to oxidize 2-methoxyphenol (guaiacol) was examined in the room-temperature ionic liquids (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) and the hexafluorophosphates of 1-butyl- and 1-octyl-3-methylimidazolium, ([bmim][PF6] and [omim][PF6]), respectively. All three biocatalysts displayed peroxidase activity when activated by an electron acceptor, tert-butyl hydroperoxide for hemin and hydrogen peroxide for MP-11 and cyt-c. Hemin required the addition of a coordinating base, pyridine or N-methylimidazole (NMI), to produce an active complex. Cyt-c did not require exogenous ligands for activity in IL, although their addition increased peroxidase activity. MP-11 could not be solubilized without an exogenous ligand, therefore, whether MP-11 was active in the absence of such ligands was not determined. Pyridine provided higher activities than NMI for the three catalysts. Hemin and MP-11 peroxidase activities were markedly higher in IL compared to molecular solvents of similar polarity, as characterized by probe solvatochromic behavior, while cyt-c activity was comparable between both types of solvents. There was no consistent preference by the catalysts for a particular IL. These observations indicate that IL are suitable media for bioelectrocatalysis.