Horseradish peroxidase in ionic liquids: Reactions with water insoluble phenolic substrates

The reactivity of horseradish peroxidase (HRP) with water insoluble phenolic compounds has been studied in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄])/water mixtures. The enzyme retained some catalytic activity up to 90% ionic liquid in water at 25 °C only at pH values higher than 9.0. Activity steadily decreased towards neutral and acidic conditions, as judged by 4-aminoantypirin/phenol activity tests. Inhibition of horseradish peroxidase under neutral acidic condition was due to the binding of fluoride anions released from tetrafluoroborate anion to the heme iron as demonstrated by the sharp UV–visible absorption transition diagnostic of the conversion from a five coordinated to a six coordinated high spin ferric heme iron. Thus, reactions with water insoluble phenols were carried out under alkaline reaction conditions and 75% [BMIM][BF₄]/water mixture. Under these conditions, the distribution of the reaction products was much narrower with respect to that observed in aqueous buffers or water/dimethylformamide or water/dimethylsulfoxide mixtures, and polymeric species other than dimers were not observed. Technical scale preparations of a novel 4-phenylphenol ortho dimer [2,2′-bi-(4-phenylphenol)] with a high yield of the desired product were obtained.