Chloride peroxidase catalyses both the ring halogenation and N-oxidation reactions of 4-chloroaniline by H2O2 and either KCl or KBr. In the absence of any halide salt only the N-oxidation reaction was observed, with the resulting conversion of 4-chloroaniline into 4-chloronitrosobenzene. The N-oxidation reaction proceeded even more rapidly in the presence of Cl- or Br-, in spite of the fact that ring halogenation was also a rapid reaction. The enhancement of N-oxidation was highly dependent on the pH of the media and displayed an optimum in the region of pH 3.5-4.0. No rate enhancement was observed above pH 5.5. KF partially inhibited the rate of N-oxidation in a pH-dependent manner. On the basis of calculated catalytic-centre activity the N-oxidation reaction was the major reaction at pH 3.5 or higher, whereas the ring-halogenation reaction became the major reaction below pH 3.5. In the presence of high concentrations of 4-chloroaniline relative to H2O2 the reaction intermediate, 4-chlorophenylhydroxylamine, was detected for the first time in a chloride peroxidase-catalysed reaction with this arylamine substrate. These findings were interpreted on the basis of current knowledge concerning the mechanism of action of chloride peroxidase.

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