Enzymatic nitration of phenols

Soybean peroxidase (SBP), an acidic peroxidase from the seed coat, has been shown to be an effective catalyst for the oxidation of a wide variety of organic compounds including phenols, non-phenolic aromatic compounds, aromatic amines, and polycyclic aromatic hydrocarbons. We recently demonstrated that SBP could also oxidize inorganic compounds, e.g. bromide for the halogenation of aromatic substrates [Enzyme Microb. Technol. 26 (2000) 337]. In the present study, we expand the repertoire of SBP-catalyzed oxidation of inorganic species and demonstrate that SBP can catalyze the nitration of a variety of phenols in the presence of H2O2 and sodium nitrite. Using 4′-hydroxy-3′-methylacetophenone as a model phenolic substrate, the influence of various reaction parameters, including the nature of organic co-solvent, pH, and peroxide concentration, on enzyme activity and stability were assessed. Nitration was directed to both ortho and para positions on the phenol, the latter occurring simultaneously with elimination of the ketone-containing substituent on the ring. Several other phenols were effective nitration substrates including 4-hydroxy-1-indanone, 7-hydroxycoumarin, and 2-hydroxy-5-methylbenzaldehyde. Nitration was also observed with horseradish peroxidase, lipoxidase, lactoperoxidase, chloroperoxidase, and the peroxidase from Coprinus cinereus.