Implication of manganese (III), oxalate, and oxygen in the degradation of nitroaromatic compounds by manganese peroxidase (MnP)

Abstract.

The fungal ligninolytic enzyme manganese peroxidase (MnP) is known to function by oxidizing Mn(II) to Mn(III), a powerful oxidant. In this work, an abiotic system consisting of Mn(III) in oxalate buffer under aerobic conditions (Mn(III)/oxalate/O₂ system) was shown to be capable of extensively transforming 2-amino-4,6-dinitrotoluene (2A46DNT) – one of the main reduction products of 2,4,6-trinitrotoluene (TNT). No significant transformation occurred in the presence of other organic acids or under anaerobic conditions. The Mn(III)/oxalate/O₂ system was also able to transform other nitroaromatic compounds such as 2-nitrotoluene, 4-nitrotoluene, 2,4-dinitrotoluene, TNT – the latter to a lesser extent –, and their reduction derivatives. The Mn(III)/oxalate/O₂ system mineralized ¹⁴C-U-ring labeled 2A46DNT slightly, while no significant mineralization of ¹⁴C-U-ring labeled TNT was observed. Unidentified ¹⁴C-transformation products were highly polar. Electron spin resonance experiments performed on the Mn(III)/oxalate/O₂ system revealed the generation of formyl free radicals (^·COO^–). The oxygen requirement for the transformation of nitroaromatic compounds suggests the involvement of superoxide free radicals (% MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBam % XvP5wqSXMqHnxAJn0BKvguHDwzZbqegm0B1jxALjhiov2DaeHbuLwB % Lnhiov2DGi1BTfMBaebbnrfifHhDYfgasaacH8YjY-vipgYlH8Gipe % c8Eeeu0xXdbba9frFj0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXd % ar-Jb9hs0dXdbPYxe9vr0-vr0-vqpWqaaeaabiGaaiaacaqabeaada % abauaaaOqaaiabb+eapnaaDaaaleaacqaIYaGmaeaacqGHsislkiab % gwSixdaaaaa!3F2C! ${\rm O}_2^{ - \cdot }$ ), produced through autoxidation of ^·COO^– by molecular oxygen. The implication of such a Mn(III)/oxalate/O₂ system in the MnP-catalyzed degradation of nitroaromatic pollutants by white-rot fungi is further discussed.