Radical Mechanism of Laccase-Catalyzed Catechol Ring-Opening

Abstract

Enzyme-catalyzed reactions are a prominent field of research in green chemistry. Laccase is a multicopper oxidase, which we used to study the oxidation of catechol. A mechanism for this ring-opening reaction is also proposed. A o-benzosemiquinone radical was the initial nascent product of catechol oxidation during the catalytic reaction. This radical underwent two reaction pathways: (1) formation of an intramolecular adduct, which gave a carbon-centered furan-derived radical trapped by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO); (2) formation of an intermolecular adduct producing dimeric and trimeric oligomers, as resolved by mass spectrometry. Products of the furan-like intermediate were also characterized by 1 H-NMR. Simultaneously, a hydroxyl radical (•OH) originating from the water solvent was identified by 17 O-isotope tracing. The kinetics of this radical were also evident with substrates including 3-and 4-methyl catechol, but not with resorcinol and hydroquinone isomers, 3-and 4-nitro catechol, and 2,3-dihydroxynaphthalene. The mechanism of selective 620 CHEN Ming et al.: Radical Mechanism of Laccase-Catalyzed Catechol Ring-Opening No.3

Degradation of Azo Dye (Acid Orange 7) in a Microbial Fuel Cell: Comparison Between Anodic Microbial-Mediated Reduction and Cathodic Laccase-Mediated Oxidation

“…The formation of phenyl diazene radical and loss of N 2 was also observed by Chivukula and Renganathan (1995) for degradation of phenolic azo dyes by P. oryzae laccase. The attack of dioxygen on phenolic ring cleavage by laccase is widely seen in oxidation of lignin products and catechol (Crestini and Argyropoulos, 1998;Chen et al, 2017). Laccase oxidation of dye is through highly reactive free radicals that are involved in the above reactions and since they are nonspecific a wide number of products are formed.…”

Intramolecular Annulation of Gossypol by Laccase to Produce Safe Cottonseed Protein

“…Moreover, the removal of the DMPO-• 16 OH radical signal by the 17 O isotope unambiguously precluded other oxygen radicals during the process. Although laccase reduces oxygen to water in the trinuclear copper center, water is another substrate of the oxidation, which is a consequence of the laccase-initiated radical process (Shinde et al, 2009;Chen et al, 2017). The fact that everlasting radicals led to a series of nonenzymatic chain reactions could be helpful for discovering the reason for the broad substrate selectivity of laccase.…”

Fungal Biotransformation of Insoluble Kraft Lignin into a Water Soluble Polymer

“…However, an aromatic ring opening would signify biodegradation, whereas the results obtained in our study using other analysis methods indicated structural modifications involving aromatic rings rather than biodegradation, for example, oxygenation leading to quinone formation, intramolecular oxidative cross-linking, and cyclization. Based on the literature, laccases are most likely involved in reactions shown in Figure A,G,H, and they can also possibly be involved in cyclization. − Most of these reactions form reactive intermediates whose further processing can lead to cross-linking.…”