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 by1H-NMR. Simultaneously, a hydroxyl radical (∙OH) originating from the water solvent was identified by17O-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 activation and ring-opening at the C4-C5 site is discussed. This reaction is distinct from intra- and extra-diol ring-cleavages catalyzed by catechol dioxygenase. These results are meaningful for mimicking laccase catalysis to further protein design.