The presence of the phenol gossypol has severely limited the utilization of cottonseed meal and oil in the food and animal feed industries. Highly efficient means of biodegradation of gossypol and an understanding of the cytotoxicity of its degradation products remain outside current knowledge and are of universal interest. In this work, we showed for the first time that laccase can catalyze the intramolecular annulation of the aldehyde and hydroxyl groups of gossypol for the o-semiquinone radical and originate the released ·OH radical. It was further found that the oxidation of aldehyde groups significantly decreases reproductive toxicity and hepatotoxicity. These results indicate a novel detoxification pathway for gossypol and reveal the crucial role played by radical species in cyclization. This discovery could facilitate the development of safe, convenient, and low-cost industrial methods for the detoxification of cotton protein and oil resources.

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