Nonphotochemical Base-Catalyzed Hydroxylation of 2,6-Dichloroquinone by H2O2 Occurs by a Radical Mechanism

详细信息    查看全文

• 作者：Stefan Franzen ; Koroush Sasan ; Bradley E. Sturgeon ; Blake J. Lyon ; Benjamin J. Battenburg ; Hanna Gracz ; Rania Dumariah ; Reza Ghiladi
• 刊名：The Journal of Physical Chemistry B
• 出版年：2012
• 出版时间：February 9, 2012
• 年：2012
• 卷：116
• 期：5
• 页码：1666-1676
• 全文大小：597K
• 年卷期：v.116,no.5(February 9, 2012)
• ISSN：1520-5207

文摘

Kinetic and structural studies have shown that peroxidases are capable of the oxidation of 2,4,6-trichlorophenol (2,4,6-TCP) to 2,6-dichloro-1,4-benzoquinone (2,6-DCQ). Further reactions of 2,6-DCQ in the presence of H₂O₂ and OH^{鈥?/sup> yield 2,6-dichloro-3-hydroxy-1,4-benzoquinone (2,6-DCQOH). The reactions of 2,6-DCQ have been monitored spectroscopically [UV鈥搗isible and electron spin resonance (ESR)] and chromatographically. The hydroxylation product, 2,6-DCQOH, has been observed by UV鈥搗isible and characterized structurally by 1H and 13C NMR spectroscopy. The results are consistent with a nonphotochemical base-catalyzed oxidation of 2,6-DCQ at pH > 7. Because H₂O₂ is present in peroxidase reaction mixtures, there is also a potential role for the hydrogen peroxide anion (HOO鈥?/sup>). However, in agreement with previous work, we observe that the nonphotochemical epoxidation by H₂O₂ at pH < 7 is immeasurably slow. Both room-temperature ESR and rapid-freeze-quench ESR methods were used to establish that the dominant nonphotochemical mechanism involves formation of a semiquinone radical (base -catalyzed pathway), rather than epoxidation (direct attack by H₂O₂ at low pH). Analysis of the kinetics using an Arrhenius model permits determination of the activation energy of hydroxylation (E_a = 36 kJ/mol), which is significantly lower than the activation energy of the peroxidase-catalyzed oxidation of 2,4,6-TCP (E_a = 56 kJ/mol). However, the reaction is second order in both 2,6-DCQ and OH鈥?/sup> so that its rate becomes significant above 25 掳C due to the increased rate of formation of 2,6-DCQ that feeds the second-order process. The peroxidase used in this study is the dehaloperoxidase-hemoglobin (DHP A) from Amphitrite ornata, which is used to study the effect of a catalyst on the reactions. The control experiments and precedents in studies of other peroxidases lead to the conclusion that hydroxylation will be observed following any process that leads to the formation of the 2,6-DCQ at pH > 7, regardless of the catalyst used in the 2,4,6-TCP oxidation reaction.}