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Metal-Independent Decomposition of Hydroperoxides by Halogenated Quinones: Detection and Identification of a Quinone Ketoxy Radical
Ben-Zhan Zhu, Guo-Qiang Shan, Chun-Hua Huang, Balaraman Kalyanaraman, Li Mao, Yu-Guo Du and Jack Halpern
Proceedings of the National Academy of Sciences of the United States of America
Vol. 106, No. 28 (Jul. 14, 2009), pp. 11466-11471
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/40483972
Page Count: 6
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We have shown recently that halogenated quinones could enhance the decomposition of hydroperoxides and formation of alkoxyl/hydroxyl radicals through a metal-independent mechanism. However, neither the proposed quinone enoxy radical intermediate, nor the major reaction products were unambiguously identified. In the present study, one of the major reaction products between 2,5-dichloro-1,4-benzoquinone (DCBQ) and t-butylhydroperoxide (t-BuOOH) was isolated and purified by semipreparative HPLC, and identified as 2-hydroxy-3-t-butoxy-5-chloro-1 f 4-benzoquinone [CBQ(OH)-O-t-Bu], which is the rearranged isomer of the postulated quinone-peroxide reaction intermediate. The formation of CBQ(OH)-O-t-Bu was found to be inhibited by the spin trapping agent 5,5-dimethyl-1-pyrroline /V-oxide (DMPO), and concurrently, a new DMPO adduct with 1-chlorine isotope peak clusters at m/z 268 was observed. Further electron spin resonance (ESR) spintrapping, 1 H-NMR and HPLC/Fourier transform ion cyclotron resonance (FTICR) mass spectrometric studies with oxygen-17-labeled and unlabeled hydrogen peroxide strongly suggest that the radical trapped by DMPO is a carbon-centered quinone ketoxy radical, which is the spin isomer of the proposed oxygen-centered quinone enoxy radical. Analogous results were observed when DCBQ was substituted by other halogenated quinones. This study represents the first detection and identification of an unusual carbon-centered quinone ketoxy radical, which provides direct experimental evidence to further support and expand our previously proposed mechanism for metal-independent decomposition of hydroperoxides by halogenated quinones.
Proceedings of the National Academy of Sciences of the United States of America © 2009 National Academy of Sciences