title: Mechanism of metal-independent decomposition of organic hydroperoxides and formation of alkoxyl radicals by halogenated quinones.
abstract: The metal-independent decomposition of organic hydroperoxides and the formation of organic alkoxyl radicals in the absence or presence of halogenated quinones were studied with electron spin resonance (ESR) and the spin-trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO). We found that 2,5-dichloro-1,4-benzoquinone (DCBQ) markedly enhanced the decomposition of tert-butylhydroperoxide (t-BuOOH), leading to the formation of the DMPO adducts with t-butoxyl radicals (t-BuO* and methyl radicals *CH(3)). The formation of DMPO/t-BuO* and DMPO/*CH(3) was dose-dependent with respect to both DCBQ and t-BuOOH and was not affected by iron- or copper-specific metal chelators. Comparison of the data obtained with DCBQ and t-BuOOH with those obtained in a parallel study with ferrous iron and t-BuOOH strongly suggested that t-BuO* was produced by DCBQ and t-BuOOH through a metal-independent mechanism. Other halogenated quinones were also found to enhance the decomposition of t-BuOOH and other organic hydroperoxides such as cumene hydroperoxide, leading to the formation of the respective organic alkoxyl radicals in a metal-independent manner. Based on these data, we propose a mechanism for DCBQ-mediated t-BuOOH decomposition and formation of t-BuO*: a nucleophilic attack of t-BuOOH on DCBQ, forming a chloro-t-butylperoxyl-1,4-benzoquinone intermediate, which decomposes homolytically to produce t-BuO*. This represents a mechanism of organic alkoxyl radical formation not requiring the involvement of redox-active transition metal ions.
authors.affiliation: State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, People's Republic of China. bzhu@rcees.ac.cn
authors.forename: Ben-Zhan
authors.initials: BZ
authors.lastname: Zhu
authors.forename: Hong-Tao
authors.initials: HT
authors.lastname: Zhao
authors.forename: Balaraman
authors.initials: B
authors.lastname: Kalyanaraman
authors.forename: Jun
authors.initials: J
authors.lastname: Liu
authors.forename: Guo-Qiang
authors.initials: GQ
authors.lastname: Shan
authors.forename: Yu-Guo
authors.initials: YG
authors.lastname: Du
authors.forename: Balz
authors.initials: B
authors.lastname: Frei
mesh_terms: D001555:Benzene Derivatives / Q000737:chemistry; D016227:Benzoquinones / Q000737:chemistry; D002614:Chelating Agents / Q000494:pharmacology; D003497:Cyclic N-Oxides / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D005609:Free Radicals; D007477:Ions; D008670:Metals / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011809:Quinones / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization; D013113:Spin Labels
chemical_list: D001555:Benzene Derivatives; D016227:Benzoquinones; D002614:Chelating Agents; D003497:Cyclic N-Oxides; D005609:Free Radicals; D007477:Ions; D008670:Metals; D011809:Quinones; D013113:Spin Labels; C004532:quinone; C071926:2,6-dichlorobenzoquinone; C017245:5,5-dimethyl-1-pyrroline-1-oxide; C007164:cumene hydroperoxide