title: Molecular mechanism for metal-independent production of hydroxyl radicals by hydrogen peroxide and halogenated quinones.
abstract: We have shown previously that hydroxyl radicals (HO*) can be produced by H2O2 and halogenated quinones, independent of transition metal ions; however, the underlying molecular mechanism is still unclear. In the present study, using the electron spin resonance secondary radical spin-trapping method, we found that tetrachloro-1,4-benzoquinone (TCBQ), but not its corresponding semiquinone anion radical, the tetrachlorosemiquinone anion radical (TCSQ*-), is essential for HO* production. The major reaction product between TCBQ and H2O2 was identified by electrospray ionization quadrupole time-of-flight mass spectrometry to be the ionic form of trichlorohydroxy-1,4-benzoquinone (TrCBQ-OH), and H2O2 was found to be the source and origin of the oxygen atom inserted into the reaction product TrCBQ-OH. On the basis of these data, we propose that HO* production by H2O2 and TCBQ is not through a semiquinone-dependent organic Fenton reaction but rather through the following mechanism: a nucleophilic attack of H2O2 to TCBQ, forming a trichlorohydroperoxyl-1,4-benzoquinone (TrCBQ-OOH) intermediate, which decomposes homolytically to produce HO*. This represents a mechanism of HO* production that does not require 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: Balaraman
authors.initials: B
authors.lastname: Kalyanaraman
authors.forename: Gui-Bin
authors.initials: GB
authors.lastname: Jiang
mesh_terms: D006219:Halogens; D006861:Hydrogen Peroxide / Q000737:chemistry*; D017665:Hydroxyl Radical; D008670:Metals; D011809:Quinones / Q000737:chemistry*
chemical_list: D006219:Halogens; D008670:Metals; D011809:Quinones; D017665:Hydroxyl Radical; D006861:Hydrogen Peroxide