Molecular mechanism for metal-independent production of hydroxyl radicals by hydrogen peroxide and halogenated quinones

We have shown previously that hydroxyl radicals (HO{bullet}) 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{bullet}⁻), is essential for HO{bullet} 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{bullet} 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{bullet}. This represents a mechanism of HO{bullet} production that does not require redox-active transition metal ions.