We have shown previously that hydroxyl radicals (HO ^• ) can be produced by H ₂ O ₂ 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 H ₂ O ₂ was identified by electrospray ionization quadrupole time-of-flight mass spectrometry to be the ionic form of trichlorohydroxy-1,4-benzoquinone (TrCBQ-OH), and H ₂ O ₂ 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 H ₂ O ₂ and TCBQ is not through a semiquinone-dependent organic Fenton reaction but rather through the following mechanism: a nucleophilic attack of H ₂ O ₂ 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.

A peer-reviewed open access journal published by the National Institute of Environmental Health Sciences

Most chemiluminescence (CL) reactions usually generate only one-step CL, which is rarely dependent on the highly reactive and biologically/environmentally important hydroxyl radicals ( ^• OH). Here, we show that an unprecedented two-step CL can be produced by the carcinogenic tetrachloro-1,4-benzoquinone (also known as p -chloranil) and H ₂ O ₂ , which was found to be well-correlated to and directly dependent on its two-step metal-independent production of ^• OH. We proposed that ^• OH-dependent formation of quinone-dioxetane and electronically excited carbonyl species might be responsible for this unusual two-step CL production by tetrachloro-1,4-benzoquinone/H ₂ O ₂ . This is a unique report of a previously undefined two-step CL-producing system that is dependent on intrinsically formed ^• OH. These findings may have potential applications in detecting and quantifying ^• OH and the ubiquitous polyhalogenated aromatic carcinogens, which may have broad biological and environmental implications for future research on these types of important species.