Hydroperoxide-Independent Generation of Spin Trapping Artifacts by Quinones and DMPO: Implications for Radical Identification in Quinone-Related Reactions

Quinones, as highly redox active molecules in biology, are believed to react with hydroperoxides to produce highly reactive ^•OH, assuming that radical adducts are exclusively formed by the addition of free radicals to the spin trap as detected by the electron paramagnetic resonance (EPR) methodology. Here, direct formation of the same DMPO adduct as that formed by genuine radical trapping of ^•OH is discovered, while quinones (i.e., 1,4-benzoquinone (BQ), methyl-BQ (2-Me-BQ, 2,5-Me-BQ, 2,6-Me-BQ), and chlorinated-BQ (2-Cl-BQ, 2,5-Cl-BQ, 2,6-Cl-BQ)) meet with the spin trap 5,5-dimethyl-1-pyrroline <i>N</i>-oxide (DMPO), independent of peroxides. According to differences in alcohol-derived adducts (e.g., DMPO-CH₂OH or DMPO-OCH₃) while alcohol is attacked by ^•OH or DMPO^•+, a nonradical mechanism is proposed for the BQ/DMPO system. This is further evidenced by the mass spectrometry data in which DMPO-OCH₃ has been identified in BQ (or chlorinated-BQ)/DMPO systems. ¹⁷O incorporation experiments verify that hydroxyl oxygen in DMPO-OH originates from water. The DMPO-OH adduct might be formed via direct oxidation and water substitution or one-electron oxidation and nucleophilic addition. This study provides a peroxide-independent alternative route leading to DMPO-OH adduct in quinone-based systems, which has profound implications for assessing adverse health effects and even biogeochemical impacts of quinones if EPR is applied.