Photo-Enhanced Production of the Spin Adduct 5,5-Dimethyl-1-pyrroline-N-oxide/·OH in Aqueous Menadione Solutions

The production of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO)/·OH in aqueous solutions of menadione and DMPO is enhanced by fluorescent room light. The formation of DMPO/·OH requires oxygen and water, is enhanced by superoxide dismutase, and occurs to a much smaller extent for benzoquinone than for menadione. This process is assigned as photo-initiated redox cycling of the menadione, which causes oxidation of DMPO to DMPO+·and reduction of oxygen to superoxide. DMPO+·reacts with water to produce DMPO/·OH. Although DMPO/·OOH was not observed in the menadione solutions, the possibility that some of the DMPO/·OH was produced by decomposition of DMPO/·OOH cannot be ruled out. There is no evidence for participation of hydroxyl radicals. Because benzosemiquinone is less readily oxidized than the semiquinone of menadione, redox cycling is less favorable for benzoquinone than for menadione and smaller quantities of DMPO/·OH are produced by photoexcitation of benzoquinone than of menadione.