Generation of SO₃^-and OH radicals in SO₃^2- reactions with inorganic environmental pollutants and its implications to SO₃^2-toxicity

Electron spin resonance (ESR) spin trapping and high performance liquid chromatography (HPLC) with electron chemical detection were utilized to investigate the generation of free radicals in reactions of sulfite (SO₃^2-) with inorganic environmental pollutants. The spin trap used was 5,5-dimethyl-1-pyrroline N-oxide (DMPO). Incubation of SO₃^2- with nitrite (NO₂^-) generated sulfur trioxide anion radical (SO₃^-), whose yield approached saturation levels in approximately four minutes. Fe²⁺ promoted SO₃^- formation. Molecular oxygen was required for radical generation. This was demonstrated by experiments carried out in an argon environment as well as by oxygen consumption measurements. Transition metal ions, CrO₄^2-, VO₂⁺, Fe³⁺, Mn²⁺, Ni²⁺, and Fe²⁺ enhanced SO₃^- generation from SO₃^2- either through direct SO₃^2- oxidation by metal ions or by metal ions-catalyzed SO₃^2- oxidation by molecular oxygen. Incubation of SO₃^2- with H₂O₂ generated both SO₃^-and OH radicals as verified by spin trapping competition measurements using ethanol and formate as OH radical scavengers. HPLC measurements showed that OH radicals generated by reaction of SO₃^2- with H₂O₂ caused 2′-deoxyguanine hydroxylation to generate 8-hydroxy-2′-deoxyguanine, a DNA damage marker. The implications of SO₃^- and OH radical formation in relation to SO₃^2- toxicity are discussed.