First Direct and Unequivocal Electron Spin Resonance Spin-Trapping Evidence for pH-Dependent Production of Hydroxyl Radicals from Sulfate Radicals

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Abstract

Recently, the sulfate radical (SO₄•-) has been found to exhibit broad application prospects in various research fields such as chemical, biomedical, and environmental sciences. It has been suggested that SO₄•- could be transformed into a more reactive hydroxyl radical (•OH); however, no direct and unequivocal experimental evidence has been reported yet. In this study, using an electron spin resonance (ESR) secondary radical spin-trapping method coupled with the classic spin-trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and the typical •OH-scavenging agent dimethyl sulfoxide (DMSO), we found that •OH can be produced from three SO₄•--generating systems from weakly acidic (pH = 5.5) to alkaline conditions (optimal at pH = 13.0), while SO₄•- is the predominant radical species at pH •OH-generating systems strongly supports the above conclusion. This is the first direct and unequivocal ESR spin-trapping evidence for •OH formation from SO₄•- over a wide pH range, which is of great significance to understand and study the mechanism of many SO₄•--related reactions and processes. This study also provides an effective and direct method for unequivocally distinguishing •OH from SO₄•-.

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