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

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 <i>N</i>-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 < 5.5. A comparative study with three typical ^•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^•^-.