Electron spin resonance (ESR) spin trapping using 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) is commonly applied for quantitative analysis of hydroxyl radical. For better understanding of the analysis, we investigated kinetics related to formation and decay of hydroxyl radical spin adduct of DMPO compared with that of 3, 3, 5, 5-tetramethyl-1-pyrroline N- oxide (M4PO) and 5-(diphenylphosphinoyl)-5-methyl-1-pyrroline N-oxide (DPPMPO). in our study where hydroxyl radical was generated by sonolysisof water, we found that (1) DMPO-OH formation was saturated even though hydroxyl radical was generated continuously and (2) the concentration of DMPO-OH decreased in inverse proportion to time after cessation of ultrasound irradiation, suggesting that the decay is a second-order reaction. Similar results were obtained in an experiment of DMPO-OH generated by photolysisof H2O2. Other than DMPO, M4PO, and DPPMPO also trapped hydroxyl radical but the spin trap efficiency was less than that of DMPO. Furthermore, M4PO-OH and DPPMPO-OH decayed more quickly than DMPO-OH. From this, we conclude that DMPO is more suitable for quantification of hydroxyl radical than M4PO and DPPMPO but results from the quantitative analysis must be interpreted with consideration of the kinetics related to formation and decay of DMPO-OH, especially in quantification oflarge amount of hydroxyl radical generation.
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