Dimethyl pyridine N oxide (DMPO) was used as a free radical trapper. EPR signals were detected with a Bruker e‐scan EPR spectrometer (Burker, Karlsruhe, Germany) as previous report (Ohsawa et al., 2007). We produced hydroxyl radical (OH˙) by the Fenton reaction in the mixture of 0.25 mm H2O2 and 0.1 mm FeSO4 in the presence of 0.1 mm DMPO. The spin traps, DMS (0.1, 1, or 10 mm), and MsrA/nonactive MsrA (1 μm) were added before the Fe (II) and H2O2. Samples (20 μL) were loaded into a quartz tube, and the EPR spectra were recorded at room temperature. The EPR microwave power was set to 4.88 mW. The modulation frequency was 9.76 GHz. The time constant was 81.92 ms, and conversion time was 81.92 ms. A sweep time of 41.94 s was used. Each sample was scanned once. A sweep width of 100 G was used for experiments with DMPO. The receiver gain was set to 3.17 × 103. Simulation and fitting of the EPR spectra were performed using the Bruker WinEPR program.