The role of the DMPO-hydrated electron spin adduct in DMPO-OH spin trapping

Time-resolved in situ radiolysis ESR (electron spin resonance, equivalently EPR, electron paramagnetic resonance) studies have shown that the scavenging of radiolytically produced hydroxyl radical in nitrous oxide-saturated aqueous solutions containing 2 mM DMPO is essentially quantitative (94% of the theoretical yield) at 100 μs after the electron pulse [1]. This result appeared to conflict with earlier results using continuous cobalt-60 γ radiolysis and hydrogen peroxide photolysis, where factors of 35 and 33% were obtained, respectively [2], [3]. To investigate this discrepancy, nitrogen-saturated aqueous solutions containing 15 mM DMPO were cobalt-60 γ irradiated (dose rate = 223 Gy/min) for periods of 0.25–6 min, and ESR absorption spectra were observed ∼ 30 s after irradiation. A rapid, pseudo-first-order termination reaction of the protonated DMPO-hydrated electron adduct (DMPO-H) with DMPO-OH was observed for the first time. The rate constant for the reaction of DMPO-H with DMPO-OH is 2.44 × 102 (± 2.2 × 101) M−1 s−1. In low-dose radiolysis experiments, this reaction lowers the observed yield of DMPO-OH to 44% of the radiation-chemical OH radical yield (G = 2.8), in good agreement with the earlier results [2], [3]. In the absence of the DMPO-H radical, the DMPO-OH exhibits second-order radical termination kinetics, 2kT = 22 (± 2) M−1 s−1 at initial DMPO-OH concentrations ≥ 13 μM, with first-order termination kinetics observed at lower concentrations, in agreement with earlier literature reports [4].