The role of the DMPO-hydrated electron spin adduct in DMPO-(OH)-O-center dot spin trapping

Time-resolved in situ radiolysis ESR (electron spin resonance, equivalently EPR, electron paramagnetic resonance) studies have shown that the scavengi ng of radiolytically produced hydroxyl radical in nitrous oxide-saturated a queous solutions containing 2 mM DMPO is essentially quantitative (94% of t he theoretical yield) at 100 mus after the electron pulse [1]. This result appeared to conflict with earlier results using continuous cobalt-60 gamma radiolysis and hydrogen peroxide photolysis, where factors of 35 and 33% we re obtained, respectively [2,3]. To investigate this discrepancy, nitrogen- saturated aqueous solutions containing 15 mM DMPO were cobalt-60 gamma irra diated (dose rate = 223 Gy/min) for periods of 0.25-6 min, and ESR absorpti on spectra were observed similar to 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 cons tant for the reaction of DMPO-H with DMPO-OH is 2.44 x 10(2) (+/- 2.2 x 10( 1)) 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 yiel d (G = 2.8), in good agreement with the earlier results [2,3]. In the absen ce of the DMPO-H radical, the DMPO-OH exhibits second-order radical termina tion kinetics, 2k(Gamma) = 22 (+/- 2) M-1 s(-1) at initial DMPO-OH concentr ations greater than or equal to 13 muM, with first-order termination kineti cs observed at lower concentrations, in agreement with earlier literature r eports [4]. (C) 2001 Elsevier Science Inc.