Reactivity of Superoxide Radical Anion and Hydroperoxyl Radical with α-Phenyl-N-tert-butylnitrone (PBN) Derivatives

Nitrones have exhibited pharmacological activity against radical-mediated pathophysiological conditions and as analytical reagents for the identification of transient radical species by electron paramagnetic resonance (EPR) spectroscopy. In this work, competitive spin trapping, stopped-flow kinetics, and density functional theory (DFT) were employed to assess and predict the reactivity Of O-2(center dot-) and HO2 center dot with various para-substituted alpha-phenyl-N-tert-butyinitrone (PBN) spin traps. Rate constants of O-2(center dot-) trapping by nitrones were determined using competitive UV-vis stopped-flow method with phenol red (PR) as probe, while HO2 center dot trapping rate constants were calculated using competition kinetics with 5,5-dimethylpyrroline N-oxide (DMPO) by employing EPR spectroscopy. The effects of the para substitution on the charge density of the nitronyl-carbon and on the free energies of nitrone reactivity with O-2(center dot-) and HO2 center dot were computationally rationalized at the PCM/B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level of theory. Theoretical and experimental data show that the rate Of O-2(center dot-) addition to PBN derivatives is not affected by the polar effect of the substituents. However, the reactivity of HO2 center dot follows the Hammett equation and is increased as the substituent becomes more electron withdrawing. This supports the conclusion that the nature of HO2 center dot addition to PBN derivatives is electrophilic, while the addition Of O-2(center dot-) to PBN-type compounds is only weakly electrophilic.