Reactivity of superoxide radical anion and hydroperoxyl radical with alpha-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)(-) and HO(2)() with various para-substituted alpha-phenyl-N-tert-butylnitrone (PBN) spin traps. Rate constants of O(2)(-) trapping by nitrones were determined using competitive UV-vis stopped-flow method with phenol red (PR) as probe, while HO(2)() 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)(-) and HO(2)() 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)(-) addition to PBN derivatives is not affected by the polar effect of the substituents. However, the reactivity of HO(2)() follows the Hammett equation and is increased as the substituent becomes more electron withdrawing. This supports the conclusion that the nature of HO(2)() addition to PBN derivatives is electrophilic, while the addition of O(2)(-) to PBN-type compounds is only weakly electrophilic.