Superoxide Radical Anion Adduct of 5,5-Dimethyl-1-pyrroline N-Oxide. 6. Redox Properties

Cyclic nitrones have been employed for decades as spin trapping reagents for the detection and identification of transient radicals, and have been employed as pharmacological agent against ROS-mediated toxicity. The short half-life of the nitrone-superoxide adducts limits the application of nitrones in biological millieu, and therefore investigaton of the redox properties of the superoxide adducts is important. Moreover, computational investigation of the redox properties of the nitrones and their corresponding spin adducts may provide new insights into the nature of their pharmacological activity against ROS-induced toxicity. In general, electron withdrawing group substitution at the C-5 position results in higher EAs and IPs making these substituted nitrones more susceptible to reduction but more difficult to oxidize compared to DMPO. One-electron reduction and oxidation of nitrones both resulted in elongated N-C-2 bonds indicating the tendency of radical anion and cation forms of nitrone to undergo ring-opening. The EAs and IPs of various O-2(center dot-) adducts indicate that DEPMPO-O2H is the most difficult to reduce and oxidize compared to the O-2(center dot-) adducts of DMPO, EMPO, and AMPO. In general, nitroxides gave higher EAs compared to nitrones making them more suceptible to reduction. One-electron oxidation of nitroxides leads to elongation of the N-C-2 bond but not for their reduction. The energetics of redox reaction of O-2(center dot-) adducts was also explored. Results indicate that the reduction of O-2(center dot-) adducts With O-2(center dot-) is preferred followed by their oxidation by O-2 and then by O-2(center dot-), but the maximum difference between these free energies of redox reactions ill aqueous solution is only 0.21 kcal/mol. The preferred decomposition pathways for the one-electron oxidation and reduction of nitroxides was also explored, and formation of potentially biologically active products such as NO2, H2O2, and hydroxamic acid was predicted.