Superoxide Radical Anion Adduct of 5,5-Dimethyl-1-pyrroline N-Oxide (DMPO). 1. The Thermodynamics of Formation and Its Acidity

Abstract

The nitrone 5,5-dimethyl-1-pyrroline N-oxide (DMPO) has been the most widely used spin trap for the detection of transient free radicals in chemical, biological, and biomedical research using electron paramagnetic resonance (EPR) spectroscopy. A density functional theory (DFT) approach was used to predict the thermodynamics of formation of the superoxide anion/hydroperoxyl radical (O2*-/*O2H) adduct of DMPO as well as its pK(a) in aqueous systems. At the B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level, we predicted (in the gas phase and with a polarizable continuum model (PCM) for water) three conformational minima for both the DMPO-O2- and DMPO-O2H adducts. Using DFT and the PCM solvation method, the pK(a) of DMPO-O2H was predicted to be 14.9 +/- 0.5. On the basis of free energy considerations, the formation of DMPO-O2H at neutral pH proceeds via initial addition of O2*- to DMPO to form the DMPO-O2- adduct and then subsequent protonation by water (or other acidic sources) to form DMPO-O2H. Under acidic conditions, the addition of *O2H to DMPO is predicted to be more exoergic than the addition of O2*- and is consistent with available experimental kinetic data.

Antioxidant properties of plant extracts: an EPR and DFT comparative study of the reaction with DPPH, TEMPOL and spin trap DMPO

“…Another change is the decrease of N bond angles, which are close to the planar structure in DMPO and close to the pyramidal NH 3 structure in the DMPO-OH-H. These results are in agreement with previous results 41 and again show the reliability of the method.…”

“…For the adduct formation, the energy released in the reaction is -342.1 kJ mol -1 as calculated from reaction DMPO + OH· → DMPO-OH·. Villamena et al 41 reported this value using B3LYP/6-31+G(d,p)//B3LYP/6-31+G(d) level as -306.0 kJ mol -1 , and this difference may be due to an underestimation of the N-oxyl energy for the B3LYP method, according to the authors.…”

Superoxide Radical Anion Adduct of 5,5-Dimethyl-1-pyrroline N-Oxide. 4. Conformational Effects on the EPR Hyperfine Splitting Constants

“…As shown in Figure 1, two major conformations were initially considered in which the -OH group of the hydroperoxyl moiety is pointing toward and away from the N-O group, and were assigned as conformers I and II, respectively. Potential energy plots of the various DMPO-O 2 H conformations gave a total of 6 minima, i.e., conformations A-F (see Figure 1) consistent to that previously reported 16. Optimized geometry and vibrational frequencies of all stationary points were obtained for the various minima at the B3LYP/6-31G* level of theory 30.…”

Observation of superoxide production during catalysis of Bacillus subtilis oxalate decarboxylase at pH 4

“…The trapping rate of superoxide with BMPO is only 0.24 M −1 s −1 at pH7.4 [72]. However, trapping yields with many cyclic nitrones increase dramatically in the presence of hydroperoxyl radical at low pH [73], possibly also with superoxide when the spin trap can be protonated [48]. Our EPR and mass spectral results confirm the trapping of the hydroperoxyl radical unequivocally, suggesting that the trapped hydroperoxyl is formed by the protein during the reaction and released into solution.…”

“…Experimental detection of O 2 •− at low pH is made difficult by the acid-catalyzed disproportionation of superoxide which occurs at very fast rates [46]. Yet, the trapping efficiency of the hydroperoxyl radical with cyclic nitrones (such as DMPO or BMPO) increases with decreasing pH, which mitigates the problem [47, 48]. …”