Superoxide radical anion adduct of 5,5-dimethyl-1-pyrroline W-oxide (DMPO). 1. the thermodynamics of formation and its acidity

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 (O ₂./.O ₂H) 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-O ₂ ^- and DMPO-O ₂H adducts. Using DFT and the PCM solvation method, the pK _a of DMPO-O ₂H was predicted to be 14.9 ± 0.5. On the basis of free energy considerations, the formation of DMPO-O ₂H at neutral pH proceeds via initial addition of O ₂. to DMPO to form the DMPO-O ₂ ^- adduct and then subsequent protonation by water (or other acidic sources) to form DMPO-O ₂H. Under acidic conditions, the addition of .O ₂H to DMPO is predicted to be more exoergic than the addition of O ₂. and is consistent with available experimental kinetic data.