Superoxide Radical Anion Adduct of 5,5-Dimethyl-1-pyrroline N-Oxide (DMPO). 3. Effect of Mildly Acidic pH on the Thermodynamics and Kinetics of Adduct Formation

The nitrone, 5,5-dimethylpyrroline N-oxide (DMPO), is a commonly used spin trap for the detection of superoxide radical anion (O2•-) using electron paramagnetic resonance spectroscopy. This work investigates the reactivity of DMPO to O2•- in mildly acidic pH (5.0−7.0). Mild acidity is characteristic of acidosis and has been observed in hypoxic systems, e.g., ischemic organs and cancer cells. Although the established pKa for O2•- is 4.8, the pKa for DMPO is unknown. The pKa of the conjugate acid of DMPO was determined to be 6.0 using potentiometric, spectrophotometric, 1H and 13C NMR, and computational methods. 1H and 13C NMR were employed to investigate the site of protonation. An alternative mechanism for the spin trapping of O2•- in mildly acidic pH was proposed, which involves protonation of the oxygen to form the N-hydroxy imino cation and subsequent addition of O2•-. The exoergicity of O2•- addition to protonated DMPO was rationalized using density functional theory (DFT) at the PCM/B3LYP/6-31+G**//B3LYP/6-31G* level of theory.