Effect of pH on superoxide/hydroperoxyl radical trapping by nitrones: an EPR/kinetic study

The pH dependence of the apparent rate constant k(Tap) for O-2(-center dot)/HO2 center dot trapping by four nitrones was examined. In each case, kinetic curves were obtained after treatment of EPR spectra of the spin adduct formed using both singular value decomposition and pseudo-inverse deconvolution methods. Modelling these curves led to evaluate k(Tap) at various pH values. Analysis of the pH dependence of k(Tap) permitted the determination of the rate constants for the spin trapping of O-2(-center dot) and of HO2 center dot separately. Whatever the nitrone, our results clearly show that the EPR signals of the nitrone/superoxide spin adducts observed in aqueous media were essentially due to the trapping of the protonated species HO2 center dot.