A new kinetic approach to the evaluation of rate constants for the spin trapping of superoxide/hydroperoxyl radical by nitrones in aqueous media

A new kinetic approach to the evaluation of rate constants for the spin trapping of superoxide/hydroperoxyl radical by nitrones in buffered media is described. This method is based on a competition between the superoxide trapping by the nitrone and the spontaneous dismutation of this radical in aqueous media. EPR spectra are recorded as a function of time at various nitrone concentrations, and kinetic curves are obtained after treatment of these spectra using both singular value decomposition and pseudo-inverse deconvolution methods. Modelling these curves permits the determination of the rate constants k(T) and k(D) for the superoxide trapping and the adduct decay reactions, respectively. Kinetics parameters thus obtained with six nitrones, namely the 2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole N-oxide ( EMPO) 1, the 5-diethoxyphosphoryl-5-methyl-3,4-dihydro-5H-pyrrole N-oxide ( DEPMPO) 2, the 5,5-dimethyl-3,4-dihydro-5H-pyrrole N-oxide ( DMPO) 3, the 1,3,5-tri[(N-(1-diethylphosphono)-1-methylethyl)-N-oxy-aldimine]benzene (TN) 4, the N-benzylidene-1-ethoxycarbonyl-1-methylethylamine N-oxide (EPPN) 5, and the N-[(1-oxidopyridin-1-ium-4-yl)methylidene]-1-ethoxycarbonyl-1-methylethylamine N-oxide (EPPyON) 6, indicate that cyclic nitrones trapped superoxide faster than the linear ones. However, the low kT values obtained for compounds 1-6 show that there is still a need for new molecules with better spin trapping capacities.