EPR spectra of DMPO spin adducts of superoxide and hydroxyl radicals in pyridine.

Electron spin resonance spectroscopy and the spin trapping technique were used to study the formation of the superoxide radical in pyridine. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) was employed as a trapping agent. Superoxide radical was generated using chemical (potassium superoxide) and photochemical methods with anthralin, benzanthrone, rose bengal, 1,8-dihydroxyanthraquinone and zinc tetraphenylporphyrine as photoactive pigments. Hyperfine coupling (hf) constants for DMPO/O2.- were determined to be aN = 12.36 G, a beta H = 9.85, G, a gamma H = 1.34 G. The aN and a beta H hf constants are in good agreement with values calculated from a previously determined relationship between hf constants and solvent acceptor number (Reszka et al., (1992) Free Radical Res. Commun., in press). When concentrated hydrogen peroxide was added to DMPO in pyridine a similar EPR spectrum was observed. It is suggested that in this case the DMPO/.O2H adduct is formed by nucleophilic addition of H2O2 to DMPO to give a hydroxylamine, followed by oxidation to the respective nitroxide. The EPR spectrum observed when tetrapropylammonium hydroxide and H2O2 were added to DMPO in pyridine had hf couplings aN = 13.53 G, a beta H = 11.38 G, a gamma H = 0.79 G and it was assigned to a DMPO/.OH adduct. This assignment was based on similarity of this spectrum to the one produced by UV photolysis of hydrogen peroxide and DMPO in aqueous solution and subsequent transfer to pyridine.