Photosensitization by harmine: An ESR spin trapping study on the generation of the superoxide anion radical

Harmine was in situ UV-irradiated in the presence of 5.5-dimethyl-1-pyrroline-N-oxide (DMPO), alpha-phenyl-N-tert-butyl nitrone (PBN), or alpha-(4-pyridyl-1-oxide) N-tert-butyl nitrone (4-POBN) as spin traps and observed by electron spin resonance (ESR) spectroscopy. The superoxide radical (O-2(.-)) was detected as the corresponding DMPO, PBN, or 4-POBN spin adduct in dimethylsulfoxide (DMSO) or acetonitrile solution, The detection of these spin adducts was prevented by the addition of superoxide dismutase (SOD) to the solution, Also, the O-2. adduct formation was inhibited by the addition of antioxidants as cysteine methyl ester or ascorbic acid, in a dose-dependent manner. The studies carried out in aqueous buffered solution did not allow superoxide radical adduct detection. In the presence of DMPO as a spin trap, the DMPO-OH spin adduct was detected, on the other hand, classical scavengers of the hydroxyl radical such as mannitol or glycerol abolished DMPO-OH spill adduct detection, Other scavengers such as ethanol. DMSO, or sodium azide inhibited the trapping of hydroxyl radicals and resulted in the formation of new radical adducts. No PBN or 4-POBN spin adducts were detected in aqueous solution, The origin of the hydroxyl radical is discussed.