For quantitative investigations of hydrogen-radical donation, stable radicals have the advantage that their concentrations are readily and directly measurable. Among them a stable free radical 1,1-diphenyl-2-picrylhydrazyl (DPPH) 1) was investigated as a reactive hydrogen acceptor 2) and further found to be useful for the antioxidant determination.3) Since then DPPH has been mainly used to examine radical scavenging activity of antioxidative vitamins and polyhydroxy aromatic compounds 4,5) based on the reactions in Fig. 1. Besides DPPH-scavenging by reductive hydrogen transfer between various donors and DPPH, non-reductive DPPH-decomposing and -scavenging were found to be induced by reacting with tertiary hydroperoxides 6) and by radical-binding at the para position in the phenyl rings of DPPH, 7) respectively.However, nowadays non-reductive DPPH-scavenging has hardly applied for biochemical and physicochemical purposes. It has been reported that peroxyradicals are unique among reactive oxygen species implicated in the production of DNA damage because they possess an extremely long half-life (order of seconds) and are predicted to have a relatively greater chemical selectivity in its reactions as compared with other radical intermediates. A stable radical 1,1-diphenyl-2-picrylhydrazyl (DPPH) has long been used as a convenient method for the antioxidant assay of biological materials such as cysteine, glutathione, ascorbic acid, tocopherol and polyhydroxy aromatic compounds (hydroquinone, pyrogallol, etc). In this study, non-reductive scavenging of DPPH was investigated by electron spin resonance (ESR) analyses for the purpose of developing a useful method for quantitative determination of peroxyradical. Since DPPH was degraded in the presence of peroxyradical derived from UV-irradiated benzoylperoxide and the peroxyradical-induced degradation of DPPH was inhibited by the addition of a spin trapping agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), it is concluded that DPPH is non-reductively scavenged by peroxyradical. Therefore, it is suggested that DPPH could be a useful agent for the quantitative measurement of peroxyradical.Key words 1,1-diphenyl-2-picrylhydrazyl (DPPH); non-reductive scavenging; peroxyradical; electron spin resonance (ESR) Fig. 1. The Reaction Scheme between DPPH and the Conjugated Group of Ascorbic Acid (a) or Hydroquinone (b)

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