@article{oai:repo.qst.go.jp:00082254,
 author = {Nakanishi, Ikuo and Shoji, Yoshimi and Ohkubo, Kei and Ozawa, Toshihiko and Ken-ichiro, Matsumoto and Fukuzumi, Shunichi and Ikuo, Nakanishi and Yoshimi, Shoji and Kei, Ohkubo and Toshihiko, Ozawa and Kenichiro, Matsumoto},
 issue = {Suppl.1},
 journal = {Free Radical Biology and Medicine},
 month = {Mar},
 note = {Background: The radical-scavenging reaction of antioxidants is known to be significantly affected by pH, solvents, and metal ions.  2,2-Diphenyl-1-picrylhydrazyl (DPPH) radical has been used to evaluate the radical-scavenging activity of antioxidants as a reactivity model of reactive oxygen species.  However, the insolubility of the DPPH radical in water has precluded its use in aqueous media.  Recently, we have succeeded in solubilizing the DPPH radical in water using β-cyclodextrin (β-CD).  This enables us to evaluate the scavenging activity of the DPPH radical by antioxidants in aqueous media.  We report herein the effect of reaction media on the scavenging reaction of the DPPH radical by ascorbic acid, a representative water-soluble antioxidant.
Methods: 25 mL of boiling water (Milli-Q) was added to the mixture containing the DPPH radical (0.15 mmol) and β-CD (0.70 mmol), and the suspension was cooled to room temperature.  The filtration of the suspension by a membrane filter (pore size: 0.22 µm) gave a deep violet solution of the DPPH radical.  The rates of the scavenging reaction of the DPPH radical were determined by monitoring the absorbance change due to the DPPH radical using a stopped-flow technique on a UNISOKU RSP-1000-02NM spectrophotometer.
Results: The second-order rate constant (k) of the reaction between ascorbic acid and the β-CD-solubilized DPPH radical was determined to be 5.4 × 103 M–1 s–1 in phosphate buffer (0.05 M, pH 7.0) at 25 °C.  The k value increased with increasing the pH value.  Furthermore, magnesium perchlorate significantly accelerated this reaction.  On the other hand, when water was replaced by deuterium oxide to prepare the phosphate buffer (0.05 M, pD 7.0), the k value significantly decreased to be 1.5 × 103 M–1 s–1.
Conclusion: The scavenging reaction of the DPPH radical by ascorbic acid is significantly affected by reaction media.},
 pages = {55--56},
 title = {Effect of Reaction Media on the Scavenging Reaction of Water-Solubilized 2,2-Diphenyl-1-picrylhydrzyl Radical by Ascorbic Acid},
 volume = {165},
 year = {2021}
}