@misc{oai:repo.qst.go.jp:00084047,
 author = {Ikuo, Nakanishi and Yoshimi, Shoji and Kei, Ohkubo and Toshihiko, Ozawa and Matsumoto, Ken-ichiro and Shunichi, Fukuzumi and Ikuo, Nakanishi and Yoshimi, Shoji and Kei, Ohkubo and Toshihiko, Ozawa and Kenichiro, Matsumoto},
 month = {Aug},
 note = {A stable radical, 2,2-diphenyl-1-picrylhydrzyl radical (DPPH•), has been frequently used to estimate the activity of antioxidants as a reactivity model of reactive oxygen species for more than six decades [1]. However, the insolubility of DPPH• in water has precluded its use in aqueous media without cosolvents, such as ethanol. On the other hand, we succeeded in solubilizing DPPH• in water by complexation with β-cyclodextrin (β-CD) [2]. We report herein the redox reactivity of β-CD-solubilized DPPH• (DPPH•/β-CD) in aqueous buffer solutions.
The cyclic voltammograms of DPPH•/β-CD were recorded in phosphate buffer solutions (0.1 M) under various pH conditions (pH 8.3, 7.2, and 6.1).  Quasi-reversible and reversible redox waves were observed for the one-electron oxidation and reduction of DPPH•/β-CD, respectively. The one-electron reduction potentials determined from the reversible redox couples linearly shifted to the positive direction as the pH of the solution decreased, while the quasi-reversible one-electron oxidation waves remained unchanged. Thus, DPPH•/β-CD undergoes one-electron reduction more easily under acidic conditions than under basic conditions. The pH effect on the reactivity of DPPH•/β-CD toward antioxidants was also examined in buffer solutions (0.05 M, pH 6.0–9.1). Ascorbic acid (AscH2), a representative water-soluble antioxidant, efficiently scavenged DPPH•/β-CD in phosphate buffer solution. The second-order rate constants (k) determined for the reaction between AscH2 and DPPH•/β-CD by monitoring the time course change of the absorbance at 527 nm due to DPPH•/β-CD increased with increasing the pH values of the solution [3]. When AscH2 was replaced by Trolox, a water-soluble analog of α-tocopherol (vitamin E), a similar pH dependence on the k values was observed, suggesting that such a pH dependence may be caused by the reactivity of DPPH• in aqueous solutions. The detailed redox mechanism of DPPH• in aqueous media will also be discussed.

References:
[1] M. S. Blois, Nature, 191, 1199–1200 (1958).
[2] I. Nakanishi, K. Ohkubo, K. Imai, M. Kamibayashi, Y. Yoshihashi, K. Matsumoto, K. Fukuhara, K. Terada, S. Itoh, T. Ozawa, S. Fukuzumi, Chem. Commun., 51, 8311–8314 (2015).
[3] I. Nakanishi, Y. Shoji, K. Ohkubo, K. Fukuhara, T. Ozawa, K. Matsumoto, S. Fukuzumi, J. Clin. Biochem. Nutr., 68, 115–122 (2021)., ISMAR-APNMR-NMRSJ-SEST 2021},
 title = {Redox Reactivity of 2,2-Diphenyl-1-picrylhydrazyl (DPPH) Radical Solubilized by beta-Cyclodextrin in Aqueous Buffer Solutions},
 year = {2021}
}