pH Effect on the Scavenging Rates of Ascorbic Acid against Selected Radicals in Phosphate Buffer Solutions

The pH of the reaction media significantly affects the radical-scavenging reaction of antioxidants. The deprotonation of OH groups in the antioxidant molecules occurs depending on the pH of the solution to produce corresponding anion species, which are stronger electron donors as compared to the parent antioxidants. We report herein the pH effect on the reaction rates of ascorbic acid (AscH2) with two radicals, which are water-solubilized 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical by beta-cyclodextrin (β-CD) [1] and 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) radical used as reactivity models of reactive oxygen species.Upon mixing of AscH2 with the bera-CD-solubilized DPPH radical in phosphate buffer solution (0.05 M, pH 7.0) on a UNISOKU RSP-1000-02NM stopped-flow spectrophotometer at 25 degree-C, the absorption band at 527 nm due to the DPPH radical decreased rapidly. From the time course change of the absorbance at 527 nm, the second-order rate constant (kH) for the reaction of AscH2 with the DPPH radical was determined to be 5.6 x 10^3^ M^-1^ s^-1^ [2]. The kH values increased with increasing the pH of the solution in the pH range 6.0-8.0. The kH value for the reaction between AscH2 and the PTIO radical was determined in a similar manner to be 2.4 x 103 10^3^ M^-1^ s^-1^ under the same experimental conditions. However, no pH effect on the kH value was observed in the case of the PTIO radical. Thus, the pH dependence on the kH value observed for the AscH2-DPPH radical system is not due to the reactivity of AscH2 but to that of the DPPH radical.[1] 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 (2015) 8311-8314.[2] I. Nakanishi, Y. Shoji, K. Ohkubo, T. Ozawa, K. Matsumoto, S. Fukuzumi, J. Clin. Biochem. Nutr. 68 (2021) 116-122.28th Annual Conference of the Society for Redox Biology & Medicine (SfRBM 2021