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Recently, Li has reported a new and simple antioxidant assay in vitro using 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide radicals (PTIO), which is one of the nitronyl nitroxide radicals and has been used for detection of nitric oxide [1]. The purple color of the solution of PTIO used as a reactivity model of reactive oxygen species is bleached by the hydrogen-transfer from antioxidants to PTIO. However, little is known about the kinetics of the reaction between antioxidants and PTIO. We report herein the mechanism of the reaction of ascorbic acid (AscH2), a representative water-soluble antioxidant, with PTIO in a phosphate buffer solution.
Upon mixing of AscH2 with PTIO in phosphate buffer solution (0.05 M, pH 7.0), the absorption band at 560 nm due to PTIO decreased rapidly. From the time course change of the absorbance at 560 nm, the second-order rate constant (k(H)) of the reaction of AscH2 with PTIO was determined to be 2.4 x 10^3 M-1 s-1. When D2O was used to prepare the phosphate buffer solution (0.05 M, pD 7.0) instead of H2O, the second-order rate constant (k(D)) was significantly decreased to be 1.9 x 10^2 M-1 s-1. Thus, the kinetic isotope effect (KIE, k(H)/k(D)) was calculated to be 12.8 [2]. If an electron-transfer process is involved in the hydrogen transfer from AscH2 to PTIO, the reaction rate would be significantly accelerated by redox-inactive metal ions. However, such an acceleration effect was not observed by addition of 0.05 M Mg(ClO4)2 on the AscH2–PTIO system. These results demonstrate that AscH2 scavenges PTIO via a one-step hydrogen-atom transfer rather than an electron transfer in phosphate buffer solution.
[1] X. Li, J. Agric. Food Chem. 65 (2017) 6288-6297.
[2] I. Nakanishi, Y. Shoji, K. Ohkubo, T. Ozawa, K. Matsumoto, S. Fukuzumi, Chem. Commun. 56 (2020) 11505-11507.
Mechanism of the Scavenging Reaction of a Nitronyl Nitroxide Radical by Ascorbic Acid in Phosphate Buffer Solutions
