Effects of reaction environments on radical-scavenging mechanisms of ascorbic acid

Ikuo Nakanishi; Yoshimi Shoji; Kei Ohkubo; Kiyoshi Fukuhara; Toshihiko Ozawa; Ken-Ichiro Matsumoto; Shunichi Fukuzumi

doi:10.3164/jcbn.20-147

Effects of reaction environments on radical-scavenging mechanisms of ascorbic acid

J Clin Biochem Nutr. 2021 Mar;68(2):116-122. doi: 10.3164/jcbn.20-147. Epub 2021 Jan 16.

Authors

Ikuo Nakanishi¹, Yoshimi Shoji¹, Kei Ohkubo^{1

2}, Kiyoshi Fukuhara³, Toshihiko Ozawa⁴, Ken-Ichiro Matsumoto¹, Shunichi Fukuzumi^{5

6}

Affiliations

¹ Quantitative RedOx Sensing Group, Department of Basic Medical Sciences for Radiation Damages, National Institute of Radiological Sciences (NIRS), Quantum Medical Science Directorate, National Institutes for Quantum and Radiological Science and Technology (QST), Inage-ku, Chiba 263-8555, Japan.
² Institute for Advanced Co-Creation Studies, Open and Transdisciplinary Research Initiatives, Osaka University, Suita, Osaka 565-0871, Japan.
³ School of Pharmacy, Showa University, Shinagawa-ku, Tokyo 142-8555, Japan.
⁴ Nihon Pharmaceutical University, Kitaadachi-gun, Saitama 362-0806, Japan.
⁵ Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea.
⁶ Faculty of Science and Engineering, Meijo University, Nagoya, Aichi 468-8502, Japan.

Abstract

The effects of reaction environments on the radical-scavenging mechanisms of ascorbic acid (AscH₂) were investigated using 2,2-diphenyl-1-picrylhydrazyl radical (DPPH^•) as a reactivity model of reactive oxygen species. Water-insoluble DPPH^• was solubilized by β-cyclodextrin (β-CD) in water. The DPPH^•-scavenging rate of AscH₂ in methanol (MeOH) was much slower than that in phosphate buffer (0.05 M, pH 7.0). An organic soluble 5,6-isopropylidene-l-ascorbic acid (iAscH₂) scavenged DPPH^• much slower in acetonitrile (MeCN) than in MeOH. In MeOH, Mg(ClO₄)₂ significantly decelerated the DPPH^•-scavenging reaction by AscH₂ and iAscH₂, while no effect of Mg(ClO₄)₂ was observed in MeCN. On the other hand, Mg(ClO₄)₂ significantly accelerated the reaction between AscH₂ and β-CD-solubilized DPPH^• (DPPH^•/β-CD) in phosphate buffer (0.05 M, pH 6.5), although the addition of 0.05 M Mg(ClO₄)₂ to the AscH₂-DPPH^•/β-CD system in phosphate buffer (0.05 M, pH 7.0) resulted in the change in pH of the phosphate buffer to be 6.5. Thus, the DPPH^•-scavenging reaction by iAscH₂ in MeCN may proceed via a one-step hydrogen-atom transfer, while an electron-transfer pathway is involved in the reaction between AscH₂ and DPPH^•/β-CD in phosphate buffer solution. These results demonstrate that the DPPH^•-scavenging mechanism of AscH₂ are affected by the reaction environments.

Keywords: antioxidant; ascorbic acid; hydrogen transfer; radical; reaction mechanism.