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

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.