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

The effects of reaction environments on the radical-scavenging mechanisms of ascorbic acid (AscH) 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 AscH 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 (iAscH) scavenged DPPH much slower in acetonitrile (MeCN) than in MeOH. In MeOH, Mg(ClO) significantly decelerated the DPPH-scavenging reaction by AscH and iAscH, while no effect of Mg(ClO) was observed in MeCN. On the other hand, Mg(ClO) significantly accelerated the reaction between AscH 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(ClO) to the AscH-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 iAscH in MeCN may proceed via a one-step hydrogen-atom transfer, while an electron-transfer pathway is involved in the reaction between AscH and DPPH/β-CD in phosphate buffer solution. These results demonstrate that the DPPH-scavenging mechanism of AscH are affected by the reaction environments.