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

Abstract

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