Aluminium ion-promoted radical-scavenging reaction of methylated hydroquinone derivatives

Abstract

The effect of the aluminium ion (Al3+) on the scavenging reaction of a 2,2-diphenyl-1-picrylhydrazyl radical (DPPH center dot), as a reactivity model of reactive oxygen species, with hydroquinone (QH(2)) and its methylated derivatives (Me(n)QH(2), n = 1-4) was investigated using stopped-flow and electrochemical techniques in a hydroalcoholic medium. The second-order rate constants (k) for the DPPH center dot-scavenging reaction of the hydroquinones increased with the increasing number of methyl substituents. Upon addition of Al3+, the k values significantly increased depending on the concentration of Al3+. Such an accelerating effect of Al3+ on the DPPH center dot-scavenging rates of the hydroquinones results from the remarkable positive shift of the one-electron reduction potential (E-red) of DPPH center dot in the presence of Al3+. These results demonstrate that Al3+, a strong Lewis acid, can act as a radical-scavenging promoter by stabilising the one-electron reduced species of the radical, although Al3+ is reported not only to act as a pro-oxidant but also to strongly interact with biomolecules, showing toxicities.