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

The effect of the aluminium ion (Al³⁺) on the scavenging reaction of a 2,2-diphenyl-1-picrylhydrazyl radical (DPPH^•), as a reactivity model of reactive oxygen species, with hydroquinone (QH₂) and its methylated derivatives (Me_nQH₂, n = 1-4) was investigated using stopped-flow and electrochemical techniques in a hydroalcoholic medium. The second-order rate constants (k) for the DPPH^•-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^•-scavenging rates of the hydroquinones results from the remarkable positive shift of the one-electron reduction potential (E_red) of DPPH^• 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.