Aluminium Ion-Promoted Radical-Scavenging Reaction of Methylated Hydroquinone Derivatives

The effect of aluminium ion (Al3+) on the scavenging reaction of a 2,2-diphenyl-1-picrylhydrazyl radical (DPPH●), as a reactivity model of reactive oxygen species, by hydroquinone (QH2) and its methylated derivatives (MenQH2, 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 increasing the 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 (Ered) of DPPH● in the presence 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