Effect of Metal Ions on the Radical-Scavenging Rates of Hydroquinones

Biologically important hydroquinones are known to show an efficient radical-scavenging activity as in the case of vitamin E, since they have phenolic hydroxyl groups. When the radical-scavenging reactions of phenolic antioxidants involve an electron-transfer process, their rates may be significantly accelerated in the presence of metal ions. Thus, we examined the effect of metal ions on the radical-scavenging rates of hydroquinones in this study.Upon addition of p-hydroquinone (QH2) to a deaerated acetnitrile (MeCN) solution of 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical at 298 K, the absorption band at 518 nm due to DPPH radical disappeared immediately. This suggests that QH2 efficiently scavenges DPPH radical. From the decay of the absorbance at 518 nm due to DPPH radical, the second-order rate constant (k) was determined as 8.7 M-1 s-1. When 0.1 M of Mg(ClO4)2 was added to the QH2-DPPH radical system in deaerated MeCN, no effect of Mg2+ on the k values was observed. Thus, there may be no contribution of electron transfer from QH2 to DPPH radical in the scavenging reaction, which may thereby proceed via a one-step hydrogen atom transfer process. Substituent effect on the k values as well as on the one-electron oxidation potentials of p-hydroquinones in the presence and absence of metal ions will also be discussed, providing a valuable insight into the roles of metal ions in biological redox reactions