The role of electron-transfer and H-atom donation on the superb antioxidant activity and free radical reaction of curcumin.

Antioxidant activity of curcumin has been thoroughly studied to declare the conflicting conclusions about the site of curcumin reactivity and the reaction mechanisms in ROS scavenging. Data confirmed that the antioxidant activity of curcumin's enol isomer (CurE) is not only higher than keto isomer (CurK) but also more than trolox. We found that two phenolic OH play a major role in the antioxidant activity for the both of CurE and CurK tautomers. Interestingly;each one of CurE and CurK isomer is capable to reduce at least two free radicals by two possible labile phenolic O-H sites of reaction. CurK isomer of curcumin reacts by H-atom transfer mechanism but CurE has tendency to react predominantly by electron transfer. Therefore;data proposed that in neutral condition such as inside the cells where;CurK and CurE tautomerism of curcumin equilibrium present;both of H-atom and electron transfer mechanisms engage in reducing free radicals.