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

► Each molecule of curcumin is capable to reduce at least two free radicals. ► Antioxidant activity of curcumin's enol isomer (CurE) is more than keto isomer (CurK). ► Data suggest that two phenolic OH are responsible for the curcumin's activity. ► CurE and CurK reduce radicals via electron and H-atom transfer actions;respectively. ► CurE is responsible for the superb antioxidant activity of curcumin at pH 7.4.