Role of Phenolic O-H and Methylene Hydrogen on the free Radical Reactions and Antioxidant Activity of Curcumin

Priyadarsini, Indira K and Maity K , Dilip and Naik, GH and Sudheer Kumar, M and Unnikrishnan, MK and Satav, JG and Mohan, Hari (2003) Role of Phenolic O-H and Methylene Hydrogen on the free Radical Reactions and Antioxidant Activity of Curcumin. Free Radical Biology & Medicine, 35 (5). pp. 475-484.

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To understand the relative importance of phenolic O-H and the CH-H hydrogen on the antioxidant activity and the free radical reactions of Curcumin, (1,7-bis[4-hydroxy-3- ethoxyphenyl]-1,6-heptadiene-3,5-dione), biochemical, physicochemical, and density functional theory (DFT) studies were carried out with curcumin and dimethoxy curcumin (1,7-bis[3, 4-dimethoxy phenyl]-1,6-heptadiene-3,5-dione). The antioxidant activity of these compounds was tested by following radiation-induced lipid peroxidation in rat liver microsomes, and the results suggested that at equal concentration, the efficiency to inhibit lipid peroxidation is changed from 82% with curcumin to 24% with dimethoxy curcumin. Kinetics of reaction of (2,2�-diphenyl-1-picrylhydrazyl) DPPH, a stable hydrogen abstracting free radical was tested with these two compounds using stopped-flow spectrometer and steady state spectrophotometer. The bimolecular rate constant for curcumin was found to be �1800 times greater than that for the dimethoxy derivative. Cyclic voltammetry studies of these two systems indicated two closely lying oxidation peaks at 0.84 and 1.0 V vs. SCE for curcumin, while only one peak at 1.0 V vs. SCE was observed for dimethoxy curcumin. Pulse radiolysis induced one-electron oxidation of curcumin and dimethoxy curcumin was studied at neutral pH using •N3 radicals. This reaction with curcumin produced phenoxyl radicals absorbing at 500 nm, while in the case of dimethoxy curcumin a very weak signal in the UV region was observed. These results suggest that, although the energetics to remove hydrogen from both phenolic OH and the CH2 group of the �-diketo structure are very close, the phenolic OH is essential for both antioxidant activity and free radical kinetics. This is further confirmed by DFT calculations where it is shown that the �OH hydrogen is more labile for abstraction compared to the �CH2 hydrogen in curcumin. Based on various experimental and theoretical results it is definitely concluded that the phenolic OH plays a major role in the activity of curcumin.

Item Type: Article
Uncontrolled Keywords: Curcumin, Dimethoxy curcumin, Antioxidant, Free radicals, Phenols, unsaturated,diketones
Subjects: Pharmacy > MCOPS Manipal > Pharmacy Practice
Depositing User: KMC Manipal
Date Deposited: 13 Jul 2011 09:57
Last Modified: 13 Jul 2011 09:58

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