Role of the reacting free radicals on the antioxidant mechanism of curcumin

Density functional theory is used to study the antioxidant mechanism of curcumin. Five different mechanisms are considered: single electron transfer (SET), radical adduct formation (RAF), H atom transfer from neutral curcumin (HAT), H atom transfer from deprotonated curcumin (HAT-D), and sequential proton loss electron transfer (SPLET). The influence of the environment is investigated for polar and non-polar surroundings. The apparent contradictions among previous experimental results are explained by the role of the nature of the reacting free radical on the relative importance of the above mentioned mechanism. It is proposed that the curcumin + DPPH reaction actually takes place mainly through the SPLET mechanism, while the reaction with OCH3, and likely with other alkoxyl radicals, is governed by the HAT mechanism. Branching ratios for the OCH3 + curcumin reaction are reported for the first time. The calculated overall rate constants for this reaction are 1.16 × 1010 (benzene) and 5.52 × 109 (water) L mol−1 s−1. The role of phenolic groups on the antioxidant activity of curcumin has been experimentally confirmed.