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Molecules 2014, 19 20096 5. Curcumin Reactivity Curcumin has three reactive functional groups: one diketone moiety, and two phenolic groups [6]. Important chemical reactions associated with the biological activity of curcumin are the hydrogen donation reactions leading to oxidation of curcumin, reversible and irreversible nucleophilic addition (Michael reaction) reactions, hydrolysis, degradation and enzymatic reactions. All these have significant role in different biological activities of curcumin. They are discussed briefly. 5.1. Reactions with ROS Curcumin has been found to be an excellent scavenger of most ROS, a property that bestows curcumin with antioxidant activity in normal cells. ROS consists of both free radical oxidants and molecular oxidants [33,35–41]. Free radical oxidants participate in hydrogen abstraction and also in electron transfer reactions. All three active sites of curcumin can undergo oxidation by electron transfer and hydrogen abstraction. Detailed investigations by different groups have confirmed that during free radical reactions, the most easily abstractable hydrogen from curcumin is from the phenol-OH group, resulting in formation of phenoxyl radicals, which are resonance stabilized across the keto-enol structure (Scheme 3). Scheme 3. Possible sites of attack of free radical oxidants with curcumin and stabilization of phenoxyl intermediate and its regeneration by ascorbic acid. H3CO OO OH OCH3 OH OCH3 O OCH3 OH H3CO OH O OH Curcumin OCH3 R OH -H / H+, e- H donor / Ascorbic acid H3CO OH O O H3CO OO O phenoxyl radical For example the reaction of peroxyl radicals with curcumin produces curcumin phenoxyl radicals, which are less reactive than the peroxyl radicals and thereby cause protection from ROS-induced oxidative stress [34,35,38]. The regeneration reaction of phenoxyl radicals back to curcumin by water soluble antioxidants like ascorbic acid, impart the molecule with a chain breaking antioxidant ability like vitamin E [38]. Reported in the literature are scavenging reactions of several other free radical ROS such as hydroxyl radicals, superoxide radicals and alkoxy radicals by curcumin [36–39]. The reaction of curcumin with superoxide radicals has been found to be as efficient as well known lipid soluble antioxidants and the reaction also leads to catalytic degradation of superoxide in which curcumin acts as a superoxide dismutase mimic [37].PDF Image | Curcumin: From Extraction to Therapeutic Agent
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