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Molecules 2014, 19 20099 The methylenic hydrogen of the diketo/enol moiety of curcumin can also act as a nucleophile and participate in Michael addition reactions with stronger electrophiles [58], but such reactions may not have significance in biological systems. Chemically modified curcumin derivatives have been prepared by condensation/addition reactions like e.g., semicarbazone derivatives and oxime derivatives of curcumin [59,60]. These stable products prepared independently have been examined for anti-cancer activity. In most of these studies, it has been reported that these derivatives are more cytotoxic to cancer cells than free curcumin. This prompts us to speculate that probably the glutathione conjugate of curcumin may also act as a cytotoxic agent and contribute to the overall anti-tumor activity of curcumin. 6. Chemistry of Curcumin-Metal Ion Interactions Curcumin forms strong complexes with most of the known metal ions. The α,β-unsaturated β-diketo moiety of curcumin is an excellent chelating agent. In the last one and half decades, many papers have been published on metal-curcumin complexes [61–95]. Curcumin is a monobasic bidentate ligand and forms stable complexes with almost all the metals and non-metals. In general, stable structures with 2:1 (ligand:metal) stoichiometry are observed. There are very few reports on 3:1 ligand:metal complexes, e.g., an octahedral complex reported with Fe3+ [64]. Metal coordination of curcumin occurs through the enolic group, where the enolic proton is replaced by the metal ion and the o-methoxy phenolic moiety remains intact in the complexes. The metal-oxygen bond is characterized by IR spectroscopy signals at 455 cm−1 and the carbonyl peaks in the complexes show a small shift of ~10 cm−1 on coordination to metals. Changes in NMR chemical shifts of curcumin have also been reported on metal coordination. The shifts however depend on the affinity and thermodynamic stability of the resulting complexes. In the case of strong complexes, the resonances of protons attached to the double bonds of the alkyl chain show significant downfield shifts, while the enolic protons show negligible shifts in the 1H-NMR spectra [34], and the 13C-NMR spectrum shows down- and up-field shfts of carbons near the coordination site. There are several papers published in the literature on complexes of curcumin with transition metals like Fe3+, Mn2+, Ni2+, Cu2+, Zn2+, Pb2+, Cd2+, Ru3+, Re3+ and many others. Complexes with non-transition metal ions and rare earth ions like Al3+, Ga3+, Sm3+, Eu3+, Dy3+, Y3+, Se2+ and metal oxides like VO2+ have also been synthesized. The structure and physical properties of these complexes depend on the nature of the metal ion, as well as the stoichiometry of the reaction conditions, which in turn decides their stability and reactivity. Stable 2:1 complexes of some transition metals can be prepared by mixing stoichiometric amounts of curcumin and metal salts in suitable organic solvents and refluxing for few hours, the complex can be separated as a precipitate, and purified either by column chromatography and repeated crystallization. The general chemical structure of 2:1 complexes of curcumin with metals is given in Figure 1a.PDF Image | Curcumin: From Extraction to Therapeutic Agent
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