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Molecules 2014, 19 20095 ground state is 10.77 D. It is a hydrophobic molecule with a logP value of ~3.0. It is almost insoluble in water and readily soluble in polar solvents like DMSO, methanol, ethanol, acetonitrile, chloroform, ethyl acetate, etc. It is sparingly soluble in hydrocarbon solvents like cyclohexane and hexane. The absorption spectrum of curcumin has two strong absorption bands, one in the visible region with maximum ranging from 410 to 430 nm and another band in the UV region with maximum at 265 nm region. The molar extinction coefficient of curcumin in methanol is 55,000 dm3·mol−1·cm−1 at 425 nm. Curcumin is a weak Brönsted acid, with three labile protons, and accordingly three pKas have been estimated corresponding to three prototropic equilibria (Scheme 2). Both NMR and absorption spectrometry have been used to estimate the pKa. The first pKa in the pH range of 7.5 to 8.5 changes curcumin from yellow to red. The chemical reactivity and solubility of the anionic curcumin, i.e., in the basic pH range increases and this form of curcumin is more water soluble than the neutral form. The absorption maximum of fully deprotonated (red in colour) curcumin in alkaline pH (>pH 10) is at 467 nm and the molar extinction coefficient is 53,000 dm3·mol−1·cm−1. There is still a debate which one of the three, i.e., the enolic OH or the phenolic OH is the most acidic. Although calculations indicate that the enolic OH is the most acidic group, the pH dependent spectral changes are difficult to distinguish between the two protons. From 1H-NMR studies Borsari et al. [34] proposed the pKa of 12.5 for the deprotonation of the enolic proton and another pKa at 13.6 for the phenolic protons. These values for enolic protons however differ significantly from those reported by other methods. With the availability of other spectroscopic techniques, it should be possible to resolve these differences in the estimation of pKas in the future. Scheme 2. Keto-enol tautomerism, Prototropic equilibria and degradation products of curcumin. Site 1 H3CO OH Site 2 O OH OCH3 enol form HO H3CO OH HCO 3 OH O O- HO pKa2 =8.5 to 10.4 O O- OCH3 O- pKa3 =9.5 to 10.7 O O- OCH3 O- H3CO OO OH keto form OCH3 HO Ferulic acid Vanillin Ferulic aldehyde Feruloyl methane & Other minor products pKa1 =7.7 to 8.5 OCH3 H3CO -O Aqueous curcumin solutions can be prepared by adding surfactants, lipids, albumins, cyclodextrins, biopolymers etc. Micellar solutions using surfactants are the best suited for preparing high concentration curcumin solutions in water [33]. However while using aqueous surfactant solutions in biological systems, care must be taken by performing proper control experiments, as surfactants can interfere in biological studies.PDF Image | Curcumin: From Extraction to Therapeutic Agent
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