Studies on curcumin and curcuminoids. XXXIV. Photophysical properties of a symmetrical, non-substituted curcumin analogue

The Molecular Basis for the Pharmacokinetics and Pharmacodynamics of Curcumin and Its Metabolites in Relation to Cancer

“…The tendency of the keto↔enol equilibrium to shift toward the enol tautomer in these solvent systems has been confirmed experimentally by Fourier transform infrared spectroscopy (FT-IR) (Kolev et al, 2005), nuclear magnetic resonance spectroscopy (NMR) (Roughley and Whiting, 1973;Unterhalt, 1980;Gorman et al, 1994;Khopde et al, 2000), fluorescence spectroscopy (Khopde et al, 2000;Nardo et al, 2008), and absorption spectroscopy (Khopde et al, 2000;Nardo et al, 2008) as well as computationally (Kolev et al, 2005;Balasubramanian, 2006;Payton et al, 2007;Galano et al, 2009). In nonpolar solvents, the steadystate absorption spectra of curcumin in cyclohexane (Nardo et al, 2008), toluene , benzene (Khopde et al, 2000), and others (Tønnesen et al, 1995) exhibit a concrete red band or shoulder, which has been ascribed to the diketo tautomer (Khopde et al, 2000;Nardo et al, 2008Nardo et al, , 2009. Curcumin hence adopts both the enol and diketo tautomeric form in nonpolar environments (Balasubramanian, 2006), albeit the diketo tautomer is quantitatively exiguous relative to the enol tautomer (Kolev et al, 2005), and exclusively the enol tautomeric form in solvents with increasing « r (Fig.…”

“…The enolic proton and carbonyl oxygen are therefore unavailable for H-bonding to other molecules. The intramolecular H-bonding is perturbed in weakly as well as strongly H-bonding solvents by the polar solvent molecules, as a result of which the enol tautomer adopts an open conformation (Emsley, 1984;Toullec, 1990;Khopde et al, 2000;Nardo et al, 2008), thereby abrogating the symmetry of the semiaromatic ring (Nardo et al, 2008(Nardo et al, , 2009 (Fig. 1B).…”

“…Curcumin's electrons are delocalized over the entire molecule (Balasubramanian, 2006;Nardo et al, 2009), accounting for the lower transition energies of the p-electrons and therefore a considerably red-shifted absorption maximum (Kunwar et al, 2006;Ingolfsson et al, 2007;Hung et al, 2008;Karewicz et al, 2011) compared with feruloyl methane (Tønnesen et al, 1995), the exact half of the curcumin molecule. If indeed curcumin inserts into the membrane in a planar configuration parallel to the bilayer normal, then curcumin fluorescence should have been quenched by 16-bromohexadecanoic acid as well, i.e., in the most hydrophobic region of the membrane, in addition to the abovementioned quenching by 11-bromoundecanoic acid (Karewicz et al, 2011).…”

“…Moreover, such an orientation would place the highly polar central region of curcumin, where even the b-carbons and the C a bear a slightly positive and negative charge, respectively (Balasubramanian, 2006), in the most hydrophobic part of the membrane (Bemporad et al, 2005), leading to thermodynamic instability. The adoption of the closed cis-enol conformation or diketo tautomerization, as has been described for curcumin in solvents (Khopde et al, 2000;Kolev et al, 2005;Balasubramanian, 2006;Nardo et al, 2008Nardo et al, , 2009) that are chemically analogous to the bilayer core (Kragh-Hansen, 1981), or hydration of the b-diketone moiety by comigration of water molecules (Bemporad et al, 2005) might ameliorate the thermodynamic instability, but would not resolve it.…”

Self-assembled silk fibroin nanoparticles loaded with binary drugs in the treatment of breast carcinoma

“…Curcumin is a natural compound with diphenolic groups derived from turmeric Curcuma longa, which is widely used in chemotherapy/radiosensitizing of cancer [17][18][19] and other biomedical fields 20,21 with almost no toxicity, 22,23 which has potential as a promising candidate in clinical trials for cancer treatment. However, owing to its poor …”