The Effect of Solvent on Tautomerism, Acidity and Radical Stability of Curcumin and Its Derivatives Based on Thermodynamic Quantities

The stability of curcumin and some of its derivatives in terms of diketo/enol tautomerism was studied in a number of solvents by means of standard density functional theory calculations. The ratio of diketo to enol forms has been investigated in the studied solvents. The active sites for deprotonation of curcumin in different liquid solutions were also investigated in order to explore the acidic properties of curcumin. The solvent presents a dominant role on the acidic property of curcumin so that the hydrogen of the enolic hydroxyl group is more acidic in ethanol and water, but the hydrogen of the phenolic hydroxyl group is more acidic in other studied solvents. It has also been revealed that the radical derived from the phenolic hydroxyl group is much more stable than the radical derived from the enolic hydroxyl group. Calculations also show that the abilities of other derivatives to scavenge free radicals are comparable with curcumin in all of the studied liquid solutions.