Theoretical study on physicochemical properties of curcumin.

Curcumin is a yellow-orange pigment, which has attracted considerable attention due to its wide spectrum of biological and pharmacological activities. In spite of much effort devoted on curcumin, there still exist some open questions concerning its fundamental physicochemical properties. The present study suggests that the DFT and TD-DFT calculations are useful to answer these questions. Firstly, the thermodynamic as well as spectral parameters support that curcumin exists predominantly in enol form in solution. Secondly, the calculated absorption spectra of curcumin anions provides direct evidence that the lowest pK(a) of curcumin corresponds to the dissociation of enolic proton, which not only reconciles the controversy on this topic, but also has important implications on the proton-transfer/dissociation-associated radical-scavenging mechanisms of curcumin.