Stabilization of diketo tautomer of curcumin by premicellar cationic surfactants: A spectroscopic, tensiometric and TD-DFT study

The interaction of aqueous curcumin with cationic surfactants of varying chain lengths and head group, stabilizing its beta-diketo tautomer and exhibiting an UV band around 355 nm, has been studied in buffered aqueous solutions in the pH range of 2.00-7.00 by UV-visible and fluorescence spectroscopy, and surface tensiometry under optimal experimental conditions of submicellar concentration of the surfactants. Time Dependent Density Functional Theory (TD-DFT) has also been used to predict the molecular structure and excitation energies of the interaction product in the ground state and the nature of the interaction. As the surfactant concentration increases, the curcumin-cationic complex forms premicellar aggregates where the curcumin exists in a highly polar microenvironment before shifting its location to the nonpolar core of the surfactant micelles. The strength of the interaction increases with the chain length of the surfactant and also changes on changing the head group of the surfactant. An observed secondary salt effect on the interaction indicates an unusual involvement of a proton in the interaction. Based on the experimental and theoretical evidences the following mechanism has been proposed for the stabilization of the beta-diketo form: on the approach of the cationic surfactant, a protonation of the methylenic central carbon atom of curcumin takes place simultaneously breaking the pi-conjugation and facilitating the detachment of the enolic proton paving the way for an ion dipole binding between the cationic head group and the electron-rich beta-diketo oxygen atoms. Hydrophobic interaction between the tail of the surfactant and the nonionic curcumin molecule assists the electrostatic interaction. (C) 2013 Elsevier B.V. All rights reserved.