The Role of Charge in the Surfactant-Assisted Stabilization of the Natural Product Curcumin

Abstract

Colloidal solutions of surfactants that form micelles or vesicles are useful for solubilizing and stabilizing hydrophobic molecules that are otherwise sparingly soluble in aqueous solutions. In this paper we investigate the use of micelles and vesicles prepared from ionic surfactants for solubilizing and stabilizing curcumin, a medicinal natural product that undergoes alkaline hydrolysis in water. We identify spectroscopic signatures to evaluate curcumin partitioning and deprotonation in surfactant mixtures containing micelles or vesicles. These spectroscopic signatures allow us to monitor the interaction of curcumin with charged surfactants over a wide range of pH values. Titration data are presented to show the pH dependence of curcumin interactions with negatively and positively charged micelles and vesicles. In solutions of cationic micelles or positively charged vesicles, strong interaction between the Cur(-1) phenoxide ion and the positively charged surfactants results in a change in the acidity of the phenolic hydrogen and a lowering of the apparent lowest pK(a) value for curcumin. In the microenvironments formed by anionic micelles or negatively charged bilayers, our data indicates that curcumin partitions as the Cur(0) species, which is stabilized by interactions with the respective surfactant aggregates, and this leads to an increase in the apparent pK(a) values. Our results may explain some of the discrepancies within the literature with respect to reported pK(a) values and the acidity of the enolic versus phenolic protons. Hydrolysis rates, quantum yields, and molar absorption coefficients are reported for curcumin in a variety of solutions.

Visible light sensitization of TiO2 nanoparticles by a dietary pigment, curcumin, for environmental photochemical transformations

“…7a shows that the adsorption of RuL 3 on TiO 2 steadily decreases from pH 3 to pH 9 while that of curcumin is much less affected by the pH change in the same condition, which indicates that the binding between curcumin and TiO 2 is only slightly affected by pH. Since curcumin is much less acidic than RuL 3 (pK a z 8.89), 47 the electrostatic repulsion between the negatively charged TiO 2 surface and the deprotonated curcumin should be signicant only at pH > 9. As a result, the adsorption of curcumin is signicantly higher than that of RuL 3 in the pH range of 3-9 (see Fig.…”

Encapsulation of curcumin in cyclodextrin-metal organic frameworks: Dissociation of loaded CD-MOFs enhances stability of curcumin

“…Several approaches have been tried to improve the delivery of curcumin in its intact hydrophobic form. These include several encapsulation-based systems as polymer nanoparticles (Bisht et al, 2007), phospholipids (Maiti, Mukherjee, Gantait, Saha, & Mukherjee, 2007), polyethylene glycol conjugates (Safavy et al, 2007), and surfactants (Wang, Leung, Kee, & English, 2010). In addition, curcumin can be stabilized by forming metallocomplexes with divalent cations prepared in a water/glycerol system (Zebib, Mouloungui, & Noirot, 2010).…”

Binding and stabilization of curcumin by mixed chitosan–surfactant systems: A spectroscopic study

“…Despite of its highly promising features as a health-promoting agent, poor aqueous solubility of curcumin (∼11 ng/ml in plain aqueous buffer pH 5.0) [9] in neutral aqueous medium remains a major trouble in its bioavailability, clinical efficiency and metabolism [10]. A number of attempts have been made to increase the aqueous solubility and stability and hence the bioavailability of curcumin through encapsulation of curcumin in surfactant micelles [11][12][13], phospholipids [14], cyclodextrine [15], hydrogel [16], liposomes [17], polymeric micelles [18], nanoparticles [19,20], etc. The curcumin solubility and stability can also be improved by chemical alteration [9], complexation or interaction with macromolecules [21,22].…”

“…Both the solvent dependent keto-enol tautomerism of the ␤-diketone moiety and cis-trans isomerism of the conjugate carbon atom chain might contribute to the complex behaviour of curcumin [24]. The absorption and fluorescence properties are of great significance in providing information on the interaction of curcumin with various micro-heterogeneous systems and biomimetic systems [11][12][13][14][15][16][17][18][21][22][23]. Because of the highly promising pharmacological activity of curcumin and consequent low bioavailability in this paper we analyse the absorption and fluorescence properties of curcumin in chitosan-surfactant system to understand the interactions, distribution and localization of the dye in such a biomimetic system (Scheme 2).…”