Influence of ethonium on the adsorption and keto-enol tautomerism of curcumin on the surface of fumed silica

Curcumin, a poorly soluble bioactive polyphenol of plant origin, exists in two tautomeric (ketone and enol) forms, the ratio of which can influence the pharmacological effects of related. At present, the current task is the development of medicinal nanocomposites with a standardized content of tautomeric forms of curcumin. The aim of the work was to find optimal conditions for the effective adsorption of ketone and enol tautomers of curcumin on the surface of a silica enterosorbent in the form of supramolecular complexes with the antiseptic cationic surfactant ethonium. The shift of the keto-enol tautomeric equilibrium of curcumin as a result of its adsorption on fumed silica was studied using UV-visible spectroscopy and quantum chemical calculations depending on the concentration of the ethonium in solution. Negligible adsorption of curcumin on silica from aqueous solutions (? 5 %) is radically increased due to the formation of soluble supramolecular complexes with this surfactant. Removal percentage of curcumin reaches a maximum value of 96 % in premicellar ethonium solutions CET = 3?10–5–1?10–3 М, and predominantly the keto tautomer adsorbs on silica surface under these conditions. Changes in the spectral characteristics of the keto tautomer in the supramolecular complex curcumin – ethonium, namely the bathochromic shift of absorption band, indicate the polarization of its carbonyl group upon interaction with ethonium. The magnitude of the bathochromic shift is greater for keto tautomer adsorbed on silica (12 nm) than in ethonium solution (5 nm), which indicates a significant polarizing effect of the sorbent surface. From micellar ethonium solutions of СMСET = 2?10–3 М curcumin is adsorbed in the enol form. The amount of its adsorption at CET ? 1?10–3 М gradually decreases to 12 %, which is explained by a decrease in the concentration of surface silanol groups capable of dissociation, compared to the original hydrophilic silica, as a result of the adsorption of ethonium, which concentration under these conditions significantly exceeds that of curcumin. The results obtained can be used in the creation of new, more effective drugs with a standardized content of ketone and enol forms of curcumin, as well as in the development of methods for sorption-photometric and visual-test determination of curcumin directly in the sorbent phase.