Keto-enol tautomerism of curcumin in the preparation of nanobiocomposites with fumed silica

The tautomerism of curcumin (Cur) in water-ethanol solutions in the presence of fumed silica was studied by UV-visible spectroscopy. The results showed that the enol tautomer exists at an ethanol concentration in solution >50%, and with an increase in the water content, the tautomeric equilibrium shifts towards the formation of the keto tautomer. Quantum-chemical calculations (solvation model SM 5.42/6-31G (d), GAMESSPLUS) of various curcumin isomers confirmed that the existence of curcumin keto tautomer in aqueous solution is more thermodynamically favorable. The ratio of keto and enol forms also depends on the dielectric constant of water-ethanol solutions: at ε < 45, only the enol form of curcumin exists, while at ε > 45, the relative amount of the keto tautomer increases in proportion to the dielectric constant. Curcumin tautomers adsorb on fumed silica in different ways. At a low curcumin concentration in the initial solutions (<1.5 × 10^-4 M), only the enol tautomer forms a monolayer on the sorbent surface, apparently due to its planar structure. The keto tautomer, characterized by a bent structure, begins to adsorb only at a concentration of Cur > 1.5 × 10^-4 M, being a component of molecular aggregates with coplanar geometry.

Adsorption of caffeic acid on titanium dioxide: a spectroscopic study

Caffeic acid is an ortho-phenol found in vegetable tissues presenting important properties such as carcinogenesis inhibitor, anti-oxidant, anti-viral, anti-inflammatory and anti-rheumatic actions. It was observed that caffeic acid was not degraded in daylight during the adsorption on TiO(2) at pH 4.8. The adsorption fit very well to a Brunauer-Emmett-Teller isotherm equation with a monolayer coverage of 68.15 mg(CA)g(-1)(TiO2 and saturation coverage of 195.4 mg(CA)g(-1)(TiO2). A strong adsorption of caffeic acid was verified on TiO(2) for the dry solid obtained from the mixture. The Raman and IR spectroscopies revealed that the adsorption should occur through the interaction of the diphenol oxygens with contribution of CC double bond of the acrylic group, however, the carboxylic acid group did not have participation in the adsorption.