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.

Thermal transformation of bioactive caffeic acid on fumed silica seen by UV-Vis spectroscopy, thermogravimetric analysis, temperature programmed desorption mass spectrometry and quantum chemical methods.

Thermochemical studies of hydroxycinnamic acid derivatives and their surface complexes are important for the pharmaceutical industry, medicine and for the development of technologies of heterogeneous biomass pyrolysis. In this study, structural and thermal transformations of caffeic acid complexes on silica surfaces were studied by UV-Vis spectroscopy, thermogravimetric analysis, temperature programmed desorption mass spectrometry (TPD MS) and quantum chemical methods. Two types of caffeic acid surface complexes are found to form through phenolic or carboxyl groups. The kinetic parameters of the chemical reactions of caffeic acid on silica surface are calculated. The mechanisms of thermal transformations of the caffeic chemisorbed surface complexes are proposed. Thermal decomposition of caffeic acid complex chemisorbed through grafted ester group proceeds via three parallel reactions, producing ketene, vinyl and acetylene derivatives of 1,2-dihydroxybenzene. Immobilization of phenolic acids on the silica surface improves greatly their thermal stability.