Deprotonation Mechanism and Acidity Constants in Aqueous Solution of Flavonols: a Combined Experimental and Theoretical Study

Four flavonols, namely quercetin, morin, kaempferol, and myricetin, were studied using spectrophotometry (UV vis) in aqueous solution. The study was performed varying the pH to analyze the stability of these compounds, and to estimate their acidity constants. In addition, the deprotonation mechanisms were studied using computational chemistry within the density functional theory framework. The calculations were performed in aqueous solution using the SMD continuum model, and the results are reported as deprotonation energies. Our results show that both quercetin and myricetin are highly unstable at basic pH. Kaempferol, on the other hand, is much more stable, and morin is the only one among the studied compounds that was not affected by pH. In spite of this inconvenience, their acidity constants were estimated through analysis of their decomposition kinetics, correcting the spectra accordingly, and obtaining a correlation of values between the experimentally observed pK(a) and the calculated Delta G of successive deprotonations.