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

Abstract

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 pKa and the calculated ΔG of successive deprotonations.

Theoretical Study of Primary Antioxidant Action Thermodynamics

“…We have confirmed the importance of mutual presence of C2=C3 double bond and 3-OH group in flavonoids -these are responsible for low B-ring O-H BDE(A) in 3-O -anions. Although the attention was focused only on one selected reaction pathway, obtained results support the experimentally observed different antioxidant activity of deprotonated flavonoids in comparison to the parent species (Lemanska et al 2001, Musialik et al 2009, Álvarez-Diduk et al 2013. For the studied environments, thermodynamically favored radical anions were identified.…”

“…HAT, SET-PT and SPLET, were studied for mono-substituted phenols, thiophenols, tocopherols, chromans and flavonoids. Because flavonoids and other polyphenols contain several phenolic OH groups with different acidities, their antioxidant action in aqueous solutions depends on pH, since their deprotonated forms show considerably different effect (Lemanska et al 2001, Musialik et al 2009, Álvarez-Diduk et al 2013. Therefore, our recent work is devoted to the study of hydrogen atom transfer and deprotonation of flavonoid phenoxide anions.…”

Oxidation of flavonoids by hypochlorous acid: reaction kinetics and antioxidant activity studies

“…Band I (330-450 nm) is associated with the cinnamoyl (B+C rings) moiety, and Band II (240-300 nm) with the A-ring benzoil system [20,21]. The position of absorption maxima depends on the number and type of substituents, on the kind of solvent, and on pH.…”

“…The concentration ratio of deprotonated to fully protonated flavonoid increases in the following order: kaempferol < rutin< myricetin < quercetin [21,29]. It has been found that for flavonols with at least two OH groups in ring B, the first deprotonation takes place from this ring.…”

Theoretical study of the energetics of carboxylic O–H bond cleavage in the para- and meta-substituted benzoic acid derivatives

“…deprotonation of a compound affects its overall antioxidant effect (Lemańska et al, 2001;Halliwell and Gutteridge, 1989). Many naturally occurring antioxidants are polyphenolic substances possessing a different number of OH groups and showing various acidities (Álvarez-Diduk et al, 2013). Moreover, some antioxidants have also a carboxylic COOH group attached either directly to the aromatic ring (gallic acid, vanillic acid or syringic acid), or present in another fragment of the molecule (e.g.…”