Determination of dissociation constants of flavonoids by capillary electrophoresis

Abstract

Ionization constants of some flavanols (catechin and epicatechin) and flavonols (kaempherol, fisetin, morin, and quercetin) are determined by capillary zone electrophoresis (CZE). This technique allows the determination of pK(a) values until about 12. The pK(a) values obtained are compared with those calculated by the SPARC computational program. This program predicts the microscopic and macroscopic pK(a) values and the order of deprotonation of the different -OH groups. While for catechin and epicatechin the first ionizable OH group occurs in ring 1 and the second ionizable group in ring 2, in flavonols the first deprotonation occurs in ring 2 and the second in ring 1.

The influence of stereochemistry on the antioxidant properties of catechin epimers

“…Figure 3 presents the pH-dependent TEAC profiles of the catechin epimers under study. From the plots presented, it follows that the radical scavenging activity of catechin * Prediction of pKa was done using calculated DE and the QSAR of catechin epimers; the equation of the QSAR defined was pKa = 0.0658DE-13.4276; R = 0.975; P = 0.00109 a [29]; b [30], capital letters in parentheses refer to the identification of A and B ring in catechin molecules, respectively; c [31]; d [14], for DE and BDE values, the numbers in parentheses refer to the position of the most easily deprotonated OH moiety; for BDE(A) and IP(A) values, the descriptors in parenthesis refer to the type of monoanion; e [32] epimers generally increases with an increase in pH of the surrounding medium. These results are in accordance with the previous study on the pH-dependent radical scavenging capacity of some green tea catechins [14].…”

Chiral separation of catechin by capillary electrophoresis using mono‐, di‐, tri‐succinyl‐β‐cyclodextrin as chiral selectors

“…However, at pH 9.8 and 11.0, (6)-catechin has more than one ionized hydroxyl group because its pK a values are 8.77, 9.97, and 11.99. 29 Therefore, the migration times of (6)-catechin dramatically increase at pH 9.8 and 11.0 due to the increase of its electrophoretic mobility which direction is reverse to the EOF. However, the EOF is also dependent on the running buffer pH.…”

Micellar Electrokinetic Chromatography with Laser-Induced Fluorescence Detection for Separation of Red and Yellow Historical Dyes

“…Kaempferol (peak a) and apigenin (peak c) eluted much later than morin, with significant peak overlap at all applied potentials (18, 20 and 25 kV). The overlapping peaks were not unexpected, given that the pKa 1 values are so similar for these dyes [38][39][40]. Herrero-Martinez [40] reported the following pKa values (the numbers in parentheses refer to the position of hydroxyl group): kaempferol: 7.11 (7) and 9.09 (4'), quercetin: 7.04 (7) and 8.55 (4'), morin: 6.93 (7) and 8.32 (6').…”

“…The overlapping peaks were not unexpected, given that the pKa 1 values are so similar for these dyes [38][39][40]. Herrero-Martinez [40] reported the following pKa values (the numbers in parentheses refer to the position of hydroxyl group): kaempferol: 7.11 (7) and 9.09 (4'), quercetin: 7.04 (7) and 8.55 (4'), morin: 6.93 (7) and 8.32 (6'). At pH = 9.24 (Figure 2), the elution order for these dyes was: kaempferol (peak a), quercetin (peak e) and morin (peak f); but at pH = 8.0, kaempferol (peak d) and apigenin (peak c) gave two poorly resolved peaks that were shifted toward the end of the electropherogram.…”