Acidity Constants of Hydroxyl Groups Placed in Several Flavonoids: Two Flavanones, Two Flavones and Five Flavonols

Acidity constants (pKa) of nine flavonoids have been spectrophotometrically determined and the obtained values carefully contrasted with those derived from various experimental approaches given in literature, as well as with the ones predicted by Percepta and SPARC programs. To obtain robust pKa values, titrations in both pH ways, from acid to base and vice-versa, have been performed. The well-known instability of most flavonoids in basic media originates some discrepancies between obtained pKa by both titration ways and also with those coming from different experimental approaches previously published. This study tries to overcome the mentioned lack of integrity of several flavonoids avoiding measurements at high pH values or designing titrations at higher speed, which are able to cover the whole pH range with a minimum damage of studied compound. Suitable spectro-photometric titrations in pure water and, for instance, in methanol/water mixtures have been carried out. Achieved results allow the pKa assignation to hydroxyl groups belonging to two flavanones (Hesperetin and Naringenin), two flavones (Apigenin and Crysin) and five flavonols (Fisetin, Galangin, Kaempherol, Morin and Quercetin).