Determination of the ionisation constants of isoflavones by capillary electrophoresis

The ionisation constants of a group of important dietary phytoestrogens were investigated using capillary electrophoresis. The variation in electrophoretic mobilities with pH of the isoflavones genistein, daidzein, biochanin A, formononetin and glycitein along with the related compounds dihydrogenistein and O-desmethylangolensin was examined in sodium carbonate:bicarbonate buffer across the pH range 8.5 –10.5. The pKa values were determined by performing nonlinear regression on the resulting plot of electrophoretic mobility vs. pH. Two markers of known pKa, namely benzoic acid and resorcinol, were included to improve accuracy. The influence of isoflavone structure on electrophoretic behaviour and pKa has been explained. Dominant effects over the measured pH range were attributed to substitution at the 4′-position.