Microscale determination of dissociation constants of multivalent pharmaceuticals by capillary electrophoresis.

Dissociation constants (pKaS) of acidic, basic, and amphoteric pharmaceuticals were exactly determined by capillary electrophoresis. A general equation for use in calculation of pKaS from solute mobilities observed at different pHs was derived to be suitable for a multivalent compound with close pKaS such as angiotensins, which are bioactive octa- or decapeptides. The obtained pKa values were highly consistent with the values determined by conventional methods. For verapamil, the detection limit was 9.8 microM (49 fmol), the total analysis time was 12 min, and the relative standard deviation of the obtained pKa value was 0.11%. This method was also applicable to a mixture of two components, e.g., a bioactive compound and the prodrug, because they have different mobilities and are separated from each other.