Determination of acid–base dissociation constants of very weak zwitterionic heterocyclic bases by capillary zone electrophoresis

Thermodynamic acid–base dissociation (ionization) constants (pKa) of seven zwitterionic heterocyclic bases, first representatives of new heterocyclic family (2,3,5,7,8,9-hexahydro-1H-diimidazo[1,2-c:2′,1′-f][1,3,2]diazaphosphinin-4-ium-5-olate 5-oxides), originally designed as chiral Lewis base catalysts for enantioselective reactions, were determined by capillary zone electrophoresis (CZE). The pKa values of the above very weak zwitterionic bases were determined from the dependence of their effective electrophoretic mobility on pH in strongly acidic background electrolytes (pH 0.85–2.80). Prior to pKa calculation by non-linear regression analysis, the CZE measured effective mobilities were corrected to reference temperature, 25 °C, and constant ionic strength, 25 mM. Thermodynamic pKa values of the analyzed zwitterionic heterocyclic bases were found to be particularly low, in the range 0.04–0.32. Moreover, from the pH dependence of effective mobility of the bases, some other relevant characteristics, such as actual and absolute ionic mobilities and hydrodynamic radii of the acidic cationic forms of the bases were determined.