Determination of acid–base dissociation constants of azahelicenes by capillary zone electrophoresis



CZE was employed to determine acid–base dissociation constants (pKa) of ionogenic groups of azahelicenes in methanol (MeOH). Azahelicenes are unique 3-D aromatic systems, which consist of ortho-fused benzene/pyridine units and exhibit helical chirality. The pKa values of pyridinium groups of the studied azahelicenes were determined from the dependence of their effective electrophoretic mobility on pH by a nonlinear regression analysis. The effective mobilities of azahelicenes were determined by CZE at pH range between 2.1 and 10.5. Thermodynamic pKa values of monobasic 1-aza[6]helicene and 2-aza[6]helicene in MeOH were determined to be 4.94 ± 0.05 and 5.68 ± 0.05, respectively, and pKa values of dibasic 1,14-diaza[5]helicene were found to be equal to 7.56 ± 0.38 and 8.85 ± 0.26. From these values, the aqueous pKa of these compounds was estimated.