Determination of dissociation constants of cytokinins by capillary zone electrophoresis

A method for the pK(a) determination, based on mobility data measured by ca pillary zone electrophoresis, was applied to cytokinins and their analogs. The combination of charged mobility standards with an uncharged electroosmo sis marker, injected in the uncoated capillary simultaneously with the meas ured substances, allows one to minimize the number of runs, reduce their du ration and, in addition, to inform on the run-to-run stability of electroos mosis and on contingent side-effects. pK(a) values of investigated cytokini ns and their analogs ranged from 2.8 to 4.0 at 25 degrees C in the phosphat e and acetate buffers of ionic strength 0.015 M. Standard deviations of the constants, obtained by the non-linear fitting of equations for the pK(a) c alculation, were 3-5-times lower than standard deviations from the linear f itting or from the point-to-point calculation utilizing the Hendersson-Hase lbalch equation. The equation of Boltzman sigmoid offers two checks on reli ability of effective mobilities that serve as the raw data in the pK(a) cal culation. (C) 2000 Elsevier Science B.V. All rights reserved.