Determination of acidity constants of cytosine and uracil derivatives by capillary zone electrophoresis

5 ABSTRACT Capillary zone electrophoresis was applied to determination of acid-base dissociation constants (pKa) of ionogenic groups of newly synthesized 5-azacytosine and 6-chlorouracil derivatives. Cytosine derivatives show strong antiviral activity and they contain one basic amino group and one acidic hydrogenphosphonate group. Uracil derivative is potential antineoplastic and it involves one acidic enol form of NH-CO group. The pKa of these ionogenic groups were determined by nonlinear regression analysis of effective mobilities dependence on pH. The pKa of 6-chlorouracil was also determined by linear regression analysis of the dependence of inverse value of effective mobility on the activity of H+ ions. Effective mobilities were measured by capillary zone electrophoresis performed in the series of BGEs in a broad pH range (2.0-10.5), at constant ionic strength (25 mM) and standard temperature (25 řC). Thermodynamic pKa of amino group was estimated to be in the range 2.69-3.19; pKa of hydrogenphosphonate group achieved values from 6.80 to 7.07. Thermodynamic acidity constant of enol form of NH-CO group determined by linear regression analysis was equal to 5.71 and by nonlinear regression analysis it was equal to 5.90. SUBJECT WORDS: analytical chemistry, capillary electrophoresis, separation methods..