Determination of Acid-base Dissociation Constants of Selected β-Blockers Using Capillary Zone Electrophoresis, RP-HPLC and ¹H NMR

Abstract

The acid-base dissociation constant (pK_a) is an important physicochemical parameter of weakly acid or weakly basic pharmaceuticals, which influences solubility, absorption, distribution, metabolism and elimination of drugs. pK_a of four standard β-blockers were firstly determined by capillary zone electrophoresis (CZE). Method is based on measurement of the effective electrophoretic mobility of an ionisable compound in a series of electrolyte solutions of various pHs. This dependence was fitted by a sigmoidal curve, where pK_a is the value of pH in the sigmoid inflection point. Then, pK_a was measured by reversed-phase high-performance liquid chromatography (RP-HPLC) using the sigmoidal dependence of the retention factor on pH of the mobile phase (methanol/phosphate buffer), where pK_a is the value of pH in the sigmoid inflection point. Thirdly, determination of pK_a using ¹H nuclear magnetic resonance (¹H NMR) was based on measurement of chemical shifts of selected protons of studied substances as a function of solution pH (methanol-d₄/D₂O). Values of pK_a obtained for water/methanol mixtures were recalculated for water medium. Experimentally obtained values of pK_a were compared with values calculated by SPARC, Marvin 5.8.2 and ACD I-Lab programs and literature values.