Determination of absolute mobility and dissociation constant of lovastatin using capillary electrophoresis and empirical equation of ion mobility

In capillary electrophoresis, determination of the basic physical and chemical properties of compounds, such as absolute mobility (m0) and dissociation constant (pKa), is of great practical significance. This is because the aforementioned properties are often used for the qualitative or quantitative analyses of the relevant compounds toward their application as potential drugs. Lovastatin is a potential drug candidate that can reduce the levels of cholesterol and low-density lipoprotein cholesterol in the blood, as well as prevent atherosclerosis and coronary heart disease. For a more convenient and rapid investigation of the properties and applications of lovastatin, it is necessary to determine its m0 and pKa values. However, existing research on capillary electrophoresis for lovastatin and other related drugs focus on their quantitative determination, and their action mechanism and functions. Unfortunately, there are very few studies aimed at the determination of the m0 and pKa values of lovastatin. Based on related studies, this paper herein proposed a novel method to determine m0 and pKa of lovastatin. The present study mainly included a calculation method and experimental verification. The calculation method was based on capillary zone electrophoresis (CZE) and the empirical formula of ion mobility. First, on the basis of the empirical formula, the calculation formula for m0 was derived from the relationship between the actual mobility (mact), effective mobility (meff) and m0. Second, for a monovalent acid (HA), according to the calculation formula for m0 part, considering the hydrogen ion concentration as the independent variable and the reciprocal of meff as the dependent variable, a straight line was obtained on the coordinate axis. From the slope of this straight line, the dissociation equilibrium constant Ka was obtained directly, and pKa was calculated easily. After the derivation of m0 and pKa in the theoretical part, the feasibility and reliability of this method were verified by using it to determine the m0 and pKa values of several organic acids and bases (barbituric acid, benzoic acid, benzylamine, phenol, and m-cresol) in the experimental part. Note that for the buffer system with pH<6.0, reverse capillary electrophoresis was used for the determination of pKa, because this technique helped shorten the migration time and facilitates the detection of analytes that could not reach the cathode. After obtaining m0 and pKa, the theoretical reference values for these parameters were obtained by PeakMaster 5.1. The experimental data were well consistent with the theoretical m0 and pKa values. The standard deviation (SDs) of m0 and pKa were less than 6.0% and 6.2%, respectively. From the correlation coefficient (R) of the linear regression equation, it was found that the linear regression lines of pKa fit well, indicating the excellent reliability of this method. Finally, with this simple and reliable method, dimethyl sulfoxide (DMSO) was used as a marker for electroosmotic flow to determine the m0 and pKa values of lovastatin (-1.70×10-8 m2/(V·s) and 9.00, respectively). This method is suitable for the determination of m0 and pKa of acidic and basic analytes. The method has high accuracy and is expected to play an indispensable role in drug analysis.