Physicochemical Characterization of the Strychnos Alkaloids by Capillary Zone Electrophoresis

A capillary zone electrophoresis (CZE) method has been developed for investigating the physicochemical characteristics of five Strychnos alkaloids in Strychnos nux-vomica L. Firstly, the dissociation constants of the five Strychnos alkaloids were determined, based on the relation between the effective mobility of the solutes and the buffer pH. The mathematical relationship was strictly deduced from the fundamental electrophoretic theory and the dissociation equilibrium. Secondly, an equation describing the relation between the migration time of alkaloids of similar structure and their molecular weights was developed and used to predict the migration order and to calculate the electrosomotic velocity. The results predicted by the theory agreed with those from experiments.

Determination of pKa values of 2-amino-2-oxazolines by capillary electrophoresis

The dissociation constants of new 2-amino-2-oxazolines were determined by capillary electrophoresis (CE) as a new technique. A method based on a linear model has been used in the CE determination. A series of eight 2-amino-2-oxazolines are investigated to determine their ionization constant. Among them, three new oxazolines synthesized are presented. The Ka values were obtained from the plots of reciprocal effective mobility against inverse concentrations of protons. The potentiometric method (PM) was performed as a comparative method. No significant differences were observed between the determined dissociation constants using both methods. Thus, the pKa values have been found to vary between 8.55 and 8.68.

Rapid screening of pKa values of pharmaceuticals by pressure-assisted capillary electrophoresis combined with short-end injection

A method applying pressure-assisted capillary electrophoresis combined with short-end injection has been developed for the rapid screening of the pKa values of pharmaceuticals. The electrophoretic separation is performed on a short capillary length with short-end injection under an applied pressure, and the effective mobility is measured in a series of 10 different buffers with constant ionic strength (I = 0.05). The application of pressure not only reduces migration times, particularly in lower pH buffers, but also improves the repeatability of effective mobility measurements. The influence of pressure on the effective mobility was investigated at various pH values. It was observed for the first time that an increase in pressure resulted in a slight decrease in the effective mobility when the pH was above the pKa for acidic analytes, whereas an increased effective mobility with increasing pressures was observed when the pH was below the pKa. However, the observed effective mobility shift by the applied pressure did not significantly affect the determined pKa values. The determined pKa values were in good agreement with published data. Furthermore, a stacking condition was applied to increase the sensitivity, and a concentration down to 2 microM could readily be detected with UV detection using a 50 microm I.D. capillary. This technique is particularly suitable for measurement of pKa values for compounds with poor aqueous solubility. The method also omits the commonly used preconditioning steps with sodium hydroxide and water. The exclusion of excessive preconditioning steps and the use of pressure reduces the total cycling analysis time, and makes it possible to determine the pKa in less than 40 min per compound without loss of accuracy.

Semi-empirical relationships between effective mobility, charge, and molecular weight of pharmaceuticals by pressure-assisted capillary electrophoresis: applications in drug discovery

Relationships between effective mobility (m(eff)), calculated charge (Z(c)), and molecular weight (MW) are semi-empirically derived for pharmaceuticals using pressure-assisted capillary electrophoresis (PACE). We determined the m(eff) at 12 different pH points (2.0-11.4) of 66 pharmaceutical-like compounds ranging in MW from 79 to 825 g/mol. Plots of the observed m(eff) values versus Z(c)/MW(x ) (where x is a fractional coefficient) gave linear relationships. For anions, it was found that the best correlation (R(2) = 0.9666) exists when the fractional coefficient is equal to 0.4920, resulting in the equation m(eff) = 0.1853 (Z(c)/MW (0.4920)). For cations, the best linear relationship (R(2) = 0.9861) gave the equation m(eff) = 0.3888 (Z(c)/MW (0.6330)). The m(eff), Z(c)/MW(x) relationships were then applied to: (i) developing a technique for selecting an appropriate pH to achieve optimal separation of pharmaceuticals and (ii) determining the maximum charge of a molecule in the pH range of determination of negative log of the dissociation constants (pK(a)) by PACE, thus enabling the correct choice of model equation to be automated without structure analysis.

Determination of dissociation constant of phosphinate group in phosphinic pseudopeptides by capillary zone electrophoresis

Capillary zone electrophoresis (CZE) was used for determination of dissociation constant of phosphinate group in phosphinic pseudopeptides, i.e. peptides where one peptide bond is substituted by phosphinic acid moiety -PO2--CH2-. The dissociation constants were determined for a set of newly synthesized pseudopeptides derived from a structure N-Ac-Val-Ala(psi)(PO2--CH2)Leu-His-NH2 by nonlinear regression of experimentally measured pH dependence of their effective electrophoretic mobilities. CZE experiments were carried out in Tris-phosphate background electrolytes in the pH range 1.4-3.2. The pseudopeptides were synthesized as a mixture of four diastereomers, the separation of which was achieved in most cases. Moreover, differences of the effective mobilities of the pseudopeptide diastereomers enabled simultaneous determination of the dissociation constant of their phosphinate group without necessity of previous isolation of individual isomers.

Medium-throughput pKa screening of pharmaceuticals by pressure-assisted capillary electrophoresis

A fast screening method for the determination of the dissociation constants (pKa) of acidic, basic, and multivalent compounds was developed by using pressure-assisted capillary electrophoresis (PACE). External air pressure was applied to shorten the analysis time. The separation efficiency decreases as air pressure increases. However, it was found that air pressure does not affect the measurement of electrophoretic mobility and pKa significantly when it is less than 2 psi. The method was evaluated in terms of accuracy, precision, and ruggedness by using a set of 48 compounds with literature pKa values ranging from 2 to 10. The difference between the measured pKa values and literature values is less than 0.2 units. The throughput is approximately 20 compounds per day with a 12-point measurement ranging from pH 2.5 to 11. It was demonstrated that this method is applicable for pKa screening of pharmaceuticals with diverse chemical structures.