Determination of the basicity of Mannich ketones by capillary electrophoresis

Application of capillary electrophoresis in determination of acid dissociation constant values

The chemical groups undergoing protonation or deprotonation in solution are described by the acid dissociation constant value, the key parameter for physicochemical characterization of biologically- and pharmacologically-important compounds. Capillary electrophoresis (CE) proved to be suitable technique for its determination: it enables automated and accurate measurements even for minute amount of sample, does not require the information about concentration, and handle both the impure and complex samples. In this review, a number of contributions reporting on the application of CE in pKa prediction has been summarized and critically discussed. The reader will find herein the brief introduction of theory, summary of all works published in the last decade, considerations on the most important innovations and achievements, and the discussion of pKa-related issues as e.g. the role of pKa-shifts in the chiral separation mechanism or the elucidation of migration order reversals observed during CE-mediated separations.

Measurement of pKa values of newly synthesized heteroarylaminoethanols by CZE

Heteroarylaminoethanol derivates are drugs which affect sympathetic nervous system and are used for medications of hypertension. In solutions they behave like weak bases and their pK(a) values represent important information on their potential biological uptake, pharmacological activity and in vivo biodisponsibility. This article brings the measurement of pK(a) values of the series of seven new important heteroarylaminoethanols, compounds with potential vasodilating, beta-adrenolytic and antioxidant activity, by capillary zone electrophoresis (CZE) with diode-array detection. It has been shown that capillary zone electrophoresis measurements of pK(a) can be easily performed with very small quantities of studied substances, and, due to CZE separation power, the purity of samples is not of key importance. Moreover, the CZE method is fast and reliable, providing that suitable operational conditions are selected. The method is based on the measurement of the effective mobility curves within a suitable pH range and related regression analysis where pK(BH)(+) and electrophoretic mobility of BH(+) are explicitly involved. The selection of sufficient operational buffers is of key importance for accurate and reproducible results, and, this article brings step by step the consideration procedure involved in this process. Further, this paper brings principles of least square regression analysis of non-linear function corresponding to exact explicit formula for mobility curve of monovalent weak base.

Determination of acid-base dissociation constants of amino- and guanidinopurine nucleotide analogs and related compounds by capillary zone electrophoresis

CZE has been applied for determination of acid-base dissociation constants (pKa) of ionogenic groups of newly synthesized amino- and (amino)guanidinopurine nucleotide analogs, such as acyclic nucleoside phosphonate, acyclic nucleoside phosphonate diesters and other related compounds. These compounds bear characteristic pharmacophores contained in various important biologically active substances, such as cytostatics and antivirals. The pKa values of ionogenic groups of the above compounds were determined by nonlinear regression analysis of the experimentally measured pH dependence of their effective electrophoretic mobilities. The effective mobilities were measured by CZE performed in series of BGEs in a broad pH range (3.50-11.25), at constant ionic strength (25 mM) and temperature (25 degrees C). pKa values were determined for the protonated guanidinyl group in (amino)guanidino 9-alkylpurines and in (amino)guanidinopurine nucleotide analogs, such as acyclic nucleoside phosphonates and acyclic nucleoside phosphonate diesters, for phosphonic acid to the second dissociation degree (-2) in acyclic nucleoside phosphonates of amino and (amino)guanidino 9-alkylpurines, and for protonated nitrogen in position 1 (N1) of purine moiety in acyclic nucleoside phosphonates of amino 9-alkylpurines. Thermodynamic pKa of protonated guanidinyl group was estimated to be in the range of 7.75-10.32, pKa of phosphonic acid to the second dissociation degree achieved values of 6.64-7.46, and pKa of protonated nitrogen in position 1 of purine was in the range of 4.13-4.89, depending on the structure of the analyzed compounds.

Rapid pKa estimation using vacuum‐assisted multiplexed capillary electrophoresis (VAMCE) with ultraviolet detection

A rapid approach for estimating the pK(a) value of small organic molecules was developed using vacuum-assisted multiplexed capillary electrophoresis (VAMCE) with ultraviolet detection. The VAMCE method employed a 96-capillary array, arranged in a standard 8 x 12 microtiter plate configuration, with each row of capillaries filled with 12 individual running buffers of equal ionic strength (I = 50 mM) covering a pH range from 2.2 to 10.7. A separate compound was injected hydrodynamically into each row of capillaries allowing the estimation of pK(a) values for eight compounds in a single run. The application of a vacuum during the separation generated a bulk fluid flow and allowed the electrophoretic separation to be completed within 5 min. The complete VAMCE method, conditioning, and electrophoretic separation was optimized to allow the pK(a) estimation for between 128 to 168 compounds in an 8-h period. The VAMCE method provided a reliable approach for estimating pK(a) values both within- and between-day. The pK(a) values for a series of 96 compounds estimated by VAMCE agreed well with some of literature pK(a) values with an average absolute difference of 0.22 log units.

Determination of acidity constants of enolisable compounds by capillary electrophoresis

Research on the structure-activity relationships of molecules with acidic carbon atoms led us to undertake a feasibility study on the determination of their acidity constants by capillary electrophoresis (CE). The studied molecules had diverse structures and were tetronic acid, acetylacetone, diethylmalonate, Meldrum's acid, 3-methylrhodanine, nitroacetic acid ethyl ester, pyrimidine-2,4,6-trione, 3-oxo-3-phenylpropionic acid ethyl ester, 1-phenylbutan-1,3-dione, 5,5-dimethylcyclohexan-1,3-dione and homophthalic anhydride. The p Ka range explored by CE was therefore very large (from 3 to 12) and p Ka values near 12 were evaluated by mathematical extrapolations. The analyses were carried out in CZE mode using a fused silica capillary grafted (or not) with hexadimethrine. Owing to the electrophoretic behaviour of these compounds according to the pH, their acidity constants could be evaluated and appeared in perfect agreement with the literature data obtained, a few decades ago, by means of potentiometry, spectrometry or conductimetry. The p Ka of homophthalic anhydride and 3-methylrhodanine were evaluated for the first time.