Determination of ionization constants of N-imidazole derivatives, aromatase inhibitors, using capillary electrophoresis and influence of substituents on pKa shifts

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.

Determination of 2-methylimidazole and 4-methylimidazole in caramel colors by capillary electrophoresis

The use of chemical preservative compounds is common in the food products industry. Caramel color is the most usual additive used in beverages, desserts, and breads worldwide. During its fabrication process, 2- and 4-methylimidazole (MeI), highly carcinogenic compounds, are generated. In these cases, the development of reliable analytical methods for the monitoring of undesirable compounds is necessary. The primary procedure for the analysis of 2- and 4-MeI is using LC- or GC-MS techniques. These procedures are time-consuming and require large amounts of organic solvents and several pretreatment steps. This prevents the routine use of this procedure. This paper describes a rapid, efficient, and simple method using capillary electrophoresis (CE) for the separation and determination of 2- and 4-MeI in caramel colors. The analyses were performed using a 75 μm i.d. uncoated fused-silica capillary with an effective length of 40 cm and a running electrolyte consisting of 160 mmol L(-1) phosphate plus 30% acetonitrile. The pH was adjusted to 2.5 with triethylamine. The analytes were separated within 6 min at a voltage of 20 kV. Method validation revealed good repeatability of both migration time (<0.8% RSD) and peak area (<2% RSD). Analytical curves for 2- and 4-MeI were linear in the 0.4-40 mg L(-1) concentration interval. Detection limits were 0.16 mg L(-1) for 4-MeI and 0.22 mg L(-1) for 2-MeI. The extraction recoveries were satisfactory. The developed method showed many advantages when compared to the previously used method.

Dialkylimidazolium ionic liquids hydrolyze cellulose under mild conditions

The average molecular weight of cellulose derived from filter paper, poplar, and Avicel decreases by up to two orders of magnitude during typical mild dissolution protocols using ionic liquids (ILs). About an order of magnitude greater cellulose depolymerization rate during ionic liquid dissolution occurs in 1-butyl-3-methylimidazolium chloride (BmimCl) and 1-ethyl-3-methylimidazolium chloride (EmimCl) compared to 1-ethyl-3-methylimidazolium acetate (EmimOAc), and, unintuitively, greater IL purity results in greater cellulose depolymerization. The following data support the mechanism of cellulose hydrolysis to be acid-catalyzed: (i) increase in number of reducing ends following cellulose dissolution in IL; (ii) addition of N-methylimidazolium base suppresses cellulose depolymerization during dissolution in IL; (iii) small amounts of glucose and traces of hydroxymethyl furfural are present following cellulose dissolution in IL. The acid is presumably synthesized via IL decomposition to generate a carbene and proton, consistent with hypothesis derived from molecular modeling. Titration experiments conducted here measure the amount of acid synthesized to be in the 4000 ppm range for high-purity BmimCl IL during mild processing conditions for cellulose dissolution. This data is relevant for understanding the extent of IL decomposition during biomass dissolution.

Spectroscopic study of 2-, 4- and 5-substituents on pKa values of imidazole heterocycles prone to intramolecular proton-electrons transfer

New 2-(1H-imidazol-2-yl)phenols (L1Et-L8tBuPt) bearing a phenolic proton in the vicinity of the imidazole base were prepared and characterized. Experimental studies of the dependence of their protonation/deprotonation equilibrium on substituent identities and intramolecular hydrogen bonding tendencies were carried out using electronic absorption spectroscopy at varying pH values. In order to make comparison, 2-(anthracen-10-yl)-4,5-diphenyl-1H-imidazole (L9Anthr) bearing no phenolic proton and 4,5-diphenyl-2-(4,5-diphenyl-1H-imidazol-2-yl)-1H-imidazole (L10BisIm) bearing two symmetrical imidazole base fragments were also prepared and experimentally investigated. DFT calculations were carried out to study frontier orbitals of the investigated molecules. While electron-releasing substituents produced increase in protonation-deprotonation pK(a)s for the hydroxyl group, values for the imidazole base were mainly affected by polarization of the imidazole ring aromaticity across the 2-imidazole carbon and the 4,5-imidazole carbons axis of the imidazole ring. It was concluded that electron-releasing substituents on the phenol ring and/or electron-withdrawing substituents on 4,5-imidazole carbons negatively affects donor strengths/coordination chemistries of 2-(1H-imidazol-2-yl)phenols, and vice versa. Change of substituents on the phenol ring significantly altered the donor strength of the imidazole base. The understanding of pK(a) variation on account of electronic effects of substituents in this work should aid the understanding of biochemical properties and substituent environments of imidazole-containing biomacromolecules.

Capillary electrophoresis for pharmaceutical analysis

This chapter describes the application of capillary electrophoresis (CE) to pharmaceutical analysis. The areas of pharmaceutical analysis covered are enantiomer separation, analysis of small molecules such as amino acids or drug counter-ions, pharmaceutical assay, related substances determinations, and physiochemical measurements such as log P and pKa of compounds. The different electrophoretic modes available and their advantages for pharmaceutical analysis are described. Recent applications of CE for each subject area are tabulated with electrolyte details. Information on electrolyte choice and method optimization to obtain optimal separations is included.

Mass spectrometry analysis of new chemical entities for pharmaceutical discovery

In this Section, we review the applications of mass spectrometry for the analysis and purification of new chemical entities (NCEs) for pharmaceutical discovery. Since the speed of synthesis of NCEs has dramatically increased over the last few years, new high throughput analytical techniques have been developed to keep pace with the synthetic developments. In this Section, we review both novel, as well as modifications of commonly used mass spectrometry techniques that have helped increase the speed of the analytical process. Part of the review is devoted to the purification of NCEs, which has undergone significant development in recent years, and the close integral association between characterization and purification to drive high throughput operations. At the end of the Section, we review potential future directions based on promising and exciting new developments.

Determination of pKa values of benzoxa-, benzothia- and benzoselena-zolinone derivatives by capillary electrophoresis

Acidity constants of benzoxa-, benzothia- and benzoselena-zolinone derivatives were determined by capillary electrophoresis, potentiometry and spectrophotometry experiments. These three analytical techniques gave pK(a) results that were in good agreement. A convenient, accurate and precise method for the determination of pK(a) was developed to measure changes in acidity constants induced by heteroatom or 6-benzoyl substituted derivatives. pK(a) values were determined simultaneously for two compounds characterized by different electrophoretic mobility (micro(e)) and pK(a) value and in the presence of an analogous neutral marker.

Capillary electrophoresis for the analysis of small-molecule pharmaceuticals

This paper reviews the application of CE to the analysis of small-molecule pharmaceuticals. The areas of pharmaceutical analysis covered are enantiomer separation, the analysis of small molecules such as amino acids or drug counter-ions, pharmaceutical assay, determination of related substances and physicochemical measurements such as log P and pK(a) of compounds. The different electrophoretic modes available and their advantages for pharmaceutical analysis are described. Recent applications of CE for each subject area are tabulated with electrolyte details.

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.

Enantioseparation of chiralN-imidazole derivatives by electrokinetic chromatography using highly sulfated cyclodextrins: Mechanism of enantioselective recognition

Baseline separation of ten new substituted [1-(imidazo-1-yl)-1-phenylmethyl)] benzothiazolinone and benzoxazolinone derivatives, with one chiral center, was achieved by CD-EKC using highly sulfated CDs (alpha, beta, gamma highly S-CDs) as chiral selectors. The influence of the type and concentration of the chiral selectors on the enantioseparations was investigated. The highly S-CDs exhibit a very high enantioselectivity power since they allow excellent enantiomeric resolutions compared to those obtained with the neutral CDs. The enantiomers were resolved with analysis times inferior to 2.5 min and resolution factors R(s) of 3.73, 3.90, 1.40, and 4.35 for compounds 1, 2, 3, and 5, respectively, using 25 mM phosphate buffer at pH 2.5 containing either highly S-alpha-CD, highly S-beta-CD, and highly S-gamma-CD (3 or 4% w/v) at 298 K, with an applied field of 0.30 kV/cm. The determination of the enantiomer migration order for the various analytes and the study of the analyte structure-enantioseparation relationships display the high contribution of the interactions between the analytes phenyl ring and the CDs to the enantiorecognition process. The thermodynamic study of the analyte-CD affinities permits us to improve our knowledge about the enantioseparation mechanism.

Enantiomeric analysis of baclofen analogs by capillary zone electrophoresis, using highly sulfated cyclodextrins: Inclusion ionization constant pKa determination

Using cyclodextrin-capillary zone electrophoresis (CD-CZE), baseline separation of baclofen phaclofen, saclofen, and hydroxy-saclofen, potent gamma-aminobutyric acid(B) (GABA(B)) agonist or antagonists was achieved. A method for the enantioresolution of those analogs of GABA was developed using anionic cyclodextrins (highly sulfated CD or highly S-CD) as chiral selectors and capillaries dynamically coated with polyethylene oxide (PEO). With charged CDs we observed good resolutions due to the large electrophoretic mobility of these chiral selectors opposite to the mobility of the solutes. The highly S-alpha-CD and S-beta-CD were found to be complementary and the most effective complexing agent, allowing good enantiomeric resolution in short runtimes. The complete resolution was obtained using 25 mM phosphate buffer at pH 2.5 containing 3% w/v of highly S-alpha-CD or S-beta-CD at 25 degrees C with an applied field of 0.30 kV/cm. The apparent binding constants of the inclusion complexes were evaluated and the migration order was determined. A comparison was possible to investigate the importance of the anionic group of the molecules in the separations. The pK(a) values were determined for all four compounds in order to explain relative electrophoretic migration of the solutes.