Achievement of enantioselectivity of non-polar primary amines by a non-chiral crown ether

Some thoughts about enantioseparations in capillary electrophoresis

In this overview the goal of the authors was to analyze from the historical perspective the reasons of success and failure of chiral capillary electrophoresis. In addition, the current trends are analyzed, unique advantages of capillary electrophoresis are highlighted and some future directions are discussed.

Contemporary theory of enantioseparations in capillary electrophoresis

The first separation of enantiomers in capillary electrophoresis (CE) counts slightly longer than three decades. Fast development of the practice and theory of chiral CE occurred in the past 30 years and today one can consider this technology to have a solid and mature theoretical background. The goal of the present review is not only to summarize the history and contemporary theory of enantioseparations by using CE but also to present the authors personal view where shall we head to with this attractive technology not only from the viewpoint of separation of enantiomers but also for better understanding the mechanisms of non-covalent (enantioselective) interactions in chemistry, biology, medicine and related disciplines.

Capillary electrophoresis separation of peptide diastereomers that contain methionine sulfoxide by dual cyclodextrin-crown ether systems

A dual-selector system employing achiral crown ethers in combination with cyclodextrins has been developed for the separation of peptide diastereomers that contain methionine sulfoxide. The combinations of the crown ethers 15-crown-5, 18-crown-6, Kryptofix® 21 and Kryptofix® 22 and β-cyclodextrin, carboxymethyl-β-cyclodextrin, and sulfated β-cyclodextrin were screened at pH 2.5 and pH 8.0 using a 40/50.2 cm, 50 μm id fused-silica capillary and a separation voltage of 25 kV. No diastereomer separation was observed in the sole presence of crown ethers, while only sulfated β-cyclodextrin was able to resolve some peptide diastereomers at pH 8.0. Depending on the amino acid sequence of the peptide and the applied cyclodextrin, the addition of crown ethers, especially the Krpytofix® diaza-crown ethers, resulted in significantly enhanced chiral recognition. Keeping one selector of the dual system constant, increasing concentrations of the second selector resulted in increased peak resolution and analyte migration time for peptide-crown ether-cyclodextrin combinations. The simultaneous diastereomer separation of three structurally related peptides was achieved using the dual selector system.

Experimental design-guided development of a stereospecific capillary electrophoresis assay for methionine sulfoxide reductase enzymes using a diastereomeric pentapeptide substrate

A capillary electrophoresis method has been developed and validated to evaluate the stereospecific activity of recombinant human methionine sulfoxide reductase enzymes employing the C-terminally dinitrophenyl-labeled N-acetylated pentapeptide ac-KIFM(O)K-Dnp as substrate (M(O)=methionine sulfoxide). The separation of the ac-KIFM(O)K-Dnp diastereomers and the reduced peptide ac-KIFMK-Dnp was optimized using experimental design with regard to the buffer pH, buffer concentration, sulfated β-cyclodextrin and 15-crown-5 concentration as well as capillary temperature and separation voltage. A fractional factorial response IV design was employed for the identification of the significant factors and a five-level circumscribed central composite design for the final method optimization. Resolution of the peptide diastereomers as well as analyte migration time served as responses in both designs. The resulting optimized conditions included 50mM Tris buffer, pH 7.85, containing 5mM 15-crown-5 and 14.3mg/mL sulfated β-cyclodextrin, at an applied voltage of 25kV and a capillary temperature of 21.5°C. The assay was subsequently applied to the determination of the stereospecificity of recombinant human methionine sulfoxide reductases A and B2. The Michaelis-Menten kinetic data were determined. The pentapeptide proved to be a good substrate for both enzymes. Furthermore, the first separation of methionine sulfoxide peptide diastereomers is reported.

Computational modeling of capillary electrophoretic behavior of primary amines using dual system of 18-crown-6 and β-cyclodextrin

Using capillary electrophoresis (CE) three chiral primary amine compounds 1-aminoindan (AI), 1-(1-naphthyl)ethylamine (NEA) and 1,2,3,4-tetrahydro-1-naphthylamine (THAN), exhibited only partial or no separation when β-cyclodextrin (βCD) was used as chiral selector. The use of 18-crown-6 (18C6) as a second additive with βCD resulted in an enhanced separation. A molecular modeling study, using molecular mechanics and the semiempirical PM6 calculations, was used to help explaining the mechanism of the enantiodifferentiation and to predict the separation process. Optimization of the structures of the complexes by the PM6 method indicate that the poor separation obtained in the presence of the βCD chiral selector alone is due to the small binding energy differences (ΔΔE) of 4.7, 1.1 and 1.2 kcal mol(-1) for AI, NEA and THAN, respectively. In the presence of 18C6 it was suggested that a sandwich compound between 18C6, amine and βCD is formed. Theoretical calculations show that a significant increase in the binding energy is obtained for the sandwich compounds indicating strong hydrophobic and van der Waals interactions that show enhanced enantiodifferentiation.

Enantioseparations by using capillary electrophoretic techniques. The story of 20 and a few more years

This paper provides the author's insight on the past, present and future of performing enantioseparations using capillary electrophoretic (CE) techniques. These techniques are discussed from the historical point of view, as well as based on their potential as the separation techniques of today and the future. The overview covers mechanistic as well as practical aspects of CE techniques.

Rapid Enantioseparation of 1-Aminoindan by Microchip Electrophoresis with Linear-Imaging UV Detection

Chiral separations of 1-aminoindan (AI) by cyclodextrin electrokinetic chromatography (CDEKC) were investigated on microfluidic quartz chips. By using a microchip electrophoresis (MCE) instrument equipped with a linear-imaging UV detector, the separation process of the enantiomeric compounds was observed. When sulfated beta-cyclodextrin was employed as a chiral selector, the baseline separation of AI could be achieved within 1 min with a high repeatability. The relative standard deviation of the migration time was less than 6%. The fastest separation was achieved in 14 s, utilizing a separation length of only 6.1 mm. These results show that the MCE analysis employing a linear imaging UV detector has a significant potential for fast chiral analysis.

Application of tetraoxadiaza-crown ether derivatives as chiral selector modifiers in capillary electrophoresis

Two new diaza-crown ether derivatives (R-1, RS-1) have been synthesized from 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane and tested as potential chiral selectors in capillary electrophoresis (CE) for the chiral separation of five amino acid derivatives. The individual use of the selectors did not lead to chiral differentiation. However, they enhanced the enantioselective effect of different cyclodextrins in dual selector systems. In this paper, we report the effect of different substituted diaza-crown ether derivatives on the separation results obtained in dual systems with cyclodextrins.

Rapid chiral on-chip separation with simplified amperometric detection

The enantiomers of adrenaline, noradrenaline, ephedrine and pseudoephedrine were separated by capillary electrophoresis on a micromachined device. Detection was carried out with a new two-electrode amperometric detector, eliminating the need for individual counter and reference electrodes. Separation of the isomers was achieved by employing carboxymethyl-beta-cyclodextrin as chiral selector in the buffer, partly with the additional inclusion of the crown ether 18-crown-6. Plate numbers of up to 20,000, chiral resolutions of 2.5 and detection limits of the order of 10(-7) M were achieved. All separations were completed in less then 3 min.

Mechanistic study of enantiomeric recognition of primary amino compounds using an achiral crown ether with cyclodextrin by capillary electrophoresis and nuclear magnetic resonance

A model and theoretical equations are presented to investigate the enantiomeric recognition mechanism of primary amino compounds using an achiral crown ether with cyclodextrin by capillary electrophoresis (CE) and nuclear magnetic resonance (NMR). Association constants were calculated from CE and 1H NMR experiment results on the basis of the model and the equations. The key step of chiral recognition was identified from those values. Using CE analyses of three primary amino compounds [1-(1-naphthyl)ethylamine; 1-aminoindan; 1,2,3,4-tetrahydro-1-naphthylamine], the key step was identified with the equilibrium where the complex of a primary amino compound and 18-crown-6 becomes associated with 2,6-di-O-methyl-beta-cyclodextrin for all the three compounds. From the 1H NMR analyses of 1-(1-naphthyl)ethylamine, the key step was identified with the equilibrium where the complex of 1-(1-naphthyl)ethylamine and 18-crown-6 becomes associated with beta-cyclodextrin.

Enantioseparations in capillary electromigration techniques: recent developments and future trends

This review summarizes the current status of enantioseparations using capillary electromigration techniques and gives the authors insights on the selected fundamental aspects and future trends in this field. The most recent developments in the field of chiral separations using capillary electrophoresis (CE) and capillary electrochromatography (CEC) are summarized. The status of chiral electromigration techniques is evaluated tacking into account the most recent developments in related techniques such as chiral HPLC, GC and SFC.

Recent progress in chiral separation principles in capillary electrophoresis

This review summarizes recent developments in the field of chiral separations by electromigration techniques including capillary zone electrophoresis (CZE), capillary gel electrophoresis (CGE), isotachophoresis (ITP), electrokinetic chromatography (EKC), and capillary electrochromatography (CEC). This overview focuses on the development of new chiral selectors and the introduction of new techniques rather than applications of already established selectors and methods. The mechanisms of the different chiral separation principles are discussed.

Enantiomeric separations of cationic and neutral compounds by capillary electrochromatography with monolithic chiral stationary phases of beta-cyclodextrin-bonded negatively charged polyacrylamide gels

Enantiomeric separation by capillary electrochromatography with beta-cyclodextrin-bonded negatively charged polyacrylamide gels was examined. The columns used are capillaries filled with a negatively charged polyacrylamide gel, a so-called monolithic stationary phase, to which allyl carbamoylated beta-CD (AC-beta-CD) derivatives covalently bind. The capillary wall is activated first with a bifunctional reagent to make the resulting gel bind covalently to the inner surface of the fused-silica tubing. Enantiomeric separations of 15 cationic compounds were achieved using the above-mentioned columns and mobile phases of 200 mmol l(-1) Tris-300 mmol I(-1) boric acid buffer (pH 7.0 or 9.0) or 200 mmol l(-1) Tris-300 mmol l(-1) boric acid buffer (pH 7.0) containing an achiral crown ether (18-crown-6). Enantiomeric separations of two neutral compounds were also achieved using 200 mmol l(-1) Tris-300 mmol l(-1) boric acid buffer (pH 9.0) as a mobile phase. High efficiencies of up to 150,000 plates m(-1) were obtained. Both the within- and between-run reproducibilities of retention time and separation factor were good. The reproducibilities of retention time and separation factor for three different columns prepared from a different batch of monomers were acceptable. The gel-filled capillaries were stable for at least 3 months with intermittent use, utilizing the mobile phase of 200 mmol I(-1) Tris-300 mmol I(-1) boric acid buffer (pH 9.0).

Enantioselective determination by capillary electrophoresis with cyclodextrins as chiral selectors

This review surveys the separation of enantiomers by capillary electrophoresis using cyclodextrins as chiral selector. Cyclodextrins or their derivatives have been widely employed for the direct chiral resolution of a wide number of enantiomers, mainly of pharmaceutical interest, selected examples are reported in the tables. For method optimisation, several parameters influencing the enantioresolution, e.g., cyclodextrin type and concentration, buffer pH and composition, presence of organic solvents or complexing additives in the buffer were considered and discussed. Finally, selected applications to real samples such as pharmaceutical formulations, biological and medical samples are also discussed.

Enhancement of chiral recognition by formation of a sandwiched complex in capillary electrophoresis

For chiral primary amino compounds not separable by cyclodextrins alone, chiral recognition was successfully achieved by the formation of a sandwiched complex of the non-chiral 18-crown-6, the chiral amine and cyclodextrin (CD) [18-crown-6+amino compound+CD]. The separation of 1-methyl-3-phenylpropylamine and 1,2,3,4-tetrahydro-1-naphthylamine racemates showed the special function of the non-chiral 18-crown-6 on chiral recognition. By formation of the sandwiched complex, the chiral center of 1-methyl-3-phenylpropylamine was successfully recognized, and resolution of 1,2,3,4-tetrahydro-1-naphthylamine dramatically increased. In these studies, the mobility differences of the enantiomers were evaluated as a function of the concentration of cyclodextrins with and without the 18-crown-6, and as a function of the concentration of the 18-crown-6. In addition, the separations by this method were compared to those by the chiral 18-crown-6 reagent.

Capillary electrophoresis of drugs: current status in the analysis of pharmaceuticals

The current status of capillary electrophoresis (CE) in pharmaceutical analyses is reviewed with about 300 references, mainly from 1996 until 1999. This article covers the use of CE for assay and purity determination of the main component, analysis of natural medicines, antisense DNA, peptides, and proteins. Analysis of hydrophobic and/or electrically neutral drugs by electrokinetic chromatography, capillary electrochromatography and nonaqueous CE is critically evaluated. Detailed techniques for the separation of enantiomers are given in the text with some actual applications. Furthermore, this review includes sensitivity and regulatory aspects for the actual use of CE in new drug applications (NDA). The analytical validation required for CE in NDA is also treated.

Enantiomeric separation by capillary electrophoresis using a crown ether as chiral selector

This paper reviews chiral separations of primary amines by capillary electrophoresis and crown ether as chiral selector. Two possible mechanisms of chiral recognition by host-guest complexation are discussed: (i) The substituents of the crown ether act as barriers for the guest compounds, and (ii) lateral electrostatic interactions between host and guest occur. Experimental conditions affecting the separation are discussed in detail. A literature overview of practical applications is presented as well. More than 80 different primary amines were analyzed, whereupon the majority could be resolved using a screening method. It is shown that a synergistic effect on the resolution of chiral amines is observed when the chiral crown ether and cyclodextrins are simultaneously used in the same buffer system. This approach opens interesting perspectives for further method optimization.