Recent innovations in the use of charged cyclodextrins in capillary electrophoresis for chiral separations in pharmaceutical analysis

A review is presented on the use of charged cyclodextrins (CDs) as chiral selectors in capillary electrophoresis (CE) for the separation of analytes in pharmaceutical analysis. An overview is given of theoretical models that have been developed for a better prediction of the enantiomeric resolution and for a better understanding of the separation mechanism. Several types of charged CDs have been used in chiral capillary electrophoretic separation (anionic, cationic, and amphoteric CDs). Especially the anionic CDs seem to be valuable due to the fact that many pharmaceutically interesting compounds can easily be protonated (e.g., amine groups). For that reason several anionic CDs are now commercially available. Cationic and amphoteric CDs are less common in chiral analysis and only a few are commercially available. Attention is paid to the most common synthesis routes and the characterization of the CDs used in chiral capillary electrophoretic separations. The degree of substitution in the synthesized CDs may vary from one manufacturer to another or even from batch to batch, which may have a detrimental effect on the reproducibility and ruggedness of the separation system. In Sections 4, 5, and 6 the applications of anionic, cationic, and amphoteric CDs for the chiral separation in CE are described. Many interesting examples are shown and the influence of important parameters on the enantioselectivity is discussed.     

Review of aqueous chiral electrokinetic chromatography (EKC) with an emphasis on chiral microemulsion EKC

The separation of enantiomers using electrokinetic chromatography (EKC) with chiral microemulsions is comprehensively reviewed through December 1, 2006. Aqueous chiral EKC separations based on other pseudostationary phases such as micelles and vesicles or on other chiral selectors such as CDs, crown ethers, glycopeptides, ligand exchange moeities are also reviewed from both mechanistic and applications perspective for the period of January 2005 to December 1, 2006.     

Advances in the Use of Cyclodextrins as Chiral Selectors in Capillary Electrokinetic Chromatography: Fundamentals and Applications

Capillary electrokinetic chromatography is generally recognized as a versatile and robust capillary electromigration technique for the separation of enantiomers. In this mode, one or more chiral selectors are added to the background electrolyte acting as pseudostationary phases. Within the various chiral selectors that have been applied to enantioseparations in capillary electrokinetic chromatography, cyclodextrins are by far the most often used selectors because of their versatility, structural variety and commercial availability. This is reflected in the large number of applications of cyclodextrins to analytical enantioseparations that have been reported between January 2012 and July 2016, the period of time covered by this review. Many of these applications cover aspects of life sciences such as drug analysis, bioanalysis or food analysis. Despite the large number of commercially available cyclodextrins, new derivatives have been developed in order to achieve altered enantioselectivities or to further broaden the application range. Cyclodextrins have also been used to demonstrate the validity of theoretical models of electromigration as well as complex formation equilibria in enantioseparations. Finally, recent studies for an understanding of the molecular basis of the chiral recognition between cyclodextrins and the analytes are discussed.

     

Enantioseparation of α-Amino Acids by Capillary Electrophoresis Using L-Cysteine-Modified Cyclodextrins as Chiral Selectors

Capillary electrophoresis (CE) is a powerful separation technique that was used in a wide range of analytical chemical applications. Cyclodextrins(CDs) are the most commonly used chiral selectors in chiral capillary electrophoresis at the present time. Under neutral conditions, however, native CDs are neutral and usually applicable only for the enantioseparation of charged analyses. To overcome this defect we modified α- and β-CD with a L-cysteine moiety and used the CD derivatives as chiral selectors for the separation of a-amino acid enantiomers by the ligand exchange mode.     

Chiral Stationary Phases Based on Small Molecules: An Update of the Last 17 Years

A literature survey covering the report on Pirkle-type chiral stationary phases (CSPs) from January 2000 to March 2017 is presented in this review. More than 200 CSPs comprising small molecules as chiral selectors covalently bound to the chromatographic support have been reported in this period. The chemical nature of these new chiral selectors, new insights into the development strategies and their applications in liquid chromatography were emphasized.     

毛细管区带电泳法拆分手性药物萘普生和氟联苯丙酸

 70-72 -------------------------------------------------------------------------------- 以β-环糊精(CD)作为手性选择剂 ,用毛细管区带电泳法成功地拆分了两种弱酸性药物萘普生(naproxen)和氟联苯丙酸(flurb iprofen),并比较了4种环糊精［β-环糊精(β-CD)、二甲基-β-环糊精( DM-β-CD)、羟丙基-β-环糊精(HP-β-CD)和三甲基-β-环糊精( TM-β-CD)］对手性拆分的影响,同时测定了萘普生对映体在不同环糊精中的出峰次 序。通过实验,发现对于此类化合物拆分的最佳pH值为5左右,即接近于该类化合物的pK a值。该方法适用于酸性手性药物的拆分。     

Chiral capillary electrophoresis-mass spectrometry: modes and applications

A review is presented to highlight several approaches for coupling capillary electrophoresis (CE) and electrospray ionization-mass spectrometry (ESI-MS) for analysis of chiral compounds. A short discussion of commercially available CE-MS instruments and interface design is followed by a detail review on various modes of chiral CE-MS. In general, for each CE-MS mode, the capabilities, applications and limitations for chiral analysis have been pointed out. The first mode, chiral capillary zone electrophoresis-mass spectrometry (CZE-MS) in which neutral derivatized cyclodextrins (CDs) are used is possible using either column coupling with voltage switching or a partial-filling technique (PFT). However, some applications of direct coupling of CZE-MS mode are also reported. The second mode is a chiral electrokinetic chromatography-mass spectrometry (EKC-MS) in which a charged chiral selector such as a sulfated beta-CD or a vancomycin could be conveniently employed. This is because these chiral selectors have a significantly higher countercurrent electrophoretic mobility which prevents the entrance of these selectors into the mass spectrometer. The combination of counter-migration and PFT demonstrates that this synergism could be successfully applied to chiral analysis of a broader range of compounds. It is well-known that the on-line coupling of micellar electrokinetic chromatography to mass spectrometry (MEKC-MS) is problematic because the high surface activity and nonvolatile nature of conventional surfactant molecules lower the electrospray ionization efficiency. However, a recent report demonstrates that this hyphenation is now possible with the use of molecular micelles. Various MEKC-ESI-MS parameters that can be used to optimize both chiral resolution and ESI response are discussed. Finally, two recent examples that demonstrate the feasibility of using either open-tubular or packed chiral CEC with MS are reviewed. This survey will attempt to cover the state-of-the-art on various modes of CE-MS from 1998 up to 2002.     

Recent developments in chiral analysis of β‐blocker drugs by capillary electromigration techniques

The latest developments in chiral analysis of β‐blocker drugs by capillary electromigration techniques are reviewed in this article. Following the previous review by Aturki et al. [Electrophoresis 2011, 32, 2602–2628], this review includes the papers published during the period from January 2011 to December 2013. During this time, some novel chiral selectors were reported and applied to improve the enantioseparation of β‐blocker drugs and structurally related compounds. These chiral selectors include CDs and their derivatives, macrocyclic antibiotics, tartrate complexs, the monolithic molecularly imprinted polymer, and the polymeric surfactants. In addition, this article summarizes the methodological improvements for enhancing sensitivity in chiral analysis of β‐blockers and structurally related compounds by CE. The involved authors described the use of online sample preconcentration techniques to increase the detection sensitivity in the enantiomeric analysis of a broad range of samples.     

The role of cyclodextrins in chiral capillary electrophoresis

The members of the enantiomeric pairs frequently show rather different biological effects, so their chiral selective synthesis, pharmacological studies and analysis are necessary. CE has unique advantages in chiral analysis. The most frequently used chiral selectors are CDs in this field. This paper gives a short view on the advantages on CE in direct chiral separations, emphasizing the role of CDs. The reason for the broad selectivity spectra of CDs is discussed in detail. The physical background of chiral selective separations is briefly shown in CE. Their interaction mechanisms are shortly defined. The general trend of their use is statistically evaluated. Most frequently used CDs and CD derivatives are characterized. Advantages of ionizable CDs and single-isomer derivatives are shown. The general trend of their use is established.     

Enantioseparation of chiral N-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.     

Synthesis and application of single-isomer 6-mono(alkylimidazolium)-beta-cyclodextrins as chiral selectors in chiral capillary electrophoresis

Novel single isomers of positively charged beta-CDs were prepared via nucleophilic substitution of 6-monotosyl-beta-CD with alkylimidazoles to afford 6-mono(alkylimidazolium)-beta-CD tosylates and then 6-mono(alkylimidazolium)-beta-CD chlorides by anion exchange. The chiral resolution abilities of these cationic CDs were studied by CE using dansyl (Dns)-amino acids as model analytes. From the experimental results, it was found that both resolution and selectivity of these cationic CDs were dependent on the following parameters: concentration of chiral selectors, pH of the running buffer, counteranions of the CDs, side chain length of the n-alkyl-imidazolium cation, temperature of the capillary column, and organic modifier used. The concentration of chiral selectors required for enantioseparation varied from 3 to 30 mM. The BGE pH played an important role in the resolution of Dns-amino acids. For acidic BGEs, chiral resolution increased with pH (4.0-6.0) and reached a local maximum at pH 6.0. However, better resolutions were obtained with basic phosphate buffer at pH 9.6. Methanol was found to be an effective organic modifier for the resolution of Dns-amino acids by CE.     

A multichannel electrophoresis microchip platform for rapid chiral selector screening

This study presents a four-channel electrophoresis chip platform, featuring double-cross hydrostatic sample injection, for rapid chiral selector screening. This platform needs only five electrodes to drive microchip electrophoresis in four separate channels for screening four chiral selectors at a time. To demonstrate the performance of this screening platform, eight neutral CDs and their derivatives as chiral selectors were screened towards two FITC-labeled chiral compounds. The screening could be accomplished in less than 2 min. Dimethyl-beta-CD and hydroxypropyl-alpha-CD was demonstrated to be the appropriate selectors for FITC-norfenefrine and FITC-baclofen, respectively. The established platform is easy to operate and suitable for rapid screening process, which is expected to be a potential platform for high-throughput screening of chiral selectors.     

Enantioseparation of chiral benzimidazole derivatives by electrokinetic chromatography using sulfated cyclodextrins

Baseline separation of 18 new substituted benzimidazole derivatives, potent AMP‐activated protein kinase (AMPK) activators, with one chiral center, was achieved by CD‐EKC using sulfated and highly sulfated CDs (SCDs and HS‐CDs) as chiral selectors. The influence of the type and concentration of the chiral selectors on the enantioseparations was investigated. The SCDs 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 around 6 min using 25 mM phosphate buffer at pH 2.5 containing either β‐S‐CD, HS‐β‐CD, HS‐γ‐CD (3 or 4% w/v) at 25°C, with a voltage of 20 kV. The apparent association constants of the inclusion complexes were calculated. The study of the solute structure‐enantioseparation relationships seems to show the high contribution of the interactions between the solutes phenyl ring and the CDs to the enantiorecognition process. The optimized method was briefly validated (LOD less than 1%) and the purity of enantiomers of compound 3 was determined. The enantiomer migration shows reversal order depending on the kind of CD.     

The Use of Dual Cyclodextrin Chiral Selector Systems in the Enantioseparation of Pharmaceuticals by Capillary Electrophoresis: An Overview

Cyclodextrin (CD) derivatives are the most efficient and frequently used chiral selectors (CSs) in capillary electrophoresis (CE). There are situations when the use of a single CD as CS is not enough to obtain efficient chiral discrimination of the enantiomers; in these cases, sometimes this problem can be resolved using a dual CD system. The use of dual CD systems can often dramatically enhance enantioseparation selectivity and can be applied for the separation of many analytes of pharmaceutical interest for which enantioseparation by CE with another CS systems can be problematic. Usually in a dual CD system an anionic CD is used together with a neutral one, but there are situations when the use of a cationic CD with a neutral one or the use of two neutral CDs or even two ionized CDs can be an efficient solution. In the current review we present general aspects of the use of dual CD systems in the analysis of pharmaceutical substances. Several examples of applications of the use of dual CD systems in the analysis of pharmaceuticals are selected and discussed. Theoretical aspects regarding the separation of enantiomers through simultaneous interaction with the two CSs are also explained. Finally, advantages, disadvantages, potential and new direction in this chiral analysis field are highlighted.     

Separation of enantiomers of ephedrine by capillary electrophoresis using cyclodextrins as chiral selectors: comparative CE, NMR and high resolution MS studies

The enantiomer migration order (EMO) of ephedrine was investigated in the presence of various CDs in CE. The molecular mechanisms of chiral recognition were followed for the ephedrine complexes with native α- and β-CD and heptakis(2,3-di-O-acetyl-6-O-sulfo)-β-CD (HDAS-β-CD) by CE, NMR spectroscopy and high-resolution MS. Minor structural differences were observed between the complexes of ephedrine with α- and β-CD although the migration order of enantiomers was opposite when these two CDs were applied as chiral selectors in CE. The EMO was also opposite between β-CD and HDAS-β-CD. Significant structural differences were observed between ephedrine complexes with the native CDs and HDAS-β-CD. The latter CD was advantageous as chiral CE selector not only due to its opposite electrophoretic mobility compared with that of the cationic chiral analyte, but also primarily due to its enhanced chiral recognition ability towards the enantiomers of ephedrine.     

Chiral separations by capillary electrophoresis: Recent developments and applications

This paper provides an overview of the different classes of chiral selectors that are used in CE. The main properties of every class are described, together with the mechanism of enantioseparation. Newly introduced selectors are also discussed. Pharmaceutical and biomedical applications published from January 2004 till March 2005 are summarized.     

Native and substituted cyclodextrins as chiral selectors for capillary electrophoresis enantioseparations: Structures,features, application,and molecular modeling

CDs are cyclic oligosaccharides consisting of α-d -glucopyranosyl units linked through 1,4-linkages, which are obtained from enzymatic degradation of starch. The coexistence of hydrophilic and hydrophobic regions in the same structure makes these macrocycles extremely versatile as complexing host with application in food, cosmetics, environmental, agriculture, textile, pharmaceutical, and chemical industries. Due to their inherent chirality, CDs have been also successfully used as chiral selectors in enantioseparation science, in particular, for CE enantioseparations. In the last decades, multidisciplinary approaches based on CE, NMR spectroscopy, X-ray crystallography, microcalorimetry, and molecular modeling have shed light on some aspects of recognition mechanisms underlying enantiodiscrimination. With the ever growing improvement of computer facilities, hardware and software, computational techniques have become a useful tool to model at molecular level the dynamics of diastereomeric associate formation to sample low-energy conformations, the binding energies between the enantiomer and the CD, and to profile noncovalent interactions contributing to the stability of CD/enantiomer association. On this basis, the aim of this review is to provide the reader with a critical overview on the applications of CDs in CE. In particular, the contemporary theory of the electrophoretic technique and the main structural features of CDs are described, with a specific focus on techniques, methods, and approaches to model CE enantioseparations promoted by native and substituted CDs. A systematic compilation of all published literature has not been attempted.     

Capillary Zone Electrophoresis Resolutions of 2,4‐Dinitrophenyl Labeled Amino Acids Enantiomers by N‐MethylatedAmino‐β‐Cyclodextrins

Abstract

Capillary zone electrophoresis resolutions of 2,4‐dinitrophenyl labeled amino acids (DNP‐AAs) enantiomers using three N‐methylated amino‐β‐cyclodextrins (CDs) [6^I‐deoxy‐6^I‐monomethylamino‐β‐CD (M‐A‐βCD), 6^I‐deoxy‐6^I‐dimethylamino‐β‐CD (diM‐A‐βCD), 6^I‐deoxy‐6^I‐trimethylammonium‐β‐cyclodextrin (triM‐A‐βCD)] as chiral selectors were investigated. These cationogenic selectors, differing in ionization and steric properties, exhibited clear differences in their enantioselectivities.

The differences in enantioresolution observed under identical acid‐base conditions (pH 5.2), providing comparable effective charges/mobilities of the CDs, e.g., excellent separations of single enantiomeric couples (triM‐A‐βCD, M‐A‐βCD), multicomponent mixtures of enantiomers (M‐A‐βCD), and mixtures of positional isomers (M‐A‐βCD, diM‐A‐βCD), indicated the importance of structural parameters (different degrees of methylation) of the studied chiral selectors in the separation mechanism.

The differences in enantioresolution observed under various acid base conditions (pH 5.2 and 9.6), providing significant differences of effective charges/mobilities of CDs, e.g., a dramatic decrease in enantioresolution as well as achiral resolution with uncharged M‐A‐βCD and preserved resolution with permanently charged triM‐A‐βCD, indicated the importance of charge of the studied chiral selectors in the separation mechanism.

The present study clearly showed that the studied CD derivatives have great potential as chiral selectors in capillary zone electrophoresis separations of DNP‐AAs and that their effective use is related to the character of the analyte (structure, hydrophobicity) as well as to working conditions (pH).     

毛细管电泳手性分离中的协同效应

综述了毛细管电泳手性分离中的协同效应。介绍了毛细管电泳手性分离中双手性选择剂的应用情况。表明用CDs/CDs.CDs/crown组成的双选择剂及聚合环糊精衍生物,聚合手性胶束体系有可能改善难拆分的对映体物质的分离效果,展示了协同效应的毛细管电泳拆分复杂物质对映体中的应用前景。     

Enantioseparation of chiral allenic acids by micellar electrokinetic chromatography with cyclodextrins as chiral selector

Enantioseparation of chiral aryl allenic acids by micellar electrokinetic chromatography (MEKC) with cyclodextrins (CDs) as chiral selectors was described. The screen of chiral selectors (beta-CD, gamma-CD, and hydroxypropyl (HP)-gamma-CD) showed that the enantioseparation was not only dependent on the type of CD but also the presence of 2-propanol in the buffer. In order to optimize the operational parameters, the effect of the concentration of CDs, sodium dodecyl sulfate (SDS), and 2-propanol, as well as the buffer ionic strength and pH on enantioseparation were studied. It was proved that the concentration of CDs, 2-propanol, and the buffer ionic strength were the critical parameters. Under optimal conditions, baseline separations of all seven allenic acid enantiomers were achieved. Furthermore, the method validation in terms of repeatability, linearity, limit of detection (LOD), and limit of quantitation (LOQ) were performed. Using the present method, the optical purity of a nonracemic sample with the enantiomeric excess (e.e.%) value of 99.65% was determined.