Enantioseparation In Capillary Electrophoresis Research Articles

Amino acids-based deep eutectic solvents as additives for improved enantioseparation in capillary electrophoresis.

In this study, several amino acids deep eutectic solvents were prepared using L-valine and L-leucine as hydrogen bond acceptors, and L-lactic acid and glycerol as hydrogen bond donors. These amino acids' deep eutectic solvents were first used as buffer additives to construct several synergistic systems along with maltodextrin in capillary electrophoresis for the enantioseparations of four racemic drugs. Compared with single maltodextrin system, the separations of model drugs in the synergistic systems were significantly improved. Some key parameters affecting chiral separation such as maltodextrin concentration, deep eutectic solvent concentration, buffer pH, and applied voltage were optimized. In order to further understand the specific mechanism of the amino acids deep eutectic solvents in improving chiral separation, we first calculated the binding constants of maltodextrin with enantiomers using the capillary electrophoresis method in the two separation modes, respectively. We also used molecular simulation to calculate the binding free energy of maltodextrin with enantiomers. It is the first time that amino acids deep eutectic solvents were used for enantioseparation in capillary electrophoresis, which will greatly promote the development of deep eutectic solvents in the field of chiral separation.

Use of chiral ionic liquid as additive for synergistic enantioseparation of basic drugs in capillary electrophoresis

This work presents a synergistic system for enantioseparation in capillary electrophoresis (CE) with a chiral ionic liquid (CIL) based on D-10-camphorsulfonic acid as additive and carboxymethyl-β-cyclodextrin (CM-β-CD) as the chiral selector. The proposed method showed excellent enantioseparation performance towards sixteen chiral drugs. In contrast to the single CM-β-CD system, the notably improved resolution (Rs) and selectivity factor (α) of model drugs were observed in synergistic system. Several key parameters such as CIL concentration, CM-β-CD concentration, buffer pH and separation voltage were investigated, after which Statistical Product and Service Solutions (SPSS) was used to prove the potential synergistic effect. The nuclear magnetic resonance (NMR) results further demonstrated the function of the CIL and the superiority of synergistic system. Finally, chiral impurity determination of chlorpheniramine maleate sample was successfully carried out using the established method.

Investigation on improvement of enantioseparation in capillary electrophoresis based on maltodextrin by chiral ionic liquids.

Taking advantage of chiral ionic liquids, this study deals with the improvement of the enantioseparation performance of a traditional chiral selector (maltodextrin) in capillary electrophoresis. Herein, two polyhydroxy compound-based chiral ionic liquids, namely tetramethylammonium-D-gluconic acid and tetramethylammonium-shikimic acid were designed and utilized as additives for chiral separation for the first time. The synergistic systems provided much better enantioseparations of twelve model drugs compared to the single maltodextrin system. These model analytes contained analgesics, antidepressants, antiallergic drugs, antifungal drugs, antihypertensive drugs, and antiparkinsonian drugs. After optimizing the separation conditions, the chiral recognition mechanism was probed by means of ultraviolet spectroscopy, nuclear magnetic resonance, and molecular modeling. The results of spectroscopic and computational analyses were in good consistency with enantioseparation outcomes. Finally, the proposed method was successfully used for the determination of the enantiomeric purity of duloxetine hydrochloride.

Evaluation of an ionic liquid chiral selector based on sulfobutylether-β-cyclodextrin in capillary electrophoresis

Herein, an ionic liquid chiral selector based on sulfobutylether-β-cyclodextrin (SBE-β-CD) was prepared for enantioseparation in capillary electrophoresis (CE) for the first time. Compared to native SBE-β-CD, ionic liquid (SBECDIL) exhibited much more superior enantioselectivity towards several model drugs. Differentiating with the previous research, the introduction of ionic liquid in this study did not prolong the migration time obviously for most analytes. Factors affecting the enantioseparations including ionic liquid concentration, type and proportion of organic solvents, buffer pH and applied voltage were investigated in detail. Furthermore, the established method was successfully used to determinate the enantiomeric purity of amlodipine besylate (AML). In addition, capillary electrophoresis, nuclear magnetic resonance (NMR) and molecular modeling were employed to probe into the chiral recognition mechanism. Several interactions jointly promoted the enantioseparations in CE.

The Use of Antibiotics as Chiral Selectors in Capillary Electrophoresis: A Review.

Chirality is becoming an essential issue in modern pharmaceutical research as regulatory agencies emphasize the safety and efficiency of enantiomers in drug development. The development of efficient and reliable chiral separation methods became a necessity in the last 30 years, and capillary electrophoresis (CE), due to its relatively low costs and “green” features, is attracting increased attention. Cyclodextrin (CD) and their derivatives are the most frequently used chiral selectors (CSs) in CE, however, the use of antibiotics as CSs represents an interesting alternative. Various classes of antibiotics (aminoglycosides, ansamycins, glycopeptides, lincosamides, macrolides, tetracyclines) have been used more or less successfully for the enantio-separation of pharmaceuticals. Antibiotics offer the possibility of a multitude of potential interactions (electrostatic, inclusion, hydrogen bonding, etc.) due to their chemical diversity, allowing the enantio-separation of analytes with a wide range of structural characteristics. This article aims to review the application of various classes of antibiotics in the CE enantio-separation of pharmaceuticals. Antibiotic physiochemical characteristics, variables impacting enantio-separation, advantages, and disadvantages when certain antibiotics are used as CSs in CE are also explored.

Structural characterization of methyl-β-cyclodextrins by high-performance liquid chromatography and nuclear magnetic resonance spectroscopy and effect of their isomeric composition on the capillary electrophoresis enantioseparation of daclatasvir

The separation of daclatasvir and its R,R,R,R-enantiomer was studied by capillary electrophoresis using various randomly methylated β-CDs and the single isomer heptakis(2,6-di-O-methyl)-β-CD (2,6-DM-β-CD) as chiral selectors in an acidic background electrolyte. Opposite enantiomer migration order was observed for randomly substituted CDs compared to 2,6-DM-β-CD as well as methylated β-CDs with different composition according to the specifications of the manufacturers. HPLC and NMR analyses confirmed that the presence of a high 2,6-DM-β-CD content in the CDs enables to achieve the migration order R,R,R,R-enantiomer > daclatasvir. In contrast, products with low 2,6-DM-β-CD isomer content and/or the presence of a large amount of methylated CD isomers, in which d-glucopyranose moieties are not substituted in either position 2 or 6, displayed the opposite enantiomer migration order daclatasvir > R,R,R,R-enantiomer. The study indicated the importance of the type and composition of derivatized CDs on chiral separations in capillary electrophoresis as well as the importance of proper quality control for cyclodextrin manufacturers. Moreover, the observed migration order could be rationalized based on the composition and substitution pattern of the CDs.

Effects of amino acid-derived chiral ionic liquids on cyclodextrin-mediated capillary electrophoresis enantioseparations of dipeptides

The synergistic effect of chiral ionic liquids composed of tetraalkylammonium ions and the amino acids Asn, Asp or Pro on the enantioseparations of dipeptides mediated by β-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin in capillary electrophoresis was studied. Addition of a chiral ionic liquid resulted in a concentration-dependent increase in the enantioresolutions compared to the sole presence of a cyclodextrin in the background electrolyte. The extent varied with the tetraalkylammonium cation (tetramethylammonium versus tetrabutylammonium) as well as the amino acid component of the ionic liquid. The presence of a chiral ionic liquid did not counteract the pH-dependent reversal of the enantiomer migration order of the dipeptides Ala-Phe, Ala-Tyr and Phe-Phe when increasing the pH of the background electrolyte from 2.5 to 3.5. Comparing the effect of a chiral ionic liquid based on Asp with the addition of equimolar concentrations of the individual components of the ionic liquid, a diverse picture was observed. In some cases, higher resolution values were obtained with the chiral ionic liquid, while for other cases superior enantioseparations were obtained upon separate addition of the amino acid component and a tetraalkylammonium chloride. With regard to the stereochemistry of the amino acid, a superior effect was typically observed using the l-configured amino acid, but in some cases higher resolution values were found in the presence of d-Asp. The rationale for the diverse observations is not obvious and may be due to the zwitterionic nature of analytes as well as the amino acid component of the chiral ionic liquid.

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.

Recent advance of novel chiral separation systems in capillary electrophoresis

Chiral analysis has been an important research field in modern separation science because the enantiomers of a racemic compound often show different or even opposite bioactivities. A variety of analytical techniques have been adopted for chiral analysis over the past few decades. In comparison with conventional chromatographic methods (e. g., high-performance liquid chromatography (HPLC), gas chromatography (GC)), capillary electrophoresis (CE) has multiple advantages such as high separation efficiency, low cost, and diverse separation modes, which have made it one of the most promising analytical techniques for enantioseparation in recent years. The simplest process for CE chiral separation is the addition of a chiral selector (e. g., cyclodextrins and their derivatives, polysaccharides, antibiotics, proteins, crown ethers, chiral exchangers, chiral ionic liquids) in a running buffer to create a chiral separation environment. However, with the ever-increasing number of chiral products in the modern industrial society, satisfactory enantioseparation cannot always be achieved with conventional CE methods. Hence, scientists are endeavoring to improve CE chiral methods. The availability of various fundamental operational modes such as capillary zone electrophoresis (CZE), micellar electrokinetic chromatography (MEKC), ligand-exchange capillary electrophoresis (LECE), non-aqueous capillary electrophoresis (NACE), and capillary electrochromatography (CEC) has enabled researchers to realize flexible design of high-performance CE chiral separation systems by altering the CE operations, especially by the modification of various advanced materials. For example, ionic liquids (ILs) are a group of organic salts whose melting points are below 100℃, or more often, close to room temperature. ILs have been demonstrated to be effective modifiers in chiral CE because of their unique physical and chemical properties such as high conductivity, exceptional chemical and thermal stabilities, as well as excellent solubility in both organic and inorganic solvents. Besides, it is feasible to design and synthesize various task-specific ILs by altering their anion-cation combinations. ILs have been employed for CE enantioseparation through various modes such as achiral IL-modified conventional enantioseparation systems, chiral IL synergistic separation systems, chiral IL LECE systems, and IL-based MEKC, or by the development of novel IL chiral selectors. Nanoparticles are another class of materials that have received considerable interest for use in chiral CE. Nanoparticles have many advantages such as unique size effect, good chemical stability, significant mechanical strength, as well as ease of modification. Several studies have demonstrated that the combination of chiral selectors with nanomaterials such as gold nanoparticles, Fe3O4 magnetic nanoparticles, carbon nanotubes, and mesoporous silica nanomaterials is a promising approach to establish an EKC system or a CEC system. In this review, we summarize the current state-of-the-art of novel CE chiral separation systems, including enantioseparation systems based on achiral or chiral ILs, nanomaterials, metal-organic frameworks (MOFs), and deep eutectic solvents, as well as chiral plug-plug partial filling CE. Another important topic of research in chiral CE is the exploration of enantiorecognition mechanisms. Modern mechanistic studies focus on the applications of advanced analytical techniques such as nuclear magnetic resonance (NMR) or molecular simulations with computer technology, instead of the conventional chromatography- or CE-based thermodynamic methods. For example, nuclear Overhauser effect spectroscopy (NOESY) and rotating-frame Overhauser enhancement spectroscopy (ROESY) have attracted attention because they provide critical information about the spatial proximity of the functional groups of chiral selectors and enantiomers. Molecular simulations have also become popular because of their powerful ability to evaluate the selector-selectand interactions, in addition to enabling visualization of the complex structures. The main objective of this paper is to provide a comprehensive review of state-of-the art of CE techniques in the field of chiral analysis, especially during the period 2015-2019. Existing problems with these techniques and future perspectives are also presented.

Use of Gamithromycin as a Chiral Selector in Capillary Electrophoresis

In this short communication, we report the use of a second-generation macrolide antibiotic, gamithromycin (Gam), as a novel chiral selector for enantioseparation in capillary electrophoresis (CE). A preliminary analysis of the experiment results shows that Gam is especially suitable for the separation of chiral primary amines. Factors influencing enantioseparations were systematically investigated including the composition of the background electrolyte (BGE), concentration of Gam, the type and proportion of organic solvents, applied voltage, etc. In particular, N-Methylformamide (NMF) was successfully used as a non-aqueous solvent for Gam, and shown to be extremely effective for the separation of primaquine (PMQ) and 1-aminoindan (AMI) when used alone or mixed with other commonly used non-aqueous solvents (e.g. methanol). To our knowledge this was also the first application of NMF as a non-aqueous solvent for antibiotic chiral selectors in CE. The best separations were obtained with 100 mM Tris, 125 mM H3BO3 and 80 mM Gam in methanol/NMF (25:75) solvent for PMQ and AMI, or 80-100 mM Gam in methanol for the other model analytes. Among the analytes, the resolution (Rs) of amlodipine (AML) reached up to 15.65, which is to our knowledge the highest value ever reported in CE studies for this compound (except for using molecularly imprinted polymers technique).

Investigation of fusidic acid as a chiral selector in capillary electrophoresis

In this short communication, we report the use of a steroid antibiotic, Fusidic Acid (Fus), as a novel chiral selector for enantioseparation in capillary electrophoresis (CE). Fus can be classified into the antibiotic chiral selector family but does not belong to any of the previously reported subfamilies. Factors influencing Fus in CE enantioseparations were systematically investigated including the composition of the background electrolyte (BGE), buffer pH, type and proportion of organic modifier, concentration of Fus, etc. The best separations were obtained with 50 mM phosphate buffer (pH 8.0) containing 60–70 mM Fus and 50% methanol. A preliminary analysis on the separation mechanism showed that Fus is especially suitable for the chiral analytes containing a rigid planar structure. The development of Fus as a novel chiral selector can be considered as a valuable work not only because of its unique enantioselectivity, but also because there are a number of steroids in nature that are structurally similar to Fus. This work indicates that the screening of high-efficient chiral selectors from these steroids is an attractive and promising research area.

Synthesis and application of tetramethylammonium-carboxymethylated-β-cyclodextrin: A novel ionic liquid in capillary electrophoresis enantioseparation

In this study, a carboxymethylated-β-cyclodextrin-based chiral ionic liquid (CIL), tetramethylammonium-carboxymethylated-β-cyclodextrin (TMA-CM-β-CD) was successfully designed and synthesized. This cyclodextrin-based ionic liquids (ILs) was used as the sole chiral selector in capillary electrophoresis, and it is very interesting to find that the chiral separation capability can be remarkably improved when a conventional cyclodextrin chiral selector evolved into an IL chiral selector. The ionic liquid showed satisfactory separation performance towards twelve tested drugs. A series of parameters affecting the enantioseparation, such as the type and proportion of organic modifier, buffer pH, chiral selector concentration, as well as applied voltage were systematically investigated. Additionally, the molecular docking program Autodock was applied to further demonstrate the mechanism of chiral recognition and the enhanced enantioselectivity of TMA-CM-β-CD, which kept in agreement with our experimental results.

Evaluation of an ionic liquid chiral selector based on clindamycin phosphate in capillary electrophoresis.

Recently, increasing attention has been given to the research on chiral ionic liquids (CILs) in chiral separation field; however, only a few literatures focus on the exploration of CILs as the sole chiral selector. In this study, an ionic liquid chiral selector based on antibiotic, namely tetramethylammonium clindamycin phosphate (TMA-CP), was originally synthesized and subsequently utilized for enantioseparation in capillary electrophoresis (CE). Remarkably improved separations of eight racemic analytes were achieved in TMA-CP system in contrast to the clindamycin phosphate (CP) system. The optimal separation conditions were determinated by systematic experiments on several crucial parameters including the type and proportion of organic modifier, CIL concentration, buffer pH, and applied voltage. Additionally, molecular modeling with AutoDock was applied to probe into the chiral recognition mechanism of the ionic liquid chiral selectors, which well corresponded with the experimental results. It is the first time that antibiotic-based ionic liquid was exploited as favorable sole chiral selector in CE, and this strategy has paved a new way for development of novel ionic liquids chiral selectors based on antibiotics. Graphical abstract.

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.

Synthesis and application of amino alcohol-derived chiral ionic liquids, as additives for enantioseparation in capillary electrophoresis

In this study, three functionalized chiral ionic liquids (CILs) derived from l-valinol, l-prolinol and l-phenylalaninol, namely N,N,N-trimethyl-l-valinol-bis(trifluoromethanesulfon)imide ([TMLV]+[Tf2N]−, CIL1), N,N-dimethyl-l-prolinol-bis(trifluoromethanesulfon)imide ([DMLP]+[Tf2N]−, CIL2) and N,N,N-trimethyl-l-phenylalaninol-bis(trifluoromethanesulfon)imide ([TMLP]+[Tf2N]−, CIL3), were synthesized and subsequently utilized for enantiomeric separation in capillary electrophoresis (CE) with 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) as chiral selector for the first time. Compared with traditional single HP-β-CD separation system, the synergistic system exhibited substantially improved separations of six tested drugs. Using the CIL1/HP-β-CD as a model system, the influence of crucial parameters including the type and proportion of organic modifier, CILs concentration, HP-β-CD concentration and buffer pH was investigated in detail. Additionally, molecular modeling with AutoDock was applied to elucidate the enhanced enantioselectivity in the presence of CILs, which has certain guiding value in predicting the migration order of the enantiomers and studying the interactions important for the chiral recognition.

Synthesis and application of amino triazolium-modified lactobionic acid as chiral selector in capillary electrophoresis

In this paper, a chiral selector, N-(4-H-1, 2, 4-triazolium)-lactobionamides (LA-ATM), was synthesized and applied for enantioseparation in capillary electrophoresis (CE) for the first time. Compared with lactobionic acid (LA) separation system, enhanced enantioseparation of five tested drugs was achieved in this modification system. In order to achieve good chiral separation, several parameters such as chiral selector concentration, buffer pH, applied voltage as well as the type and proportion of organic modifier were systematically investigated. Molecular modeling was applied to demonstrate the chiral recognition mechanism of the LA-ATM, which well supported the experimental results. Furthermore, a mathematical equation built up based on the molecular mechanics calculations was used in predicting resolution of tested drugs using LA or LA-ATM mediated CE, the predicted result was well correlated with the experimental result.

Cyclodextrins as Chiral Selectors in Capillary Electrophoresis Enantioseparations.

Due to their structural variability and their commercial availability, cyclodextrins are the most frequently used chiral selectors in capillary electrophoresis. A variety of migration modes can be realized depending on the characteristics of the cyclodextrins and the analytes. The basic considerations regarding the development of a chiral CE method employing cyclodextrins as chiral selectors are briefly discussed. The presented examples illustrate the separation modes of an acidic and a basic analyte with native and charged cyclodextrin derivatives as a function of the pH of the background electrolyte and the concentration of the cyclodextrin.

Enhanced enantioselectivity of native α‐cyclodextrins by the synergy of chiral ionic liquids in capillary electrophoresis

In the cyclodextrins family, the native α-cyclodextrin has almost been abandoned in capillary electrophoresis chiral separation due to its much weaker enantioselectivity compared with β-cyclodextrin and their derivatives. In this work, several amino acid chiral ionic liquids were selected to establish synergistic enantioseparation systems with native α-cyclodextrin. Enhanced enantioselectivities were observed in the chiral ionic liquids/α-cyclodextrin synergistic systems compared with single α-cyclodextrin system. A series of comparison experiments were performed to demonstrate the superiority of the synergistic systems. Primary parameters affecting the enantioseparation were systematically optimized, including the type and concentration of chiral ionic liquids, α-cyclodextrin concentration, buffer pH, and applied voltage. Best separations of the model enantiomers were obtained in a 20mM Tris/H3 PO4 buffer at pH 2.5 containing 3% (m/v) α-cyclodextrin and 30mM tetramethylammonium-l-arginine. The results show that the α-cyclodextrin is also worth our attention when selecting chiral selectors for capillary electrophoresis enantioseparation of specific racemic compound.

Investigation of maltodextrin-based synergistic system with amino acid chiral ionic liquid as additive for enantioseparation in capillary electrophoresis.

The combined use of chiral ionic liquids (ILs) and chiral selectors in capillary electrophoresis (CE) to establish a synergistic system has proven to be an effective approach for enantioseparation. In this article, tetramethylammonium-L-arginine, a kind of amino acid chiral IL, was applied to investigate its potential synergistic effect with maltodextrin in CE enantioseparation. The established maltodextrin-based synergistic system showed markedly improved enantioseparations compared with the single maltodextrin system. Parameters such as the chiral IL concentration, maltodextrin concentration, buffer pH, applied voltage, and capillary temperature were optimized. Satisfactory enantioseparation of the five studied drugs, including nefopam, duloxetine, ketoconazole, cetirizine, and citalopram was achieved in 50 mM Tris-H3 PO4 buffer solution (pH 3.0) containing 7.0% (m/v) maltodextrin and 60 mM tetramethylammonium-L-arginine. In addition, the chiral configuration of tetramethylammonium-L-arginine was also investigated to demonstrate the existence of a synergistic effect between chiral ILs and maltodextrin.

Establishment and molecular modeling study of maltodextrin-based synergistic enantioseparation systems with two new hydroxy acid chiral ionic liquids as additives in capillary electrophoresis

Discovering more superior performance of ionic liquids for the separation science has triggered increasing interest. In this work, two new Hydroxy acid-based chiral ionic liquids (CILs) (tertramethylammonium-d-pantothenate (TMA-d-PAN), tertramethylammonium-d-quinate (TMA-d-QUI)) were designed and first used as additives to establish the maltodextrin-based synergistic systems for enantioseparation in capillary electrophoresis (CE). Compared to traditional single maltodextrin chiral separation system, significantly improved separations of all tested drugs in the CIL/Maltodextrin synergistic systems were obtained. Some parameters (CIL concentration, maltodextrin concentration, buffer pH, and applied voltage) in the TMA-d-PAN/Maltodextrin synergistic system have been examined and optimized for analytes. The molecular docking software AutoDock was applied to simulate the recognition process and surmise feasible resolution mechanism in the Maltodextrin/CILs synergistic systems, which has certain guiding value.