Separation of alpha-hydroxy acid enantiomers by high performance capillary electrophoresis using copper(II)-L-amino acid and copper(II)-aspartame complexes as chiral selectors in the background electrolyte

Versatile capillary electrophoresis method for the direct chiral separation of aliphatic and aromatic α-hydroxy acids, β-hydroxy acids and polyhydroxy acids using vancomycin as chiral selector

Hydroxy acids (HAs) are ubiquitous in nature and play significant roles in various industrial and biological processes. Most HAs harbor at least one chiral center, therefore the development of efficient chiral analysis techniques for HA stereoisomers is of crucial importance across a wide range of fields. A capillary electrophoresis (CE) method was developed for the chiral analysis and quantification of aliphatic and aromatic α‑hydroxy acid (AHA) enantiomers, aliphatic β‑hydroxy acid (BHA) enantiomers and aliphatic polyhydroxy acid (PHA) stereoisomers. Using a modified partial filling-counter current method with indirect UV detection, high resolution (Rs) was achieved with vancomycin as a chiral selector added to the background electrolyte composed of 10 mM of benzoic acid/L-histidine at pH 5 using a polyacrylamide-coated capillary. This method could be readily applied to the determination of the enantiomers of 12 aliphatic AHAs, 4 aromatic AHAs, 3 aliphatic BHAs, as well as to the determination of the stereoisomers of tartaric acid, 2,3-dihydroxybutanoic acid, 2,3,4,5-tetrahydroxypentanoic acid, and 2,3,4,5,6-pentahydroxyhexanoic acid without the need for sample derivatization. Finally, our study provides a robust and versatile strategy for the chiral and stereoselective analysis of a broad range of hydroxy acid compounds.

Synthesis and applications of sulfopropyl ether γ-cyclodextrin polymer as chiral selector in capillary electrophoresis

A novel sulfopropyl ether γ-cyclodextrin polymer (SPE-γ-CDP) through polycondensating sulfated cyclodextrins (SCDs) was synthesized. This synthesis approach also has the potential of preparing other derived cyclodextrins (CDs) polymers. The polymerized SCDs took on both the properties of SCDs and certain characteristics of polymers, such as chiral selectivity and high viscosity. Synthesis parameters, including reactions sequence, sulfation, and polycondensation conditions were investigated systematically. The product was characterized by elemental analysis, infrared spectroscopy (IR), and indirect UV detections prior to use as background electrolytes additive. The separation conditions, including the concentration of SPE-γ-CDP, the concentration and pH of running buffer, separation voltage, as well as the additional organic solution were optimized during chiral separation of neutral, acidic, and basic enantiomers in capillary electrophoresis (CE). SPE-γ-CDP was proven to be an effective chiral resolving agent in CE with the advantages of simple synthesis process, low cost, similar ratio of charge-to-mass, low current, great reproducibility, and reusability. Graphical Abstract Synthesis and applications of sulfopropyl ether γ-cyclodextrin polymer.

Chiral separation using capillary electromigration techniques based on ligand exchange principle

Over the last couple of decades, researchers have developed diverse chiral separation methods emerged from a few chiral separation principles. This review article is primarily focused on the application of chiral ligand-exchange (CLE) principle in capillary electromigration techniques, such as capillary electrophoresis (CE) and capillary electrochromatography (CEC). First, the most commonly used CLE-CZE separation mode by using different kinds of central ions, such as Cu(II), Zn(II), borate ion, and other metal ions, has been introduced. Meanwhile, several kinds of surfactants have been applied as the micelle-forming agents in the CLE micellar electrokinetic chromatography mode. The highlight of recent research of CLE-CEC is the exploitation of novel columns for chiral separation. Then, two kinds of capillary columns, packed capillary and monolithic capillary column, have been briefly described. Finally, the effective application of these chiral separation methods has been presented, including the application in life science and food analysis area.

Chiral metal-ion complexes for enantioseparation by capillary electrophoresis and capillary electrochromatography: a selective review

This review gives an overview about chiral separation by capillary electrophoresis and capillary electrochromatography using different chiral metal-ion complexes. The topic enantioseparation is still of big interest for chiral drugs and natural compounds. Regarding chiral drugs it is often the case that the enantiomers differ in activity. The chiral separation principle of ligand-exchange (LE) can be enabled for liquid chromatography, capillary electrophoresis as well as for capillary electrochromatography. Ligand-exchange can be applied particularly for chiral amino acids, amino alcohols or α-hydroxy acids. Examples and applications are given along with the latest developments.

Enantioseparation by MEKC using a ligand exchange-based chiral pseudostationary phase

In this paper, a new ligand-exchange -MEKC mode, based on the design of a unique lipohilic species (4'-octadecylneamine derivative), which served both as micelle-forming surfactant (by its hydrophobic part) and central ion-complexing ligand (by its hydrophilic part) is described. The CMC of the used lipophilic neamine derivative was first determined by surface tension measurements. Subsequent NMR experiments were performed in order to investigate the Cu(II) binding properties of the neamine micellar phase. The enantioseparation properties of both the octadecylneamine derivative-Cu(II) MEKC and the native neamine-Cu(II) CE systems were evaluated and compared using the tryptophan racemate as a probe analyte. The effects of several different electrophoretic conditions on the enantiomer migration behavior in the ligand-exchange-MEKC mode were examined. The developed methodology was also applied to the enantioseparation of other analytes such as 1-methyl-tryptophan, 3,5-diiodo-tyrosine and 1-naphtyl-alanine.

Chiral separations by CE employing CDs

This paper summarizes the history of chiral separations done by using electromigration methods with CDs. Several enantioresolution mechanisms and a wide number of chiral selectors have been applied to the separation of optical isomers by CE. Among them inclusion-complexation with CDs or their derivatives played a very important role in CE. Since the beginning our group was involved in studying method optimization for enantiomer resolution by using these chiral selectors. One of our publications was the basis for further development in the field, at least for us. New chiral selectors, development of theory, new methodological approaches and a wide number of practical applications are the main results achieved in the last almost 25 years using CE as an enantioseparative technique.

Validated chiral separation of M9, a Mannich ketone compound, by chiral ligand-exchange chromatography

Enantioseparation of the Mannich ketone M9, a potential antifungal compound, was examined using chiral ligand-exchange chromatography. The chiral mobile phase contained complexes of Cu(II) with the optically active selector L-aspartame (APM) and the organic modifier methanol. The separation was optimized with respect to the concentration of the Cu(II)-(L-APM) complexes, pH of mobile phase, methanol content, and column temperature. A baseline separation (R(s) = 3.08) was achieved for enantiomers of M9 under optimal conditions, and the analysis was accomplished in eleven minutes. The developed method was extensively validated. The sample stability, linearity, precision (method repeatability and intermediate precision) and accuracy, and the limits of detection and quantification of the developed method were studied. The proposed method was shown to be accurate and suitable for the quantitative determination of each enantiomer of M9.

Enantioseparation by ligand-exchange using particle-loaded monoliths: Capillary-LC versus capillary electrochromatography

Particle-loaded monoliths containing a polymethacrylamide backbone were prepared by suspending a silica-based chiral phase in the mixture of the monomers followed by in-situ polymerization in the capillary. As chiral selector l-4-hydroxyproline chemically bonded to 3 microm silica particles was used following the separation principle of ligand-exchange. Electrolytes containing Cu(II) ions were used. Amino acid enantiomers were separated by capillary-LC and CEC, whereby the latter showed the better resolution properties. For the chiral separation of alpha-hydroxy acids the EOF was reversed by copolymerizing diallyldimethylammonium chloride instead of vinylsulfonic acid as charge providing agent. Short columns of 6 cm were found to be sufficient in the case of CEC for baseline separations of amino acids with alpha values up to 5.

Ligand exchange capillary electrophoresis by cyclodextrin derivatives, a powerful tool for enantiomeric separations

Five pure CD derivatives synthesized in our laboratory were used as chiral selectors in the presence of copper(II) ion. Three enantiomeric pairs of amino acids were submitted to separation experiments in CE, by exploiting the ligand exchange mechanism. The results obtained in the investigated systems, together with those of the analogous systems previously studied, clearly show the usefulness of this technique in chiral separations. By comparing the ligand exchange CE results with potentiometric results, either reported elsewhere or studied here for the first time (system Cu/CDampy/tyrosine), it has been possible to rationalise the separation results. The importance of the availability of pure selectors, and to characterise them both spectroscopically and thermodynamically is discussed.

Mechanism of enantioseparation ofDL-pantothenic acid in ligand exchange capillary electrophoresis using a diol–borate system

Borate complexes formed in the ternary system at pH 9.2 containing borate, (S)-3-amino-1,2-propanediol (SAP), and DL-pantothenic acid (DL-PTA) were identified by 13C and 11B NMR, and it is confirmed that the binary complexes, [B(OH)2(SAP)], [B(SAP)2]+ [B(OH)2(D- or L-PTA)]2-, and [B(D- or L-PTA)2]3- (including [B(D-PTA)(L-PTA)]3-), and the ternary complexes, [B(SAP)(D- or L-PTA)]-, coexist at equilibrium in the ternary system. Thermodynamic experiments by variable-temperature 11B NMR revealed that the ternary complex, [B(SAP)(D-PTA)]-, is entropically more stable than [B(SAP)(L-PTA)]-. Because two geometrical isomers are possible for the respective ternary complexes, semi-empirical molecular orbital calculations were performed by PM5, PM3, and AM1 methods in order to obtain the optimized structures. It is indicated from the calculated heats of formation and experimentally obtained thermodynamic parameters that the (S)-isomer is more probable for the respective ternary complexes with D- and L-PTA. In the optimized structure of (S)-[B(SAP)(D-PTA)]- in water, the SAP and D-PTA ligands were oppositely oriented to form a rather linear structure, while the diastereomer, (S)-[B(SAP)(L-PTA)]-, had a folded structure. Because such a difference in the solvated structure of the ternary complexes can give a different electrophoretic velocity in CE, the enantioseparation of DL-PTA in CE is reasonably attributed to a difference in the observed electrophoretic mobility for the equilibrated ternary systems containing the respective ternary complexes.

Chiral ligand exchange micellar electrokinetic chromatography using borate anion as a central ion

Three compounds having 1,2-diol structure (1-phenyl-1,2-ethanediol, 3-phenoxy-1,2-propanediol, and 3-benzyloxy-1,2-propanediol) were enantioseparated by ligand exchange MEKC using (5S)-pinanediol (SPD) as a chiral selector and borate anion as a central ion together with SDS. When (S)-1,2-propanediol, (S)-1,2,4-butanetriol, or (S)-3-tert-butylamino-1,2-propanediol were used as the chiral ligand instead of SPD, these three compounds were not enantioseparated. When borate was replaced with 2-aminoethane-1-sulfonate or N-cyclohexyl-3-aminopropanesulfonate, no chiral separation was achieved. Therefore, the hydrophobic interaction between the chiral selector and the chiral analytes within the transient diastereomeric complex may play an important role in the enantioseparation achieved by the proposed method.

Chiral separation of halogenated amino acids by ligand-exchange capillary electrophoresis

The chiral separation of halogenated amino acids by ligand-exchange CE is described. Halogenated amino acids attracted increasing interest in recent years because of their physiological activities. Different chiral selectors, as there are L-4-hydroxyproline, L-histidine, and N-alkyl derivatives of L-4-hydroxyproline in form of their copper(II) complexes, are compared for their chiral recognition ability for halogenated amino acids. The influence of various parameters, such as selector concentration, pH, organic modifier, and field strength, on the resolution was investigated. All halogenated amino acids investigated were baseline-separated under optimized conditions.

Chiral separation based on ligand-exchange capillary electrophoresis using a copper(II)-L-ornithine ternary complex as selector

A ligand-exchange capillary electrophoresis was explored, with L-ornithine as the ligand and copper(II) as the central ion. Its applicability was demonstrated with underivatized and dansyl amino acids, a dipeptide, and drugs with amino alcohol structure. The enantioselectivity was found to be strongly dependent on pH and copper(II)-L-Orn complex concentration. Due to the adsorption of the positively charged species onto the capillary inner walls, the chiral separation selectivity is very high while the efficiency is relatively low. Permanent 1,3-propanediamine-coated capillaries show an improved separation efficiency and theoretical plate numbers increasing from 10(4) to 10(5). Similar phenomena were observed when sodium dodecyl sulfate (SDS) micelles were added to the copper(II) complex solution. The poor separation efficiency of chiral compounds in uncoated capillaries may result from the low rate of the ligand-exchange reactions, and the high enantioselectivity may derive from the complexing process in the adsorbed phase.

Coordination properties of 3-functionalized β-cyclodextrins. Thermodynamic stereoselectivity of copper(ii) complexes of the A,B-diamino derivative and its exploitation in LECE

The bis-amino AB derivative of beta-cyclodextrin on the secondary rim was synthesised and spectroscopically characterised by different techniques. Its binary systems both with protons and copper(II) were thermodynamically characterised by pH-metric potentiometry. In addition the ternary systems with each of the enantiomers of tryptophan and alanine were investigated. A thermodynamic stereoselectivity was observed for the tryptophan enantiomers and this was exploited to separate them by capillary electrophoresis through a ligand exchange mechanism (LECE). LECE separation of racemates of phenylalanine and tyrosine was also obtained.

Enantioselective ligand exchange in modern separation techniques

As a follow-up to a series of review articles on enantioselective ligand exchange chromatography, the present contribution critically evaluates achievements in this area of active and successful research which have been reported in the scientific since 1992. Also discussed is enantioselective ligand exchange in electromigration techniques which have developed especially fruitfully during the last decade.

Influence of structure and chirality of the selector on the chiral recognition of amino acids using ligand-exchange capillary electrophoresis

The dependence of the chiral recognition ability and enantiomer migration order on the structure, substitution pattern and chirality of chiral selectors used in ligand-exchange capillary electrophoresis is investigated. As chiral selectors different N-alkyl derivatives of proline and hydroxyproline as their copper(II) complexes are used. The influence of the position and conformation of the hydroxy group in the hydroxyproline derivatives and of the structure and chirality of the side chain on enantioselectivity is investigated. Furthermore, the effect of surfactants such as sodium dodecyl sulfate and cetyl trimethyl ammonium bromide on resolution and enantiomer migration order is studied. The investigations were carried out with three aromatic amino acids as model compounds.

Direct chiral resolution of tartaric acid in food products by ligand exchange capillary electrophoresis using copper(II)-D-quinic acid as a chiral selector

Chiral resolution of native DL-tartaric acid was performed by ligand-exchange capillary electrophoresis using copper(II)-D-quinic acid as a chiral selector. Factors affecting chiral resolution, migration time, and peak area of tartaric acid were studied. The running conditions for optimum separation of tartaric acid were found to be 1 mM copper(II) sulfate-10 mM D-quinic acid (pH 5.0) with an effective voltage of -15 kV at 30 degrees C, using direct detection at 250 nm, and resolution of racemic tartaric acid was approximately 1.3. With this system, chiral resolution of DL-tartaric acid in food products was conducted successfully.

Chiral separation by chromatographic and electromigration techniques. A review

This review gives a survey of different chiral separation principles and their use in high-performance liquid chromatography (HPLC), gas chromatography (GC), supercritical fluid chromatography (SFC), thin-layer chromatography (TLC), capillary electrophoresis (CE) and capillary electrochromatography (CEC) highlighting new developments and innovative techniques. The mechanisms of the different separation principles are briefly discussed and some selected applications are shown.

Direct chiral resolution of malic acid in apple juice by ligand-exchange capillary electrophoresis using copper(II)-L-tartaric acid as a chiral selector

Chiral resolution of native DL-malic acid was achieved by ligand-exchange capillary electrophoresis using copper(II)-L-tartrate as a chiral selector. Factors affecting chiral resolution, migration time, and peak area of malic acid were studied. The running conditions for optimum separation of malic acid were found to be 1 mM copper(II) sulfate-1 mM L-tartrate (pH 5.1) with an effective voltage of -20 kV at 30 degrees C, using direct detection at 280 nm, and resolution (Rs) of racemic malic acid was approximately 4. With this system, D- and L-malic acids in apple juice were analyzed successfully.

Chiral ligand-exchange capillary electrophoresis

This review summarizes the application of capillary electrophoresis and capillary electrochromatography for the chiral separation of various substance classes using the principle of ligand exchange. The application of this principle to various substance classes is reported.

Enantioseparation of hydroxy acids on easy-to-prepare continuous beds for capillary electrochromatography

This paper deals with the enantioseparation of hydroxy acids by ligand-exchange capillary electrochromatography. A chiral continuous bed was easily prepared by in situ polymerization of monomers, including an L-4-hydroxyproline derivative. This phase showed chiral recognition for several hydroxy acids, in addition to amino acids.

Inspection of the reversal of enantiomer migration order in ligand exchange micellar electrokinetic capillary chromatography

Enantiometers of D,L-phenylalanine were separated by capillary electrophoresis based on the principle of ligand exchange. Copper (II) complex of 4-hydroxy-L-proline was used as chiral selector. The separation and the migration order of D- and L-phenylalanine were strongly affected by adding an anion surfactant sodium dodecylsulfate (SDS). Without SDS in the electrolyte, the separation was also carried out but the resolution was very small. With SDS added into the electrolyte, the resolution decreased with increasing concentration of SDS until 5.0 mM. When the concentration of SDS in the electrolyte was over 5.0 mM, inversion of the migration order of DL-phenylalanine was observed and the resolution was also increased with increasing concentration up to 20 mM. It was interesting to find that the inversion of the migration order took place not only in the enantioscparation but also in the positional isomers. A family of a fluorinated amino acid, o-, m- and p-fluoro-D,L-phenylalanine was separated and the inversion of the migration order is discussed.

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.

Application of ligand-exchange capillary electrophoresis to the chiral separation of alpha-hydroxy acids and beta-blockers

The application of the principle of ligand-exchange capillary electrophoresis to two substance classes is described. As chiral selector N-(2-hydroxyoctyl)-L-4-hydroxyproline-copper(II) complex was used. This principle was applied to the chiral separation of alpha-hydroxy acids and drugs containing amino alcohol structure such as beta-blockers. The enantioselectivity was found to be strongly dependent on pH corresponding to the optimal conditions for complex formation for each structure class.

Chiral separation of unmodified amino acids with non-aqueous capillary electrophoresis based on the ligand-exchange principle

A ligand exchange mechanism in non-aqueous capillary electrophoresis was employed for the separation of eight unmodified amino acids using chiral complexes of copper(II) with L-proline and L-isoleucine. The electrophoretic medium consisted of 25 mM ammonium acetate and 1 M acetic acid in methanol. We were able to completely separate the enantiomeric pairs of each of the investigated racemic amino acids. We also report the optimization of the separation parameters, such as pH*, composition of the complex, and concentration of the complexing agents.

Chiral separations in capillary electrophoresis

The marked increase in the number of communications on the utilization of electrophoresis for practical chiral separations within the last three years is the most evident, and the most important fact. It reveals that the basic period of intensive research in the field is finished. The search for chiral selectors discriminating racemates in a reasonably analytical manner and the study of both the mechanism and physicochemical aspects of the chiral discrimination process were the main features of that period. Here, we review the state of the art in the field and state the references of the related literature up to the end of 1998.

Chiral separation of sympathomimetics by ligand exchange capillary electrophoresis

A rapid and simple approach to the chiral separation of sympathomimetic drugs with amino alcohol structure by ligand exchange capillary electrophoresis is described. An N-(2-hydroxyoctyl)-L-4-hydroxyproline/copper(II) complex is used as chiral selector. Thirteen sympathomimetics were resolved, nine with baseline resolution. The influence of pH and composition of the electrolyte on resolution was investigated. The optimal pH for complexation of these amino alcohols was found to be 12.

Capillary electrophoresis enantioseparation of drugs using beta-cyclodextrin polymer: intramolecular synergistic effect

Enantiomers of eight basic compounds of pharmaceutical interest were studied by capillary electrophoresis (CE) using a soluble cyclodextrin (CD) polymer and native CD as chiral selectors. The effects of the polymer concentration and the pH of the buffer solution on resolution were examined. It was found that the beta-CD polymer shows higher stereoselectivity than its parent cyclodextrin. Such improvement is due to the structural difference between beta-CD and beta-CD polymer. These results suggested the existence of an intramolecular synergistic effect in CE.

Separation of chiral compounds by capillary electrophoresis

This review presents the different chiral selectors used in capillary electrophoresis (CE) for the separation of enantiomers. The use of charged cyclodextrins, crown ethers, polysaccharides, proteins, natural and synthetic micelles, macrocyclic antibiotics and ergot alkaloids is discussed in detail. Neutral native and derivatized cyclodextrins are not treated because several review articles have already been published on this topic. Recent developments like the application of two chiral selectors in the same background electrolyte are highlighted.