Martin G. Schmid

Chiral separation principles in capillary electrophoresis

Capillary electrophoretic techniques for the separation of enantiomers are reviewed. The techniques used for chiral separation include capillary zone electrophoresis, electrokinetic chromatography, isotachophoresis, capillary gel electrophoresis and capillary electrochromatography. The separation principles and the chiral recognition mechanism are discussed and a comprehensive collection of applications to drugs and other compounds of interest is given in tables.

Chiral separation by capillary electromigration techniques

This review gives an overview of chiral separation principles and their applications in capillary electromigration techniques. The basic chiral separation principles are explained and the mechanisms discussed. Recent developments and new techniques in CZE and capillary electrochromatography (CEC) are highlighted. New chiral selectors among cyclodextrins, crown ethers, carbohydrates, macrocyclic antibiotics, proteins, chiral ion-pairing reagents, chiral surfactants and chiral metal ion complexes and their chiral recognition ability are discussed. Recent advances in chip technology for chiral separation and new approaches regarding improvement of detection sensitivity are presented. Due to the tremendous number of publications dealing with applications, in this review only recent applications are summarized.

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.

Chiral separation of amino acids by ligand‐exchange capillary electrochromatography using continuous beds

A chiral ligand‐exchange phase for capillary electrochromatography based on continuous bed technology was developed. The chiral stationary phase is prepared by a one‐step in situ copolymerization procedure using methacrylamide, piperazine diacrylamide, vinylsulfonic acid and N‐(2‐hydroxy‐3‐allyloxypropyl)‐L‐4‐hydroxyproline. These chiral continuous beds are inexpensive and easy to prepare. They also have several advantages over silica‐based packed capillaries. Since the bed is covalently attached to the capillary wall, no frit is required. The applicability of this new approach to the chiral separation of underivatized amino acids is demonstrated.

A new easy-to-prepare homogeneous continuous electrochromatographic bed for enantiomer recognition

Completely homogeneous polyacrylamide‐based gels were used for capillary electrochromatography (CEC) of drug enantiomers. Like continuous beds (also called continuous polymer rods, silica rods, monoliths) they do not require frits to support the bed because it is covalently linked to the capillary wall. A long lifetime is an important feature of the beds. The gel matrices can be prepared in any laboratory and for specific interactions they can be derivatized with appropriate ligands. The application range is, therefore, broad. For chiral electrochromatography, negatively and positively charged polyacrylamide gels copolymerized with 2‐hydroxy‐3‐allyloxy‐propyl‐β‐cyclodextrin (allyl‐β‐CD) were prepared. The latter monomer was synthesized from β‐CD and allylglycidyl ether by a very simple one‐step procedure. Eight acidic, neutral and basic drug compounds were resolved into their enantiomers, most of them with baseline separation. Interestingly, the resolution is independent of the electroendosmotic velocity, i.e., rapid analyses will not give low resolution. Upon increasing this velocity, the plate height for the fast enantiomer did not change (or decreased slightly), whereas that for the slow enantiomer increased. Only the last term in the van Deemter equation contributed significantly to the total plate height. The composition of the gel was chosen such that the „pores”︁ became large enough to guarantee a satisfactory electroendosmotic flow (EOF). This open gel structure explains why acetone diffused as in free solution, i.e., independently of the presence of the gel matrix. This finding also indicates that the separation of small molecules in polyacrylamide gels cannot be explained by „molecular‐sieving”︁, but rather by some type of adsorption (”︁aromatic adsorption”︁?).

Chiral Separation Principles in Chromatographic and Electromigration Techniques

Almost half of the drugs in use today are chiral. It is well established that the pharmacological activity is mostly restricted to one of the enantiomers (eutomer). There can be qualitative and quantitative differences in the activity of the enantiomers. In many cases, the inactive enantiomer (distomer) shows unwanted side effects or even toxic effects. Even if the side effects are not that drastic, the distomer has to be metabolized and this represents an unnecessary burden for the organism. Therefore, the development of methods for the separation of enantiomers, both on analytical and preparative scale, has become increasingly important.Chromatographic techniques such as thin layer chromatography (TLC), gas chromatography (GC), supercritical fluid chromatography (SFC), and above all high-performance liquid chromatography (HPLC) have been used for enantiomer separation for about two decades. More recently, electromigration techniques, such as capillary electrophoresis and capillary electrochromatography, have been shown to be powerful alternatives to chromatographic methods. This review gives a short overview of different chiral separation principles and their application. Several new developments are discussed.

Capillary zone electrophoretic separation of the enantiomers of dipeptides based on host-guest complexation with a chiral crown ether

Abstract The enantiomeric separation of racemic glycyldipeptides and diastereomeric dipeptides by using capillary zone electrophoresis and (+)-18-crown-6-tetracarboxylic acid (18C6H4) as a chiral selector added to the electrolyte is described. The separation of dipeptides with two stereogenic centres into four peaks by using capillary zone electrophoresis is reported for the first time. Chiral discrimination is attributed to the formation of a diastereomeric host-guest complex which leads to different interactions for each enantiomer. Owing to the differences in stability of the complexes, the four optical isomers elute at different migration times, allowing the chiral separation. The influence of buffer composition, crown ether concentration and the addition of organic modifiers was studied. All glycyldipeptides were resolved and for most of the diastereomeric dipeptides four baseline-separated peaks were observed.

Application of ligand-exchange capillary electrophorersis to the chiral separation of α-hydroxy acids and β-blockers

Abstract 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 α-hydroxy acids and drugs containing amino alcohol structure such as β-blockers. The enantioselectivity was found to be strongly dependent on pH corresponding to the optimal conditions for complex formation for each structure class.

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 by chromatographic and electromigration techniques using ligand-exchange as chiral separation principle

This article gives a short overview of the application of the principle of chiral ligand-exchange in HPLC, CE, and CEC. Since its introduction by Davankov, more than thousand articles have appeared in this field. Citing all these papers would extend the scope of this review—it would fill several books. Therefore only some milestones are mentioned in this article and it will focus on our own activities in this field. Some new developments are mentioned, and selected biochemical and biomedical application are presented.