Single isomer cyclodextrins as chiral selectors in capillary electrophoresis

Since decades, cyclodextrins are one of the most powerful selectors in chiral capillary electrophoresis for the enantioseparation of diverse organic compounds. This review concerns papers published over the last decade (from 2009 until nowadays), dealing with the capillary electrophoretic application of single isomer cyclodextrin derivatives in chiral separations. Following a brief overview of their synthetic approaches, the inventory of the neutral, negatively and positively charged (including both permanently ionic and pH-tunable ionizable substituents) and zwitterionic CD derivatives is presented, with insights to underlying structural aspects by NMR spectroscopy and molecular modeling. CE represents an ideal tool to study the weak, non-covalent supramolecular interactions. The published methods are reviewed in the light of enantioselectivity, enantiomer migration order and the fine-tuning of enantiodiscrimination by the substitution pattern of the single entity selector molecules, which is hardly possible for their randomly substituted counterparts. All the reviewed publications herein support that cyclodextrin-based chiral capillary electrophoresis seems to remain a popular choice in pharmaceutical and biomedical analysis.

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.

Evaluation of newly synthesized and commercially available charged cyclomaltooligosaccharides (cyclodextrins) for capillary electrokinetic chromatography

A highly new charged cyclodextrin (CD) derivatives, (6-O-carboxymethyl-2,3-di-O-methyl)cyclomaltoheptaoses (CDM-beta-CDs), was synthesized and characterized as anionic reagents for capillary electrophoresis (CE) in an electrokinetic chromatography mode of separation. Substitution with dimethyl groups at the secondary hydroxyl sites of the CD is aimed at influencing the magnitude and selectivity of analyte-CD interactions, while substitution by carboxymethyl groups at the primary hydroxyl sites provides for high charge and electrophoretic mobility. Full regioselective methylation at the secondary hydroxyl sites was achieved in this work, while substitution at the primary hydroxyl sites generated a mixture of multiply charged products. The separation performance of CDM-beta-CD was evaluated using a variety of analyte mixtures. The results obtained from commercially available negatively charged cyclodextrins, heptakis(2,3-di-O-methyl-6-O-sulfo)cyclomaltoheptaose (HDMS-beta-CD) and O-(carboxymethyl)cyclomaltoheptaose (CM-beta-CD) with an average degree of substitution one (DS 1), were compared to CDM-beta-CD using a sample composed of eight positional isomers of dihydroxynaphthalene. Four hydroxylated polychlorobiphenyl derivatives, a group of chiral and isomeric catchecins, and chiral binaphthyl compounds were also separated with CDM-beta-CD. The effect of adding neutral beta-cyclodextrin (beta-CD) into the running buffer containing charged cyclodextrins was investigated and provided evidence of significant inter-CD interactions. Under certain running buffer conditions, the charged cyclodextrins also appear to adsorb to the capillary walls to various degrees.

Electrophoretic enantiomer separations at high pH using the new, single-isomer octakis(2,3-dimethyl-6-O-sulfo)-gamma-cyclodextrin as chiral resolving agent

The latest, single-isomer, sulfated gamma-cyclodextrin, the sodium salt of octakis(2,3-dimethyl-6-O-sulfo)-gamma-cyclodextrin that is stable in basic media was used to separate the enantiomers of neutral, weak acid and weak base analytes by capillary electrophoresis in high pH aqueous background electrolytes. The effective mobilities and separation selectivities were found to follow trends similar to those observed earlier in acidic aqueous background electrolytes. Octakis(2,3-dimethyl-6-O-sulfo)-gamma-cyclodextrin proved to interact with all three analyte types less strongly than other single-isomer sulfated cyclodextrins do under comparable conditions.

Capillary electrophoretic separation of enantiomers in a high-pH background electrolyte by means of the single-isomer chiral resolving agent octa(6-O-sulfo)-gamma-cyclodextrin

The new, alkali-stable, single-isomer, sulfated gamma-cyclodextrin, the sodium salt of octa(6-O-sulfo)-gamma-cyclodextrin (OS) was used for the first time to separate the enantiomers of non-ionic, acidic, basic and ampholytic analytes by capillary electrophoresis in high-pH aqueous background electrolytes. The effective mobilities and separation selectivities were found to follow trends similar to those observed earlier in acidic aqueous background electrolytes. OS proved to be a broadly applicable chiral resolving agent and afforded adequate peak resolution values with short separation times for a number of non-ionic, weak acid, weak base and ampholytic analytes.

Recent developments in chiral capillary electrophoresis and applications of this technique to pharmaceutical and biomedical analysis

This paper provides an overview of the current status of chiral capillary electrophoresis (CE). The emphasis is placed on the application of CE in chiral separation of various racemic compounds. During the last two years about 280 papers, several review articles, and two entire issues, edited by S. Fanali (Electrophoresis 1999, 20, 2577-2798, and H. Nishi and S. Terabe (J. Chromatogr. A 2000, 879, 1-471.) have been devoted to chiral CE. Enantiomeric separations of various compounds, e.g., pharmaceuticals, drug candidates, drugs and related metabolites in biological fluids, amino acids, di- and tri peptides, pesticides and fungicides, have been performed using different chiral selectors. Native and derivatized cyclodextrins continue to be the most widely used chiral selectors. Other chiral selectors such as natural and synthetic chiral micelles, crown ethers, chiral ligands, proteins, oligo- and polysaccharides, and macrocyclic antibiotics have also been applied to chiral CE separations.

Fundamental aspects of chiral separations by capillary electrophoresis

A review is presented that surveys the basic theory of direct separation of enantiomers by capillary electrophoretic (CE) techniques. These separations are based on the formation of diastereomeric complexes between the enantiomeric analytes and a chiral selector added to the electrolyte solution. The review covers a comprehensive treatment of the equations needed for optimization of selectivity coefficients, resolution and analysis time in the zone electrophoretic mode. In this context, it takes into account combined equilibria of complexation and protonation/deprotonation as well as complexation and paritition into micelles. On the basis of these equations, the benefits of charged selectors and the optimization potential inherent to pH tuning can be documented. In addition, the review deals with some basic aspects of chiral isoelectric focusing and briefly discusses indirect enantioseparation. In a subsequent section a survey is given on particularfeatures of the various types of chiral selectors. Finally, the recent developments in preparative enantioseparation in continuous free-flow system and by use of isoelectric membranes are discussed.

Mechanistic study on the opposite migration order of clenbuterol enantiomers in capillary electrophoresis with beta-cyclodextrin and single-isomer heptakis(2,3-diacetyl-6-sulfo)-beta-cyclodextrin

Opposite migration order was observed for the enantiomers of the chiral beta2-adrenergic drug clenbuterol (CL) in capillary electrophoresis (CE) when resolved with native beta-cyclodextrin (beta-CD) and heptakis (2,3-diacetyl-6-sulfo)-beta-CD (HDAS-beta-CD). The possible mechanisms of the affinity reversal of the CL enantiomers depending on the structure of the CD were studied using 1H-nuclear magnetic resonance (1H-NMR) spectrometry and one-dimensional rotating frame nuclear Overhauser and exchange spectrometry (1-D ROESY). Significant differences were observed between the structure of the (+/-)-CL complexes with beta-CD and HDAS-beta-CD.

Separation selectivity patterns in the capillary electrophoretic separation of anionic enantiomers by octakis-6-sulfato-gamma-cyclodextrin

The capillary electrophoretic separation of anionic enantiomers with multiply-charged, single-isomer, anionic resolving agents was reexamined with the help of the charged resolving agent migration model. Three general model parameters were identified that influence the shape of the separation selectivity and enantiomer mobility difference curves: parameter b, the binding selectivity (K(RCD)/K(SCD)), parameter s, the size selectivity (micro0RCD/micro0SCD), and parameter a, the complexation-induced alteration of the analyte's mobility (micro0SCD/micro0). Function analysis of the model indicates that there are six unique separation selectivity vs. resolving agent concentration patterns: in two of the patterns, separation selectivity asymptotically increases to the limiting value set by parameter b; in two other patterns, separation selectivity passes a local maximum and asymptotically decreases to the limiting value set by parameter b; and in the last two patterns, separation selectivity passes a local maximum, decreases to unity, then, after reversal of the intrinsic migration order, asymptotically increases to the limiting value set by parameter b. Though the patterns with asymptotically increasing selectivities were observed in earlier work, this paper reports the first experimental verification of the existence of the local selectivity maximum during the capillary electrophoretic separation of the enantiomers of several weak acids in high pH background electrolytes with octakis-6-sulfato-gamma-cyclodextrin as the resolving agent.

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.

Capillary electrophoretic separation of the enantiomers of organophosphates with a phosphorus stereogenic center using the sodium salt of octakis(2,3-diacetyl-6-sulfo)-gamma-cyclodextrin as resolving agent

The sodium salt of the single-isomer, chiral resolving agent, octakis(2,3-diacetyl-6-sulfo)-gamma-cyclodextrin (ODAS-gammaCD) has been used for the capillary electrophoretic separation of the enantiomers of alkylarylphosphates which carry a phosphorus-based stereogenic center. The effective mobilities and separation selectivities were measured at different ODAS-gammaCD and methanol concentrations to find the conditions under which the minor enantiomers could be adequately quantitated in samples obtained by chemical resolution of the racemic mixtures. This work extends the utility of ODAS-gammaCD to a hitherto unexplored field, the capillary electrophoretic separation of the enantiomers of organophosphorus compounds.