Cyclodextrins, modified, chiral separation using

Lin et al. [95] used capillary electrophoresis with dual cyclodextrin systems for the enantiomer separation of miconazole. A cyclodextrin-modified micellar capillary electrophoretic method was developed using mixture of /i-cyclodextrins and mono-3-0-phenylcarbamoyl-/j-cyclodextrin as chiral additives for the chiral separation of miconazole with the dual cyclodextrins systems. The enantiomers were resolved using a running buffer of 50 mmol/L borate pH 9.5 containing 15 mmol/L jS-cyclodextrin and 15 mmol/L mono-3-<9-phcnylcarbamoyl-/j-cyclodextrin containing 50 mmol/L sodium dodecyl sulfate and 1 mol/L urea. A study of the respective influence of the /i-cyclodcxtrin and the mono-3-(9-phenylcarbamoyl-/i-cyclodextrin concentration was performed to determine the optical conditions with respect to the resolution. Good repeatability of the method was obtained. 

Anions and uncharged analytes tend to spend more time in the buffered solution and as a result their movement relates to this. While these are useful generalizations, various factors contribute to the migration order of the analytes. These include the anionic or cationic nature of the surfactant, the influence of electroendosmosis, the properties of the buffer, the contributions of electrostatic versus hydrophobic interactions and the electrophoretic mobility of the native analyte. In addition, organic modifiers, e.g. methanol, acetonitrile and tetrahydrofuran are used to enhance separations and these increase the affinity of the more hydrophobic analytes for the liquid rather than the micellar phase. The effect of chirality of the analyte on its interaction with the micelles is utilized to separate enantiomers that either are already present in a sample or have been chemically produced. Such pre-capillary derivatization has been used to produce chiral amino acids for capillary electrophoresis. An alternative approach to chiral separations is the incorporation of additives such as cyclodextrins in the buffer solution. 

Based on the theory, the separation of enantiomers requires a chiral additive to the CE separation buffer, while diastereomers can also be separated without the chiral selector. The majority of chiral CE separations are based on simple or chemically modified cyclodextrins. However, also other additives such as chiral crown ethers, linear oligo- and polysaccharides, macrocyclic antibiotics, chiral calixarenes, chiral ion-pairing agents, and chiral surfactants can be used. Eew non-chiral separation examples for the separation of diastereomers can be found. 

Capillary gas chromatography (GC) using modified cyclodextrins as chiral stationary phases is the preferred method for the separation of volatile enantiomers. Fused-silica capillary columns coated with several alkyl or aryl a-cyclo-dextrin, -cyclodextrin and y-cyclodextrin derivatives are suitable to separate most of the volatile chiral compounds. Multidimensional GC (MDGC)-mass spectrometry (MS) allows the separation of essential oil components on an achiral normal phase column and through heart-cutting techniques, the separated components are led to a chiral column for enantiomeric separation. The mass detector ensures the correct identification of the separated components [73]. Preparative chiral GC is suitable for the isolation of enantiomers [5, 73]. 

Chiral mobile-phase modifiers 133 Separation of dansylated amino acids using /3-cyclodextrin in mobile phase and reversed-phase column... 

The FITC labeling method was also applied to chiral separations of amino acids on a microchip to determine the enantiomeric ratios of amino acids found on a meteorite [27], Since biotic amino acids are normally single enantiomers, chiral separations of amino acids are not truly clinical in nature, but illustrate the potential for chiral separations of small molecules of clinical interest. Ma-thies and co-workers used this technique to search for evidence of life in extraterrestrial environments. Enantiomeric forms of Val, Ala, Glu, and Asp could be discriminated by addition of a-, (3-, or y-cyclodextrin (CD) to the run buffer. Improved resolution with faster separations was found with respect to conventional CE. This method has been modified, by addition of SDS to the buffer, to perform cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) [28]. Increasing the SDS concentration decreased the magnitude of elec-troosmotic flow (EOF), increasing the effective migration distance, and therefore the resolution on the microchips. 

CE is playing a major role in the separation of chiral compounds, a field that is gaining increasing attention in pharmaceutical sciences as well as in forensic toxicology (Lurie, 1994 Novotny et al., 1994 Ward, 1994). The chirally active selectors used in CE include optically active complexes such as Cu(II)-l-histidine, Cu(II)-aspartame, cyclodextrins, modified CDs, bile salts, crown ethers, and proteins (bovine serum albumin, aracid glycoprotein, etc.). 

The first separations of enantiomers in GC on cyclodextrin modified column were carried out by Sybilska et al. in 1983 [5], They applied a formamide solution of a-cyclodextrin as a stationary phase in the classical packed column. The column allowed an efficient separation of chiral monoterpenes - a- and P-pinenes into enantiomers. This system of using CDs in GC is characterised by obtaining high enantioselectivity factors, so enantioseparation is still possible for receiving not very efficient packed columns. Unfortunately, the columns appeared to be not very stable at higher temperatures. 

Fig. 2 Influence of cyclodextrin type on the chiral resolution of the basic drug tocainide and related substances using 40 mM sodium phosphate (pH 3.0) containing (a) 20 mM -CD and (b) 50 mM methyl- -CD and 50 mM heptakis (2,6 di-O-methyl-jS-CD. [Adopted with kind permission from D. Beider and G. Schomburg, Chiral separations of basic and acidic compounds in modified capillaries using cyclodextrin-modified capillary zone electrophoresis, J. Chromatogr. A 666 351 (1994).]...

Fig. 4 Schematic representation of the interaction of a representative drug substance with cyclodextrin. A wide range of noncharged and charged (anionic, cationic, and amphoteric) cyclodextrins have been used as chiral selectors, as well as for the optimized separation of nonchiral compounds using capillary electrophoresis. Cationic and amphoteric cyclodextrins are less commonly used in chiral analysis, and only a few are commercially available. The degree of substitution of a cyclodextrin 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. (Modified from Ref. 169.)...

Methods development for chiral analyses has been one of the most challenging separation problems for the analytical chemist in the pharmaceutical industry. Racemic drug substances have a variety of chemical structures and several chiral selectors are available for the analyst to choose in order to obtain the enan-tioselectivity needed for chiral resolution. To alleviate this problem, a fast capillary electrophoresis procedure for the enantiomeric separation of acidic and basic compounds using native and modified cyclodextrins has been described (200). The technique is called cyclodextrin array chiral analysis. A generalized optimi-... 

Nielsen, M. W. F. Chiral separation of basic drugs using cyclodextrin-modified capillary zone electrophoresis. 

Tesaf ova et al. [87] used a modified version of SIMUL, which they called SIMULMIC to simulate the separation of neutral analytes in a system with a neutral cyclodextrin and anionic micelles. A number of systems were examined in which various combinations of the inlet and outlet vials and the capillary itself were filled with cyclodextrin. Simulation results were used to examine the micellar/cyclodextrin boundary at various times and concentrations although no simulation results for a chiral separation were reported. To the best of our knowledge, this is the only dynamic simulation of an electrokinetic chromatography (EKC) separation to date. 

B, C and D are from different genetically modified soya bean oils, and Chromatogram E is from an unmodified soya bean oil. This example shows the use of the unique character of the cyclodextrin phase for separating materials that are neither chiral, or of such a size that inclusion plays a significant part in the retention mechanism. 

Since micellar electrokinetic chromatography (MEKC) was first introduced in 1984, it has become one of major separation modes in capillary electrophoresis (CE), especially owing to its applicability to the separation of neutral compounds as well as charged ones. Chiral separation is one of the major objectives of CE, as well as MEKC, and a number of successful reports on enantiomer separations by CE and MEKC has been published. In chiral separations by MEKC, the following two modes are normally employed (a) MEKC using chiral micelles and (b) cyclodextrin (CD)-modified MEKC (CD/MEKC). 

The enantiomers of azepinoindole and its 7V-desmethyl metabolites were well resolved on a 8-cyclodextrin-modified silica column (A = 231 run). A 1/99 r-butyl alcohol/water (10 mM sodium phosphate buffer at pH 7 with 15 mM jS-cyclodextrin and 2 mM triethylamine) mobile phase was used to generate the chiral separation. The assay calibration range covered 100-500 ng/mL. Peaks were somewhat tailed. Elution was complete in 25 min [584]. 

A few reports are available on chiral separations of pollutants using this modality of liquid chromatography. The separated chiral pollutants are 2-(2-chlorophenoxy)propionic acid and 2-(4-chlorophenoxy)propionic acid on n-alkyl-)8-D-glucopyranoside [17], ibuprofens on vancomycin [18] and PCBs on y-cyclodextrin [19]. Marina etal. [20] reported chiral separations of polychlorinated biphenyls (PCBs) 45, 84, 88, 91, 95, 132, 136, 139, 149, 171, 183 and 196 by MEKC using cyclodextrin chiral selectors. Mixtures of and y-cyclodextrins were used as chiral modifiers in a 2-(yV-cyclohexylamino)ethanesulfonic acid (CHES) buffer containing urea and sodium dodecyl sulfate (SDS) micelles. A mixture of PCBs 45, 88, 91, 95, 136, 139, 149 and 196 was separated into all 16 enantiomers in an... 

Jiang T-F, Lv Z-H, Wang Y-H (2007) Chiral separation of ephedrine alkaloids from Ephedra sinica extract and its medicinal preparation using cyclodextrin-modified capillary zone electrophoresis. J Anal Chem 62 85-89. doi 10.1134/S1061934807010170... 

Nidsen, M. W. E "Chiral Separations of Basic Drugs Using Cyclodextrin-modified Capillary Zone Electrophoresis." Analytical Chemistry, 651993,885-893. 

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