Chiral compounds capillary electrophoresis

Enantioresolution in capillary electrophoresis (CE) is typically achieved with the help of chiral additives dissolved in the background electrolyte. A number of low as well as high molecular weight compounds such as proteins, antibiotics, crown ethers, and cyclodextrins have already been tested and optimized. Since the mechanism of retention and resolution remains ambiguous, the selection of an additive best suited for the specific separation relies on the one-at-a-time testing of each individual compound, a tedious process at best. Obviously, the use of a mixed library of chiral additives combined with an efficient deconvolution strategy has the potential to accelerate this selection. 

Liu, L., Nussbaum, M.A. Systematic screening approach for chiral separations of basic compounds by capillary electrophoresis with modihed cyclodextrins. J. Pharm. Biomed. An. 1999,19, 679-694. 

Chiral separation of drng molecules and of their precursors, in the case of synthesis of enantiomerically pure drugs, is one of the important application areas of HPLC in pharmaceutical analysis. Besides HPLC, capillary electrophoresis (CE) is another technique of choice for chiral separations. Chapter 18 provides an overview of the different modes (e.g., direct and indirect ones) of obtaining a chiral separation in HPLC and CE. The direct approaches, i.e., those where the compound of interest is not derivatized prior to separation, are discussed in more detail since they are cnrrently the most frequently used techniques. These approaches require the use of the so-called chiral selectors to enable enantioselective recognition and enantiomeric separation. Many different molecnles have been nsed as chiral selectors, both in HPLC and CE. They can be classified into three different groups, based on their... 

The aim of this chapter is to give an overview of chiral separations of pharmaceutical compounds by means of HPLC. Capillary electrophoresis, which is the most popular technique besides HPLC for performing chiral separations at the analytical level, will also be briefly discussed. A second reason to discuss chiral separation in CE in short is the large overlap in the chiral selectors applied in both techniques. 

Liu, L., Osborne, L. M., and Nussbaum, M. A. (1996). Development and validation of a combined potency assay and enantiomeric purity method for a chiral pharmaceutical compound using capillary electrophoresis. J. Chromatogr. A 745(1—2), 45 —52. 

Williams, R. C., Edwards, J. R, and Ainsworth, C. R. (1994). Analysis of diastereoisomer impurities in chiral pharmaceutical compounds by capillary electrophoresis. Chromatographia 38, 441-446. 

Rudaz, S., Calleri, E., Geiser, L., Gherkaoui, S., Prat, J., and Veuthey, J. L. (2003). Infinite enantiomeric resolution of basic compounds using highly sulfated cyclodextrin as chiral selector in capillary electrophoresis. Electrophoresis 24, 2633—2641. 

S Fanali, G Caponecchi, Z Aturki. Enantiomeric resolution by capillary zone electrophoresis use of pepsin for separation of chiral compounds of pharmaceutical interest. J Microcolumn Sep 9 9—14, 1997. 

Sensors for the detection of enantiomers are of great interest, as so far the on-line monitoring of production processes and medical diagnostics using standard chemical analytical methods is not possible. Quite often only one enantiomer of a chiral compound is actually a bioactive therapeutic. Therefore a proper analysis of the final product is essential. Currently, this involves separation techniques like liquid chromatography, GC and capillary electrophoresis, and determination of enantiomeric purity with circular dichro-ism and specific rotation. These are all off-line procedures and therefore no real-time analysis can be performed. Sensing devices for the distinction of different enantiomers would be a much cheaper, faster and easier-to-use alternative for this task, amenable to automation. 

The chiral recognition mechanisms in NLC and NCE devices are similar to conventional liquid chromatography and capillary electrophoresis with chiral mobile phase additives. It is important to note here that, to date, no chiral stationary phase has been developed in microfluidic devices. As discussed above polysaccharides, cyclodextrins, macrocyclic glycopeptide antibiotics, proteins, crown ethers, ligand exchangers, and Pirkle s type molecules are the most commonly used chiral selectors. These compounds... 

New developments in analytical chemistry in the coming years will undoubtedly elicit new separation techniques that will allow easier isolation of methyl-branched hydrocarbons, irrespective of the branching position or array of unsaturated hydrocarbons often found in complex mixtures. Since chiral columns for separation of long-chain hydrocarbons do not yet exist, other separation techniques such as capillary electrophoresis or LC-GC-MS with the possibility of trapping compounds or classes of compounds could soon become available. Such separation and recovery methods will certainly allow better behavioral experimentation with isolated natural compounds/mixtures, an essential step to enable the acquisition of causative evidence to support the current correlative data. 

The two forms of capillary array electrophoresis are emerging as powerful methods for the determination of enantiomeric purity of chiral compounds in a truly high-through-put manner. Various modifications are possible, for example, detection systems based on UV/Vis, MS, or electrical conductivity. Moreover, chiral selectors in the CE electrolyte are not even necessary if the mixture of enantiomers is first converted into diastereo-mers, for example, using chiral fluorescent-active derivatization agents [51,57]. 

Capillary electrophoresis has been used for the analysis of chiral pollutants, e.g., pesticides, polynuclear-aromatic hydrocarbons, amines, carbonyl compounds, surfactants, dyes, and other toxic compounds. Moreover, CE has also been utilized to separate the structural isomers of various... 

Fig. 1 Flow diagram indicating starting point and optimization procedure for enantiomeric separation of basic, acidic, and neutral compounds. [Adopted with kind permission from M. M. Rogan and K. D. Altria, Introduction to the theory and applications of chiral capillary electrophoresis, Beckman primer Vol. IV, p. 22, Part No. 726388.]...

Since micellar electrokinetic chromatography (MEKC) was hrst 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)-modilied MEKC (CD-MEKC ... 

Marina, M.L. Crego, A.L. Capillary electrophoresis A good alternative for the separation of chiral compounds of environmental interest. J. Liq. Chromatogr. Relat. Technol. 1997, 20, 1337-1365. 

In this context, there has been a considerable development of enantioselective synthetic methodologies, which have now reached a high degree of diversity and complexity. Simultaneously, this trend has created an intensive demand for stereoselective separation techniques and analytical assays for precise determination of the enantiomeric purity of chiral compounds. The development of chiral stationary phases (CSPs) or chiral selectors for gas chromatography (GC), hquid chromatography (LC) and capillary electrophoresis (CE) rapidly opened a new dimension in the area of separation technologies. 

Electrophoretic methods are widely used alternatives for the analytical determination of the enantiomeric purity of chiral compounds [194]. Due to the high elTi-ciency of capillary electrophoresis, separations can be achieved even when very low selectivities are observed. At a preparative scale, these methods are well established for the purification of proteins and cells [195] but there is very little published on enantioselective separations. Only recently, some interest in chiral preparative applications has been manifested. Separation of the enantiomers ofterbu-taline [196] and piperoxan [197] have been reported by classical gel electrophoresis using sulfated cyclodextrin as a chiral additive, while the separation of the enantiomers of methadone could be successfully achieved by using free-fluid isotachophoresis [198] and by applying a process called interval-flow electrophoresis [199]. 

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