Chiral-specific phase

Chiral Hplc Columns. There are about 40 commercially available chiral columns which are suitable for analytical and preparative purposes (57). In spite of the large number of commercially available chiral stationary phases, it is difficult and time-consuming to obtain good chiral separation. In order to try a specific resolution meaninghilly, a battery of chiral hplc columns is necessary and this is quite expensive. 

Cyclodextrin stationary phases utilize cyclodextrins bound to a soHd support in such a way that the cyclodextrin is free to interact with solutes in solution. These bonded phases consist of cyclodextrin molecules linked to siUca gel by specific nonhydrolytic silane linkages (5,6). This stable cyclodextrin bonded phase is sold commercially under the trade name Cyclobond (Advanced Separation Technologies, Whippany, New Jersey). The vast majority of all reported hplc separations on CD-bonded phases utilize this media which was also the first chiral stationary phase (csp) developed for use in the reversed-phase mode. 

Synthetic chiral adsorbents are usually prepared by tethering a chiral molecule to a silica surface. The attachment to the silica is through alkylsiloxy bonds. A study which demonstrates the technique reports the resolution of a number of aromatic compoimds on a 1- to 8-g scale. The adsorbent is a silica that has been derivatized with a chiral reagent. Specifically, hydroxyl groups on the silica surface are covalently boimd to a derivative of f -phenylglycine. A medium-pressure chromatography apparatus is used. The racemic mixture is passed through the column, and, when resolution is successful, the separated enantiomers are isolated as completely resolved fiactions. Scheme 2.5 shows some other examples of chiral stationary phases. 

The resolution of enantiomers by liquid chromatography using chiral stationary phases is based on the formation of reversible diastereomeric complexes of different stability between the sample and stationary phase. Since the formation of the complexes is strongly dependent on the structure of the sample, there are no universal chiral stationary phases. The specific advantages of TLC for enantiomeric separations result from its low cost, convenience and speed (10,97,98). The main limitation, particularly with respect to column liquid chromatography, is the small number of phases currently available. 

The development of a plethora of HPLC CSPs in the 1980s and 1990s has, to a large extent, made the use of chiral mobile-phase additives (CMPAs) redundant in most modem pharmaceutical analytical laboratories [23]. Before this period, chiral selectors were used routinely as additives in HPLC, but are now only used for a small number of specific applications [23]. CMPAs are used to form... 

The most popular and commonly used chiral stationary phases (CSPs) are polysaccharides, cyclodextrins, macrocyclic glycopeptide antibiotics, Pirkle types, proteins, ligand exchangers, and crown ether based. The art of the chiral resolution on these CSPs has been discussed in detail in Chapters 2-8, respectively. Apart from these CSPs, the chiral resolutions of some racemic compounds have also been reported on other CSPs containing different chiral molecules and polymers. These other types of CSP are based on the use of chiral molecules such as alkaloids, amides, amines, acids, and synthetic polymers. These CSPs have proved to be very useful for the chiral resolutions due to some specific requirements. Moreover, the chiral resolution can be predicted on the CSPs obtained by the molecular imprinted techniques. The chiral resolution on these miscellaneous CSPs using liquid chromatography is discussed in this chapter. 

There are three possible approaches to the separation of chiral species by CE (1) addition of chiral selectors to the buffer, (2) use of a chiral stationary phase, and (3) precolumn derivatization. These correspond to the approaches in HPLC, and the separation mechanisms are described in Section 2.8. In the first approach, additives are added to CZE, CGE, or MECC buffers to effect the separation. In the second approach, chiral selectors can be immobilized on the capillary wall, although that is a difficult process. Alternatively, capillaries filled with enantiospecific packings can be employed for CEC. In the third approach, enantiomers are derivatized with chirally specific reagents prior to CZE or MECC. Addition of chiral selectors to the buffer is the most common approach. 

Although classical column chromatography over chiral stationary phases has been used for several years, HPLC has until recently been regarded mainly as an analytical tool. However, methods have now been developed and many papers have been published on the various applications of particular stationary phases in preparative HPLC, which may be the method of choice when the preparation of small amounts of material are required for screening purposes, for use in biological assays, or as standards in purity assays. The use of chromatographic techniques may be quicker and less risky than custom synthesis and resolution and can guarantee material to a defined specification. 

Chemically bonded phases may also be tailored to a specific separation problem. A case in point is the synthesis of chiral stationary phases for the separation of optical isomers. Another application of polar bonded phases, which is beyond the scope of this book,... 

Enantiomeric excess can be detected in a number of ways. Direct observation of optical activity, that is, determination of the specific rotation [a]d of a compound, is cumbersome. Biochemical detection is also possible, although methodologies are generally specific to individual compounds or compound types. Indirect measurement through a chromatographic procedure, for example, gas chromatography or gas chromatography-mass spectrometry on a chiral stationary phase, has wide applicability and is very sensitive. 

The chirality of DNA was applied to selective separation. A DNA aptamer as a new target-specific chiral selector for HPLC was investigated by Michaud et al. [119,120]. They showed that a tailor-made chiral stationary phase based on a DNA aptamer with known stereospecific binding for the D-enantiomer of the oligopeptide, arginine-vasopressin, exhibits enantioselectivity between the d- and L-peptides. This DNA-based target-specific aptamer chiral stationary phase provides a powerful tool for the resolution of small (bioactive) molecule enantiomers. 

In recent years several sensitive and specific methods for the simultaneous determination of ephedrine alkaloids in plant material have been published. These include thin-layer chromatography (292,426), gas chromatography (251), straight-phase and reversed-phase high-performance liquid chromatography (253, 255, 302, 355, 427), isotachophoresis (303, 356), and 13C-NMR (304). Resolution of enantiomeric alkaloids by HPLC has been achieved on chiral stationary phases (417, 418) or after derivatization with a chiral agent on an achiral stationary phase (419). Chromatographic separation and analytical detection of... 

Separation of optically active isomers is one of the most important areas of HPLC apphcation in the pharmaceutical industry. Since most of biological systems are predominantly homochiral, different enantiomers of the same drug could have different effect and potency, and the development of enan-tioselective analytical (and preparative) separation methods is very important. Detailed description of chiral HPLC separation is given in Chapter 22 of this book here we only briefly review the specifics of distinctive types of chiral stationary phases (CSP). 

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