Chiral selectors and enantiomers

The stability constants of the complexes formed between chiral selector and enantiomers are given by the following equations ... 

The most sensitive enantioselective separation technique is capillary zone electrophoresis. Here, the detectors utilized are not sensitive enough to be able to detect the enantiomers. In the case of sensors, amperometric biosensors have been found to be most sensitive.264 A better enantioselectivity was found for potentiometric, enantioselective membrane electrodes because a direct interaction between the chiral selector and enantiomer occurred.282 285... 

Davankov and other researchers made substantial contributions to impart the three-point interaction model with modern interpretation [24-26]. As pointed out by Davankov et al., it is required (but not necessarily sufficient) for the chiral selector to recognize the enantiomers to have at least three configuration-dependent active points, which are different in nature, on both chiral selector and enantiomer molecules. The active points on chiral selector must be complementary to and be able to simultaneously interact with those on enantiomer molecules. It is possible that two of the three required interactions can be repulsive if the third one is strong enough to promote the formation of diastereomeric associates between chiral selector and selectand [25]. Davankov et al. used the left- and right-hand model to vividly demonstrate that with the assistance of achiral surface, two-point or even one-point interaction is sufficient for chiral recognition. They treated these cases as expansion of TPI model rather than contradictions to it and asserted that the model is also applicable to CSPs based on proteins and polysaccharides. In some instances, achiral elements, such as solvent molecules and sorbent surfaces, may also participate in the chiral recognition process [24, 25]. 

The three interactions between the chiral selector and enantiomers considered do not need to be attractive, as some of them may be repulsive. Only the resulting sum for one (the favored diastereomeric combination) should produce a more favorable binding. Two interactions may be repulsive if the third one is strong enough to promote the formation of one of the two possible diastereomeric selector-ligand complexes. 

Both CSPs and CMPAs are based on the formation of transient diastereomeric complexes between the chiral selectors and enantiomers during chromatographic procedure. The diastereomeric complexes can be separated due to their different properties [79]. 

Recently, the separation of some milligram quantities of terbutaline by classical gel electrophoresis has been reported [194]. A sulfated cyclodextrin impregnated on the agarose gel was used as a chiral selector and the complete resolution was achieved in 5 h. Analogously, small amounts of enantiomers can be isolated using thin-layer... 

A compromise among all the properties mentioned herein should be established, depending on the technique used and on the particular application. Preparative separation of enantiomers is still an open subject which requires further investigation in the search of new chiral selectors and techniques well adapted to large scale processes. 

Phinney et al. [Ill] investigated the application of citrus pectins, as chiral selectors, to enantiomer separations in capillary electrophoresis. Successful enantioreso-lution of primaquine and other antimalarials, was achieved by utilizing potassium polypectate as the chiral selector. Changes in pH, chiral additive concentration, and capillary type were studied in relation to chiral resolution. The effect of degree of esterification of pectin materials on chiral recognition was evaluated. 

CE has been established as a very efficient technique for the separation of drug enantiomers. Chiral separations in CE are also based on the formation of diastereomeric complexes between the enantiomers and a chiral selector. The main advantages of the technique are its high efficiency, short analysis times, versatility due to the great variety of chiral selectors that can be added to the BGE, short equilibration times required when changing the chiral selector and low consumption of selector. °... 

The chiral recognition of enantiomers can be of three types (i) desionoselective, (ii) ionoselective, or (iii) duoselective, in which only the non-dissociated, the dissociated or both forms (charged and uncharged), respectively, of the enantiomers selectively interact with the chiral selector. In the case of ionoselective and duoselective interactions, a reversal of the migration order of the enantiomers is theoretically possible by the appropriate selection of CD concentration and the pH of the BGE. The addition of organic modifier to the BGE can also change selectivity by modifying the solubility of the chiral selector and/or of the solute, the complex equilibrium, the conductivity of the BGE and the electroendos-motic flow (EOE) level. Several other factors, such as the temperature, the type and the concentration of the BGE, and the level of the EOE can influence the separation. 

More than 100 CSPs are commercially available nowadays, which should make the separation of any pair of enantiomers feasible. However, the enantiorecognition mechanisms involved in the chiral recognition between the analytes and the CSPs are complex and therefore the selection of the appropriate CSPs, depending on the structure of the analyte, is a difficult task. A common approach to develop a new enantioseparation is the stepwise trial-and-error approach based on detailed consideration of the enantiorecognition mechanisms between the chiral selector and the analyte, or on the analyst s experience, or on the consultation of literature or databases. However, this approach is time-consuming and often unsuccessful owing to the fact that achieving enantioresolution is often purely empirical... 

CE has been applied extensively for the separation of chiral compounds in chemical and pharmaceutical analysis.First chiral separations were reported by Gozel et al. who separated the enantiomers of some dansylated amino acids by using diastereomeric complex formation with Cu " -aspartame. Later, Tran et al. demonstrated that such a separation was also possible by derivatization of amino acids with L-Marfey s reagent. Nishi et al. were able to separate some chiral pharmaceutical compounds by using bile salts as chiral selectors and as micellar surfactants. However, it was not until Fanali first showed the utilization of cyclodextrins as chiral selectors that a boom in the number of applications was noted. Cyclodextrins are added to the buffer electrolyte and a chiral recognition may... 

Since one or more of the interactions in these systems might originate from the stationary phase, only a two- or a one-point interaction between the solute and the selector is necessary for mechanisms (2) and (3) to occur [50]. However, some of the CMPAs used in HPLC [37,40,51,52] have also been used as chiral selectors in CE [53-56], which indicates that at least one of the separation mechanisms between the selector and enantiomers is selective complex formation in the mobile phase in these cases, since there is no stationary phase present in CE. A recent example by Yuan et al. [57] is presented in Eigure 17.1. The authors introduced the use of (R)-A,A,A-trimethyl-2-aminobutanol-bis(trifluoromethane-sulfon)imidate as the chiral selector for enantioseparation in HPLC, CE, and GC. This chiral liquid serves simultaneously as a chiral selector and a co-solvent. 

Three approaches can be employed to separate peptide stereoisomers and amino acid enantiomers separations on chiral columns, separations on achiral stationary phases with mobile phases containing chiral selectors, and precolumn derivatization with chiral agents [111]. Cyclodextrins are most often used for the preparation of chiral columns and as chiral selectors in mobile phases. Macrocyclic antibiotics have also been used as chiral selectors [126]. Very recently, Ilsz et al. [127] reviewed HPLC separation of small peptides and amino acids on macrocyclic antibiotic-based chiral stationary phases. 

Based on preliminary results from Helfferich130, further developments by Davankov and co-workers5 131 133 turned the principle of chelation into a powerful chiral chromatographic method by the introduction of chiral-complex-forming synlhetie resins. The technique is based on the reversible chelate complex formation of the chiral selector and the selectand (analyte) molecules with transient metal cations. The technical term is chiral ligand exchange chromatography (CLEC) reliable and complete LC separation of enantiomers of free a-amino acids and other classes of chiral compounds was made as early as 1968 131. 

A more recently developed technique, simulated moving-bed chromatography, has been very successful for chiral separations at the preparative scale. This technique affords the separation of large amounts of enantiomers by means of developing suitable chiral selectors and mobile phases [42]. 

If L is the chiral selector and S and R the enantiomers to be determined, the following reactions take place ... 

The main components of the membrane of the enantioselective, potentiometric electrode are chiral selector and matrix. Selection of the chiral selector may be done accordingly with the stability of the complex formed between the enantiomer and chiral selector on certain medium conditions, e.g., when a certain matrix is used or at a certain pH. Accordingly, a combined multivariate regression and neural networks are proposed for the selection of the best chiral selector for the determination of an enantiomer [17]. The most utilized chiral selectors for EPME construction include crown ethers [18-21], cyclodextrins [22-35], maltodextrins 136-421, antibiotics [43-50] and fullerenes [51,52], The response characteristics of these sensors as well as their enantioselectivity are correlated with the type of matrix used for sensors construction. 

Berzas Nevado et al. [138] developed a new capillary zone electrophoresis method for the separation of omeprazole enantiomers. Methyl-/ -cyclodextrin was chosen as the chiral selector, and several parameters, such as cyclodextrin structure and concentration, buffer concentration, pH, and capillary temperature were investigated to optimize separation and run times. Analysis time, shorter than 8 min was found using a background electrolyte solution consisting of 40 mM phosphate buffer adjusted to pH 2.2, 30 mM /1-cyclodextrin and 5 mM sodium disulfide, hydrodynamic injection, and 15 kV separation voltage. Detection limits were evaluated on the basis of baseline noise and were established 0.31 mg/1 for the omeprazole enantiomers. The method was applied to pharmaceutical preparations with recoveries between 84% and 104% of the labeled contents. 

Separation of the enantiomers of pantoprazole sodium was described by Kuhn et al [14]. Bovine serum albumin (BSA) was used as the chiral selector, and different experimental parameters were investigated to obtain good resolution between the enantiomers. Increasing the concentration of BSA improved the chiral resolution, but the sensitivity of the detection system was lowered. Using a buffer system having a pH around 7.4 and addition of 1-propanol caused an enhancement to the peak shape and the resolution. This method is suitable for routine analysis. 

The macrocyclic antibiotics vancomycin [51,52] and teicoplanin [53] were successfully employed as chiral selectors for enantiomer separation in p-CEC. Warfarin,... 

Practically every type of separation that has been done by the column technique can also be carried out by thin-layer chromatography. Several papers and reviews were published on the various aspects of the technique. In addition to the books on chromatography [17,26-301, an overview of ion-exchange application of TLC was presented by Devenyi and Kalasz 311. Recent results on the separation of enantiomers have been reviewed by Mack, Hauck and Herbert (32.33) (enantiomer. separation on an RP-18 plate, impregnated with copper salt and proline derivative as chiral selectors) and Lepri, Coas and Desideri, using a microcrystalline triacetylcellulose stationary phase, or modified beta-cyclodextrins in the mobile phase 134.35). 

An analogous study on chiral resolution of racemic [Cr(ox)2(acac)]2- and [Cr(ox)2(gly)] " anions in aqueous 1,4-dioxane was performed with A-[Co(sep)] + cation as a chiral selector [314]. The A-enantiomers of both anionic complexes were identified by CD spectra. When A-[Co(en)3] trication served as a chiral selector, A-enantiomers were also eluted the A-[Co(d-l,2pn)3] and A-[Co(d-pm)3] + cations eluted A-enantiomers [314], These results are consistent with those obtained in aqueous solutions [313]. 

The most important technique for enantiomeric separation in TLC is chiral ligand-exchange chromatography (LEC). LEC is based on the copper(II) complex formation of a chiral selector and the respective optical antipodes. Differences in the retention of the enantiomers are caused by dissimilar stabilities of their diastereomeric metal complexes. The requirement of sufficient stability of the ternary complex involves five-membered ring formation, and compounds such as a-amino and a-hydroxy-acids are the most suitable. 

Chiral Selectors and Chiral Stationary Phases Employed in Liquid Chromatographic Enantiomer Separation... 

The capability of quartz microbalances for the analysis of enantiomers has been demonstrated successfully by Bodenhofer et al. [102]. Cyclopeptides have already been successful as chiral selectors in enantiomer separation by capillary electrophoresis [90, 91], Here, we showed that cyclopeptide 1 exhibits different affinities to D- and L-arginine [99], The differences in sensitivity of the quartz microbalances result in a chiral separation factor of 1.12 [76]. Sensitivity differences of the quartz microbalance coated with 1 for the two enantiomers of arginine are shown in Figure 10.13. 

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