Cyclodextrins achiral separations

FITC-labeled amino acids as model compounds were separated in sodium dodecyl sulfate/borate buffer at pH 9.4 with y-cyclodextrin as chiral discriminator. Separation speed achieved with the microchip was one order of magnitude faster than that with conventional CE. Bailey and co-workers demonstrated chiral and achiral separation of amphetamine and analogous compounds labeled with 4-fluoro-7-nitrobenzofurazane on an S-folded separation channel. Phosphate buffer with sulfated y-cyclodextrin and sodium dodecyl sulfate as additives was used. It is expected that chiral separations on microfabricated electrophoresis devices with high speed and high throughput will find its way into pharmaceutical and biomedical sciences as well as into other areas. 

Like plasma and urine, matrixes from plant or environmental sources contain a vast diversity of components. Thus, achiral-chiral LC-LC is also useful for analysis involving samples from these sources. Stalcup et al. (1991) studied the enantiomeric purity of scopolamine extracted from Datura sanguinea in both homogenized plant leaves and commercial extracts. A reverse-phase separation on a C j g column separated the scopolamine from other alkaloid and matrix components while the enantiomeric separation (also in the reverse-phase mode) was carried out on two coupled [3-cyclodextrin columns or a single acetylated (3-cyclodextrin column. The single... 

Walhagen, A., Edholm, L.E. (1991). Chiral separation of achiral stationary phases with different functionalities using P-cyclodextrin in the mobile phase and applications to bioanalysis and coupled columns. Chromatographia 32, 215-223. 

Investigations on the stereochemistry of chiral semiochemicals may be carried out by (gas) chromatographic separation of stereoisomers using chiral stationary phases, e.g. modified cyclodextrins [32]. Alter natively, formation of diastereomers (e.g. Mosher s ester or derivatives involving lactic acid etc.) may be followed by separation on conventional achiral stationary phases. Assignment of the absolute configuration of the natural product will again need comparison with an authentic (synthetic) reference sample. 

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. 

The ability to design chiral ILs in which the cation and anion is of fixed chirality represents additional tuning features of ILs. Two approaches have incorporated ILs as new stationary phases for chiral GC. One method involves the use of chiral ILs as stationary phases in WCOT GC [37]. In the second approach, chiral selectors (e.g., cyclodextrins) were dissolved in an achiral IL and the mixture coated onto the wall of the capillary colunm [38]. Both approaches can separate a variety of different analytes, but the observed enantioselectivities and efficiencies do not rival those observed with commercially available chiral stationary phases (CSPs). 

A chiral selector can also be dissolved in the IL solvent and be subsequently coated on the capillary wall [38]. In this approach, the achiral [C4CiIm]Cl was used to dissolve permethylated p-cyclodextrin (p-PM) and dimethylated P-cyclodextrin (p-DM). The chromatographic separations obtained from these two columns were compared to two commercially available CSPs based on p-PM and p-DM dissolved in polydimethylsiloxane. From a set of 64 chiral molecules separafed by fhe commercial p-PM column, only 21 of the molecules were enantioresolved by the IL-based p-PM column. Likewise, from a collecfion of 80 analytes separated by the p-DM column, only 16 analytes could be separated on the IL-based p-DM column. The authors also noted a considerable enhancement in the separation efficiency of fhe IL-based CSPs. This resulf, coupled to fhe loss of enantioselecfivify for mosf separations, suggests that the imidazolium cation may occupy the cavity of the cyclodextrin preventing the analyte-cyclodextrin inclusion complex-ation that is crucial for chiral recognition. The ability for ILs to form inclusion complexes wifh cyclodextrin molecules has been recently studied by Tran and coworkers using near-infrared spectromefry [39]. 

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 micellar solubilization may involve the use of chiral surfactants, or a combination of achiral surfactants and a chiral selector. Terabe [26] and Bereuter [25] provide a comprehensive overview of applications involving chiral surfactants such as bile salts or synthetic amino acid surfactants. The use of cyclodextrins (CD) as the chiral selector in combination with MEKC was successful for the separation of neutral racemic nonsteroidal aromatase inhibitors and barbituates [38]. Further approaches to the separation of enantiomers utilizing a combination of CD-MEKC have been described in the review by Terabe [26]. 

If the CB[n] family is to supplant the cyclodextrins as the platform of choice for basic and applied studies of molecular recognition, then a series of additional developments must take place, many of which involve the development of new synthetic procedures. First, improved methods for the separation of crude CB[n] mixture on the laboratory (i.e., up to 1 kg) to industrial (i.e., tonnes) scale must be developed. Alternatively, synthetic procedures that selectively target a specific CB[n] (e.g., CB[8] or CB[10]) would be tremendously valuable. Second, the development of high-yielding methods of functionalizing pre-formed CB[n] - particularly for CB[7], CB[8], and CB[10] - would have a dramatic impact on the field. Third, a deficiency of the CB[n] family relative to the cyclodextrins is that they are inherently achiral, which... 

Lurie IS. Separation selectivity in chiral and achiral capillary electrophoresis with mixed cyclodextrins. J Chromatogr A 1997 792 297. 

Chankvetadze et al. have demonstrated the potential of flow-counterbalanced capillary electrophoresis (FCCE) in chiral and achiral micropreparative separations [27], Unlimited increase of separation selectivity can be achieved for binary mixtures, such as (+ )-chlorpheniramine with carboxymethyl-(3-cyclodextrin chiral selector, or a- and (3-isomers of a asparatame dipeptide. The carrier of the chiral selector or pseudo-stationary phase, electroosmotic flow (EOF), pressure-driven flow, or hydrodynamic flow can be used as a counterbalancing flow to the electrophoretic mobility of the analyte or vice versa, resulting in dramatic changes of the effective mobilities of the sample mixture components [28], This approach can be used for micropreparative CE, stepwise separations, and fraction collection of multicomponent mixtures [27],... 

As shown in Eq. (2) together with the chiral recognition Kk + Kg), the other necessary requirement for enantioseparations in CE is a mobility difference between the free and the complexed analyte fXf - fjL, 0). Otherwise, it will be impossible to transfer a chiral recognition into a chiral separation. This requirement does not hold when neutral analytes are analyzed with neutral chiral selectors. In such a case, an additional buffer component is required that will assist in generating a difference between the mobilities of an analyte in its free and complexed forms with a chiral selector. This is achieved by an achiral micellar phase in cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) [9]. However, a charged CD or a chiral micellar phase can combine the... 

A slightly different approach by Roussel and Favrou [61] was taken toward understanding chiral separations by cyclodextrins and quantifying the effect of substituents. Using compounds 23-30 described above, the authors carried out chiral separations using p- and T cyclodextrins as a chiral mobile phase additive. The full factorial design methodology was applied to k o, (retention without cyclodextrin), k (+), k (-) and a for two different achiral stationary phases. In the presence of y-CD on a nonendcapped phase, equations 22-25 were derived. 

Neutral cyclodextrins are also used in micellar electrokinetic chromatography with achiral surfactants to modify their enantioselectivity, particularly for the separation of hydrophobic analytes [53,55,185-187]. Enantioselectivity in this case results from differences in the distribution of enantiomers between the micellar pseudostation-ary phase and the cyclodextrin, as well as from the different migration velocities of the cyclodextrin and micelles. Neutral enantiomers can be separated based on differences in their equilibrium constants between the electrolyte solution and a charged chiral surfactant micellar phase, if the micelle has a different electrophoretic mobility to the free enantiomers. Suitable chiral surfactants include the bile salts (section 8.3.3), long alkyl-chain amino acid derivatives (e.g. sodium N-dodecanoyl-... 

All three techniques have been unequally exemplified in the separation of heterocyclic atropisomers. GC on a 10% permethyl-(-cyclodextrin (Chirasil-Dex, Chrompack) chemically bonded to a dimethyl polysiloxane backbone (CB-(-PMCD) coupled with El—MS was employed to elucidate the structure of a hexachlorinated bipyrrole which was obtained as a byproduct during the synthesis of the achiral heptachloro-1 ( -methyl-1,2 ( -bipyrrole (02AC4287). Two baseline separated peaks were obtained in ca. 20 min confirming the chiral structure of the by-product. 

The use of a chiral additive in the mobile phase has been exemplified by Roussel and Favrou who studied the separation of several N-aryl-thiazo-line-2-thione atropisomers on an achiral column with p or y cyclodextrin in the mobile phase. It is worth recalling that the association constants between each enantiomer and the chiral selector can be determined by varying the chiral additive concentration (93CHI471, 93JIP283). 

Whereas the separation of racemates in the case of urea and TOT was achieved only by a chiral crystal lattice of the achiral or racemic host, respectively, the optically active cyclodextrins, available from the chiral pool, are able to differentiate a chiral guest within their intramolecular cavity. Therefore, they do not necessarily need the crystal lattice to form inclusion compounds. The guest is encapsulated, while is is in solution, too, if the guest by size and shape fits into the cavity of the specific cyclodextrin molecule (a- (26), P- (27), or y-cyclodextrins). 

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