Enantioseparation In Capillary Electrophoresis Research Articles

Mechanism of capillary electrophoresis enantioseparations using a combination of an achiral crown ether plus cyclodextrins

The addition of an achiral crown ether (18-crown-6) to a cyclodextrin-based separation can significantly affect the capillary electrophoresis (CE) enantioresolution of organic racemates that contain a primary amine functional group. In most cases an enhancement of the enantioseparation was observed. However, there are also cases where the addition of 18-crown-6 was detrimental to a cyclodextrin-based CE enantioseparation. The effect of concentration of the two complexing additives as well as the effect of pH and added potassium ion were examined. A specific three-body complex involving simultaneous, dual inclusion complex formation can be used to explain both the enhanced and diminished enantioselectivities observed when 18-crown-6 is added to the run buffer.

Enantioseparation in capillary electrophoresis using 2-hydroxypropyltrimethylammonium salt of β-cyclodextrin as a chiral selector

The resolving ability of the 2-hydroxypropyltrimethylammonium salt of β-cyclodextrin (TMA-β-CD) as a chiral selector in capillary electrophoresis (CE) is reported in this work in continuation of our research on this subject [Chem. Soc. Rev., 25 (1996) 141; J. Chromatogr. A, 732 (1996) 143, 183]. The composition of the chiral selector and the effect of its addition to the background electrolyte on the EOF in a fused-silica and a polyacrylamide-coated capillary were studied. The enantioseparations of neutral chiral analytes are reported first using a positively charged cyclodextrin (CD) derivative. The carrier ability of a positively charged CD derivative is used for the migration of neutral analytes in a polyacrylamide-coated capillary. Several general advantages of this mode of electrophoretic separations are illustrated together with selected possibilities for the reversal of the enantiomer migration order for neutral and anionic analytes.

Comparison and Modeling Study of Vancomycin, Ristocetin A, and Teicoplanin for CE Enantioseparations

The structurally related glycopeptide antibiotics vancomycin, ristocetin A, and teicoplanin can all be used as chiral selectors in capillary electrophoresis (CE). Both experimental and modeling studies were done to elucidate their similarities and differences. There are identifiable morphological differences in the aglycon macrocyclic portions of these three compounds. In addition, there are other structural distinctions that can affect their CE enantioselectivity, migration times, and efficiency. Teicoplanin is the most distinct of the three and is the only one that is surface active. Its aggregational properties appear to affect its enantioselectivity among other things. The similar but not identical structures of the three glycopeptides produce similar but not identical enantioselectivities. This leads to the empirically useful "principle of complementary separations", in which a partial resolution with one chiral selector can be brought to baseline with one of the others. Overall, ristocetin A appears to have the greatest applicability for CE enantioseparations.

Capillary electrophoresis and 1H NMR studies on chiral recognition of atropisomeric binaphthyl derivatives by cyclodextrin hosts

The usefulness of 1H NMR spectroscopy is illustrated for a detailed study of a selector-solute interactions which maintain enantioseparations in capillary electrophoresis. The apparent binding constants ( K a) of enantiomers and the chemical shift differences at saturation (Δδ c) were calculated using 1H NMR spectroscopy. Some problematic aspects concerning the application of NMR spectroscopy for the explanation of the chiral recognition mechanisms in separation techniques are also shortly discussed. The enantioseparation of a cationic chiral analyte using a cationic cyclodextrin derivative is reported for the first time. This result supports the possibility of enantioseparations in a selector-solute pair carrying the same net charge as recently reported for a negatively charged selector-solute pair.

Capillary electrophoretic enantiomeric separations using the glycopeptide antibiotic, teicoplanin

Teicoplanin is the third in a series of macrocyclic glycopeptide antibiotics that has been evaluated as a chiral selector in capillary electrophoresis (CE). It was used to resolve over 100 anionic racemates at low selector concentrations. Like the other related glycopeptide antibiotics, its enantioselectivity tends to be opposite to that of the ansa-type antibiotics which prefers cationic compounds, particularly amines. Factors that affect teicoplanin-based enantioseparations include the selector concentration, pH, and the concentration of the organic modifier. The temperature and the nature and strength of the buffer are also known-to affect the stability of the chiral selector as well as the enantioseparation. Teicoplanin exhibited some features that were not noted with the other glycopeptide antibiotics. For example, it aggregates (forms micelles) in aqueous solutions and this influences its enantioselectivity. Unlike the other studied glycopeptides, teicoplanin precipitates in alcohol-water mixtures. It also binds less to the capillary wall than vancomycin as evidenced by the faster electroosmotic flow velocity. The micellization of teicoplanin is pH dependent so that the effect of pH on enantiorecognition is more complex for teicoplanin than for other chiral selectors. Also it is shown that the simple model proposed to explain the role of organic modifiers in cyclodextrin-based CE enantioseparations may not apply to these and other systems. © 1996 Wiley-Liss, Inc.

About some aspects of the use of charged cyclodextrins for capillary electrophoresis enantioseparation.

Free capillary zone electrophoresis with the negatively charged polyanion of the beta-cyclodextrin sulfobutyl ether (SBE-beta-CD) as a chiral additive was used for the resolution of basic racemic drugs. High enantioselectivity was established for some racemic compounds using extremely low (micromolar) concentrations of the chiral additive. The dependencies of the migration times and the selectivity of the enantioseparation on the concentration of the chiral additive and the pH of the run buffer were studied. Examples of the chiral separation in counter-current flows of discrete zones of the chiral selector and the racemic compound as well as separation of the neutral racemic compound thalidomide in a micellar electrokinetic chromatography-like modus were demonstrated using SBE-beta-CD.