Reversal Of Enantiomer Migration Order Research Articles

Enantioselective recognition of sutezolid by cyclodextrin modified non-aqueous capillary electrophoresis and explanation of complex formation by means of infrared spectroscopy, NMR and molecular modelling

A method for the enantioseparation of sutezolid, the next analogue after linezolid and tedizolid, belonging to the truly new class of antibacterial agents, the oxazolidinones, was developed based on non-aqueous capillary electrophoresis (NACE), using a single isomer of cyclodextrins as a chiral pseudophase. During the experiment, the enantioseparation of sutezolid together with its predecessor, linezolid, both weak base antibacterial agents, was evaluated using anionic single-isomers of cyclodextrins from hydrophilic, up to hydrophobic: heptakis-(2,3-dihydroxy-6-sulfo)-β-cyclodextrin, heptakis-(2,3-diacetyl-6-sulfo)-β-cyclodextrin (HDAS-β-CD), as well as heptakis-(2,3-dimethyl-6-sulfo)-β-cyclodextrin (HDMS-β-CD), respectively. Based on the observed results, the cyclodextrins, HDAS-β-CD and HDMS-β-CD which carry the acetyl and methyl groups at the C2 and C3 positions, respectively, provided the baseline separation of sutezolid enantiomers. However, HDMS-β-CD led to a reversal of enantiomer migration order (EMO) in comparison to HDAS-β-CD. Instead, enantiomers of linezolid were separated only by HDMS-β-CD. During the experiments, different organic solvents and their mixtures in various ratios were tested. The selectivity and separation efficiency were critically affected by the nature of the buffer system, the type of organic solvent, and the concentrations of trifluoroacetic acid (TFA) in the NACE buffer system.Focusing on the desired EMO in which the eutomers (S)-sutezolid and (S)-linezolid migrated last, the highest enantioresolution using the NACE method was achieved at normal polarity mode with 45 mM HDMS-β-CD dissolved in MeOH/ACN (85:15, v/v) containing 200 mM TFA/20 mM ammonium formate. Moreover, infrared spectroscopy, NMR and molecular modelling were investigated to provide information about complex formation.

Reverse migration order of sibutramine enantiomers as a function of cyclodextrin concentration in capillary electrophoresis

The current study demonstrates the reversal of enantiomer migration order (EMO) in capillary electrophoresis (CE) based separations of sibutramines (SIB) as a function of the concentration of two types of cyclodextrin (CD), native β-CD and acetyl-β-CD. At normal working concentrations (<10 mM) of either CD, (S)-SIB migrated first. However, at CD concentrations greater than 10 mM, (R)-SIB was the first to migrate. This study describes factors involved in determining EMO for sibutramine enantiomers at low and high concentrations of CDs. The reversal of EMO could be explained in terms of the opposing effects of the stability and the limiting complex mobility of the SIB-CD complexes. The enantioseparation of SIB with methyl- and 2-hydroxypropyl-β-CD was possible based on differences in the binding constants of complexes. However, reverse EMO was not observed because of equal mobilities of SIB enantiomers complexed with methyl- and 2-hydroxypropyl-β-CD.

Separation of vinca alkaloid enantiomers by capillary electrophoresis applying cyclodextrin derivatives and characterization of cyclodextrin complexes by nuclear magnetic resonance spectroscopy

In this work, the enantiomeric separation of three vinca alkaloid enantiomers (vincamine, vinpocetine and vincadifformine) has been investigated in an aqueous capillary electrophoresis (CE) system using cyclodextrins (CDs). The investigated CDs were the native α-, β-, and γ-CDs and their hydroxypropylated, randomly methylated, carboxymethylated and sulfobutylated derivatives. The first part of this study consisted of the determination of the apparent averaged complex stability constants with the selected CDs. Several parameters, such as the nature and the concentration of the CD, were studied and were found to have a significant effect on the enantiomeric resolution for all studied compounds. All three vinca alkaloids were successfully enantioseparated with CDs where different migration orders were observed in case of several CDs depending on the cavity size or substituent of the host. Chiral separation and determination of the stability constants were also performed with NMR spectroscopy which confirmed the CE results. Averaged stoichiometries of the complexes were determined using the Job plot method resulting in a 1:1 complex irrespective of the alkaloid enantiomers or cyclodextrin derivative. The structures of the inclusion complexes were elucidated using 2D ROESY NMR spectroscopy. On the basis of NMR results reversal of enantiomer migration order was clarified in various cases.

Separation of propranolol enantiomers by CE using sulfated β‐CD derivatives in aqueous and non‐aqueous electrolytes: Comparative CE and NMR study

Separations using CE employing non-aqueous BGE are already as well established as separations in aqueous buffers. The separation mechanisms in achiral CE with non-aqueous BGEs are most likely similar to those in aqueous buffers. However, for the separation of enantiomers involving their interaction with chiral buffer additives, the interaction mechanisms might be very different in aqueous and non-aqueous BGEs. While the hypothesis regarding distinct mechanisms of enantiomer separations in aqueous and non-aqueous BGEs has been mentioned in several papers, no direct proof of this hypothesis has been reported to date. In the present study, the enantiomers of propranolol were resolved using CE in aqueous and non-aqueous methanolic BGEs with two single isomer sulfated derivatives of beta-CD, namely heptakis (2,3-diacetyl-6-sulfo)-beta-CD and heptakis (2,3-dimethyl-6-sulfo)-beta-CD. The enantiomer migration order of propranolol was inverted when an aqueous BGE was replaced with non-aqueous BGE in the case of heptakis (2,3-dimethyl-6-sulfo)-beta-CD but remained the same in the case of heptakis (2,3-diacetyl-6-sulfo)-beta-CD. The possible molecular mechanisms leading to this reversal of enantiomer migration order were studied by using nuclear overhauser effect spectroscopy in both aqueous and non-aqueous BGEs.

Mathematical Approach by a Selectivity Model for Rationalization of pH- and Selector Concentration-Dependent Reversal of the Enantiomer Migration Order in Capillary Electrophoresis

A separation selectivity model for capillary electrophoresis enantioseparations of weak bases in the presence of uncharged chiral selectors was described as a function of buffer pH and chiral selector concentration. On the basis of the selectivity at the extreme pH and selector concentration values, 15 principal cases could be distinguished describing the migration behavior of the analytes when increasing either pH or selector concentration. A pH-dependent reversal of the enantiomer migration order can be observed (1) when the complex mobility ratio is reversed due to an enantioselective complexation-induced pK(a)-shift, (2) in the case of an inversion of the ratio of the mobilities of the analyte-selector complex and the free analyte due to significantly weaker complexation of the neutral species, and (3) in the case of opposite chiral recognition of the protonated and uncharged species by the chiral selector. Reversal of enantiomer migration order as a function of the selector concentration at a fixed buffer pH is caused by the opposing effects of complexation constants and complex mobilities at the respective pH value. The model was applied to rationalize the enantioseparations of dipeptides, especially the pH-dependent reversal of the migration order of the LL- and DD-enantiomers of Ala-Tyr in the presence of heptakis-(2,6-di-O-methyl)-beta-cyclodextrin as the chiral selector based on the complexation constants of the protonated and zwitterionic forms of the analytes.

Inspection of the reversal of enantiomer migration order in ligand exchange micellar electrokinetic capillary chromatography.

Enantiometers of D,L-phenylalanine were separated by capillary electrophoresis based on the principle of ligand exchange. Copper (II) complex of 4-hydroxy-L-proline was used as chiral selector. The separation and the migration order of D- and L-phenylalanine were strongly affected by adding an anion surfactant sodium dodecylsulfate (SDS). Without SDS in the electrolyte, the separation was also carried out but the resolution was very small. With SDS added into the electrolyte, the resolution decreased with increasing concentration of SDS until 5.0 mM. When the concentration of SDS in the electrolyte was over 5.0 mM, inversion of the migration order of DL-phenylalanine was observed and the resolution was also increased with increasing concentration up to 20 mM. It was interesting to find that the inversion of the migration order took place not only in the enantioscparation but also in the positional isomers. A family of a fluorinated amino acid, o-, m- and p-fluoro-D,L-phenylalanine was separated and the inversion of the migration order is discussed.

Separation behavior of amino acid enantiomers in ligand exchange micellar electrokinetic chromatography

The separation of amino acid enantiomers using a Cu(II) complex with l-hydroxyproline as a chiral selector has been investigated by ligand exchange micellar electrokinetic chromatography. It was found that anionic surfactants like sodium dodecyl sulfate (SDS), sodium n-decyl sulfate, and sodium n-tetradecyl sulfate can not only improve the resolution of d,l-enantiomers, but also can change their migration orders. The migration order of o-, m-, and p-positional isomers of fluorophenylalanine and tyrosine was reversed when the concentration of SDS exceeded a certain concentration. Tween 20 improved the separation efficiency, but the migration time became longer. Cetyltrimethylammonium bromide can change the direction of electroosmotic flow, but it has no significant improvement on resolution. Organic modifiers cause the decline of resolution, but not the reversal of enantiomer migration order (EMO). A possible mechanism of the reversal of EMO is presented. ©1999 John Wiley & Sons, Inc. J Micro Sep 11: 534–540, 1999

Separation behavior of amino acid enantiomers in ligand exchange micellar electrokinetic chromatography

The separation of amino acid enantiomers using a Cu(II) complex with l-hydroxyproline as a chiral selector has been investigated by ligand exchange micellar electrokinetic chromatography. It was found that anionic surfactants like sodium dodecyl sulfate (SDS), sodium n-decyl sulfate, and sodium n-tetradecyl sulfate can not only improve the resolution of d,l-enantiomers, but also can change their migration orders. The migration order of o-, m-, and p-positional isomers of fluorophenylalanine and tyrosine was reversed when the concentration of SDS exceeded a certain concentration. Tween 20 improved the separation efficiency, but the migration time became longer. Cetyltrimethylammonium bromide can change the direction of electroosmotic flow, but it has no significant improvement on resolution. Organic modifiers cause the decline of resolution, but not the reversal of enantiomer migration order (EMO). A possible mechanism of the reversal of EMO is presented. ©1999 John Wiley & Sons, Inc. J Micro Sep 11: 534–540, 1999

Separation of Amino Acid Enantiomers by Micellar Electrokinetic Capillary Chromatography Using Synthetic Chiral Surfactants

A synthetic chiral surfactant was employed for the enantioselective micellar electrokinetic capillary chromatographic (MECC) separation of amino acid enantiomers derivatized with 6-aminoquinoyI- N-hydroxysuccinimidyl carbamate (AQC). The effect of surfactant concentration and buffer pH on resolution was studied, and the optimized conditions were used to evaluate the method in terms of sensitivity, reproducibility, and linearity. Resolution and α values for 12 AQC-derivatized amino acids are reported. The ability to perform both achiral and chiral separations simultaneously is illustrated in a separation of a mixture of six amino acid enantiomeric pairs, all with baseline resolution. The direct reversal of enantiomer migration order, useful in quantitation and chiral identification, is also shown. Comparisons with other N-protected amino acid derivatives are made in terms of resolution and sensitivity, and the advantages of this chiral MECC technique used in conjunction with the inherent advantages of the AQC derivatizing reagent are discussed.