Naira Burjanadze

Chiral recognition of verapamil by cyclodextrins studied with capillary electrophoresis, NMR spectroscopy, and electrospray ionization mass spectrometry

Capillary electrophoresis (CE) allows the observation of the opposite affinities of the enantiomers of (+/-)-verapamil [2-isopropyl-2,8-bis(3,4-dimethoxyphenyl)-6-methyl-6-azaoctannitrile+ ++, VP] toward beta-cyclodextrin (beta-CD) and heptakis(2,3, 6-tri-O-methyl)-beta-CD (TM-beta-CD). In addition, in the presence of beta-CD in the background electrolyte, longer migration times and lower separation factors were observed compared to TM-beta-CD. The binding constants of (+)- and (-)-VP with beta-CD and TM-beta-CD determined using (13)C NMR spectroscopy explain the results observed in CE. Electrospray ionization mass spectrometry (ESI-MS) was used as an alternative technique for the characterization of VP-CD complexes.

Separation of brompheniramine enantiomers by capillary electrophoresis and study of chiral recognition mechanisms of cyclodextrins using NMR-spectroscopy, UV spectrometry, electrospray ionization mass spectrometry and X-ray crystallography.

Opposite migration order was observed for the enantiomers of brompheniramine [N-[3-(4-bromphenyl)-3-(2-pyridyl)propyl]-N,N-dimethylamine] (BrPh) in capillary electrophoresis (CE) when native beta-cyclodextrin (beta-CD) and heptakis(2,3,6-tri-O-methyl)-beta-CD (TM-beta-CD) were used as chiral selectors. NMR spectrometry was applied in order to obtain information about the stoichiometry, binding constants and structure of the selector-selectand complexes in solution. The data were further confirmed by UV spectrometry and electrospray ionization mass spectrometry. The structure of the complexes in the solid state was determined using X-ray crystallography performed on the co-crystals precipitated from the 1:1 aqueous solution of selector and selectand. This multiple approach allowed an elucidation of the most likely structural reason for a different affinity (binding strength) of BrPh enantiomers towards beta-CD and TM-beta-CD. However, the question about a force responsible for the opposite affinity pattern of BrPh enantiomers towards these CDs could not be answered definitely.

Comparative enantioseparations with native β‐cyclodextrin and heptakis‐(2‐O‐methyl‐ 3,6‐di‐O‐sulfo)‐β‐cyclodextrin in capillary electrophoresis

Twenty‐three cationic chiral analytes were resolved in capillary electrophoresis using native β‐cyclodextrin and single isomer heptakis‐(2‐O‐methyl‐3,6‐di‐O‐sulfo)‐β‐cyclodextrin as chiral selectors. For 12 of 16 chiral analytes resolved with both chiral selectors the enantiomer migration order was opposite. In selected cases the structure of cyclodextrin‐analyte complexes in aqueous solution was investigated using one‐dimensional transverse rotating frame nuclear Overhauser and exchange spectroscopy. It was found that in contrast to mainly inclusion‐type complexes between chiral analytes and β‐cyclodextrin, external complexes are formed between the chiral analytes and structurally crowded, highly charged heptakis‐(2‐O‐methyl‐3,6‐di‐O‐sulfo)‐β‐cyclodextrin.

Mechanistic study of opposite migration order of dimethindene enantiomers in capillary electrophoresis in the presence of native β-cyclodextrin and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin

The possible mechanisms of the opposite affinity pattern of the enantiomers of dimethindene [(R,S)-N,N-dimethyl-3[1(2-pyridyl)ethyl]indene-2-ethylamine] (DIM) towards native beta-cyclodextrin (beta-CD) and heptakis(2,3,6-tri-O-methyl-)-beta-CD (TM-beta-CD) were studied using capillary electrophoresis (CE), NMR spectrometry, electrospray ionization mass spectrometry (ESI-MS) and X-ray crystallography. NMR spectrometry allowed to estimate the stoichiometry of the complex and to determine the binding constants. As found using ESI-MS, together with more abundant 1:1 complex, a complex with 1:2 stoichiometry may also be present in a rather small amount in a solution of DIM and beta-CD. One-dimensional ROESY experiments indicated that the geometry of the complexes of DIM with native beta-CD depends on the ratio of the components in the solution. In the 1:1 solution of DIM and beta-CD the complex may be formed by inclusion of the indene moiety of DIM into the cavity of beta-CD on the primary side and into the cavity of TM-beta-CD into the secondary side. The most likely structural reason for lower affinity of the enantiomers of DIM towards the cavity of TM-beta-CD compared to native beta-CD could be elucidated. The indene moiety does not enter the cavity of TM-beta-CD as deeply as the cavity of beta-CD. This may be the most likely explanation of significantly higher affinity constants of DIM enantiomers towards the latter CD compared to the former one. The marked difference between the structure of the complexes may also be responsible for the opposite affinity pattern of the DIM enantiomers towards beta-CD and TM-beta-CD.

Mechanistic study on the opposite migration order of the enantiomers of ketamine with α- and β-cyclodextrin in capillary electrophoresis

The enantiomer migration order of the chiral anaesthetic drug ketamine [RS-2-(2-chlorophenyl)-2-(methylamino)cyclohexanone] was studied by capillary electrophoresis using various cyclodextrins as chiral selectors. The opposite migration order of the enantiomers of ketamine was observed when native α- and β-cyclodextrins were used as chiral selectors in capillary electrophoresis. The possible mechanisms of the opposite enantiomer migration order were investigated by employing electrospray ionization mass spectrometry, 1 H-NMR spectrometry, one dimensional rotating frame nuclear Overhauser and exchange spectroscopy, X-ray crystallography, and molecular modeling techniques. As this study indicates, capillary electrophoresis can be used as a reliable experimental technique for examination of the correctness of the results of molecular modeling calculations. In addition, based on X-ray crystallographic analysis it was confirmed that (-)-ketamine as a free base possesses S configuration.

Enantioseperation of chiral phenothiazine derivatives in capillary electrophoresis using cyclodextrin type chiral selectors

SummaryThe enantioseparation of chiral phenothiazine derivatives has been studied in capillary electrophoresis (CE) using various cyclodextrins (CD) such as native α, β, and γ-CD and some of their neutral and charged derivatives. Comparative binding studies were performed using nuclear magnetic resonance (NMR) spectrometry. The advantages and disadvantages of the binding parameters obtained in both techniques (NMR and CE) from the viewpoint of separation optimization are discussed.

Enantioseparation of warfarin by capillary electrophoresis with UV and LIF detection using single and dual cyclodextrin type chiral selectors

SummaryThe enantioseparation of warfarin (WF) by capillary electrophoresis (CE) has been studied using various cyclodextrins (CD) such as native α,β and γ-CD, several neutral and randomly and selectively substituted charged CD derivatives. The enantiomer migration order of WF was determined with individual CDs. Various, CD combinations were used in order to enhance or diminish the separation factor in a designed way.

Enantioseparation of chiral vasodilator drug isoxsuprine in high-performance liquid chromatography and capillary electrophoresis

Two independent methods using high-performance liquid chromatography (HPLC) on polysaccharide type chiral stationary phase (CSP) and capillary electrophoresis (CE) with native and derivatised cyclodextrins (CD) have been proposed for the enantioseparation of chiral vasodilator drug isoxsuprine (ISP). The methods have been compared from the viewpoint of separation characteristics (efficiency, sensitivity, analysis time, costs, etc.).

Enantioseparation of the anticoagulant drug phenprocoumon in capillary electrophoresis with UV and laser-induced fluorescence detection and application of the method to urine samples.

The enantioseparation of phenprocoumon (PhC) in capillary electrophoresis (CE) has been studied using various cyclodextrins (CDs) such as native α, β and γ‐CD and several neutral and randomly, as well as selectively substituted charged CD derivatives. Reversal of the enantiomer migration order was observed when using heptakis(2,3,6‐tri‐O‐methyl (TM)‐β‐CD as a chiral selector compared to all other CDs used. The detection of PhC was performed using either UV or laser‐induced fluorescence (LIF) detection. The limit of detection (LOD) observed with LIF detection was ca. 20 times lower compared to UV. The method has been applied to the analysis of urine samples of the patient under treatment with PhC in combination with other drugs such as ramipril, hydrochlorothiazide, and nifedipine.

Enantioseparation of tetramisole by capillary electrophoresis and high performance liquid chromatography and application of these techniques to enantiomeric purity determination of a veterinary drug formulation of L-levamisole

The enantioseparation of the chiral anthelmintic drug tetramisole was studied by capillary electrophoresis using various cyclodextrins as chiral selectors. In particular, the following aspects have been addressed: (1) the enantiomer affinity pattern of tetramisole towards various cyclodextrins; (2) development of a capillary electrophoretic method for quantitative determination of the enantiomeric impurity, i.e. of the R-(+) enantiomer, in S-(–)-tetramisole (levamisole); (3) application of the capillary electrophoretic method for the determination of the enantiomeric impurity of R-(+)-tetramisole in commercial samples and in a veterinary drug formulation of S-(–)-tetramisole; and (4) comparison of capillary electrophoretic and traditional pharmacopoeial polarimetric methods for the enantiomeric impurity determination in S-(–)-tetramisole. Opposite affinity order of tetramisole enantiomers was observed towards different cyclodextrins, depending on the nature of the substituents and the substitution pattern. In addition, an enantioseparation of tetramisole was developed by high performance liquid chromatography on polysaccharide type chiral stationary phase. This method was compared to the aforementioned capillary electrophoretic method. Preliminary validation results indicated that capillary electrophoresis represents a flexible, fast, and inexpensive technique for determination of the enantiomeric impurity of R-(+)-tetramisole in the commercial samples and drug formulations of S-(–)-tetramisole (levamisole). Both of the commercial samples as well as the veterinary drug formulation of levamisole contained R-(+)-tetramisole as enantiomeric impurity. Capillary electrophoresis represents a superior method for enantiomeric impurity determinations compared to the traditional polarimetric method still used in the major pharmacopoeias. CE offers the advantages of miniaturised techniques, as well as higher peak efficiency, flexibility, and shorter method development time compared to HPLC.