Sole Chiral Selector Research Articles

Development of cyclodextrin-electrokinetic chromatography-based strategies for the separation of ketorolac enantiomers

The development of analytical strategies enabling the chiral separation of ketorolac by electrokinetic chromatography is presented in this work. This anti-inflammatory drug is commercialised as racemic mixture but its physiological activity resides in the S-enantiomer. Different experimental conditions were investigated at acidic and neutral pH, including the use of cyclodextrins (CD) or amino acid-based ionic liquids (AAILs) ([TMA][L-Cit], [TMA][L-Arg], [TMA][L-Harg], [TBA][L-Cit], [TBA][L-Arg], and [TBA][L-Harg]) as the sole chiral selectors or the combination of a CD with AAILs or their cationic components (tetramethylammonium (TMA-OH) or tetrabutylammonium (TBA-OH)). The best conditions in terms of migration times and enantiomeric resolutions were obtained using 2 mM heptakis(2,3,6-tri-O-methyl)-β-CD (TM-β-CD) in 50 mM phosphate buffer at pH 7.0 or 2 mM TM-β-CD with 60 mM TMA-OH in 50 mM formate buffer at pH 3.0. The presence of TMA-OH in the separation medium affected considerably the electroosmotic flow (EOF) mobility which, in turn, significantly shortened the migration times for ketorolac enantiomers at pH 3.0 due to an inversion in the EOF. The multivariate optimization of the experimental conditions for the single system and for the combined system with TMA-OH enabled to obtain the separation of ketorolac enantiomers in less than 11 min with enantiomeric resolutions of 2.5 and 1.9, respectively. The single system was selected to evaluate its analytical characteristics in terms of limits of detection/quantification, accuracy, and precision, according to the International Council for Harmonization (ICH) guidelines, which resulted adequate to be applied to the analysis of pharmaceutical formulations.

Evaluation of deep eutectic solvents chiral selectors based on lactobionic acid in capillary electrophoresis.

Recently, deep eutectic solvents (DESs) have attracted considerable interest in analytical chemistry. This work described the enantioseparations of twenty amino alcohol drugs with several DESs based on lactobionic acid (LA) as the sole chiral selector in capillary electrophoresis (CE) firstly. Compared to the single LA system and the ionic liquid/LA synergistic system, the DES system exhibited considerably improved separations. The influences of some key parameters on separations were investigated in detail. This work also experimentally demonstrated that the carboxyl group was indispensable in the process of chiral recognition. The mechanisms of the improvements of DESs on enantioseparations were studied via ultraviolet spectroscopy. Furthermore, the proposed method was used to determine the enantiomeric purity of propranolol hydrochloride successfully. This is the first time that chiral DESs were utilized as the sole chiral selectors in CE, and this strategy has opened up a new prospect for the use of DESs in enantioseparation.

Novel supramolecular deep eutectic solvent (SUPRADES) as a sole chiral selector in capillary electrophoresis for the enantiomeric separation of fluorine-substituted amphetamine analogs

In this study, a novel supramolecular deep eutectic solvent consisting of sulfated-β-CD and citric acid (S-β-CD-CA) is reported for the first time. This innovative system was evaluated as a sole chiral selector in capillary electrophoresis for the enantioseparation of six fluorine-substituted amphetamine analogs, yielding remarkable outcomes. Baseline separations of all amphetamine analogs under study were achieved in less than 21.00 min using the S-β-CD-CA as the chiral selector. It was observed that the addition of 0.050 % v/v S-β-CD-CA into the background electrolyte resulted in the baseline separation of five out of the six fluorine-substituted amphetamine analogs, while in the case of the para-substituted amphetamine analog, 4-fluoramphetamine (4-FA), a higher percentage (0.15 % v/v) was required to achieve baseline enantioseparation. These findings emphasized the potential of this new supramolecular system in providing a class of solvents with promising chiral recognition properties.

Use of single and dual systems of γ-cyclodextrin or γ -cyclodextrin/L-Carnitine derived ionic liquid for the enantiomeric determination of cysteine by electrokinetic chromatography. A comparative study

Two electrokinetic chromatography methods based on the use of γ-CD as the sole chiral selector or the combination of γ-CD and L-Carnitine methyl ester bis(trifluoromethane)sulfonimide were developed and validated in this work for the stereoselective determination of cysteine. Sample preparation included the use of 9-fluorenylmethoxycarbonyl as derivatizing reagent. To achieve the best enantioresolution in a short analysis time, the influence of several experimental and instrumental variables like the buffer pH, separation voltage, and temperature was investigated. Enantiomeric resolutions of 2.9 and 5.9 in approximately 7 min were obtained using γ-CD or γ-CD/L-Carnitine methyl ester bis(trifluoromethane)sulfonimide as chiral selectors, respectively. After evaluating the analytical characteristics of both EKC methods, their potential was demonstrated through the stereoselective quantitation of cysteine in dietary supplements. The enantiomeric impurity D-cysteine was detectable in one of the samples analyzed. The first analytical methods enabling the stereoselective quantitation of cysteine in real samples by CE are described in this work.

Tetraalkylammonium-l-tartrate ionic liquids as sole chiral selectors in capillary electrophoresis

In the field of separation science, the use of tartaric acid-based ionic liquids (TTILs) for chiral separation has not yet been reported. In this work, six novel mono- and di-tetraalkylammonium l-tartrate ILs (TAA-l-TTs) with different alkane chain length were evaluated for their potential as sole chiral selectors in capillary electrophoresis (CE). In comparison with conventional l-tartaric acid (l-TT) chiral selector, these IL-type chiral selectors exhibit remarkably improved enantioseparation performance, which is even better than most previously reported TT ester-type chiral selectors. Major factors influencing CE separation were investigated, including the background electrolyte (BGE) composition, ILs concentration, type and proportion of organic modifier, buffer pH, etc. The results show that the l-TT-based IL-type chiral selectors prefer a weak acid buffer (150 mM boric acid at pH 4.5) with a high proportion of methanol (80%-90%). The best resolutions (Rs) were obtained when the IL concentration was set at 60 mM. We further carried out a series of comparative experiments and a molecular simulation study to explore the enantiorecognition mechanism of TTILs. The result indicates that the TAA-l-TTs cannot be considered as a simple combination of TAA salt and l-TT. The dissociation degree of TTILs may be an important factor influencing the enantiorecognition process.

Enantiomeric separation of homocysteine and cysteine by electrokinetic chromatography using mixtures of γ-cyclodextrin and carnitine-based ionic liquids

Two new chiral ionic liquids based on the ester of the non-protein amino acid carnitine as cationic counterpart were synthesized, characterized and evaluated as chiral selectors for the enantiomeric separation of homocysteine and cysteine derivatized with 9-fluorenylmethoxycarbonyl chloride by electrokinetic chromatography. The use of both ionic liquids, l-carnitine methyl ester bis(trifluoromethane)sulfonimide and l-carnitine methyl ester l-(+)-lactate, as sole chiral selectors in the separation buffer, did not allow the enantiomeric separation of the studied amino acids. However, the combined use of l-carnitine methyl ester bis(trifluoromethane)sulfonimide ionic liquid with γ-CD forming a dual system led to the enantiomeric separation of both analytes and showed the existence of a strong synergistic effect. On the contrary, the dual system γ-CD plus l-carnitine methyl ester l-(+)-lactate did not improve the enantiomeric separations with respect to those obtained using the CD alone. The influence of different experimental variables such as buffer composition and pH, and ionic liquid concentration was investigated. Also, the nature of the anionic moiety was evaluated by comparing the results obtained with both ionic liquids when combined with γ-CD. The use of 2 mM γ-CD combined with 5 mM l-carnitine methyl ester bis(trifluoromethane)sulfonimide in a 50 mM phosphate buffer at pH 7.0 enabled to achieve the simultaneous enantiomeric separation of homocysteine and cysteine with high resolution values (>6.0) in analysis times close to 12 min.

Synthesis, application and molecular modeling study of ionic liquid functionalized lactobionic acid, 3-methyl-1-(3-sulfopropyl)-1H-imidazol-3-ium lactobionate, as a chiral selector in capillary electrophoresis.

Recently, ionic liquids have been widely used for chiral separation. However, several papers have reported the use of ionic liquids as the sole chiral selector. In this work, a novel capillary electrophoresis method using the ionic liquid 3-methyl-1-(3-sulfopropyl)-1H-imidazol-3-ium lactobionate (MSI-LA) as a chiral selector was developed for the enantioseparation of propranolol, metoprolol, bisoprolol, esmolol, atenolol, sotalol, terbutaline and clenbuterol. Method optimization was conducted and the optimized method (40 mM borax buffer, v/v 30% methanol, pH 8.5, 160 mM MSI-LA; +10 kV voltage; capillary temperature, 15 °C; 50 mbar/5 s injection; detection wavelength, 230 nm) was successful for the baseline separation of the eight basic drugs. The LA (lactobionic acid) system was compared. The possible mechanisms of enantioseparation were investigated using molecular modeling. Both the cation and anion of MSI-LA provided interactions between the chiral selector and analytes, and finally enhanced chiral recognition capability compared with LA.

Synthesis and application of tetramethylammonium-carboxymethylated-β-cyclodextrin: A novel ionic liquid in capillary electrophoresis enantioseparation

In this study, a carboxymethylated-β-cyclodextrin-based chiral ionic liquid (CIL), tetramethylammonium-carboxymethylated-β-cyclodextrin (TMA-CM-β-CD) was successfully designed and synthesized. This cyclodextrin-based ionic liquids (ILs) was used as the sole chiral selector in capillary electrophoresis, and it is very interesting to find that the chiral separation capability can be remarkably improved when a conventional cyclodextrin chiral selector evolved into an IL chiral selector. The ionic liquid showed satisfactory separation performance towards twelve tested drugs. A series of parameters affecting the enantioseparation, such as the type and proportion of organic modifier, buffer pH, chiral selector concentration, as well as applied voltage were systematically investigated. Additionally, the molecular docking program Autodock was applied to further demonstrate the mechanism of chiral recognition and the enhanced enantioselectivity of TMA-CM-β-CD, which kept in agreement with our experimental results.

Synthesis of a chiral ionic liquid, cholinium-clindamycin phosphate, as sole chiral selector in capillary electrophoresis

Recently, in separation science, ionic liquids (ILs) have been commonly used as modifiers for buffer solutions, dynamic coating solutions, or coating solutions on carriers in capillary electrophoresis. However, only several papers have reported the use of chiral ILs as the sole chiral selector. In this paper, a chiral ionic liquid, cholinium-clindamycin phosphate (Ch-CP), was synthesized and employed as a sole chiral selector in capillary electrophoresis (CE). A series of parameters affecting the separation were optimized, including chiral selector concentration, buffer pH, proportion of organic modifier, as well as the applied voltage. Under the optimal conditions, compared to clindamycin phosphate (CP), the IL selector showed better enantioseparation capability and improved peak shapes for five racemic drugs. In addition, Molecular docking program Autodock was employed to elucidate the chiral recognition mechanism of Ch-CP, the computing results conformed to the experimental results.

Investigation of the synergistic effect of chiral ionic liquids as additives in non-aqueous capillary electrophoresis for enantioseparation

In this work, tetraalkylammonium amino acid ionic liquids (TAA-AAILs) were first applied to non-aqueous capillary electrophoresis (NACE) to establish synergistic systems with a conventional chiral selector, native β-cyclodextrin (β-CD). Excellent enantioseparations of some dansyl-amino acid (Dns-AA) samples were achieved. A series of comparison experiments and a molecular docking study were performed to validate the synergistic effect of TAA-AAILs and β-CD in NACE. Several interesting results were observed compared with previously reported chiral ILs-related aqueous CE studies. In particular, the direct enantioselectivity of TAA-AAILs was observed for the first time by using it as sole chiral selector in NACE. This was an encouraging finding because it was the first direct and convincing evidence that AAILs were able to participate in the enantiorecognition process in the conventional chiral selectors-based synergistic systems. The new TAA-AAILs synergistic NACE system was further optimized in terms of alkyl chain length, TAA-AAILs concentration, β-CD concentration, electrolyte composition and applied voltage, etc. Best enantioseparations of Dns-AAs were obtained when 100 mM β-CD and 10 mM tetramethylammonium-l-arginine (TMA-l-Arg) were added in an NMF buffer containing 50 mM Tris and 35 mM CA (apparent pH 7.85) under UV detection. The applied voltage was set at 30 kV. The method was then successfully employed for the determination of enantiomeric impurities of a real AA sample. This work proved that the use of chiral ILs as additives in NACE is a promising approach for enantioseparation.

Enantiomeric separation of ivabradine by cyclodextrin-electrokinetic chromatography. Effect of amino acid chiral ionic liquids

A chiral methodology was developed for the first time to ensure the quality control of ivabradine, a novel anti-ischemic and heart rate lowering drug commercialized as a pure enantiomer. With this aim, electrokinetic chromatography (EKC) was employed and the enantiomeric separation of ivabradine was investigated using different anionic and neutral cyclodextrins (CDs) and amino acid-based chiral ionic liquids (CILs) as sole chiral selectors. Baseline separation was only achieved with sulfated CDs, and the best enantiomeric resolution was obtained with sulfated-γ-CD. Under the optimized conditions, ivabradine enantiomers were separated in 6 min with a resolution of 2.7. Nuclear magnetic resonance experiments showed a 1:1 stoichiometry for the enantiomer-CD complexes and apparent and averaged equilibrium constants were determined. The combined use of sulfated-γ-CD and different CILs as dual separation systems was investigated, resulting in a significant increase in the resolution. The use of 5 mM tetrabutylammonium-aspartic acid ([TBA][L-Asp]) in 50 mM formate buffer (pH 2.0) containing 4 mM sulfated-γ-CD were considered the best conditions in terms of resolution and migration times for ivabradine enantiomers. Nevertheless, as no inversion of the enantiomer migration order was observed when combining CILs and sulfated-γ-CD and a good enantiomeric resolution and efficiency were obtained using just sulfated-γ-CD as the sole chiral selector, the analytical characteristics of this method were evaluated, showing good recovery (98% and 103% for S- and R-ivabradine, respectively) and precision values (RSD < 5% for instrumental repeatability, < 6% for method repeatability and < 7% for intermediate precision). The limits of detection (LODs) were 0.22 and 0.28 μg mL−1 for S- and R-ivabradine, respectively, and the method enabled to detect a 0.1% of the enantiomeric impurity, allowing to accomplish the requirements of the International Conference on Harmonisation (ICH) guidelines. Finally, the method was applied to the analysis of a pharmaceutical formulation of ivabradine. The content of R-ivabradine was below the LOD and the amount of S-ivabradine was in agreement to the labeled content.

Amino acid chiral ionic liquids combined with hydroxypropyl-β-cyclodextrin for drug enantioseparation by capillary electrophoresis

Four amino acid chiral ionic liquids were evaluated in dual systems with hydroxypropyl-β-cyclodextrin to investigate the enantioseparation by CE of a group of seven drugs as model compounds (duloxetine, verapamil, terbutaline, econazole, sulconazole, metoprolol, and nadolol). The use of two of these chiral ionic liquids (tetramethylammonium L-Lysine ([TMA][L-Lys]) and tetramethylammonium L-glutamic acid ([TMA][L-Glu])) as modifiers in CE is reported for the first time in this work whereas tetrabutylammonium L-lysine ([TBA][L-Lys]) and tetrabutylammonium L-glutamic acid ([TBA][L-Glu]) were employed previously in CE although very scarcely. The effect of the nature and the concentration of each ionic liquid added to the separation buffer containing the neutral cyclodextrin on the enantiomeric resolution and the migration time obtained for each drug, was investigated. A synergistic effect was observed when combining each chiral ionic liquid with hydroxypropyl-β-cyclodextrin in the case of the five compounds for which the cyclodextrin showed enantiomeric discrimination power when used as sole chiral selector (duloxetine, verapamil, terbutaline, econazole, sulconazole). Buffer concentration and pH, temperature and separation voltage were varied in order to optimize the enantiomeric separation of these five compounds using dual systems giving rise to resolutions ranging from 1.1 to 6.6.

Evaluation of an ionic liquid chiral selector based on clindamycin phosphate in capillary electrophoresis.

Recently, increasing attention has been given to the research on chiral ionic liquids (CILs) in chiral separation field; however, only a few literatures focus on the exploration of CILs as the sole chiral selector. In this study, an ionic liquid chiral selector based on antibiotic, namely tetramethylammonium clindamycin phosphate (TMA-CP), was originally synthesized and subsequently utilized for enantioseparation in capillary electrophoresis (CE). Remarkably improved separations of eight racemic analytes were achieved in TMA-CP system in contrast to the clindamycin phosphate (CP) system. The optimal separation conditions were determinated by systematic experiments on several crucial parameters including the type and proportion of organic modifier, CIL concentration, buffer pH, and applied voltage. Additionally, molecular modeling with AutoDock was applied to probe into the chiral recognition mechanism of the ionic liquid chiral selectors, which well corresponded with the experimental results. It is the first time that antibiotic-based ionic liquid was exploited as favorable sole chiral selector in CE, and this strategy has paved a new way for development of novel ionic liquids chiral selectors based on antibiotics. Graphical abstract.

Effect of the combined use of γ-cyclodextrin and a chiral ionic liquid on the enantiomeric separation of homocysteine by capillary electrophoresis

The enantioseparation of the non-protein amino acid homocysteine by CE was investigated in this article using eleven neutral CDs and five chiral ionic liquids as chiral selectors. Using a previous derivatization step with FMOC and the subsequent separation under neutral conditions, homocysteine enantiomers were only separated when γ-CD or (R)-N,N,N-trimethyl-2-aminobutanol-bis(trifluoromethane-sulfon)imidate (EtCholNTf2) were employed as sole chiral selectors in the separation buffer. On the one hand, γ-CD gave rise to the enantiomeric separation in 10min with a resolution value of 1.9, whereas EtCholNTf2 let to obtain a resolution value of 1.4 in more than 50min. Then, the evaluation of the combined use of both selectors was also carried out, resulting in a considerable increase in the Rs. The best enantioseparation for homocysteine was obtained when 10mM EtCholNTf2 was added to 50mM phosphate buffer (pH 7.0) containing 2mM γ-CD. In an attempt to discriminate specific chiral cation effect from the salt effect, the influence of adding LiNTf2 to the separation medium was also evaluated, resulting in lower resolution values for homocysteine when compared to those achieved with the addition of EtCholNTf2, indicating a synergistic effect between EtCholNTf2 and γ-CD. Interestingly, the enantiomer migration order changed depending on the use of a single chiral selector or dual systems. When EtCholNTf2 or γ-CD were employed as sole chiral selectors, D-enantiomer was the first-migrating enantiomer. However, an inversion in the migration order was observed when both selectors were combined in a dual system being the L-enantiomer the first-migrating one.

Tetramethylammonium-lactobionate: A novel ionic liquid chiral selector based on saccharides in capillary electrophoresis.

Chiral ionic liquids (ILs) have aroused widespread interest in separation science; however, only a few papers have reported the application of chiral ILs in CE for enantioseparation, and the use of chiral ILs as the sole chiral selector in an electrophoretic or a chromatographic system was reported in only three papers. In this study, we designed a lactobionic acid LA-based IL, tetramethylammonium-lactobionate (TMA-LA), and it is very interesting to find that the chiral separation capability can be remarkably improved when a conventional saccharide chiral selector evolved into an IL chiral selector. A comparative study of the enantiorecognition capability of three separation systems (single LA system, LA + TMA-chloride (TMA-Cl) system, and TMA-LA IL system) was also conducted, and the results showed that the use of TMA-LA IL as the sole chiral selector exhibited a remarkable superiority. A series of parameters affecting the enantioseparation, such as the type and proportion of organic modifier, buffer composition and pH, chiral selector concentration, as well as applied voltage were systematically investigated. The best enantioseparation was obtained at pH 7.6 using a 40 mM borax buffer with 40% v/v methanol, 200 mM TMA-LA, and 20 kV applied voltage. It is the first time that a saccharide-based IL is evaluated as a sole chiral selector in CE, and we hope this study would provide a new direction for the development of novel ILs chiral selectors based on conventional chiral selectors.

Use of chiral amino acid ester‐based ionic liquids as chiral selectors in CE

In this study, the applicability of a chiral ionic liquid (CIL) as the sole chiral selector in CE was investigated for the first time. In particular, five amino acid ester-based CILs were synthesized and used as additives in the BGE in order to evaluate their chiral recognition ability. The performance of these CILs as the sole chiral selectors was evaluated by using 1,1'-binaphthyl-2,2-diylhydrogenphosphate (BNP) as the analyte and by comparing the resolution values. Different parameters were examined, such as the alkyl group bulkiness and the configuration of the cation, the anion type of the CIL and its concentration, and the pH of the BGE, in order to optimize the separation of the enantiomers and to demonstrate the effect that each parameter has on the chiral-recognition ability of the CIL. Baseline separation of BNP within 13 min was achieved by using a BGE of 100 mM Tris/10 mM sodium tetraboratedecahydrate (pH 8) and a chiral selector of 60 mM l-alanine tert butyl ester lactate. The run-to-run and batch-to-batch reproducibilities were also evaluated by computing the %RSD values of the EOF and the two enantiomer peaks. In both cases, very good reproducibilities were observed, since all %RSD values were below 1%.

Chiral separation of amino acids by capillary electrophoresis with 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate as chiral selector

3-[(3-cholamidopropyl)-dimethylammoniol-1-propane sulfonate (CHAPS) can be used as an effective chiral selector for the separation of dansyl-amino acids by capillary electrophoresis (CE). While CHAPS can serve as an chiral selector, better enantiomeric separation can be performed by using CHAPS not as the sole chiral selector but as one of a [CHAPS-SDS-cyclodextrin] three-component system. In this CHAPS-SDS-CD system, enantiomeric separations of the amino acids can be readily accomplished by judiciously adjusting the pH of the solution, concentrations of CHAPS and SDS, and the concentration and type of CD. All amino acids can be baseline resolved in less than 15 minutes with resolution as high as 2.01 at pH 6.5 with 50 mM of CHAPS and 75 mM of SDS. The resolution is also dependent on the size of the CD. Substantial increase in the resolution can be readily achieved by replacing β-CD with γ-CD. For example, theR s for Leu was increased by four-folds (from 1.65 to 6.29) while the elution time still remains as short as 20 min when β-CD was replaced by γ-CD.

Chiral separation of amino acids by capillary electrophoresis with octyl-beta-thioglucopyranoside as chiral selector.

1-S-Octyl-beta-D-thioglucopyranoside (OTG) was evaluated as a chiral selector for the separation of dansyl-amino acids by capillary electrophoresis. Enantiomeric separations of the amino acids can be accomplished by judiciously adjusting the pH of the solution and the concentration of OTG. Better separation can be achieved, however, when OTG is used, not as a sole chiral selector but rather together with sodium dodecyl sulfate (SDS) and cyclodextrin (CD). Interestingly, not only can this OTG-SDS-CD system provide better separation than OTG alone but also it can separate compounds which cannot be separated when only CD or CD and SDS are used. All amino acids were baseline separated with this OTG-SDS-CD system at optimal conditions.