Chiral separations in capillary electrophoresis

Enantioselective liquid-liquid extraction of 2-cyclohexylmandelic acid enantiomers using chiral ionic liquids

Obtaining optically pure compounds in an eco-friendly and cost-efficient manner plays an important role in human health and pharmaceutical industry. Racemic separation using multistage stereoselective liquid-liquid extraction has become one of the most practical and effective approach to access homochiral enantiomers. Currently, chiral ionic liquids (CILs) with structural designability have become a promising chiral additive and enable them as adjustable candidates for racemic separation. Herein, a high-effective stereoselective liquid-liquid extraction process composed of imidazolium cations and amino acid-derived anions as the chiral additive was established for racemic 2-cyclohexylmandelic acid (CHMA) separation. We have systematically investigated the choice of organic solvent, concentration of CIL, extraction temperature, and the pH of aqueous phase. For three-stage stereoselective extraction, the maximum enantiomeric excess (e.e.) for CHMA was reached up to 40.6%. Furthermore, the mechanism of steric effect and stereoselective capacity between the CILs and racemic CHMA was discussed and simulated. We envision that the work will facilitate the development of CILs in multistage liquid-liquid extraction and promote the large-scale production of optically pure enantiomers.

The separation of cyclic diadenosine diphosphorothioate and the diastereomers of its difluorinated derivative and the estimation of the binding constants and ionic mobilities of their complexes with 2-hydroxypropyl-β-cyclodextrin by affinity capillary electrophoresis

The incorporation of phosphorothioate linkages has recently been extensively employed in therapeutic oligonucleotides. For their separation and quality control, new high-efficient and high-sensitive analytical methods are needed. In this work, a new affinity capillary electrophoresis method has been developed and applied for the separation of a potential anticancer drug, 2',3'-cyclic diadenosine diphosphorothioate (Rp, Rp) (ADU-S100), and three recently newly synthesized diastereomers of its difluorinated derivative, 3',3'-cyclic di(2'-fluoro, 2'-deoxyadenosine phosphorothioate). The separation was performed in the various background electrolytes (BGEs) within a pH range 5-9 using several native and derivatized cyclodextrins (CDs) as chiral additives of the BGE. Relatively good separations were obtained with β-, γ-, and 2-hydroxypropyl-γ-CDs in some of the BGEs tested. However, the best separation was achieved using the 2-hydroxypropyl-β-CD chiral selector at 43.5 mM average concentration in the BGE composed of 40 mM Tris, 40 mM tricine, pH 8.1. Under these conditions, all the previous four cyclic dinucleotides (CDNs) were baseline separated within 4 min. Additionally, the average apparent binding constants and the average actual ionic mobilities of the complexes of all four CDNs with 2-hydroxypropyl-β-CD in the above BGE were determined. The formed complexes were found to be relatively weak, with the average apparent binding constants in the range of 12.2-94.1 L mol-1 and with the actual ionic mobilities spanning the interval (-7.8 to -12.7) × 10-9 m2 V-1 s-1. The developed method can be applied for the separation, analysis, and characterization of the above and similar CDNs.

Supramolecular hybrids of chiral Waugh polyoxometalate with cyclodextrins

The development of novel systems for chiral polyoxometalates (POMs) is an attractive research field because of their fascinating topological structures and well-defined functions. Herein, we have developed a new reaction route for the synthesis of two unprecedented chiral Waugh POM-based supramolecular architectures. Single-crystal X-ray diffraction reveals that the architectures exhibit a wavy three-dimensional framework and bamboo-rod-connected framework upon regulating the size of the cyclodextrin and the stacking pattern of the D₃ symmetric Waugh {MnMo₉}. Solution studies using NMR, circular dichroism and isothermal titration calorimetry corroborate nicely the very weak interactions between the components. The intricate chiral microenvironment originating from the hybrid frameworks may be responsible for the selective recognition of the Λ-{MnMo₉} enantiomer. This study highlights the importance of the asymmetric configuration of the POM for designing CD/POM assemblies and understanding their chirality.

Chiral mesostructured hydroxide zinc carbonate for enantioseparation in high performance liquid chromatography

Chiral mesostructured hydroxide zinc carbonate coated silica gel (CMHZC@S) was prepared by a hydrothermal route using amino acids as symmetry-breaking agents. CMHZC@S with three levels of hierarchical chiralities, as a chiral stationary phase (CSP), showed enantiomeric separation ability for high performance liquid chromatography (HPLC).

R-VAPOL-phosphoric acid based 1H and 13C-NMR for sensing of chiral amines and acids

Enantiomers have significant importance in pharmaceuticals, biology and modern chemistry and therefore distinguishing and quantifying the enantiomeric forms is of utmost importance. Herein, we propose diphenyl-3,3'-biphenanthryl-4,4'-diyl phosphate (R-VAPOL-PA) as a promising chiral solvating agent to discriminate amines and acids of poly-functional groups such as chiral amines, amino alcohols and hydroxy acids. The methodological approach involves using the nature of hydrogen bonds and ion pairs as a mode of weak interactions to form diastereomers where the probe is associated with enantiomers. The resulting diastereomer difference in the NMR spectrum enables the chiral discrimination with a complete baseline peak separation and an accurate enantiomeric excess (ee) analysis. We also carried out density functional theory (DFT) calculations to understand the complex formation to explain enantiodiscrimination by analysing the formation and stability of different chiral complexes. The binding energy differences between enantiomeric forms revealed by DFT calculations are qualitatively in agreement with the diastereomer difference in the NMR spectrum and unequivocally establishes the suggested experimental protocol of R-VAPOL-PA-based enantiomeric discrimination.

Interrogation of fractional crystallization behavior of a newly exploited chiral resolution method for racemic 1-(pyridin-2-yl)ethylamine via DFT-D3 calculations of cohesive energy

A novel chiral separation method for 1-(pyridin-2-yl)ethylamine is developed and the underlying energetics is investigated by DFT-D3.

Studies of a pyridino-crown ether-based chiral stationary phase on the enantioseparation of biogenic chiral aralkylamines and α-amino acid esters by high-performance liquid chromatography

This paper reports the enantioseparation ability of a pyridino-18-crown-6 ether-based chiral stationary phase [(S,S)-CSP-1]. The enantiomeric discrimination of chiral stationary phase (S,S)-CSP-1 was evaluated by HPLC using the mixtures of enantiomers of various protonated primary aralkylamines [1-phenylethylamine hydrogen perchlorate (PEA), 2,3-dihydro-1H-inden-1-amine (1-aminoindan), 2,2'-(1,2-diaminoethane-1,2-diyl) diphenol (HPEN)] and perchlorate salts of α-amino acid esters [alanine benzyl ester (Ala-OBn), phenylalanine benzyl ester (Phe-OBn), phenylalanine methyl ester (Phe-OMe), phenylglycine methyl ester (PhGly-OMe), glutamic acid dibenzyl ester (Glu-diOBn), and valine benzyl ester (Val-OBn)]. The best enantioseparation was achieved in the case of PEA. The high enantioselectivity was rationalized by the strong π-π interaction of the extended π system of the aryl-substituted pyridine unit.

Separation of folinic acid diastereomers in capillary electrophoresis using a new cationic β-cyclodextrin derivative

A method for the separation of folinic acid diastereomers by capillary electrophoresis in chiral separation media was developed. Aiming to achieve a good separation of the anionic analytes, a newly synthesized cationic β-cyclodextrin derivative, mono-6-deoxy-6-piperdine-β-cyclodextrin, was applied as the chiral selector. The effect of background electrolyte pH, the concentration of the cyclodextrin additive, and organic modifier on the separation was investigated. A good separation of folinic acid diastereomers was obtained with 30 mmol/L phosphate buffer at pH 6.50 containing 6.0 mmol/L of mono-6-deoxy-6-piperdine-β-cyclodextrin in 10% acetonitrile. Based on the capillary electrophoresis data, the binding constants of each diastereomer with mono-6-deoxy-6-piperdine-β-cyclodextrin were determined. Moreover, a computational modeling study, using the semi-empirical PM3 method, was used to discuss the possible mechanism of separation of folinic acid with mono-6-deoxy-6-piperdine-β-cyclodextrin.

Optical recognition of alkyl nitrile by a homochiral iron(ii) spin crossover host

In this work, a chiral mononuclear spin crossover host1·MeCN was found to have the ability to optically recognize lactonitrile (LN) and methylglutaronitrile (MGN).

Enantioseparation of dansyl amino acids and dipeptides by chiral ligand exchange capillary electrophoresis based on Zn(II)-L-hydroxyproline complexes coordinating with γ-cyclodextrins

A chiral ligand exchange capillary electrophoresis (CLE-CE) method using Zn(II) as the central ion and L-4-hydroxyproline as the chiral ligand coordinating with γ-cyclodextrin (γ-CD) was developed for the enantioseparation of amino acids (AAs) and dipeptides. The effects of various separation parameters, including the pH of the running buffer, the ratio of Zn(II) to L-4-hydroxyproline, the concentration of complexes and cyclodextrins (CDs) were systematically investigated. After optimization, it has been found that eight pairs of labeled AAs and six pairs of labeled dipeptides could be baseline-separated with a running electrolyte of 100.0mM boric acid, 5.0mM ammonium acetate, 3.0mM Zn(II), 6.0mM L-hydroxyproline and 4.0mM γ-CD at pH 8.2. The quantitation of AAs and dipeptides was conducted and good linearity (r(2)≥0.997) and favorable repeatability (RSD≤3.6%) were obtained. Furthermore, the proposed method was applied in determining the enantiomeric purity of AAs and dipeptides. Meanwhile, the possible enantiorecognition mechanism based on the synergistic effect of chiral metal complexes and γ-CD was explored and discussed briefly.

Single-molecule force spectroscopic study on chiral recognition of cysteine derivatives immobilized on a gold substrate by using AFM tips chemically modified with optically active crown ethers

The chiral recognition of cysteine derivatives on a gold substrate was investigated using AFM tips modified with optically active crown ethers.

Chiral separation of bavachinin in Fructus Psoraleae and rat plasma by liquid chromatography using permethylated-β-CD as a chiral selector

A simple, sensitive and selective method of high-performance liquid chromatography (HPLC) has been successfully developed for separation of bavachinin enantiomers in Fructus Psoraleae and rat plasma. The separation and detection conditions of HPLC were optimized. Chiral bavachinin were separated with the mobile phase of methanol and water (70:30, v/v) at a flow rate of 1.0 mL/min. The linear ranges were in the range of 20–1000 μg/mL. The detection limits were tested as 4 ng/mL and 6 ng/mL for (+)-bavachinin and (−)-bavachinin, respectively. The method has been applied to analyze chiral bavachinin in rat plasma. HPLC–MS method was used to test the accuracy.

Chiral separation using capillary electromigration techniques based on ligand exchange principle

Over the last couple of decades, researchers have developed diverse chiral separation methods emerged from a few chiral separation principles. This review article is primarily focused on the application of chiral ligand-exchange (CLE) principle in capillary electromigration techniques, such as capillary electrophoresis (CE) and capillary electrochromatography (CEC). First, the most commonly used CLE-CZE separation mode by using different kinds of central ions, such as Cu(II), Zn(II), borate ion, and other metal ions, has been introduced. Meanwhile, several kinds of surfactants have been applied as the micelle-forming agents in the CLE micellar electrokinetic chromatography mode. The highlight of recent research of CLE-CEC is the exploitation of novel columns for chiral separation. Then, two kinds of capillary columns, packed capillary and monolithic capillary column, have been briefly described. Finally, the effective application of these chiral separation methods has been presented, including the application in life science and food analysis area.

Electrophoresis today and tomorrow: Helping biologists' dreams come true

Intensive research and development of electrophoresis methodology and instrumentation during past decades has resulted in unique methods widely implemented in bioanalysis. While two-dimensional electrophoresis and denaturing polyacrylamide gel electrophoresis in sodium dodecylsulfate are still the most frequently used electrophoretic methods applied to analyses of proteins, new miniaturized capillary and microfluidic versions of electromigrational methods have been developed. High-throughput electrophoretic instruments with hundreds of capillaries for parallel separations and laser-induced fluorescence detection of labeled DNA strands have been of key importance for the scientific and commercial success of the Human Genome Project. Another powerful method, capillary isoelectric focusing with pressurized and pH-driven mobilization, provides efficient separations and on-line sensitive detection of proteins, bacteria and viruses. Electrophoretic microfluidic devices can integrate single-cell injection, cell lysis, separation of its components and fluorescence or mass spectrometry detection. These miniaturized devices also proved the capability of single-molecule detection.

Advanced CE for chiral analysis of drugs, metabolites, and biomarkers in biological samples

An analysis of recent trends indicates that CE can show real advantages over chromatographic methods in ultratrace enantioselective determination of biologically active compounds in complex biological matrices. It is due to high separation efficiency and many applicable in-capillary electromigration effects in CE (countercurrent migration, stacking effects) enhancing significantly (enantio)separability and enabling effective sample preparation (preconcentration, purification, analyte derivatization). Other possible on-line combinations of CE, such as column coupled CE-CE techniques and implementation of nonelectrophoretic techniques (extraction, membrane filtration, flow injection) into CE, offer additional approaches for highly effective sample preparation and separation. CE matured to a highly flexible and compatible technique enabling its hyphenation with powerful detection systems allowing extremely sensitive detection (e.g. LIF) and/or structural characterization of analytes (e.g. MS). Within the last decade, more as well as less conventional analytical on-line approaches have been effectively utilized in this field and their practical potentialities are demonstrated on many new application examples in this article. Here, three basic areas of (enantioselective) drug bioanalysis are highlighted and supported by a brief theoretical description of each individual approach in a compact review structure (to create integrated view on the topic), including (i) progressive enantioseparation approaches and new enantioselective agents, (ii) in-capillary sample preparation (preconcentration, purification, derivatization), and (iii) detection possibilities related to enhanced sensitivity and structural characterization.

Effect of chirality on PVP/drug interaction within binary physical mixtures of ibuprofen, ketoprofen, and naproxen: a DSC study

We report on the thermal behavior of freshly prepared binary drug/polymer physical mixtures that contained ibuprofen, ketoprofen, or naproxen as a drug, and polyvinylpyrrolidone (PVP), hydroxyethylcellulose (HEC), or methylcellulose (MC) as excipient. At 6-10 degrees C/min heating rates the DSC detected a sharp, single endotherm that corresponds to the melting of drug. On heating physical mixtures of PVP and racemic ibuprofen or ketoprofen at lower heating rates, another endotherm was registered in front of the original one. To observe the additional endotherm, specific minimal values of the heating rate and of PVP weight fraction were needed; for ibuprofen and ketoprofen they were 1.5 and 2.0 degrees C/min, and 5 and 15% (w/w), respectively. At greater PVP weight fractions the top temperatures, T(mp), of both peaks were reduced almost linearly indicating strong solid-state interfacial reaction between the drug particles and PVP matrix. The additional endotherm was abolished at greater heating rates (2 degrees C/min for ibuprofen, 3 degrees C/min for ketoprofen), by replacing the racemate with respective S+-enantiomer and by replacing PVP with HEC and MC. Hence, the possible inclusion of enantioselective component within the PVP/drug interaction, responsible for the amorphization of physical mixture over storage, is assumed.