Synthesis, characterization, and application of chiral ionic liquids and their polymers in micellar electrokinetic chromatography

Amino acids-based deep eutectic solvents as additives for improved enantioseparation in capillary electrophoresis

In this study, several amino acids deep eutectic solvents were prepared using L-valine and L-leucine as hydrogen bond acceptors, and L-lactic acid and glycerol as hydrogen bond donors. These amino acids' deep eutectic solvents were first used as buffer additives to construct several synergistic systems along with maltodextrin in capillary electrophoresis for the enantioseparations of four racemic drugs. Compared with single maltodextrin system, the separations of model drugs in the synergistic systems were significantly improved. Some key parameters affecting chiral separation such as maltodextrin concentration, deep eutectic solvent concentration, buffer pH, and applied voltage were optimized. In order to further understand the specific mechanism of the amino acids deep eutectic solvents in improving chiral separation, we first calculated the binding constants of maltodextrin with enantiomers using the capillary electrophoresis method in the two separation modes, respectively. We also used molecular simulation to calculate the binding free energy of maltodextrin with enantiomers. It is the first time that amino acids deep eutectic solvents were used for enantioseparation in capillary electrophoresis, which will greatly promote the development of deep eutectic solvents in the field of chiral separation.

Cyclodextrin Incorporation into Covalent Organic Frameworks Enables Extensive Liquid and Gas Chromatographic Enantioseparations

The separation of enantiomers using high-performance chromatography technologies represents great importance and interest. In this aspect, β-cyclodextrin (β-CD) and its derivatives have been extensively studied as chiral stationary phases (CSPs). Nevertheless, β-CD that was immobilized on a traditional matrix often exhibited low stabilities and limited operating ranges. Recently, covalent organic frameworks (COFs) with highly ordered nanopores are emerging as promising CSPs for enantioseparations, but their practical applications are still hampered by the difficulty of monomer and COF synthesis. Herein, two β-CD-driven COFs are synthesized via a fast and facile plasma-induced polymerization combined postsynthesis modification strategy. The precisely defined COF channels enhanced the accessibility of the accommodated β-CD to the analytes and acted as robust protective barriers to safeguard the β-CD from harsh environments. Therefore, the β-CD-modified COFs can be potentially general CSPs for extensive enantioseparation in both gas chromatography and high-performance liquid chromatography, and a wide range of racemates were separated. Compared to the commonly employed commercial chiral columns, these COF-based columns exhibited comparable resolution capability and superior application versatility. This work integrates the advantages and overcomes the defects of COFs and β-CD, thus advancing COFs as platforms for chiral selector modification and giving great promise for practical chromatographic enantioseparation.

Evaluation of kasugamycin as a chiral selector in capillary electrophoresis

The discovery of novel chiral selectors always fascinates us. This work describes the chiral separation performances of a new chiral selector (kasugamycin, KAS) in capillary electrophoresis (CE) for six pairs of stereoisomers, including ephedrine and pseudoephedrine, quinine and quinidine, cinchonine and cinchonidine, and amlodipine, promethazine and ofloxacin enantiomers. Kasugamycin, an aminoglycoside antibiotic in agriculture, shows significant biological activity against rice blast with low toxicity. As it turns out, this new chiral selector possesses good CE compatibility and stereoselectivity towards model analytes. In this work, we systematically investigated several separation parameters including kasugamycin concentration, buffer pH, separation voltage and the composition of the buffer solution. A detailed discussion about the chiral recognition mechanism was made based on Statistical Product and Service Solution (SPSS) analysis, NMR experiments (1D and 2D) and molecular modeling. This is the first time that kasugamycin is utilized as a chiral selector in CE, and the development of new chiral selectors from agricultural or veterinary antibiotics deserves more attention.

Predicting the aggregation number of cationic surfactants based on ANN-QSAR modeling approaches: understanding the impact of molecular descriptors on aggregation numbers

In this work, a quantitative structure-activity relationship (QSAR) study is performed on some cationic surfactants to evaluate the relationship between the molecular structures of the compounds with their aggregation numbers (AGGNs) in aqueous solution at 25 °C. An artificial neural network (ANN) model is combined with the QSAR study to predict the aggregation number of the surfactants. In the ANN analysis, four out of more than 3000 molecular descriptors were used as input variables, and the complete set of 41 cationic surfactants was randomly divided into a training set of 29, a test set of 6, and a validation set of 6 molecules. After that, a multiple linear regression (MLR) analysis was utilized to build a linear model using the same descriptors and the results were compared statistically with those of the ANN analysis. The square of the correlation coefficient (R ²) and root mean square error (RMSE) of the ANN and MLR models (for the whole data set) were 0.9392, 7.84, and 0.5010, 22.52, respectively. The results of the comparison revealed the efficiency of ANN in detecting a correlation between the molecular structure of surfactants and their AGGN values with a high predictive power due to the non-linearity in the studied data. Based on the ANN algorithm, the relative importance of the selected descriptors was computed and arranged in the following descending order: H-047 > ESpm12x > JGI6> Mor20p. Then, the QSAR data was interpreted and the impact of each descriptor on the AGGNs of the molecules were thoroughly discussed. The results showed there is a correlation between each selected descriptor and the AGGN values of the surfactants.

Investigation on improvement of enantioseparation in capillary electrophoresis based on maltodextrin by chiral ionic liquids

Taking advantage of chiral ionic liquids, this study deals with the improvement of the enantioseparation performance of a traditional chiral selector (maltodextrin) in capillary electrophoresis. Herein, two polyhydroxy compound-based chiral ionic liquids, namely tetramethylammonium-D-gluconic acid and tetramethylammonium-shikimic acid were designed and utilized as additives for chiral separation for the first time. The synergistic systems provided much better enantioseparations of twelve model drugs compared to the single maltodextrin system. These model analytes contained analgesics, antidepressants, antiallergic drugs, antifungal drugs, antihypertensive drugs, and antiparkinsonian drugs. After optimizing the separation conditions, the chiral recognition mechanism was probed by means of ultraviolet spectroscopy, nuclear magnetic resonance, and molecular modeling. The results of spectroscopic and computational analyses were in good consistency with enantioseparation outcomes. Finally, the proposed method was successfully used for the determination of the enantiomeric purity of duloxetine hydrochloride.

Are Highly Stable Covalent Organic Frameworks the Key to Universal Chiral Stationary Phases for Liquid and Gas Chromatographic Separations?

High-performance liquid chromatography (HPLC) and gas chromatography (GC) over chiral stationary phases (CSPs) represent the most popular and highly applicable technology in the field of chiral separation, but there are currently no CSPs that can be used for both liquid and gas chromatography simultaneously. We demonstrate here that two olefin-linked covalent organic frameworks (COFs) featuring chiral crown ether groups can be general CSPs for extensive separation not only in GC but also in normal-phase and reversed-phase HPLC. Both COFs have the same 2D layered porous structure but channels of different sizes and display high stability under different chemical environments including water, organic solvents, acids, and bases. Chiral crown ethers are periodically aligned within the COF channels, allowing for enantioselective recognition of guest molecules through intermolecular interactions. The COF-packed HPLC and GC columns show excellent complementarity and each affords high resolution, selectivity, and durability for the separation of a wide range of racemic compounds, including amino acids, esters, lactones, amides, alcohols, aldehydes, ketones, and drugs. The resolution performances are comparable to and the versatility is superior to those of the most widely used commercial chiral columns, showing promises for practical applications. This work thus advances COFs with high stability as potential universal CSPs for chromatography that are otherwise hard or impossible to produce.

Effects of amino acid-derived chiral ionic liquids on cyclodextrin-mediated capillary electrophoresis enantioseparations of dipeptides

The synergistic effect of chiral ionic liquids composed of tetraalkylammonium ions and the amino acids Asn, Asp or Pro on the enantioseparations of dipeptides mediated by β-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin in capillary electrophoresis was studied. Addition of a chiral ionic liquid resulted in a concentration-dependent increase in the enantioresolutions compared to the sole presence of a cyclodextrin in the background electrolyte. The extent varied with the tetraalkylammonium cation (tetramethylammonium versus tetrabutylammonium) as well as the amino acid component of the ionic liquid. The presence of a chiral ionic liquid did not counteract the pH-dependent reversal of the enantiomer migration order of the dipeptides Ala-Phe, Ala-Tyr and Phe-Phe when increasing the pH of the background electrolyte from 2.5 to 3.5. Comparing the effect of a chiral ionic liquid based on Asp with the addition of equimolar concentrations of the individual components of the ionic liquid, a diverse picture was observed. In some cases, higher resolution values were obtained with the chiral ionic liquid, while for other cases superior enantioseparations were obtained upon separate addition of the amino acid component and a tetraalkylammonium chloride. With regard to the stereochemistry of the amino acid, a superior effect was typically observed using the l-configured amino acid, but in some cases higher resolution values were found in the presence of d-Asp. The rationale for the diverse observations is not obvious and may be due to the zwitterionic nature of analytes as well as the amino acid component of the chiral ionic liquid.

Chiral Capillary Electrokinetic Chromatography: Principle and Applications, Detection and Identification, Design of Experiment, and Exploration of Chiral Recognition Using Molecular Modeling

This work reviews the literature of chiral capillary electrokinetic chromatography from January 2016 to March 2021. This is done to explore the state-of-the-art approach and recent developments carried out in this field. The separation principle of the technique is described and supported with simple graphical illustrations, showing migration under normal and reversed polarity modes of the separation voltage. The most relevant applications of the technique for enantioseparation of drugs and other enantiomeric molecules in different fields using chiral selectors in single, dual, or multiple systems are highlighted. Measures to improve the detection sensitivity of chiral capillary electrokinetic chromatography with UV detector are discussed, and the alternative aspects are explored, besides special emphases to hyphenation compatibility to mass spectrometry. Partial filling and counter migration techniques are described. Indirect identification of the separated enantiomers and the determination of enantiomeric migration order are mentioned. The application of Quality by Design principles to facilitate method development, optimization, and validation is presented. The elucidation and explanation of chiral recognition in molecular bases are discussed with special focus on the role of molecular modeling.

Stability study of α-bromophenylacetic acid: Does it represent an appropriate model analyte for chiral separations?

The stability of α-bromophenylacetic acid (BPAA) in 50% aqueous methanol solution has been tested. CE in different running buffers was used to separate BPAA from the decomposition reaction products α-hydroxyphenylacetic (mandelic) acid and α-methoxyphenylacetic acid. Suitable CE separation of all three compounds and other product, bromide, was achieved in 60 mmol/L formate buffer (pH 3.0) at -30 kV in 50 μm (i.d.) poly(vinyl alcohol)-coated fused silica capillary (30 cm/24.5 cm) with UV detection at 200 nm. The CE method was applied to determine the reaction order of the decomposition of BPAA (0.47 mmol/L) via nucleophilic substitution in 50% aqueous methanol. The first-order reaction kinetics was confirmed by linear and non-linear regression, giving the rate constants 1.52 × 10^-4 ± 2.76 × 10^-5 s^-1 and 7.89 × 10^-5 ± 5.02 × 10^-6 s^-1, respectively. Additionally, the degradation products were identified by CE coupled to mass spectrometric (MS) detection. The CE-MS experiments carried out in 60 mmol/L formate buffer (pH 3.0) and in 60 mmol/L acetate buffer (pH 5.0) confirmed the results obtained by CE-UV. Furthermore, the stability of BPAA in polar solvents was tested by ¹H NMR experiments. Our results provide strong evidence of the instability and fast degradation of BPAA in 50% aqueous methanol indicating that BPAA is not suitable as the model analyte for chiral separations.

Recent advances in the enantioseparation promoted by ionic liquids and their resolution mechanisms

More and more various chemical media are being applied in enantioseparation; among them, ionic liquids (ILs) have attracted the long-term attention in this decade as green designable solvents. This paper provides comprehensive overview for the applications of ILs in chiral extraction, gas chromatography, liquid chromatography, capillary electrophoresis and other techniques for enantioseparation. Additionally, the important resolution mechanisms based on ILs have also been summarized and discussed. This review focuses on the latest development of enantioseparation methods by using ILs in various modes, leading to meaningful and valuable information to related fields and thus promotes further research and application of reported methods.

Chiral Selectors in Capillary Electrophoresis: Trends During 2017⁻2018

Chiral separation is an important process in the chemical and pharmaceutical industries. From the analytical chemistry perspective, chiral separation is required for assessing the fit-for-purpose and the safety of chemical products. Capillary electrophoresis, in the electrokinetic chromatography mode is an established analytical technique for chiral separations. A water-soluble chiral selector is typically used. This review therefore examines the use of various chiral selectors in electrokinetic chromatography during 2017–2018. The chiral selectors were both low and high (macromolecules) molecular mass molecules as well as molecular aggregates (supramolecules). There were 58 papers found by search in Scopus, indicating continuous and active activity in this research area. The macromolecules were sugar-, amino acid-, and nucleic acid-based polymers. The supramolecules were bile salt micelles. The low molecular mass selectors were mainly ionic liquids and complexes with a central ion. A majority of the papers were on the use or preparation of sugar-based macromolecules, e.g., native or derivatised cyclodextrins. Studies to explain chiral recognition of macromolecular and supramolecular chiral selectors were mainly done by molecular modelling and nuclear magnetic resonance spectroscopy. Demonstrations were predominantly on drug analysis for the separation of racemates.

Enantioseparation of amino alcohol drugs by nonaqueous capillary electrophoresis with a maltobionic acid-based ionic liquid as the chiral selector

This study deals with the nonaqueous capillary electrophoretic enantioseparation of twenty-two amino alcohol drugs with a maltobionic acid (MA)-based ionic liquid (tetramethylammonium maltobionic acid, TMA-MA) as the novel chiral selector.

Enantioseparation by Capillary Electrophoresis Using Ionic Liquids as Chiral Selectors

Capillary electrophoresis (CE) is one of the most widely employed analytical techniques to achieve enantiomeric separations. In spite of the fact that there are many chiral selectors commercially available to perform enantioseparations by CE, one of the most relevant topics in this field is the search for new selectors capable of providing high enantiomeric resolutions. Chiral ionic liquids (CILs) have interesting characteristics conferring them a high potential in chiral separations although only some of them are commercially available. The aim of this article is to review all the works published on the use of CILs as chiral selectors in the development of enantioselective methodologies by CE, covering the period from 2006 (when the first research work on this topic was published) to 2017. The use of CILs as sole chiral selectors, as chiral selectors in dual systems or as chiral ligands will be considered. This review also provides detailed analytical information on the experimental conditions used to carry out enantioseparations in different fields as well as on the separation mechanism involved.

Investigation of maltodextrin-based synergistic system with amino acid chiral ionic liquid as additive for enantioseparation in capillary electrophoresis

The combined use of chiral ionic liquids (ILs) and chiral selectors in capillary electrophoresis (CE) to establish a synergistic system has proven to be an effective approach for enantioseparation. In this article, tetramethylammonium-L-arginine, a kind of amino acid chiral IL, was applied to investigate its potential synergistic effect with maltodextrin in CE enantioseparation. The established maltodextrin-based synergistic system showed markedly improved enantioseparations compared with the single maltodextrin system. Parameters such as the chiral IL concentration, maltodextrin concentration, buffer pH, applied voltage, and capillary temperature were optimized. Satisfactory enantioseparation of the five studied drugs, including nefopam, duloxetine, ketoconazole, cetirizine, and citalopram was achieved in 50 mM Tris-H₃ PO₄ buffer solution (pH 3.0) containing 7.0% (m/v) maltodextrin and 60 mM tetramethylammonium-L-arginine. In addition, the chiral configuration of tetramethylammonium-L-arginine was also investigated to demonstrate the existence of a synergistic effect between chiral ILs and maltodextrin.

Renewable sources from plants as the starting material for designing new terpene chiral ionic liquids used for the chromatographic separation of acidic enantiomers

Synthesis of cheap and natural resources is an important topic in green chemistry.

Development of a high speed counter-current chromatography system with Cu(II)-chiral ionic liquid complexes and hydroxypropyl-β-cyclodextrin as dual chiral selectors for enantioseparation of naringenin

Cu(II) complexed amino acid ionic liquid, Cu(II)-[1-butyl-3-methylimidazolium][L-Pro] (Cu(II)-[BMIm][L-Pro]), was successfully adopted as chiral ligand to improve the enantioseparation efficiency in high speed counter-current chromatography (HSCCC). For the enantioseparation of intractable naringenin (NRG) racemic mixtures, Cu(II)-[BMIm][L-Pro] coupled with hydroxypropyl-β-cyclodextrin (HP-β-CD) was successfully applied as dual chiral selectors in HSCCC. The influence of important parameters, including the concentration of the chiral selectors, the pH value, and the temperature were investigated. Under optimal conditions, 4.5mg of (+)-NRG and 4.1mg of (-)-NRG were successfully separated from 10mg NRG racemic mixtures with the purity of 98%. The chiral recognition mechanism of dual chiral selectors was illuminated by the UV-vis and NMR spectra, suggesting that the enantioseparation was upon the difference of the thermodynamic stability of the quaternary complexes of Cu(II), [BMIm][L-Pro], HP-β-CD, and NRG. The results illustrated that the developed HSCCC system, based on the synergistic mechanism of Cu(II)-[BMIm][L-Pro] and HP-β-CD, exhibited better performance on enantioseparation and had great application potential in preparative chiral separation of natural products.

A remarkable chiral recognition of racemic Mosher's acid salt by naturally derived chiral ionic liquids using 19F NMR spectroscopy

A new class of d-xylose derived imidazolium-based chiral ionic liquids were designed and synthesized via simple tuning approaches.

Synthesis and properties of novel chiral imidazolium-based ionic liquids derived from carvone

A large series of novel chiral imidazolium ionic liquids were synthesized using the terpenoid carvone as the chiral substrate.

Self-assembly of isomannide-based monoesters of C18-fatty acids and their cellular uptake studies

The self-assembling behavior of oleic, elaidic and stearic acid-isomannide glycolipids is revealed.

Ionic Liquids in Capillary Electrophoresis

Recently, a great interest was drawn toward ionic liquids (ILs) in analytical separation techniques. ILs possess many properties making them excellent additives in capillary electrophoresis (CE) background electrolytes (BGE). The most important property is the charge of the dissolved ions in BGE enabling the cations to interact with deprotonated silanol groups on the capillary surface and thereby modifying the electroosmotic flow (EOF). Ionic and/or proton donor-acceptor interactions between analyte and IL are possible interactions facilitating new kinds of separation mechanisms in CE. Further advantages of ILs are the high conductivity, the environmentally friendliness, and the good solubility for organic and inorganic compounds. The most commonly used ILs in capillary electrophoresis are dialkylimidazolium-based ILs, whereas for enantioseparation a lot of innovative chiral cations and anions were investigated.ILs are reported to be additives to a normal CE background electrolyte or the sole electrolyte in CE, nonaqueous CE (NACE), micellar electrokinetic chromatography (MEKC), and in enantioseparation. An overview of applications and separation mechanisms reported in the literature is given here, in addition to the enantioseparation of pseudoephedrine using tetrabutylammonium chloride (TBAC) as IL additive to an ammonium formate buffer containing β-cyclodextrin (β-CD).

Fabrication of chiral amino acid ionic liquid modified magnetic multifunctional nanospheres for centrifugal chiral chromatography separation of racemates

As the rapid development of nanotechnology, the magnetic nanospheres modified with special chiral selective ligands show a great potentiality in enantiomeric separation. In this study, magnetic nanospheres modified with task-specific chiral ionic liquid were designed for the separation of chiral amino acids. These modified magnetic nanospheres were effective in a direct chiral separation of five racemic amino acids (D- and L-cysteine, D- and L-arginine, D- and L-leucine, D- and L-glutamine and D- and L-tryptophan). Furthermore, a new online method for complete separation of the enantiomers via the magnetic nanospheres was established with centrifugal chiral chromatography using a spiral tube assembly mounted on a type-J coil planet centrifuge. One kind of chiral compounds, D- and L-tryptophan was resolved well using this method. These results demonstrated that the modified nanospheres display a good chiral recognition ability, and can be used as a potential material for chiral separation of various racemates.

Combined use of chiral ionic liquid surfactants and neutral cyclodextrins: evaluation of ionic liquid head groups for enantioseparation of neutral compounds in capillary electrophoresis

Cyclodextrins (CDs) are most commonly used chiral selectors in capillary electrophoresis (CE). Although the use of neutral CDs and its derivatives have shown to resolve plethora of charged enantiomers, they cannot resolve neutral enantiomers. The use of ionic liquids (ILs) surfactants forming successful complex with CDs present itself an opportunity to resolve neutral enantiomers. In this work, the effect of IL head groups and their complexation ability with heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin (TM-β-CD) was studied for the separation of neutral enantiomers by CE. First, cationic IL type surfactants with different chiral head groups were synthesized. Physicochemical properties such as critical micelle concentration were determined by surface tension, whereas aggregation and polarity were determined by fluorescence spectroscopy. The complexation ability of ILs with TM-β-CD was characterized in the gas phase by CE-mass spectrometry. The influence of the type of ILs head group and its concentration on chiral resolution, resolution per unit time and selectivity were investigated for four structurally diverse neutral compounds. The binding constants of the neutral analytes to the IL-CD complex were estimated by y-reciprocal method. The hydrophobicity of the side chain of the IL head group displayed significant effect on the binding constants and enantioseparations.

Recent advances of ionic liquids and polymeric ionic liquids in capillary electrophoresis and capillary electrochromatography

Ionic liquids (ILs) and polymeric ionic liquids (PILs) with unique and fascinating properties have drawn considerable interest for their use in separation science, especially in chromatographic techniques. In this article, significant contributions of ILs and PILs in the improvement of capillary electrophoresis and capillary electrochromatography are described, and a specific overview of the most relevant examples of their applications in the last five years is also given. Accordingly, some general conclusions and future perspectives in these areas are discussed.