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

Update on chiral recognition mechanisms in separation science

The stereospecific analysis of chiral molecules is an important issue in many scientific fields. In separation sciences, this is achieved via the formation of transient diastereomeric complexes between a chiral selector and the selectand enantiomers driven by molecular interactions including electrostatic, ion-dipole, dipole-dipole, van der Waals or π-π interactions as well as hydrogen or halogen bonds depending on the nature of selector and selectand. Nuclear magnetic resonance spectroscopy and molecular modeling methods are currently the most frequently applied techniques to understand the selector-selectand interactions at a molecular level and to draw conclusions on the chiral separation mechanism. The present short review summarizes some of the recent achievements for the understanding of the chiral recognition of the most important chiral selectors combining separation techniques with molecular modeling and/or spectroscopic techniques dating between 2020 and early 2024. The selectors include polysaccharide derivatives, cyclodextrins, macrocyclic glycopeptides, proteins, donor-acceptor type selectors, ion-exchangers, crown ethers, and molecular micelles. The application of chiral ionic liquids and chiral deep eutectic solvents, as well as further selectors, are also briefly addressed. A compilation of all published literature on chiral selectors has not been attempted.

Strategies for chiral separation: from racemate to enantiomer

Chiral separation has become a crucial topic for effectively utilizing superfluous racemates synthesized by chemical means and satisfying the growing requirements for producing enantiopure chiral compounds. However, the remarkably close physical and chemical properties of enantiomers present significant obstacles, making it necessary to develop novel enantioseparation methods. This review comprehensively summaries the latest developments in the main enantioseparation methods, including preparative-scale chromatography, enantioselective liquid-liquid extraction, crystallization-based methods for chiral separation, deracemization process coupling racemization and crystallization, porous material method and membrane resolution method, focusing on significant cases involving crystallization, deracemization and membranes. Notably, potential trends and future directions are suggested based on the state-of-art "coupling" strategy, which may greatly reinvigorate the existing individual methods and facilitate the emergence of cross-cutting ideas among researchers from different enantioseparation domains.

From capillaries to microchips, green electrophoretic features for enantiomeric separations: A decade review (2013-2022)

Enantioseparation by the electromigration-based method is well-established and widely discussed in the literature. Electrophoretic strategies have been used to baseline resolve complex enantiomeric mixtures, typically using a selector substance into the background electrolyte (BGE) from capillaries to microchips. Along with developing new materials/substances for enantioseparations, it is the concern about the green analytical chemistry (GAC) principles for method development and application. This review article brings a last decade's update on the publications involving enantioseparation by electrophoresis for capillary and microchip systems. It also brings a critical discussion on GAC principles and new green metrics in the context of developing an enantioseparation method. Chemical and green features of native and modified cyclodextrins are discussed. Still, given the employment of greener substances, ionic liquids and deep-eutectic solvents are highlighted, and some new selectors are proposed. For all the mentioned selectors, green features about their production, application, and disposal are considered. Sample preparation and BGE composition in GAC perspective, as well as greener derivatization possibilities, were also addressed. Therefore, one of the goals of this review is to aid the electrophoretic researchers to look where they have not.

The potential of the use of deep eutectic solvents and amino acid-based ionic liquids to enhance the chiral discrimination ability of different chiral selectors in capillary electrophoresis

The effect of the combined use of amino acid-based ionic liquids (AAILs) and deep eutectic solvents (DESs) with either cyclodextrin- (CD) or cyclofructan- (CF) based chiral selectors for the chiral separation of amphetamine derivatives was investigated in the present study. A non-significant improvement in enantiomeric separation of target analytes was observed when AAILs were combined with either CF or CD. On the other side, a markedly improved chiral separation of enantiomers was obtained using the dual carboxymethyl-β-cyclodextrin/DES system, highlighting the existence of a synergistic effect. After the addition of 0.5% v/v of choline chloride-ethylene glycol, the resolution of the enantiomers of amphetamine, methamphetamine and 3-fluorethamphetamine, increased from 1.4, 1.1, 1.0 to 1.8, 1.8, and 1.5 min, and the analysis times increased from 19.54, 20.48, 18.71 to 35.71, 35.78 and 32.90 min, respectively. This was not the case for the CF/DES dual system, in which the separation of amphetamines worsened, indicating an antagonistic effect. In conclusion, DESs are a very promising additive in capillary electrophoresis that can improve the separation of chiral molecules in combination with CDs but not CFs.

Copper (II) Ions Induced Self-Disproportionation of Enantiomers in Capillary Electrophoresis for the Quantification of Atenolol Enantiomers

Despite the fact that the self-disproportionation of enantiomers (SDE) has been found for several decades and has been widely used in crystallization, sublimation and chromatography for the purification or separation of nonracemic compounds, the phenomenon of SDE in capillary electrophoresis (CE) has never been reported up to now. Here, a new approach to separate enantiomers in CE based on SDE was demonstrated by introducing copper (II) ions into the separation media. The enantiomers of atenolol interact with copper ions to produce positively charged complexes with different electrophoretic mobilities from the single molecules. The dynamic equilibrium between homo- or heterochiral complexes (associates) and single molecules of atenolol enantiomers supports the manifestation of SDE. Different mobilities of the single molecules and associates, and different distribution of two enantiomers between the single molecules and associates caused by their different concentrations, produce a net difference in electrodriven migration velocities of the two enantiomers. The relative movement of two enantiomers causes a zone depleted in one enantiomer at the rear end of sample segment, giving a trapezoidal CE curve with a step at the end. Quantification of enantiomers is achieved according to the step height. The analysis does not rely on the use of enantiomerically pure chiral selector and the result agrees with that obtained by conventional chiral CE using a chiral selector.

Enantioseparation and ecotoxicity evaluation of ibrutinib by Electrokinetic Chromatography using single and dual systems

In this work, two chiral methods enabling the separation of ibrutinib enantiomers were developed by Electrokinetic Chromatography. A cyclodextrin (CD) or a mixture of the CD and a chiral ionic liquid (CIL) was used as chiral selector. Using the single CD system, seven neutral and six anionic CDs were tested in a formate buffer at pH 3.0 working in positive and negative polarity, respectively. The use of sulfated-γ-CD (S-γ-CD) and negative polarity originated the best results considering analysis time and enantioresolution. The optimization of the experimental conditions allowed obtaining the separation of ibrutinib enantiomers in an analysis time of 4.2 min with an enantioresolution value of 1.5. The effect of the addition of fifteen CILs on the enantioresolution was evaluated showing that both analysis time and enantioresolution were generally increased. A mixture of S-γ-CD and [TMA][L-Lys] was selected which provided the separation of ibrutinib enantiomers in 8.1 min with an enantioresolution value of 3.3 under the same experimental conditions as in the case of using the single CD system. The enantiomeric impurity (S-ibrutinib) was the first-migrating isomer when using the single CD and the combined CD/CIL systems, as corresponds to the most desirable situation. Both chiral methods allowed the detection of the enantiomeric impurity up to a 0.1% as established by the International Council on Harmonization. After establishing the analytical characteristics of both chiral methodologies developed, they were applied to the enantiomeric determination of ibrutinib in a pharmaceutical formulation for hospital use marketed as pure enantiomer (R-ibrutinib) and to evaluate the stability and ecotoxicity of racemic ibrutinib and R-ibrutinib on Daphnia magna. The developed methodologies enabled, for the first time, the rapid chiral quantitation of ibrutinib in abiotic and biotic matrices.

Chiral ionic liquid-multi walled carbon nanotubes composite membrane applied to the separation of amino acid enantiomers

Various membranes are playing more and more important roles in the field of analytical and preparative applications of general interest, and some of them have been used in enantioresolution for amino acids (AAs) or similar bioactive molecules. In this study, a new composite membrane was prepared with chiral ionic liquid (CIL) of [BuPyro] [L-Pro] as chiral selector together with multi walled carbon nanotubes (MWCNTs) and additives through a simple way for the first time. Based on such a separation medium, the enantioresolution of amino acid enantiomers were achieved by forming ternary ligand complexes with Cu(II). It was comprehensively characterized by various ways, and key preparation conditions were discovered. After comparing the performance of three operation modes on the resolution of racemic phenylalanine, the effects of main influential factors were investigated and enantiomeric excess value (e.e.%) was 90.2% for the (D,L)-Phe aqueous solution (membrane thickness: 0.15±0.02 mm, total weight: 80 mg, CIL: 41.7%). Through effective desorption, up to 98.1% of the target was recovered. Finally, the mechanism of resolution was revealed by molecular simulation, kinetics and isotherm models, and the difference of interactive energy between ternary complexes of L-Phe-Cu(II)-CIL and D-Phe-Cu(II)-CIL was calculated as 1.56 kcal/mol. The membrane also remained stable after the post-treatment and showed good potential in chiral separation.

Utilization of ionic deep eutectic solvent as sustainable mobile phase additive in HPLC for improving the separation of biogenic amines

Biogenic amines are present in large quantities in fermented foods and are a key marker for assessing food safety. This paper proposes a novel method for high-performance liquid chromatography analysis of biogenic amines using deep eutectic solvent as mobile phase additive. After screening eight synthetic deep eutectic solvents and comparing with several common additives, deep eutectic solvents based on choline chloride and ethylene glycol showed significant effects for improving the separation of biogenic amines. A Box-Behnken design of 17 runs was used to screen and optimize the key chromatographic parameters, resulting in an expected composition of the mobile phase of 0.73% deep eutectic solvent, 65% acetonitrile, and a column temperature of 28°C. The proposed method exhibited excellent linearity (0.1-50 μg/mL, R² ≥ 0.9987), limit of detection (0.007-0.031 μg/mL), precision (1.28-5.34%) and accuracy (87.2-110.6%). The method can be applied successfully to the separation and analysis of biogenic amines in cooking wine samples. This article is protected by copyright. All rights reserved.

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.