Field Of Chiral Separations Research Articles

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.

Short bridging and partial derivatization synergistically modified β-cyclodextrin bonded chiral stationary phases for improved enantioseparation

β-Cyclodextrin (β-CD) and its derivatives have been widely employed in the field of chiral separation, but they are still faced the limitation of low enantioselectivity and complex processes. Derivatization with functional molecules or preparation as bridging dimers are the two main modifications for β-CD to obtain chiral recognition compounds. Herein, a partially derived bridged β-CD (CPI-EBCD) bonded chiral stationary phases was prepared to improve enantioseparation. The chiral recognition moiety was synthesized by a bridged β-cyclodextrin dimer using a short-chain bridging agent (ethylenediamine) and then modifying the bridged cyclodextrin with a 4-chlorophenylisocyanate (CPI) containing a benzene ring and polar group. Compared with natural β-CD, dual-chambered CPI-EBCDs have better encapsulation synergies and more recognition sites with the guest molecule, while the short flexible bridging groups make the double cavities closer and more easily recognizable as linear molecules. The introduction of derived groups CPI provided more recognition sites and more types of interactions, including π-π interaction force, hydrogen bonding effect, and dipole-dipole interaction, thus improving the enantiomer-specific chirality recognition effect. The chiral stationary phase CPI-EBCDP was obtained by connecting CPI-EDCB with mesoporous silica microspheres by simple photochemical reaction using a green non-toxic diazo resin as coupling agent, simplifying preparation process. In the reversed phase mode of liquid chromatography, CPI-EBCDP has excellent chiral recognition ability, and 12 chiral compounds are successfully isolated by optimizing mobile phase conditions, with good reproducibility and stability. The successful preparation of this new chiral stationary phase provides an important reference for the subsequent development of cyclodextrin-like chiral stationary phases.

Chiral zeolite beta used as stationary phase for enantioseparation in miniaturized open tubular capillary electrochromatography with amperometric detection

BackgroundWith the rapid growth of the demand for optically pure compounds in the fields of biology, medicine and stereospecific synthesis, it is of great importance to develop efficient, economical, simple enantioseparation and analysis methods. Open tubular capillary electrochromatography (OT-CEC) has attracted much attention in the field of chiral separation, but its column capacity and the sensitivity of common-used optical detection are relatively low. Zeolite beta nanomaterial is both enantioselective and size-selective, providing suitable chiral microenvironment for chiral recognition, and amperometric detection (AD) avoids the low sensitivity caused by the short optical path in optical detection to some extent. ResultsZeolite beta nanomaterials with different particle sizes (25, 50 and 200 nm) were synthesized, and the morphology and structure were characterized by scanning electron microscopy and X-ray diffraction. Then, a novel chiral OT column was prepared by one-step method using zeolite beta nanomaterial as chiral stationary phase, and its separation performance was characterized by miniaturized CEC with AD (mini-CEC-AD) device. Under the optimum conditions, six groups of chiral drugs achieved baseline separation. Norepinephrine enantiomers were used for evaluating the inter-day, intra-day and inter-column reproducibility of the prepared open-tubular column. The relative standard deviations of migration time, peak area, resolution and selectivity factor were within 8.7 %. The limits of detection for norepinephrine enantiomers were 0.18 μg mL−1 (S/N = 3), and the average recoveries were in range of 96.7–105.0 %. This developed method has been successfully applied to the analysis of impurity enantiomer in potassium dichromate (+)-norepinephrine injection sample. SignificanceZeolite beta nanomaterial was used as the stationary phase to prepare chiral OT columns for the first time, and this one-step preparation method is simple and easy. The introduction of zeolite beta enriches the types of chiral stationary phase materials in electrochromatographic columns, and mini-OT-CEC-AD system provides an alternative for fast enantioseparation of chiral compounds.

Optimization of enantiomer separation in flow-modulated comprehensive two-dimensional gas chromatography by response surface methodology coupled to artificial neural networks: Wine analysis case study

In spite of extensive applications of flow modulated comprehensive two-dimensional gas chromatography (FM-GG × GC) in different research areas, its application in the field of chiral separation is very limited. From a practical point of view, the establishment of experimental parameters for enantiomer separations is possibly more demanding in this case. Since the carrier gas flows in both dimensions, it affects not only the separation parameters, but also the fill/flush volumes of the modulator and its working efficiency. In this context, a multivariate design of experiment was applied to find the optimum experimental parameters of a reversed fill/flush (RFF) modulator for enantiomer separation of organic compounds present in botrytized wine samples. The results were described both with response surface methodology and artificial neural networks (ANN). The enantiomeric composition of chiral compounds present in the botrytized wines was used to identify their geographical origin, by principal component analysis (PCA). In addition, the developed one-class partial least squares (OC-PLS) model enabled recognition of the wine samples from the Tokaj wine region with 93% effectiveness in the presence of other samples.

Fabrication of cellulose derivative coated spherical covalent organic frameworks as chiral stationary phases for high-performance liquid chromatographic enantioseparation

Porous spherical silica-based chiral stationary phases (CSPs) have been commercially used in the field of chiral separation, however, the scope of their application is, to some extent, limited by the instability of silica towards mobile phase containing strong base or acid. As such, developing new matrix-based CSPs is one of the effective strategies to overcome this bottleneck in studies of chiral separation materials. In this work, we have demonstrated that stable spherical covalent organic frameworks (SCOFs) can be utilized as matrixes for the fabrication of new CSPs for the first time. Specifically, a porous imine-linked SCOF with good crystallinity, large surface area, and high chemical stability is synthesized at room temperature. Then, cellulose-tris (3,5-dimethylphenylcarbamate) (CDMPC), a typical cellulose derivative, is selected as a potential chiral selector and coated onto the robust SCOFs, giving rise to the fabrication of new CDMPC@SCOF CSPs. The as-synthesized stable SCOF-based CSPs are exploited for high-performance liquid chromatographic (HPLC) enantioseparation, showing high resolution abilities for the separation of racemic compounds such as metalaxyl, 1-(1-naphthalenyl)ethanol, epoxiconazol, trans-stilbene oxide, and so on. Moreover, the prepared SCOF-based CSPs exhibit more superior acid and base stability than those of the silica-based CSPs. Our work not only uncovers the great potential of SCOFs as matrixes for constructing novel CSPs, but also expands the application of COFs in the field of enantiomeric separation under harsh base and acid conditions.

One-pot synthesis of a novel chiral Zr-based metal-organic framework for capillary electrochromatographic enantioseparation.

A novel chiral stationary phase (CSP) of Zr-based metal-organic framework, l-Cys-PCN-224, was prepared by one-pot method and applied for the enantioseparation by capillary electrochromatography. The CSP was characterized by X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, Fourier-transform infrared spectra, nitrogen adsorption/desorption, circular dichroism spectrum, zeta-potential, and so on. The results revealed that the CSP had good crystallinity, high specific surface area (2580 m2 /g), and good thermal stability. Meanwhile, the cross-section of l-Cys-PCN-224-bonded open-tubular (OT) column was observed by a scanning electron microscope, which proved the successful bonding of l-Cys-PCN-224 particles to the inner wall. Relative standard deviations of the column efficiencies were 3.87%-9.14%, and not obviously changed after 200 runs, which indicated that l-Cys-PCN-224-bonded OT column had the better stability and reproducibility. Excellent chiral separation performance was verified with nine kinds of natural amino acids including acidic, neutral, and basic as the analytes. All amino acids studied achieved good separation with the resolution of 1.38-13.9 and selector factor of 1.11-3.71. These results demonstrated that the CSP had an excellent potential in the chiral separation field.

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.

Molecular docking and density functional theory calculations of vinpocetine and teicoplanin aglycone chiral selector

Chiral separation has been an essential issue as stereochemistry significantly influences the biological activity. In some cases, one stereoisomer may show effectiveness in pharmacology activities while the other may appear toxic. Thus, devising technology with knowledge at the fundamental level becomes crucial in the field of chiral separation. This work presented molecular docking simulation and quantum chemical calculation of vinpocetine (VP) drug stereoisomers complexation with teicoplanin aglycone (TAG) chiral selector. Docking simulation was used to find the best conformation of VP stereoisomers with TAG chiral selector followed by density functional theory (DFT) calculation at B3LYP-D3(BJ)/6–31 g(d) level of theory representing the second phase of this work. The stability of VP–TAG inclusion complexes based on binding energy differences obtained from the quantum-chemical calculation was in the order of (3R16S)VP–TAG > (3R16R)VP–TAG > (3S16S)VP–TAG > (3R16S)VP–TAG. The significant differences in the binding energy suggested that the chiral separation of VP stereoisomers can be achieved using TAG.

Chiral counter-current chromatography: A survey of its instrument, mechanism, procedure, and applications.

The chiral separation by counter-current chromatography has made great progress in the past three decades. It has become increasingly popular in the field of chiral separation, and many applications have been introduced during the last years. This review mainly focuses on the current topics, applications, and trends in chiral separation by counter-current chromatography. It contains the development of modern counter-current chromatography apparatus, theory of counter-current chromatography, overview of applications of chiral counter-current chromatography enantioseparation, its current situation, and challenges. At last, some conclusions and perspectives also have been discussed in this review.

γ-Cyclodextrin metal-organic framework supported by polydopamine as stationary phases for electrochromatographic enantioseparation

Metal-organic frameworks (MOFs) are a novel class of porous nanomaterials composed of metals and organic linkers that have been used in many fields. A novel stereoselective stationary phase for capillary electrochromatography (CEC) has been developed in this work by employing a biodegradable, easier-to-prepare and green homochiral MOFs (Cu-SD), which composed of octakis-[6-deoxy-6-(3-mercaptopropanoic sodium)]-γ-cyclodextrin (SD, sugammadex) and transition metal ion (Cu2+) supported by polydopamine as linker. The modified capillary was characterized by scanning electron microscopy (SEM), X-ray spectrometry (XRD), energy dispersive (EDS) and Fourier transform infrared spectroscopy (FT-IR). The Cu-SD modified capillary column exhibited outstanding chiral recognition separation for five Dns-DL-amino acids (the DL-forms of phenylalanine, leucine, valine, threonine, serine). In addition, the modified column showed excellent stability and reproducibility. The relative standard deviations (RSDs) of the retention time for intra-day (n = 5) and inter-day (n = 3) and between columns (n = 3) enantioseparations were less than 0.27, 1.45 and 4.88%, respectively. Conceivably, this new-type of γ-CD-MOF represents a useful chiral stationary phase in electrochromatographic separations, and it has great potential in chiral separation field.

Synthesis of a chiral metal-organic framework using zinc oxide as zinc source for enantioselective adsorption of 2,2'-furoin racemate

Chiral metal-organic frameworks (CMOFs),[Zn(L-mal)(H2O)2]n were prepared by using zinc oxide instead of the traditional metal inorganic salt as the zinc source, and their chiral separation performance was investigated. Equal amounts of ZnO and L-malic acid were dissolved in H2O, and[Zn(L-mal)(H2O)2]n were obtained by allowing the mixture to stand at room temperature for 24 h. Then, 100 mg[Zn(L-mal)(H2O)2]n as the adsorbent was added to 250 μL 1 g/L 2,2'-furoin racemate, and 4.5 mL isopropanol and 1 mL methanol were selected as the extraction solvent and elution solvent, respectively, for selective adsorption experiments. The final filtrate was analyzed by high performance liquid chromatography (HPLC). The results showed that[Zn(L-mal)(H2O)2]n exhibited better selective adsorption ability for R-2,2'-furoin with 20% enantiomeric excess (ee). This work provides a new method for the green preparation of CMOF and expands its application in the field of chiral separation.

Chiral Metal-Organic Framework d-His-ZIF-8@SiO2 Core-Shell Microspheres Used for HPLC Enantioseparations.

Chiral metal-organic frameworks (MOFs) have aroused great attention in the chiral separation field based on their excellent characteristics, including abundant topological structures, large surface area, adjustable pore/channel sizes, multiple active sites, and good chemical stability. However, the irregular morphology and nonuniformity of the synthesized MOF particles cause low column efficiency and high column backpressure for MOF-packed columns, which significantly affects their separation performance. Herein, we prepared a homochiral d-his-ZIF-8@SiO2 composite by growing of d-his-ZIF-8 on the carboxylic-functionalized SiO2 microspheres via a simple one-pot synthesis approach. The d-his-ZIF-8@SiO2 core-shell microspheres with uniform particles and narrow size distribution were applied as the chiral stationary phase (CSP) for enantioseparations in HPLC. Various racemates were separated on the d-his-ZIF-8@SiO2-packed columns with n-hexane/isopropanol as the mobile phase. Eighteen racemates including alcohol, phenol, amine, ketone, and organic acid were well resolved on the homochiral d-his-ZIF-8@SiO2 CSP. The d-his-ZIF-8@SiO2 core-shell microspheres' CSP possesses an excellent chiral resolution ability toward various racemic compounds with good reproducibility and stability. Hence, the fabrication of chiral MOF@SiO2 core-shell microspheres is an effective strategy to improve the application of homochiral MOFs as CSPs in the field of chromatography.

N-acetyl-L-cysteine modified CuFe2O4@SiO2 core-shell nanoparticles as chiral probes for recognition of chiral tyrosine

Chiral recognition exhibits the important practical significance. In this paper, the magnetic CuFe2O4@SiO2 was firstly synthesized by co-precipitation. Then, the core-shell chiral nanomaterial CuFe2O4@SiO2-NALC (NALC = N-acetyl-L-cysteine) was futher prepared by reflow in a flask. It is worth mention that the chiral nanomaterials are obtained by simple synthetic methods and using inexpensive materials. Chiral recognition effect of CuFe2O4@SiO2-NALC on L-/D-tyrosine was confirmed by UV-Vis absorption spectroscopy. CuFe2O4@SiO2-NALC is easy to separate due to the magnetic hull, which has great development prospects of the field of chiral separation.

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.

Resolution of an Atropisomeric Naphthamide by Second-Order Asymmetric Transformation: A Highly Productive Technique

Temperature-cycle-induced deracemization (TCID) has been widely studied in the field of chiral separation, ranging from fundamental research to applications. In this study, the second-order asymmetric transformation (SOAT) of 2-methoxy-1-naphthamide in an azeotropic mixture of ethyl acetate and cyclohexane is compared with TCID, in terms of process productivity. The results indicate that the volumetric productivity using SOAT was over 100-times higher than that using TCID, such that a scale-up by a factor of 10 was easily implemented.

Chiral Capillary Electrophoresis-Mass Spectrometry.

Capillary electrophoresis (CE) is a well-established and one of the most powerful separation techniques in the field of chiral separations. Its hyphenation with mass spectrometry (MS) combines both the high separation efficiency and low sample consumption of CE and the high sensitivity and structural information of MS. Thus, the outstanding chiral resolution power of CE along with the MS advantages makes CE-MS a perfect combination to achieve sensitive enantioseparations. This chapter describes three representative examples of different approaches used in the chiral analysis of amino acids in biological fluids by CE-MS. The first methodology uses the partial filling technique to avoid the entry of cyclodextrins in the MS source. The second method shows the possibility to carry out the direct coupling EKC-MS even when a relative high concentration of a native cyclodextrin is used as chiral selector. The last example illustrates an alternative strategy based on the formation of stable diastereomers between an enantiomerically pure chiral reagent and the amino acids enantiomers which can be separated in an achiral environment.

Recent Advances in Chiral Separation of Antihistamine Drugs: Analytical and Bioanalytical Methods.

Chirality is now a key area in the field of research. Researchers are well versed with analytical separations but at the same time, they are unaware of the growth in the field of chiral separation. According to the United States Food and Drug Administration (US FDA) guidelines, it is mandatory to separate chiral drugs before they are marketed. Chiral separation has gained importance in the last 10 years due to the differential biological responses of the enantiomers in chiral environment. Identical physical and chemical properties of the enantiomers pose a major challenge for the separation of chiral compounds. Further, bioanalytical method development is also necessary to throw light on the fate of separated enantiomers in biological environment. High Performance Liquid Chromatography (HPLC) and Capillary Electrophoresis (CE) are the most widely used techniques for such separation. Antihistamines are a class of drugs that are represented by a wide number of chiral compounds. Hence this review focuses on enantioseparation of chiral antihistamine drugs. It begins with a brief discussion about antihistamine drugs, chiral separation and its need for study, followed by a brief overview of the analytical and bio-analytical work carried out on different chiral antihistamine drugs. The analytical and bio-analytical techniques that are used include HPLC, CE and some new techniques.

The Way to Ultrafast, High-Throughput Enantioseparations of Bioactive Compounds in Liquid and Supercritical Fluid Chromatography.

Until less than 10 years ago, chiral separations were carried out with columns packed with 5 or 3 μm fully porous particles (FPPs). Times to resolve enantiomeric mixtures were easily larger than 30 min, or so. Pushed especially by stringent requirements from medicinal and pharmaceutical industries, during the last years the field of chiral separations by liquid chromatography has undergone what can be defined a “true revolution”. With the purpose of developing ever faster and efficient method of separations, indeed, very efficient particle formats, such as superficially porous particles (SPPs) or sub-2 μm FPPs, have been functionalized with chiral selectors and employed in ultrafast applications. Thanks to the use of short column (1–2 cm long), packed with these extremely efficient chiral stationary phases (CSPs), operated at very high flow rates (5–8 mL/min), resolution of racemates could be accomplished in very short time, in many cases less than 1 s in normal-, reversed-phase and HILIC conditions. These CSPs have been found to be particularly promising also to carry out high-throughput separations under supercritical fluid chromatography (SFC) conditions. The most important results that have been recently achieved in terms of ultrafast, high-throughput enantioseparations both in liquid and supercritical fluid chromatography with particular attention to the very important field of bioactive chiral compounds will be reviewed in this manuscript. Attention will be focused not only on the latest introduced CSPs and their applications, but also on instrumental modifications which are required in some cases in order to fully exploit the intrinsic potential of new generation chiral columns.

New frontiers and cutting edge applications in ultra high performance liquid chromatography through latest generation superficially porous particles with particular emphasis to the field of chiral separations.

About ten years after their introduction to the market (happened in 2006), the so-called second generation superficially porous particles (SPPs) have undoubtedly become the benchmark as well as, very often, the preferred choice for many applications in liquid chromatography (LC), when high efficiency and fast separations are required. This trend has interested practically all kinds of separations, with the only exception of chiral chromatography (at least so far). The technology of production of base SPPs is advanced, relatively simple and widely available. The deep investigation of mass transfer mechanisms under reversed-phase (RP) and normal-phase (NP) conditions for achiral separations has shown the advantages in the use of these particles over their fully porous counterparts. In addition, it has been demonstrated that SPPs are extremely suitable for the preparation of efficient packed beds through slurry packing techniques. However, the research in this field is in continual evolution. In this article, some of the most advanced concepts and modern applications based on the use of SPPs, embracing in particular ultrafast chiral chromatography and the design of SPPs with engineered pore structures or very reduced particle diameter, are revised. We describe modern trends in these fields and focus on those aspect where further innovation and research will be required. Graphical Abstract Word cloud of cutting edge applications of superficially porous particles in liquid chromatography.

Chip-based high-performance liquid chromatography for high-speed enantioseparations.

In this work, the first high-performance chiral liquid chromatography in packed microfluidic chips is presented. The chromatographic separation was performed on a column integrated into the microfluidic glass chip and packed with the particulate chiral stationary phase. Cellulose tris(3,5-dimethylphenylcarbamate) coated on 5-μm fully porous silica was used as chiral stationary phase material. Several racemic analytes including pharmaceutical products were baseline separated into their corresponding enantiomers under reversed-phase, polar organic and normal-phase conditions, demonstrating the versatility of the glass chip in the field of chiral separations. Van Deemter plots revealed a reduced plate height of 2.2 and a trend to enhanced mass transfer processes for solutes under low retention conditions. The utilization of very short column lengths of down to 12 mm led to ultrafast separations of enantiomers within 5 s.