Separation Of Enantiomers Research Articles

Stereoselective Processes Based on σ-Hole Interactions.

The σ-hole interaction represents a noncovalent interaction between atoms with σ-hole(s) on their surface (such as halogens and chalcogens) and negative sites. Over the last decade, significant developments have emerged in applications where the σ-hole interaction was demonstrated to play a key role in the control over chirality. The aim of this review is to give a comprehensive overview of the current advancements in the use of σ-hole interactions in stereoselective processes, such as formation of chiral supramolecular assemblies, separation of enantiomers, enantioselective complexation and asymmetric catalysis.

Super-Assembled Chiral Mesostructured Heteromembranes for Smart and Sensitive Couple-Accelerated Enantioseparation

In the context of sustainable development, chirality, especially chiral drugs, has attracted great interest in the pharmaceutical industry, yet the smart and sensitive separation of enantiomers still presents a major scientific challenge. Herein, inspired by supramolecular templating via chiral transcription nanoparticles, an artificial chiral nanochannel membrane with asymmetric structure, porosity, and abundant chiral surface is fabricated for smart and sensitive enantiomer recognition and separation. Constructed from chiral transcript mesoporous silica (CMS) super-assembled on a porous anode alumina oxide (AAO) support, the obtained heterostructured chiral membrane (CMS/AAO) exhibits enhanced enantioseparation (approximately 170% compared to the supramolecular-templated nanoparticles) among a series of amino acids with various isoelectric points (PIs). Especially for amino acids with a PI greater than 7, the couple-accelerated enantioseparation (CAE) can be achieved for the first time. Further analysis using an osmotic energy conversion test and simulations based on the Poisson-Nernst-Planck (PNP) equations confirm that the heterostructure and charge polarity are the key to achieve chiral amino acids and ion separation. We expect this work will inspire the development of multifunctional membrane systems for more sustainable and energy-efficient enantioseparation.

Enantioseparation and determination of oxypeucedanin and its application to a stereoselective analysis in Angelica Dahuricae Radix and pharmacokinetic study of rats

In this study, a new enantioseparation method was established for the quantitative analysis of the oxypeucedanin enantiomers by using cellulose tris(3,5-dichlorophenyl carbamate) stationary phase column Chiralpak IC. For this method, enantiomeric separation of oxypeucedanin was achieved with the mobile phase consisting of acetonitrile-water (60:40, v/v) at a flow rate of 0.5 mL/min by changing the type and proportion of mobile phase. And the quantitative determination of racemic oxypeucedanin in Angelica Dahuricae Radix (in vitro) and rat plasma (in vivo) were performed on above-mentioned condition by High PerformanceLiquid Chromatography combined with diode arrangement detector (HPLC-DAD) and mass spectrometry (HPLC-MS/MS). The precision, repeatability, stability, recovery were within the acceptance criteria. And the method was validated in the concentration range of 1-400 μg/mL for the two enantiomers in vitro and 0.2–600 ng/mL in vivo. After validation, the established method was successfully applied to the stereoselective analysis of racemic oxypeucedanin in Angelica dahurica from different regions and the stereoselective pharmacokinetic investigation in rat. Results showed that the (+)-oxypeucedanin was at a relative high level in Angelica dahuricae Radix and (−)-oxypeucedanin performed a higher plasma concentration, which demonstrated the difference of oxypeucedanin enantiomers both in vitro and in vivo.

Ultrafast simultaneous chiral analysis of native amino acid enantiomers using supercritical fluid chromatography/tandem mass spectrometry

In the chiral separation of amino acids, liquid chromatography has been mainly used because of the physicochemical properties of the analytes. To date, only few reports of the use of supercritical fluid chromatography (SFC) for the analysis of chiral amino acids exist, and there is much room for improvement in terms of the number of measurable amino acids, peak shape, and analysis time. In this study, we developed a novel method for the chiral analysis of native amino acids using a system combining SFC and tandem mass spectrometry. Specifically, the separation of amino acid enantiomers was investigated using a CROWNPAK CR-I(+) column with a chiral stationary phase of optically active crown ether. Methanol/water mobile phase with trifluoroacetic acid as a modifier based on supercritical carbon dioxide (CO2) was used. At a low modifier concentration of 30% for the separation of hydrophilic compounds, 18 proteinogenic amino acid enantiomers except glycine and proline were successfully separated with resolution (Rs) = 1.96–33.62 within 6.5 min. In attempt to shorten the analysis time, the flow rate was increased; using a CO2/modifier ratio of 60/40 at a flow rate of 3 mL/min, ultrafast chromatography of 17 amino acid enantiomers, except histidine, was achieved with retention time ≤ 1 min and resolution ≥ 1.5. The developed ultrafast chiral separation method was verified by analyzing a commercially available black vinegar, which detected eight kinds of d-amino acids. The present method has thus confirmed to be successful and practical in terms of both analyte coverage and throughput.

Separation of planar chiral ferrocenes by capillary electrokinetic chromatography and liquid chromatography

Capillary electrokinetic chromatography (CEKC) and liquid chromatography (LC) methods were explored for the enantiomeric separation of six unsymmetrically disubstituted ferrocene derivatives. In normal-phase mode liquid chromatography, the use of n-heptane, ethanol or isopropanol with 1% n-butylamine as mobile phase on six polysaccharide-based columns, allowed to fully separate the enantiomers of three compounds out of the six (i.e 7-chloro-N-(2-((dimethylamino)methyl)ferrocenyl)quinolin-4-amine (ferroquine) (compound 1), 1-[(1S)-(1-Aminoethyl)]-2-(diphenylphosphino)ferrocene (compound 5) and 1-[(1R)-1-(Dicyclohexylphosphino)ethyl]-2-(diphenylphosphino)ferrocene (compound 6). Among the columns used, the Lux i-Cellulose-5 was the most effective. In capillary electrokinetic chromatography, a phosphate buffer of 25 mM concentration and pH equal to 2.5 was chosen as background electrolyte, leading to cationic ferrocene derivatives. The addition of neutral cyclodextrins was undertaken first and native β- or γ-cyclodextrins were found to resolve the enantiomers of two derivatives. Then, 15 mM of anionic cyclodextrins were added to the background electrolyte. The use of SBE-β-CD, S-β-CD or S-γ-CD have allowed the separation of the enantiomers for most of the ferrocene derivatives studied with high resolution values in short migration time. For instance, for 1-(R)-2-(Diphenylphosphino)ethyldi-tert-butylphosphine ferrocene (compound 2), the migration times were less than 2 minutes and the resolution value was equal to 3.52 in short-end mode with 15 mM S-β-CD, at 25 kV and 25°C. Finally, a dual cyclodextrins system was tested using 15 mM of S-β-CD plus 15 mM HP-γ-CD in the phosphate buffer. This system allowed the improved separation of two ferrocene derivatives with an unusual resolution value equal to 41.5 in long-end mode. Overall, CEKC showed better enantioseparating power of the six chiral ferrocenes studied than liquid chromatography.

Progress in Chiral Stationary Phases Based on Proteins and Glycoproteins.

A lot of chiral stationary phases (CSPs) have been introduced for the purpose of analytical and preparative separations of enantiomers. CSPs based on proteins and glycoproteins have unique properties among those CSPs. This review article deals with the preparation of CSPs based on proteins and glycoproteins, their chiral recognition properties and mechanisms, focusing on the CSPs investigated in our group. The dealt proteins and glycoproteins are including bovine serum albumin, human serum albumin, lysozyme, pepsin, human α1-acid glycoprotein (AGP), chicken ovomucoid and chicken ovoglycoprotein (named chicken AGP).

Quality by design approach for enantioseparation of terbutaline and its sulfate conjugate metabolite for bioanalytical application using supercritical fluid chromatography

Terbutaline is mainly metabolized by sulfoconjugation stereoselectively, favoring its (S)-(+) enantiomer. Reported chiral separations of Terbutaline enantiomers were achieved by various chromatographic methods. However, the simultaneous enantioseparation of Terbutaline and the monosulfate conjugate metabolites was never reported. This study aims at shedding light on the influential factors and interactions leading to successful enantioseparation of Terbutaline and its monosulfate conjugate pairs by Supercritical Fluid Chromatography (SFC) for the first time within a Quality by Design framework using Design of Experiments. The effect of molarity of mobile phase additive, mobile phase flow rate, column temperature and back pressure were evaluated. Compared to previous reports, the response surface interestingly revealed the favorability of high temperature and high flow rate up to 2.25 ml/min for resolution of the two pairs of enantiomers on polysaccharide chiral stationary phase CHIRALPAK IC. In addition, a switch in the elution order of Terbutaline and the sulfate conjugate peak pairs was observed upon elimination of the mobile phase additive where the sulfate conjugate underwent intra-molecular ionic interactions and the change in elution order was only due to TER behavior. The multifactorial interactions would not have been detected with the common one-factor-at-a-time approach during method development, demonstrating the superiority and importance of the Analytical Quality by Design frame in enantioseparation.

Separation and determination of fluindapyr enantiomers in cucumber and tomato and by supercritical fluid chromatography tandem mass spectrometry

Fluindapyr is a pyrazolamide chiral fungicide of succinate dehydrogenase inhibitor (SDHIs) with two enantiomers. Pesticide enantiomers often exhibit different biological activities, toxicity due to their different enantioselectivity. Therefore, it is important to separate fluindapyr enantiomers and assess each enantiomer. In this study, fluindapyr enantiomers were baseline separated by supercritical fluid chromatography-mass spectrometry in 2 min. The limit of quantification (LOQ) of this method was 5 μg/kg. The developed method was applied to monitor the fluindapyr enantiomers in cucumber and tomato, the data showed that R-(-)-fluindapyr was preferentially degraded in tomato leaves, S-(+)-fluindapyr was preferentially degraded in cucumber leaves, and fluindapyr enantiomers had no enantioselective degradation behavior in two fruits. It is proved again that enantiomers have different enantioselective degradation behavior with the different plant species and even to different parts of the same plant. The enantioselectivity is likely to be caused by different biodegradation enzyme systems.

Additives in chiral packed column super/subcritical fluid chromatography: A little goes a long way

The introduction of additives has revolutionized super/subcritical fluid chromatography (SFC) by expanding the range of compounds that could be analyzed by the technique. From being considered a mere extension of gas chromatography, incorporation of a modifier, and subsequently an additive has made SFC a workhorse for chiral separations. Additives are by far the smallest component of the SFC mobile phase but can result in changing the polarity and acidity of the mobile phase, suppression of ionization, deactivation of the stationary phase, and act as an ion pairing agent. A wide variety of compounds have been tested as additives including but not limited to diethylamine, trifluoroacetic acid, ammonium acetate, and water. This review summarizes the different role played by additives in the SFC mobile phase. Further this work aims to help the reader by critically evaluating different additives used in enantiomeric separations with packed column SFC.

Advances and applications of chiral resolution in pharmaceutical field

The attention to chiral drugs has been raised to an unprecedented level as drug discovery and development strategies grow rapidly. However, separation of enantiomers is still a huge task, which leads to an increasing significance to equip a wider range of expertise in chiral separation science to meet the current and future challenges. In the last few decades, remarkable progress of chiral resolution has been achieved. This review summarizes and classifies chiral resolution methods in analytical scale and preparative scale systematically and comprehensively, including crystallization-based method, inclusion complexation, chromatographic separation, capillary electrophoresis, kinetic resolution, liquid-liquid extraction, membrane-based separation, and especially one bold new progress based on chiral-induced spin selectivity theory. The advances and recent applications will be presented in detail, in which the contents may bring more thinking to wide-ranging readers in various professional fields, from analytical chemistry, pharmaceutical chemistry, natural medicinal chemistry, to manufacturing of drug production.

Computing metasurfaces enabled chiral edge image sensing

SummaryComputing metasurfaces have shown the extraordinary ability to precisely perform optical analog operations to the input light wave, and therefore exhibit greater potentials toward sensing applications. Here, we propose a unique application of computing metasurface for chiral edge sensing by incorporating a weak-value amplification technique. The computing metasurface performs the spatial differentiation operations of phase objects and extracts the edge-enhanced images, because the phase gradient generally occurs at the edge. The chirality-induced polarization rotation acts as the preselection state and the spatial differentiation operations in the metasurface provide weak coupling. The amplified pointer shift related to the tiny polarization rotation will eventually lead to an asymmetric edge-enhanced image. Owing to the high sensitivity of the weak-value amplification, we experimentally demonstrate a high-contrast recognition of chirality by edge detection, which may have potential applications in real-time measurement and separation of chiral enantiomers.

Exploiting Self-Association to Evaluate Enantiomeric Composition by Cyclic Ion Mobility-Mass Spectrometry.

The characterization of enantiomers is an important analytical challenge in the chemical and life sciences. Thorough evaluation of the purity of chiral molecules is particularly required in the pharmaceutical industry where safety concerns are paramount. Assessment of the enantiomeric composition is still challenging and time-consuming, meaning that alternative approaches are required. In this study, we exploit the formation of dimers as diastereomeric pairs of enantiomers to affect separation by high resolution cyclic ion mobility–mass spectrometry. Using the example of (R/S)-thalidomide, we show that even though this is not an enantiomer separation, we can determine which enantiomer is in excess and obtain quantitative information on the enantiomer composition without the need for a chiral modifier. Further examples of the approach are presented, including d/l-tryptophan and (R/S)-propanolol, and demonstrate the need for mobility resolving power in excess of 400 (CCS/ΔCCS).

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.

Separation of ofloxacin enantiomers by capillary electrophoresis with fluorescence detection using deoxyribonucleic acid oligonucleotides as chiral selectors.

This study used capillary electrophoresis with fluorescence detection- and a partial-filling mode-based method for chiral separation of ofloxacin. The deoxyribonucleic acid oligonucleotides with different base sequences were studied as potential chiral selectors including deoxyribonucleic acid tetrahedron, G-quadruplex, and G-riched double-strand deoxyribonucleic acid. Under the optimized conditions, all the deoxyribonucleic acid chiral selectors exhibited excellent chiral separation capabilities with a resolution higher than 1.5. The electrophoretic behavior of the ofloxacin enantiomer might result from the intermediate conjugate with different stabilities between chiral selectors and analytes by a combination of the hydrogen bond and spatial recognition structure. Moreover, satisfactory repeatability regarding run-to-run and interday repeatability was obtained, and all the relative standard deviation values of migration times and resolutions were below 4% (n=6). Conclusively, both spatial structure and arrangement of the G bases potentiated the chiral separation capability of deoxyribonucleic acid for ofloxacin enantiomer. This work offered a stepping stone for enantioseparation using deoxyribonucleic acid as chiral selectors.

Chirality sorting using a structured caustic vector vortex field

In this work, we theoretically demonstrate that the simultaneous manipulation of both the orbital angular momentum and the polarization conversion can be realized in a structured caustic vector vortex field. Under a slight focusing condition, the structured caustic vector vortex field would be auto-focused into dual foci with specific topological charge and orthogonal polarizations, which can be manipulated through adjusting the initial state of polarization distribution and a caustic phase applied on the illumination. Furthermore, we demonstrate that the structured focal field carrying opposite spin angular momentum is suitable to sort chiral nanoparticles, in which the different energy flow directions would bring distinct dynamic behaviors to enantiomers. This finding may have potential applications in all-optical enantiopure chemical syntheses and enantiomer separations in pharmaceuticals.

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.

Chiral lateral optical force near plasmonic ring induced by Laguerre–Gaussian beam

Owing to the good adjustability and the strong near-field enhancement, surface plasmons are widely used in optical force trap, thus the optical force trap can achieve excellent performance. Here, we use the Laguerre–Gaussian beam and a plasmonic gold ring to separate enantiomers by the chiral optical force. Along with the radial optical force that traps the particles, there is also a chirality-sign-sensitive lateral force arising from the optical spin angular momentum, which is caused by the interaction between optical orbit angular momentum and gold ring structure. By selecting a specific incident wavelength, the strong angular scattering and non-chiral related azimuthal optical force can be suppressed. Thus the chiral related azimuthal optical force can induce an opposite orbital rotation of the trapped particles with chirality of different sign near the gold ring. This work proposes an effective approach for catchingand separating chiral enantiomers.

Enantioseparation of selected chiral agrochemicals by using nano-liquid chromatography and capillary electrochromatography with amylose tris(3‑chloro-5-methylphenylcarbamate) covalently immobilized onto silica

In this paper enantiomers of selected chiral agrochemicals representing various structural classes were separated by using nano-liquid chromatography (nano-LC) and capillary electrochromatography (CEC) employing a capillary column packed with silica particles containing immobilized amylose tris(3‑chloro-5-methylphenylcarbamate) (i-ADMPC) as a chiral selector (CS). Special attention was paid to peak dispersion in nano-LC and CEC instruments used in order to make comparison between these two techniques more reliable. Enantioseparations were studied utilizing methanol (MeOH) or acetonitrile-water (ACNH2O), both containing 5 mM of ammonium acetate as the mobile phases (MPs). The tested chiral stationary phase (CSP), containing 20% (w/w) of the neutral CS onto native silica, allowed the generation of sufficiently strong electroosmotic flow (EOF) to observe separation of enantiomers of studied agrochemicals in a reasonable time also in CEC mode. Modestly higher efficiencies and enantioresolutions were obtained in CEC than in nano-LC. Just a moderate preference of CEC over nano-LC in this particular study can be explained with a significant mass transfer resistance through the CSP that is caused due to high content of the CS in CSP.

Enantiomeric Separation of (R)-5-(1-Hydroxyethyl)-2-(pyridine- 2-yl)-[1, 3, 4]-thiadiazole, an Intermediate of Nafithromycin on Immobilized Amylose Based Stationary Phase

Chiral high performance liquid chromatography method for the separation of (R)-5-(1- Hydroxyethyl)-2-(pyridine-2-yl)-[1, 3, 4]-thiadiazole (R-alcohol) an intermediate of new antibiotic Nafithromycin (NFT) has been developed and validated. NFT is a ketolide class of antibiotic currently under Phase III clinical trials in India to treat community acquired bacterial pneumonia. Chiral separation of the enantiomers with resolution >2 was accomplished on Chiralpak IA column using a mixture of n-hexane and isopropanol (IPA) in the ratio of (90:10; v/v) as an elution solvent, run at a flow rate of 1.2 mL.min-1. The column was maintained at 25°C. The analytes were detected at 254 nm. The developed method was extensively validated as per the international council for harmonisation of technical requirements for pharmaceuticals for human use guideline. Sensitivity of the method was adequate to control the passage of unwanted enantiomer to next step with limit of quantification (LOQ) of the undesired enantiomer (S)-5-(1-Hydroxyethyl)-2-(pyridine- 2-yl)-[1, 3, 4]-thiadiazole (S-alcohol) of 0.37 µgmL-1. Mean recovery of the S-alcohol was 98.9±8.7%. During development influence of column oven temperature on the chiral separation was assessed.

Experimental and computational studies of enantioseparation of three profen enantiomers with a focus on quantification of the enantiomeric impurities present in the corresponding enantiopure S-profen drugs

A rapid reversed-phase high performance liquid chromatographic (HPLC) methodology for chiral separation of three profen compounds has been developed and then applied to enantiomeric impurity testing of their corresponding enantiopure drugs. The assay is specific, allowing quantitation of the enantiomeric impurities at levels of 0.0078%, 0.0105%, and 0.0416% relative to S-ibuprofen, S-naproxen, and S-ketoprofen, respectively. In order to gain a better insight into the chiral recognition mechanisms of chiral profens on an FLM Chiral NQ(2)-RH column, molecular docking studies were carried out using AutoDock 4.0 software. It was found that hydrogen bonding, hydrophobic interactions, and π-π stacking were all involved in stereoselective interactions, and the calculated binding energy (BE) obtained reflected the binding strength of each enantiomer interacting with a chiral selector. The higher the BE value, the harder it was to elute the corresponding enantiomer, which also accorded with the enantiomer elution order observed in the actual enantiomeric separation. Additionally, thermodynamic analysis revealed that the enantioseparation process at 15–40°C was driven mainly by entropic contributions. The methodology was further validated according to the International Council for Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) guideline Q2 (R1) and proved to be sensitive, linear, precise, and accurate for determining R-profen impurities in three commercially available single-enantiomer S-profen drugs. As expected, in the case of products acquired in actual pharmacies, the levels of all of the monitored impurities were found to be lower than the allowable impurity limits.