In situ fabrication of chiral covalent triazine frameworks membranes for enantiomer separation

A Homochiral Porous Organic Cage-Polymer Membrane for Enantioselective Resolution

Membrane-based enantioselective separation is a promising method for chiral resolution due to its low cost and high efficiency. However, scalable fabrication of chiral separation membranes displaying both high enantioselectivity and high flux of enantiomers is still a challenge. Here, the authors report the preparation of homochiral porous organic cage (Covalent cage 3 (CC3)-R)-based enantioselective thin-film-composite membranes using polyamide (PA) as the matrix, where fully organic and solvent-processable cage crystals have good compatibility with the polymer scaffold. The hierarchical CC3-R channels consist of chiral selective windows and inner cavities, leading to favorable chiral resolution and permeation of enantiomers; the CC3-R/PA composite membranes display an enantiomeric excess of 95.2% for R-(+)-limonene over S-(-)-limonene and a high flux of 99.9 mg h-1 m-2. This work sheds light on the use of homochiral porous organic cages for preparing enantioselective membranes and demonstrates a new route for the development of next-generation chiral separation membranes.

Chiral-induced synthesis of chiral covalent organic frameworks core-shell microspheres for HPLC enantioseparation

Two chiral covalent organic frameworks (CCOFs) core-shell microspheres based on achiral organic precursors by chiral-induced synthesis strategy for HPLC enantioseparation are reported for the first time. Using n-hexane/isopropanol as mobile phase, various kinds of racemates were selected as analytes and separated on the CCOF-TpPa-1@SiO2 and CCOF-TpBD@SiO2-packed columns with a low column backpressure (3 ~ 9 bar). The fabricated two CCOFs@SiO2 chiral columns exhibited good separation performance towards various racemates with high column efficiency (e.g., 19,500 plates m-1 for (4-fluorophenyl)ethanol and 18,900 plates m-1 for 1-(4-chlorophenyl)ethanol) and good reproducibility. Some effects have been investigated such as the analyte mass and column temperature on the HPLC enantioseparation. Moreover, the chiral separation results of the CCOF-TpPa-1@SiO2 chiral column and the commercialized Chiralpak AD-H column show a good complementarity. This study demonstrates that the usage of chiral-induced synthesis strategy for preparing CCOFs core-shell microspheres as a novel stationary phase has a good application potential in HPLC.

A free carboxyl-decorated metal-organic framework with 3D helical chirality for highly enantioselective recognition

With the judicious selection of a designed polycarboxylate derived from L-phenylalanine, (S)-5-(((1-carboxy-2-phenylethyl)amino)methyl)isophthalic acid (H3L), a novel homochiral metal-organic framework decorated with a free carboxyl, {[Cu2(HL)2(bipy)]∙2H2O}n (Cu-MOF), has been designed and synthesized in a solvothermal process. The result of single crystal X-ray diffraction analysis showed that Cu-MOF had the character of a three-dimensional structure with helical chirality. As we expected, in Cu-MOF, one accessible free carboxylic acid group on H3L pointed toward the spiral channels, and the other two -COOH groups were utilized in bonding. The enantioseparation performance of Cu-MOF was thoroughly investigated and the results showed that Cu-MOF can specifically recognize S-1-(1-naphthyl) ethanol (S-NE) with enantiomeric excess (ee) value of 99.35 %, which was much higher than the other three racemates. The appropriate size together with suitable interaction sites played an important role in enantioseparations. Inspired by the excellent chiral recognition effects towards S-NE, the chiral recognition mechanism was experimentally clarified. A fully agreement observed in 13C CP MAS NMR analysis as well as the X-ray photoelectron spectroscopy (XPS) determination revealed that a strong hydrogen bonding interaction forces existed between the hydroxyl of the optical S-NE and the decorated -COOH in the chiral framework. The control experiment further identified the decisive role of the uncoordinated carboxyl group in Cu-MOF. In addition, the strong intermolecular off-set π-π interactions between the phenyl ring involved with the coordinated COO- groups in Cu-MOF and the naphthyl ring of S-NE, was the another important factor for the specifical enantioseparation of S-enantiomer. On the basis of strong intermolecular hydrogen bonding, NE racemates were enantioselective discriminated and enantiomeric purity can be determined by means of Raman scattering spectroscopy.

Enantioselective separation and determination of ibuprofen: Stereoselective pharmacokinetics, pharmacodynamics and analytical methods

Ibuprofen (IBP), the 29th most prescribed drug in the United States in 2019, is a widely used nonsteroidal anti-inflammatory drug (NSAID) comprising two enantiomers, (R)-IBP and (S)-IBP, collectively known as (RS)-IBP. This critical review examines analytical techniques for the enantioselective separation and determination of IBP enantiomers, crucial for pharmaceutical and clinical applications. The review focuses on state-of-the-art methods, including chromatographic techniques including high-performance liquid chromatography, gas chromatography, liquid chromatography-tandem mass spectrometry, and some other techniques. This review addresses pharmacokinetics, pharmacology, and side effects of each enantiomer, ensuring safe drug usage. By consolidating diverse analytical methods and their applicability in different matrices, this review serves as a valuable resource for researchers, analysts, and practitioners in pharmaceutical analysis, pharmacology, and clinical studies.

Synergetic Multichiral Covalent Organic Framework for Enantioselective Recognition and Separation

In enantiomer recognition and separation, a highly enantioselective approach with universal applicability is urgently desired but hard to realize, especially in the case of chiral molecules. To resolve the trade-off between enantioselectivity and universality, a glutathione (GSH) and methylated cyclodextrins (MCD)-functionalized covalent organic framework (GSH-MCD COF) with porosity and abundant chiral surfaces is presented that was designed and synthesized for recognition and separation of various enantiomers. As expected, the GSH-MCD COF can be used as chiral stationary phases for the separation of various enantiomers, including aromatic alcohols, aromatic acids, amides, amino acids, and organic acids, with performance and versatility even superior to some widely used commercial chiral chromatographic columns. Furthermore, the synthesized GSH-MCD COF shows high enantioselectivity for the rapid recognition and identification of enantiomers and chiral metabolites when coupling to Raman spectroscopy. Molecular simulations suggest that the COF provides a confined microenvironment for cyclodextrins and peptides that dictates the separation and recognition capability. This work provides a strategy to synthesize synergetic multichiral COF and achieve separations and recognitions of enantiomers in complex samples.

Thermosensitive poly(N-isopropylacrylamide) hydrogel/highly internal phase emulsion porous polymer tube tip solid-phase extraction for the determination of methylimidazoles in beverage

Poly(N-isopropylacrylamide) thermosensitive hydrogel tube tip solid-phase extraction/ultra-high liquid chromatography-mass spectrometry (UPLC-MS/MS) method was developed for analysis of methylimidazoles in beverages. Thermosensitive poly(N-isopropylacrylamide) (PNIPA) hydrogel solid-phase extraction (SPE) medium was prepared on the surface of highly internal phase emulsion (HIPE) porous polymer by thermally initiated polymerization in a tube tip. The temperature sensitive SPE medium has the characteristics of high porosity and high specific surface area. When the temperature is higher than 30.0℃, it can well adsorb polar molecular, and could quickly desorb polar molecular when the temperature was less than 20.0℃. The tube tip SPE coupled with UPLC-MS/MS method was established for the determination of three polar molecules including 1-methylimidazole, 4-methylimidazole and 2-methylimidazole, with linear ranges of 2.50 - 240 μg/L, and detection limits of 1.20, 1.20 and 0.65 μg/L, respectively. The method was applied to the determination of three methylimidazoles in beverages with the spiked recoveries of 81.5%-115.5% and the RSD of 0.6%-5.0%, and the relative errors of the results with the national standard UPLC-MS/MS method were in the range of -8.5%-8.9%.

In situ growth preparation of a new chiral covalent triazine framework core-shell microspheres used for HPLC enantioseparation

The manufacturing of chiral covalent triazine framework core-shell microspheres CC-MP CCTF@SiO₂ composite is reported as stationary phase for HPLC enantioseparation. The CC-MP CCTF@SiO₂ core-shell microspheres were prepared by immobilizing chiral COF CC-MP CCTF constructed using cyanuric chloride and (S)-2-methylpiperazine on the surface of activated SiO₂ through an in-situ growth approach. Various racemates as analytes were separated on the CC-MP CCTF@SiO₂-packed column. The experimental results indicate that 19 pairs of enantiomers were well separated on the CC-MP CCTF@SiO₂-packed column, including alcohols, phenols, amines, ketones, and organic acids. Among them, there are 17 pairs of enantiomers that can achieve baseline separation with good peak shapes. Their resolution values on this chiral column are between 0.4 and 5.61. The influences of analyte mass, column temperature, and composition of the mobile phase on the resolution of enantiomers were studied. In addition, the chiral resolution ability of CC-MP CCTF@SiO₂-packed column was compared with the commercial chiral chromatographic columns (Chiralpak AD-H and Chiralcel OD-H columns) and some CCOF@SiO₂ chiral columns (β-CD-COF@SiO₂, CTpBD@SiO₂, and MDI-β-CD-modified COF@SiO₂). The CC-MP CCTF@SiO₂-packed column exhibited some unique advantages and can complement these chiral columns in chiral separations. The research results show that the CC-MP CCTF@SiO₂ chiral column offered high column efficiency (e.g., 17680 plates m^-1 for ethyl mandelate), low column backpressure (5-9 bar), high enantioselectivity, and excellent chiral resolution ability for HPLC enantioseparation with good stability and reproducibility. The relative standard deviations (RSD) (n = 5) of the retention time, and peak areas by repeated separation of ethyl mandelate are 0.23% and 0.67%, respectively. It demonstrates that the CC-MP CCTF@SiO₂ core-shell microsphere composite has great potential in enantiomeric separation by HPLC.

Enantioseparation Membranes: Research Status, Challenges, and Trends

The purity of enantiomers plays a critical role in human health and safety. Enantioseparation is an effective way and necessary process to obtain pure chiral compounds. Enantiomer membrane separation is a new chiral resolution technique, which has the potential for industrialization. This paper mainly summarizes the research status of enantioseparation membranes including membrane materials, preparation methods, factors affecting membrane properties, and separation mechanisms. In addition, the key problems and challenges to be solved in the research of enantioseparation membranes are analyzed. Last but not least, the future development trend of the chiral membrane is expected.

Chiral-Controlled Cyclic Chemiluminescence Reactions for the Analysis of Enantiomer Amino Acids

The similarity and complexity of chiral amino acids (AAs) in complex samples remain a significant challenge in their analysis. In this work, the chiral metal-organic framework (MOF)-controlled cyclic chemiluminescence (CCL) reaction is developed and utilized in the analysis of enantiomer AAs. The chiral MOF of d-Co_0.75Zn_0.25-MOF-74 is designed and prepared by modifying the Co_0.75Zn_0.25-MOF-74 with d-tartaric acid. The developed chiral bimetallic MOF can not only offer the chiral recognize sites but also act as the catalyst in the cyclic luminol-H₂O₂ reaction. Moreover, a distinguishable CCL signal can be obtained on enantiomer AAs via the luminol-H₂O₂ reaction with the control of d-Co_0.75Zn_0.25-MOF-74. The amplified difference of enantiomer AAs can be quantified by the decay coefficient (k-values) which are calculated from the exponential decay fitting of their obtained CCL signals. According to simulation results, the selective recognition of 19 pairs of AAs is controlled by the pore size of the MOF-74 and their hydrogen-bond interaction with d-tartaric acid on the chiral MOF. Furthermore, the k-values can also be used to estimate the change of chiral AAs in complex samples. Consequently, this chiral MOF-controlled CCL reaction is applied to differentiate enantiomer AAs involved in the quality monitoring of dairy products and auxiliary diagnosis, which provides a new approach for chiral studies and their potential applications.

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.

Challenges and opportunities for chiral covalent organic frameworks

As highly versatile crystalline porous materials, covalent organic frameworks (COFs) have emerged as an ideal platform for developing novel functional materials, attributed to their precise tunability of structure and functionality. Introducing chiral functional units into frameworks produces chiral COFs (CCOFs) with chiral superiorities through chirality conservation and conversion processes. This review summarises recent research progress in CCOFs, including synthetic methods, chiroptical characterisations, and their applications in asymmetric catalysis, chiral separation, and enantioselective recognition and sensing. Challenges and limitations are discussed to uncover future opportunities in CCOF research.

The progress on porous organic materials for chiral separation

Chiral compounds have similar structures and properties, but their pharmacological action is very different or even opposite. Therefore, the separation of chiral compounds has great significance in pharmaceutical and agriculture. Porous organic materials are novel crystalline porous materials, which possess high surface area, controllable pore size, and favorable functionalization. Therefore, porous organic materials are considered to be an ideal material for chiral separation. In this review, we summarized the progress of chiral porous organic materials for chiral separation in recent years. Furthermore, the applications of chiral porous organic materials as chiral separation medias (chromatography stationary phases and membrane materials) in enantioseparation were highlighted. Finally, the remaining challenges and future directions for porous organic materials in chiral separation were also briefly outlined further to promote the development of porous organic materials in chiral separation.

Accelerating Sample Preparation for the Analysis of Complex Samples

Sample preparation (that is, separation and enrichment) is a critical step in complex sample analysis that affects the sensitivity, selectivity, speed, and accuracy of analytical results, especially in rapid analysis. From chaos to order, the entropy reduction procedure of sample preparation cannot happen spontaneously. Given that sample preparation consumes over two thirds of analysis time, sample preparation becomes the bottleneck issue in analytical chemistry, resulting in the urgent necessity of developing accelerated sample preparation techniques.

Chiral fluorescence recognition of glutamine enantiomers by a modified Zr-based MOF based on solvent-assisted ligand incorporation

In this study, three types of chiral fluorescent zirconium-based metal-organic framework materials were synthesized using l-dibenzoyl tartaric acid as the chiral modifier by the solvent-assisted ligand incorporation method, which was the porous coordination network yellow material, denoted as PCN-128Y. PCN-128Y-1 and PCN-128Y-2 featured unique chiral selectivity for the Gln enantiomers amongst seven acids and the highly stable luminescence property, which were caused by the heterochiral interaction and aggregation-induced emission. Furthermore, a rapid fluorescence method for the chiral detection of glutamine (Gln) enantiomers was developed. The homochiral crystals of PCN-128Y-1 displayed enantiodiscrimination in the quenching by d-Gln such that the ratio of enantioselectivity was 2.0 in 30 seconds at pH 7.0, according to the Stern-Volmer quenching plots. The detection limits of d- and l-Gln were 6.6 × 10-4 mol L-1 and 3.3 × 10-4 mol L-1, respectively. Finally, both the maximum adsorption capacities of PCN-128Y-1 for the Gln enantiomers (Q e(l-Gln) = 967 mg g-1; Q e(d-Gln) = 1607 mg g-1) and the enantiomeric excess value (6.2%) manifested that PCN-128Y-1 had strong adsorption capacity for the Gln enantiomers and higher affinity for d-Gln.