Efficient Separation of Enantiomers Using Stereoregular Chiral Polymers

Facile synthesis of a new chiral polyimine macrocycle and its application for enantioseparation in high-performance liquid chromatography

Macrocyclic compounds such as crown ethers and cyclodextrins play an important role in the field of chromatography and show excellent separation performance. The design of simple and convenient methods for the efficient synthesis of novel chiral macrocycles for chromatographic separation is of great significance. In this work, a novel chiral polyimine macrocycle (PIMC) was designed and synthesized by the simply one-step reaction of 2,6-diformyl-4-tert-butylphenol with (S)-(-)-1,2-propanediamine. Then, it was bonded onto silica by the thiol-ene click reaction to construct a new chiral stationary phase (CSP) for high-performance liquid chromatography (HPLC). The chiral separation performance of the proposed CSP was examined by separating various racemates in the normal-phase (NP) and reversed-phase (RP) HPLC. In total, twelve and nine racemates, including ethers, esters, amines, alcohols, organic acids, ketones, and epoxides, were separated to varying degrees via NP-HPLC and RP-HPLC, respectively, Moreover, the CSP offered good chiral separation complementarity to Chiralcel OD-H and Chiralpak AD-H columns for resolution of these test racemates, and it can separate several racemic compounds that either cannot be separated or cannot be separated well be separated by the two commercially available columns. After the column was used for hundreds of injections, the relative standard deviations of the retention time and resolution were below 0.56 % and 0.45 %, respectively, showing the good reproducibility and stability of the CSP. This study provides a simple and convenient approach to synthesize a novel chiral macrocycle and CSP and also indicates the broad application prospects of such chiral PIMCs in HPLC chiral separation.

Binaphthyl-based chiral covalent organic frameworks for chiral drug separation

Separation of racemic drugs is of great importance and interest in chemistry and pharmacology. Here, we report the bottom-up synthesis of the binaphthyl-based chiral covalent organic frameworks (CCOFs), (R)-BHTP-COF. Then, high-performance liquid chromatography (HPLC) columns were prepared using (R)-BHTP-COF as a chiral stationary phase (CSP). Racemic ibuprofen was successfully baseline-separated on (R)-BHTP-COF-based CSP, and achieved excellent selectivity (α = 2.32) and chromatographic resolution (Rs = 3.39) factors. Meanwhile, the separation of six racemic drugs by the (R)-BHTP-COF-packed column exhibited high resolution, selectivity, and durability. The successful applications indicate the great potential of CCOFs as a novel stationary phase for efficient HPLC separation.

Exploring the Effects of Optically Active Solvents in Chiral Chromatography on Polysaccharide-Based Columns

Exploring the effectiveness of optically active solvents as mobile-phase modifiers in chiral liquid chromatography (LC) can offer an additional new tool to tune the chiral selectivity. Hence, the potential of l-ethyl lactate (LEL), a biobased solvent of this nature, was explored for its distinctive interactions with both the mobile phase and analytes, as anticipated from its chiral nature. The findings reveal that LEL provides distinct selectivity compared to commonly used modifiers in chiral LC. Reversed-phase LC (RPLC)-type chiral separations were therefore compared under various conditions whereby LEL was partially or completely replacing common achiral solvents such as acetonitrile (ACN) and methanol (MeOH). An increase in chiral resolution was obtained in 8 of 16 test compounds. For 5 of them a decrease was obtained, and 3 test solutes did not offer satisfactory results under any of the tested conditions on the polysaccharide columns. When LEL was combined with methanol instead of ACN, worse results were obtained, presumably due to its protic nature. Moreover, LEL demonstrates excellent compatibility with salt additives and is fully miscible with aqueous phases. Interestingly, a steeper increase in chiral resolution is observed for LEL, as compared to ACN at lower temperatures. While LEL is somewhat hindered by its higher UV absorbance, it paves the way toward more simplified chiral screening platforms, whereby chiral solutions can be found for fewer columns and greener solvents such as LEL are incorporated. Finally, to elucidate the impact of chiral interactions between the solvent and analytes, the influence of d-ethyl lactate (DEL) was compared with that of LEL. The results revealed different interactions between the stereoisomers of ethyl lactate (EL) and chiral analytes, demonstrating an influence of optically active solvents on enantioseparations.

Stereoselective Crystallization of Chiral Pharmaceuticals Aided by Cellulose Derivatives through Helical Pattern Matching

Stereoselective inhibition aided by "tailor-made" polymeric additives is an efficient approach to obtain enantiopure compounds through conglomerate crystallization. The chemical and configurational match between the side groups of polymers and the molecules of undesired enantiomer is considered to be a necessary condition for successful stereoseparation. Whereas in this contribution, we present an effective resolution of chiral pharmaceuticals by using cellulose acetates as the additives, which stereoselectively reside on the specific crystal faces of one enantiomer and inhibit its crystal nucleation and growth through helical pattern and supramolecular interaction complementarity. An investigation of nimodipine serves as a case study to highlight the novelty of this strategy wherein R-crystals exhibiting an impressive enantiomeric excess value of 97 % can be attained by employing a mere 0.01 wt % cellulose acetate. Guaifenesin and phenyl lactic acid are also well-resolved by utilizing this methodology. Our work not only brings about a brand-new design strategy for "tailor-made" additives, but will also promote the further exploration of the endless potential for utilizing natural biomolecules in chiral recognition and resolution.

Chiral Acetal-Based Stereo-Controlled Degradable Polymer Synthesis

The precise synthesis of chiral polymers remains a significant challenge in polymer chemistry, particularly for applications in advanced biomedical and electronic materials. The development of degradable polymers is important for eco-friendly and advanced materials. Here, we introduce a stereo-controlled degradable polymer via cascade enyne metathesis polymerization and enantioselective acetal synthesis through Pd-catalyzed asymmetric hydroamination. This approach allows for the creation of chiral acetal-based polymers with controlled stereochemistry and degradability, highlighting their potential for use in drug delivery and electronic applications. This concept article reviews the background, development, and potential applications of these stereo-controlled degradable polymers.

Synthesis of chiral graphene structures and their comprehensive applications: a critical review

From a molecular viewpoint, chirality is a crucial factor in biological processes. Enantiomers of a molecule have identical chemical and physical properties, but chiral molecules found in species exist in one enantiomer form throughout life, growth, and evolution. Chiral graphene materials have considerable potential for application in various domains because of their unique structural framework, properties, and controlled synthesis, including chiral creation, segregation, and transmission. This review article provides an in-depth analysis of the synthesis of chiral graphene materials reported over the past decade, including chiral nanoribbons, chiral tunneling, chiral dichroism, chiral recognition, and chiral transfer. The second segment focuses on the diverse applications of chiral graphene in biological engineering, electrochemical sensors, and photodetectors. Finally, we discuss research challenges and potential future uses, along with probable outcomes.

Helical polyisocyanide-based macroporous organic catalysts for asymmetric Michael addition with high efficiency and stereoselectivity

Porous materials have attracted interest due to their high specific surface area and rich functionality. Immobilizing organocatalysts onto porous polymers not only boosts enantioselectivity but also improves the reaction rates. In this work, a series of porous polymers C-poly-3ms with rigid polyisocyanide-carrying secondary amine pendants as building blocks were successfully prepared. And the pore size and optical activity of C-poly-3ms can be controlled by the length of the polyisocyanide blocks due to their rigid and helical backbone. C-poly-3150 demonstrated a preferred left-handed helix with a θ 364 value of -8.21 × 103. The pore size and S BET of C-poly-3150 were 17.52 nm and 7.98 m2 g-1, respectively. The porous C-poly-3150 catalyzes the asymmetric Michael addition reaction efficiently and generates the target products in satisfactory yield and excellent enantioselectivity. For 6ab, an enantiomeric excess (ee) and a diastereomeric ratio (dr) up to 99% and 99/1 could be achieved, respectively. The recovered catalyst can be recycled at least 6 times in the asymmetric Michael addition reaction while maintaining activity and stereoselectivity.

Optically active poly(diphenylacetylene)s showing solvent-dependent helix inversion accompanied by modulation of helix inversion barriers

Symmetrically substituted poly(diphenylacetylene)s bearing optically active 2-octyloxycarbonyl groups at the para-positions of the pendant phenyl rings not only show a unique solvent-dependent helix inversion to afford diastereomeric right- and left-handed helical polymers but also significant unprecedented solvent-dependent changes in the helix inversion barrier of the polymer backbone resulting in switching between static or dynamic behavior of the helical polymers at approximately room temperature depending on the solvents used.

A New Class of Chiral Polyethers and Polyesters Based on the [2.2]Paracyclophane Scaffold

Over the past decades, the research on optically active polymers (OAPs) has significantly grown, and extensive studies have been carried out on their syntheses, conformations, and applications. The most commonly used OAPs are based on natural products such as sugars or amino acids, which limits their scope. A broader range of applications can be achieved by synthesizing lab-tailored monomers, which allow precise control over structure and properties. This research developed a four-step synthetic route to a previously unreported chiral [2.2]paracyclophane-based epoxide monomer. An aluminum catalyst and an alkylammonium initiating system were applied and optimized for its polymerization to provide access to a novel class of chiral polyethers. Furthermore, we demonstrated the copolymerization viability of the (4-[2.2]paracyclophanyl)oxirane monomer using phthalic anhydride.

Chiral Linker Installation in a Metal-Organic Framework for Enantioselective Luminescent Sensing

Linker installation is a potent strategy for integrating specific properties and functionalities into metal-organic frameworks (MOFs). This method enhances the structural diversity of frameworks and enables the precise construction of robust structures, complementing the conventional postsynthetic modification approaches, by fully leveraging open metal sites and active organic linkers at targeting locations. Herein, we demonstrated an insertion of a d-camphorate linker into a flexible Zr-based MOF, PCN-700, through linker installation. The resultant homochiral MOF not only exhibits remarkable stability but also functions as a highly efficient luminescent material for enantioselective sensing. Competitive absorption and energy/electron transfer processes contribute to the sensing performance, while the difference in binding affinities dominates the enantioselectivity. This work presents a straightforward route to crafting stable homochiral MOFs for enantioselective sensing.

Rational Design of New Conjugated Polymers with Main Chain Chirality for Efficient Optoelectronic Devices: Carbo[6]Helicene and Indacenodithiophene Copolymers as Model Compounds

The unique properties of conjugated polymers (CPs) in various optoelectronic applications are mainly attributed to their different self-assembly processes and superstructures. Various methods are utilized to tune and control CP structure and properties with less attention paid to the use of chirality. CPs with main chain chirality are rare and their microscopic and macroscopic properties are still unknown. In this work, the first experimental results are provided along these lines by synthesizing a series of racemic and enantiopure CPs containing statistical and alternating carbo[6]helicene and indacenodithiophene moieties and evaluating their microscopic (optical, energy levels) and macroscopic properties (hole mobilities, photovoltaic performance). It is demonstrated that a small statistical insertion of either the racemic or enantiopure helicene into the polymer backbone finely tunes the microscopic and macroscopic properties as a function of the statistical content. The microscopic properties of the enantiopure versus the racemic polymers with the same helicene loading remain similar. On the contrary, the macroscopic properties, and more interestingly those between the two enantiomeric forms, are altered as a function of the statistical content. Once incorporated into a solar cell device, these chiral CPs display better performance in their enantiopure versus racemic forms.

Homochiral π-Rich Covalent Organic Frameworks Enabled Chirality Imprinting in Conjugated Polymers: Confined Polymerization and Chiral Memory from Scratch

Optically active π-conjugated polymers (OACPs) have garnered increasing research interest for their resemblance to biological helices and intriguing chirality-related functions. Traditional methods for synthesizing involve decorating achiral conjugated polymer architectures with enantiopure side substituents through complex organic synthesis. Here, we report a new approach: the templated synthesis of unsubstituted OACPs via supramolecularly confined polymerizations of achiral monomers within nanopores of 2D or 3D chiral covalent organic frameworks (CCOFs). We show that the chiral π-rich nanospaces facilitate the in situ enantiospecific polymerization and self-propagation, akin to nonenzymatic polymerase chain reaction (PCR) system, resulting in chiral imprinting. The stacked polymer chains are kinetically inert enough to memorize the chiral information after liberating from CCOFs, and even after treatment at temperature up to 200 °C. The isolated OACPs demonstrate robust enantiodiscrimination, achieving up to 85 % ee in separating racemic amino acids. This underscores the potential of utilizing CCOFs as templates for supramolecularly imprinting optical activity into CPs, paving the way for synthetic evolution and advanced functional exploration of OACPs.

A high-performance chiral 19F-labeled probe with an increased structural twisting

We developed a new strategy to enhance the chiral discrimination capability of 19F-labeled probes by tuning the torsion angle of the probe's backbone, allowing for the resolution of challenging analytes. Its versatility is demonstrated through the superior performance and the wide analyte scope.

Separation of enantiomers of chiral fullerene derivatives through enantioselective encapsulation within an adaptable helical cavity of syndiotactic poly(methyl methacrylate) with helicity memory

Optically active left (M)- and right (P)-handed helical syndiotactic poly(methyl methacrylate)s (M- and P-st-PMMAs) with a helicity memory enantioselectively encapsulated the racemic C60 derivatives, such as 3,4-fulleroproline tert-butyl ester (rac-1) and tetraallylated C60 (rac-2), as well as the C60-bound racemic 310-helical peptides (rac-3) within their helical cavities to form peapod-like inclusion complexes and a unique "helix-in-helix" superstructure, respectively. The enantiomeric excess (ee) and separation factor (enantioselectivity) (α) of the analyte 1 (ee = 23%-25% and α = 2.35-2.50) encapsulated within the helical cavities of the M- and P-st-PMMAs were higher than those of the analytes 2 and 3 (ee = 4.3%-6.0% and α = 1.28-1.50). The optically pure (S)- and (R)-1 were found to more efficiently induce an excess one-handed helical conformation in the st-PMMA backbone than the optically pure (S)- and (R)-1-phenylethylamine, resulting in intense mirror-image vibrational circular dichroism (VCD) spectra in the PMMA IR regions. The excess one-handed helices induced in the st-PMMAs complexed with (S)- and (R)-1 were memorized after replacement with the achiral C60, and the complexes exhibited induced electric CDs in the achiral C60 chromophore regions.

Realization of exciton-polariton optical chirality based on strong coupling between intrinsic chiral quasibound states in the continuum and monolayer WS2

Hybrid quasiparticles produced by the strong interaction between nanostructures and excitons will exhibit optical chirality when one of the coupled components is chiral. Due to the tunability of hybrid states, the coupled system has potential applications in chiral devices and chiral sensing. However, reported chiral materials including chiral molecules and three-dimensional chiral structures in the coupled system limit the application due to the weak chiroptical responses and difficult fabrication, respectively. In this paper, we design chiral quasibound states in the continuum (q-BIC) metasurface by introducing planar symmetry-breaking and z-axis perturbation into an array structure whose unit cell is a C4 rotational symmetric disk. By tuning the polarization state of the eigenmode, a significant chiroptical response is obtained in our q-BIC metasurface. Furthermore, mode splitting is observed not only in the reflection spectrum but also in the circular dichroism (CD) spectrum in the chiral q-BIC and monolayer WS2 strong coupling system, which indicates the realization of the exciton-polariton optical chirality. More importantly, one order of magnitude difference in the reflection to left and right circularly polarized light is achieved resulting in significant CD signals. Our work provides a new strategy to realize the exciton polaritons with significant chiroptical responses, which exhibits promising applications in on-chip chiral devices.

High-performance liquid chromatographic enantioseparation of azole analogs of monoterpene lactones and amides focusing on the separation characteristics of polysaccharide-based chiral stationary phases

High-performance liquid chromatography-based enantioseparation of newly prepared azole analogs of monoterpene lactones and amides was studied. Effects of additives and mobile phase composition were evaluated both in normal and polar organic modes. Applying amylose tris-(3,5-dimethylphenylcarbamate) selector in normal and polar organic modes acid and base additives were found to affect the peak profiles, without significantly influencing the enantiorecognition ability of the studied selector. In most cases, differences observed in retention times and enantioselectivities were lower than 10 and 20 % under normal phase and polar organic conditions, respectively. Under normal phase conditions decreased retention was observed for all the studied analytes with increased eluent polarity. Interestingly, enantioselectivity was only slightly (<10 %) influenced by the variation in the n-hexane/2-propanol ratio between 80/20 and 20/80 v/v. In polar organic mode, five different neat solvents (acetonitrile, methanol, ethanol, 1-propanol, and 2-propanol) were tested, and the best results were obtained with acetonitrile and ethanol in the case of Lux Amylose-1 column with enantioresolutions most often above 2. Based on results obtained with amylose and cellulose-based columns the amylose tris-(3,5-dimethylphenylcarbamate) selector is found to offer a superior performance both in normal and polar organic modes. When evaluating the possible effects of the selector immobilization, no striking differences were found in the normal phase. Usually, enantioselectivities and resolutions were higher (10-20 %), while retention factors of the first peaks were lower (20-30 %), on the coated-type column. In contrast, in polar organic mode, the retention characteristics and enantiorecognition ability of the coated and immobilized selectors were heavily affected by the nature of the polar solvent. Special attention has been paid to the history-dependent behavior of polysaccharide-based selectors. A confidence interval-based evaluation is suggested to help comparison of the histereticity observed in different systems. Several examples are shown to confirm that the recently discovered hysteresis is a common characteristic of polysaccharide-based selectors.

Chirality-induced supramolecular nanodishes: enantioselectivity and energy transfer

Self-assembly is one of the most important issues of fabricating materials with precise chiral nanostructures. Herein, we constructed a chiral assembly system from amphiphiles containing hydrophobic/hydrophilic chiral coils bonded to hexabiphenyl, exhibiting controllable enantioselectivity over various aggregation behaviors. The chiral coils aroused various steric hindrances affecting intrinsic stacking tendency and compactness, leading to different aggregating behaviors, as concluded from the self-assembly investigation. The strong π-π stacking interaction between the long hexabiphenyl groups gave rise to a relatively compact arrangement in the aqueous solution, whereas the methyl side groups on the coil segments raised steric hindrance at the rigid-flexible interface, resulting in loose stacking and formation of nanostructures with a larger curvature. Compared with the achiral molecule 1 that formed micron-sized large sheets, molecules 2-4 containing chiral coils aggregated into nanodishes, which looked exactly like mosquito-repellent incense, to overcome surface tension. The helical structures effectively amplified chirality and exhibited strong circular dichroism (CD) signals, which indicate enantioselectivity. In addition, the relatively loose packing behavior permitted their co-assembly with a dye and aided efficient energy transfer, providing a foundation for the chiral application of supramolecules. Thus, by introducing a simple methyl side group in amphiphilic molecules, asymmetric synthesis and energy transfer efficiency can be realized.

Positive Synergy between the Helical Poly(phenylacetylene) Backbones and the Helical L-Proline Oligopeptide Pendants for Enhanced Enantioseparation Properties

A series of optically active helical poly(phenylacetylene)s (PPA-Pro1, PPA-Pro3, PPA-Pro6, PPA-Pro9, and PPA-Pro12) bearing different chain lengths of L-proline oligopeptide in the side chains were obtained by polymerizing the corresponding novel phenylacetylene monomers. The monomer adopted a trans-rich helix structure when the L-proline oligopeptide chain length was longer, according to the optical activities and 2D-NMR analysis. The helical structure could be maintained and significantly influenced the polymers' helical conformation by introducing the L-proline oligopeptide to the pendants. By the way, the morphology of PPA-Pro3 was observed by atomic force microscope (AFM) on highly oriented pyrolytic graphite (HOPG), and the information on the helix direction, pitch, and chain arrangement was obtained. Also, the chiral separation properties of these polymer-based chiral stationary phases (CSPs) were investigated using high-performance liquid chromatography (HPLC). The poly(phenylacetylene)s showed enhanced enantioseparation properties toward various racemates depending on the longer chain length of the L-proline oligopeptide in the pendants and the positive synergy between the helical backbone and helical side chains. Particularly, PPA-Pro9 showed comparable or even superior enantioseparation properties for racemates 2 and 9 to four commercial columns (Daicel Chiralpak or Chiralcel AD, AS, OD, and OT), indicating that these poly(phenylacetylene)-based CSPs have potential practical values. This work presented here provides inspiration for the further development of CSPs based on a new paradigm.

A Metal-Free Helical Covalent Inorganic Polymer: Preparation, Crystal Structure and Optical Properties

Helical morphologies are widely observed in nature, however, it is very challenging to prepare artificial helical polymers. Especially, precisely understanding the structure information of artificial metal-free helical covalent inorganic polymers via single-crystal X-ray diffraction (SCXRD) analysis is rarely explored. Here, we successfully prepare a novel metal-free helical covalent inorganic polymer ({[Te(C6 H5 )2 ] [PO3 (OH)]}n , named CityU-10) by introducing angular anions (HOPO3 2- ) into traditional tellurium-oxygen chains. The dynamic reversibility of the reaction is realized through the introduction of organic tellurium precursor and the slow hydrolysis of polyphosphoric acid. High-quality and large-size single crystals of CityU-10 have been successfully characterized via SCXRD, where the same-handed helical inorganic polymer chains form a pseudo-two-dimensional layer via multiple hydrogen-bonding interactions. The left-handed layers and right-handed layers alternatively stack together through weak hydrogen bonds to form a three-dimensional supramolecular structure. The single crystals of CityU-10 are found to display promising optical properties with a large birefringence. Our results would offer new guidelines for designing and preparing new crystalline covalent polymers through tellurium-based chemistry.

Chiral measure of chiral polyhedrons

To quantify chiral shape, a tensor describing the particle shape has been proposed. This tensor, named the shape tensor (S-tensor), is an analog of the hydrodynamic tensor that relates the rotational and translational motions of particles in a liquid. The determinant of the S-tensor, named chirality measure density (CMD), was calculated for chiral tetrahedrons and octahedrons. It was found that CMD is opposite in sign when the mirror images are chiral to each other and vanishes when they are achiral. Therefore, the CMD is a good measure to distinguish the mirror images. The interaction between chiral particles was discussed in terms of the CMD.

Stimuli-responsive synthetic helical polymers

Synthetic dynamic helical polymers (supramolecular and covalent) and foldamers share the helix as a structural motif. Although the materials are different, these systems also share many structural properties, such as helix induction or conformational communication mechanisms. The introduction of stimuli responsive building blocks or monomer repeating units in these materials triggers conformational or structural changes, due to the presence/absence of the external stimulus, which are transmitted to the helix resulting in different effects, such as assymetry amplification, helix inversion or even changes in the helical scaffold (elongation, J/H helical aggregates). In this review, we show through selected examples how different stimuli (e.g., temperature, solvents, cations, anions, redox, chiral additives, pH or light) can alter the helical structures of dynamic helical polymers (covalent and supramolecular) and foldamers acting on the conformational composition or molecular structure of their components, which is also transmitted to the macromolecular helical structure.

Chiral Discrimination of Acyclic Secondary Amines by 19F NMR

Chiral aliphatic amine compounds exhibit a range of physiological activities, making them highly sought-after in the pharmaceutical industry and biological research. One notable obstacle in studying these compounds stems from the pronounced steric hindrance surrounding the nitrogen atom. This characteristic often leads to a weak affinity of acyclic secondary amines for molecular probes, making their chiral discrimination intricate. In response to this challenge, our research has unveiled a novel 19F-labeled probe adept at recognizing and distinguishing between enantiomers of these acyclic secondary amines. By strategically incorporating a single fluorine atom as the 19F label, we have managed to diminish the steric hindrance at the binding site. This alteration bolsters the probe's affinity toward bulkier analytes. As a testament to its effectiveness, we have successfully employed our probe in the chiral analysis of relevant pharmaceuticals, accurately determining their enantiocomposition.

High-Enantioselectivity Adsorption Separation of Racemic Mandelic Acid and Methyl Mandelate by Robust Chiral UiO-68-Type Zr-MOFs

Mandelic acid and its analogues are highly valuable medical intermediates and play an important role in the pharmaceutical industry, biochemistry, and life sciences. Therefore, effective enantioselective recognition and separation of mandelic acid are of great significance. In this study, two of our recently reported chiral amine-alcohol-functionalized UiO-68-type Zr-HMOFs 1 and 3 with high chemical stability, abundant binding sites, and large chiral pores were selected as chiral selectors for the enantioselective separation of mandelic acid (MA), methyl mandelate (MM), and other chiral molecules containing only one phenyl. Materials 1 and 3 exhibited excellent enantioselective separation performance for MA and MM. Especially for the separation of racemate MA, the enantiomeric excess values reached 97.3 and 98.9%, which are the highest reported values so far. Experimental and density functional theory (DFT) computational results demonstrated that the introduction of additional phenyls on the chiral amine alcohol pendants in 3 had somewhat impact on the enantioselective adsorption and separation of MA or MM compared with 1, but it was not significant. Further research on the enantioselective separation of those chiral adsorbates containing only one phenyl by material 1 indicated the crucial role of the groups directly bonded to the chiral carbons of the adsorbates in the selective separation of enantiomers, especially showing higher enantioselectivity for the adsorbates with two hydrogen-bonding groups directly bonded to its chiral carbon.

Enantioselective adsorption of ibuprofen enantiomers using chiral-active carbon nanoparticles induced S-α-methylbenzylamine

The chiral media is crucial to the chiral recognition and separation of enantiomers. In this study, we report the preparation of novel chiral carbon nanoparticles (CCNPs) via surface passivation using glucose as the carbon source and S-(-)-α-methylbenzylamine as the chiral ligand. The structures of the obtained CCNPs are characterized via FT-IR, Raman spectroscopy, DLS, XPS, XRD, TEM, and zeta potential analysis. These CCNPs could be employed as the chiral adsorbent and used for the enantioselective adsorption of the ibuprofen enantiomers. The results demonstrated that the CCNPs could selectively adsorb R-enantiomer from ibuprofen racemate solution and give an enantiomeric excess (e.e.) of about 50% under an optimal adsorption condition. Moreover, the regeneration efficiency of the CCNPs remained above e.e. of 43% after the fifth cycle. The present work confirmed that the prepared CCNPs show great potential in the enantioselective separation of ibuprofen racemate.

Chiral analysis and semi-preparative separation of metconazole stereoisomers by supercritical fluid chromatography and cytotoxicity assessment in vitro

Metconazole is one of the widely-used chiral triazole fungicides in controlling wheat leaf rust, powdery mildew, Fusarium head blight with high efficacy, and so forth. In the current work, the effects of chiral stationary phases, alcoholic modifiers, and column temperature on the chiral separation of metconazole were discussed in detail. Amylose tris(3,5-dimethylphenylcarbamate)-coated chiral stationary phase exhibited much stronger chiral recognition ability toward metconazole stereoisomers in the CO2 /ethanol mixture as compared to the others. Then, a two-step semi-preparative separation of metconazole was performed through supercritical fluid chromatography and high-performance liquid chromatography, and the enantiomeric excess values of four stereoisomers were achieved over 98%. Moreover, the enantioselective cytotoxicity of cis-metconazole against HepG2 cells has been investigated, and the order of the cell proliferation toxicity against HepG2 cells was (1R, 5S)-metconazole > (1S, 5R)-metconazole > the mixture. Briefly, this study would provide valuable information in the preparative separation of optically pure metconazole products through chromatographic techniques and their environmental risk assessment.

Photoswitchable Enantioselective and Helix-Sense Controlled Living Polymerization

A pair of enantiomeric photoswitchable PdII catalysts, alkyne-PdII /LR-azo and alkyne-PdII /LS-azo , were prepared via the coordination of alkyne-PdII and azobenzene-modified phosphine ligands LR-azo and LS-azo . Owing to the cis-trans photoisomerization of the azobenzene moiety, alkyne-PdII /LR-azo and alkyne-PdII /LS-azo exhibited different polymerization activities, helix-sense selectivities, and enantioselectivities during the polymerization of isocyanide monomers under irradiation of different wavelength lights. Furthermore, the achiral isocyanide monomer A-1 could be polymerized efficiently using alkyne-PdII /LR-azo under dark condition in a living/controlled manner. Further, it generated single right-handed helical poly-A-1m (LR-azo ), confirmed by the circular dichroism spectra and atomic force microscopy images. However, the polymerization of A-1 almost could not be initiated under 420 nm light in identical conditions of dark condition. Moreover, the photoswitchable catalyst alkyne-PdII /LR-azo exhibited high enantioselectivity for the polymerization of the racemates of L-1 and D-1, respectively. D-1 was polymerized preferentially under dark condition with a D-1/L-1 rate ratio of 70, yielding single right-handed polyisocyanides. Additionally, reversible enantioselectivity was observed under 420 nm light using alkyne-PdII /LR-azo , and the calculated polymerization rate ratio of L-1/D-1 was 57 because of the isomerization of the azobenzene moiety of the catalyst. Furthermore, alkyne-PdII /LS-azo showed opposite enantioselectivity and helix-sense selectivity during the polymerization of the racemates of L-1 and D-1.

Cellulose/amylose derivatives bearing bulky substituents as reversible fluorescent sensors for detection of Fe3

Two novel cellulose and amylose derivatives bearing bulky tris(2-benzothienylformate) pendants (Cel-3 and Amy-3) were expeditiously prepared by one-step esterification. The fluorescent sensing performance of six polysaccharide derivatives, including Cel-3/Amy-3, and other four previously prepared benzothienyl- or benzofuranyl-phenylcarbamates of cellulose and amylose (Cel-1/Amy-1, Cel-2/Amy-2), were carefully evaluated using eight metal ions, including Co2+, K+, Na+, Li+, Hg2+, Ni2+, Ca2+ and Fe3+. All six derivatives exhibited excellent fluorescence quenching property to Fe3+ ions with high sensitivity and selectivity. Especially, the limit of detection of Amy-2 with benzofuranylphenylcarbamates for Fe3+ was 3.0 μM, much lower than the maximum contaminant level for Fe3+ in the drinking water. Additionally, the six bulky derivatives displayed the interesting fluorescence "turn-off" and "turn-on" observation, indicating a desirable reversibility for Fe3+ detection. The high anti-interference ability was also observed for detection of Fe3+ on the benzothienyl/benzofuranyl derivatives of cellulose and amylose in the combined system containing Co2+, K+, Na+, Li+, Hg2+, Ni2+ and Ca2+. It suggested that the obtained polysaccharide derivatives with bulky chromophores possessed good potentials for detection of Fe3+ as high-efficient fluorescent sensors in diverse applications. The sensing mechanism for detection of Fe3+ was further proposed based on the Stern-Volmer plots and fluorescence titration analysis.

Control of One-Handed Helicity in Polyacetylenes: Impact of an Extremely Small Amount of Chiral Substituents

Controlling the one-handed helicity in synthetic polymers is crucial for developing helical polymer-based advanced chiral materials. We now report that an extremely small amount of chiral biphenylylacetylene (BPA) monomers (ca. 0.3-0.5 mol %) allows complete control of the one-handed helicity throughout the polymer chains mostly composed of achiral BPAs. Chiral substituents introduced at the 2-position of the biphenyl units of BPA positioned in the vicinity of the polymer backbones contribute to a significant amplification of the helical bias, as interpreted by theoretical modeling and simulation. The helical structures, such as the helical pitch and absolute helical handedness (right- or left-handed helix) of the one-handed helical copolymers, were unambiguously determined by high-resolution atomic force microscopy combined with X-ray diffraction. The exceptionally strong helix-biasing power of the chiral BPA provides a highly durable and practically useful chiral material for the separation of enantiomers in chromatography by copolymerization of an achiral functional BPA with a small amount of the chiral BPA (0.5 mol %) due to the robust helical scaffold of the one-handed helical copolymer.

Effect of Extra-Framework Anion Substitution on the Properties of a Chiral Crystalline Sponge

Chiral metal-organic materials, CMOMs, are of interest as they can offer selective binding sites for chiral guests. Such binding sites can enable CMOMs to serve as chiral crystalline sponges (CCSs) to determine molecular structure and/or purify enantiomers. We recently reported on the chiral recognition properties of a homochiral cationic diamondoid, dia, network {[Ni(S-IDEC)(bipy)(H2O)][NO3]}n (S-IDEC = S-indoline-2-carboxylicate, bipy = 4,4'-bipyridine), CMOM-5[NO3]. The modularity of CMOM-5[NO3] means there are five feasible approaches to fine-tune structures and properties via substitution of one or more of the following components: metal cation (Ni2+); bridging ligand (S-IDEC); linker (bipy); extra-framework anion (NO3-); and terminal ligand (H2O). Herein, we report the effect of anion substitution on the CCS properties of CMOM-5[NO3] by preparing and characterizing {[Ni(S-IDEC)(bipy)(H2O)][BF4]}n, CMOM-5[BF4]. The chiral channels in CMOM-5[BF4] enabled it to function as a CCS for determination of the absolute crystal structures of both enantiomers of three chiral compounds: 1-phenyl-1-butanol (1P1B); methyl mandelate (MM); ethyl mandelate (EM). Chiral resolution experiments revealed CMOM-5[BF4] to be highly selective toward the S-isomers of MM and EM with enantiomeric excess, ee, values of 82.6 and 78.4%, respectively. The ee measured for S-EM surpasses the 64.3% exhibited by [DyNaL(H2O)4] 6H2O and far exceeds that of CMOM-5[NO3] (6.0%). Structural studies of the binding sites in CMOM-5[BF4] provide insight into their high enantioselectivity.

Construction of metallo-helicoids with high antimicrobial activity via intermolecular coordination

Metallo-helicoids are constructed by intermolecular coordination interactions between covalent linear polymer and tritopic/hexatopic molecular templates. These metallo-polymers with helicoidal conformation exhibit high antimicrobial activities against both Gram-positive and Gram-negative pathogens.

Design of molecularly imprinted resin material with sulfonic acid functionalization for enantioseparation of (±)-cathine

In the current work, an enantioselective imprinting technique was used to develop a very selective adsorbent for the (+)-cathine ((+)-Cat) enantiomer. The phenolic sulfonamide produced from 2,4-dihydroxybenzenesulfonic acid (HBS) and (+)-Cat ((+)-Cat-HBS) was initially synthesized by triphenylphosphene activation and subsequently involved in condensation polymerization with resorcinol in the presence of formaldehyde under acidic conditions. Alkaline sulfonamide bond-breaking was subsequently employed to separate the (+)-Cat template from the polymer, and the resulting imprinted resin ((+)-CIP) displayed high selectivity for the (+)-Cat, with a capacity of 225 ± 2 mg/g. Studies of selectivity also showed that the (+)-Cat enantiomer was preferred over its counterpart because of the development of configurationally matching receptors. In addition, the produced resin was used for the enantioresolution of (±)-Cat racemate by column method, yielding a loading supernatant solution with an enantiomeric excess of (+)-Cat 50% and a recovery eluant solution with an excess of (-)-Cat 85%.

Amino alcohol-derived chiral stationary phases

An updated minireview of chiral stationary phases (CSPs) based on amino alcohols is presented. In this minireview, we focused on amino alcohols as starting materials in preparation of chiral catalysts for asymmetric organic synthesis and CSPs for chiral separations. Among the various CSPs, we summarized the important developments and applications of the amino alcohol-based Pirkle-type CSPs, ligand exchange CSPs, α-amino acid-derived amino alcohol CSPs, and symmetric CSPs from their first appearance to the present day to propose ideas for the development of new CSPs with improved performance.

Homochiral Metallacycle Used as a Stationary Phase for Capillary Gas Chromatographic Separation of Chiral and Achiral Compounds

Metallacycles are a novel class of supramolecular materials with circular structures, internal cavities, and abundant host-guest chemical properties that have exhibited good application prospects in many fields. However, to the best of our knowledge, no research on the use of metallacycles as stationary phases for gas chromatographic (GC) separations has been published yet. In this work, we report for the first time the use of a homochiral metallacycle, [ZnCl2L]2, as a stationary phase for GC separations. [ZnCl2L]2 was synthesized by reaction of (S)-(1-isonicotinoylpyrrolidin-2-yl)methyl-isonicotinate (L) with ZnCl2 via coordination-driven self-assembly. The [ZnCl2L]2-coated column displayed an excellent separation performance not only of organic isomers but also of racemic compounds. Sixteen racemates (including alcohols, esters, amino acid derivatives, ethers, organic acids, and epoxides) and 21 isomeric compounds (including positional, structural, and cis/trans-isomers) were well separated on the [ZnCl2L]2-coated column. Impressively, some racemates were resolved with high resolution values (Rs), including 1,2-butanediol diacetate (Rs = 25.86), ethyl 3-hydroxybutyrate (Rs = 20.97), 1,3-butanediol diacetate (Rs = 18.09), and threonine derivative (Rs = 18.61). Compared with the commercial β-DEX 120 column for separation of the tested racemates, the [ZnCl2L]2-coated column exhibited good enantioseparation complementarity, enabling separation of some racemates that could not be separated, or were not well resolved, by the β-DEX 120 column. In addition, many organic mixtures, such as n-alkanes, alkylbenzenes, n-alcohols, and a Grob test mixture, were also well separated on the [ZnCl2L]2-coated column. The column also has good reproducibility and thermal stability on separation. This work not only reveals the great potential of metallacycles for GC separations but also opens up a new application of metallacycles in separation science.

Recent advances of optically active helical polymers as adsorbents and chiral stationary phases for chiral resolution

Chiral resolution is very important and still a big challenge due to different biological activity and same physicochemical property of one pair (R)- and (S)-isomer. There is no doubt that chiral selectors are essentially needed for chiral resolution, which can stereoselectively interact with a pair of isomers. To date, a large amount of optically active helical polymers as chiral selectors have been synthesized via two strategies. First, the target helical polymers are derived from natural polysaccharide such as cellulose and amylose. Second, they can be synthesized by polymerization of chiral monomers. Alternatively, an achiral polymer is prepared first followed by static or dynamic chiral induction. Furthermore, a part of them is harnessed as chiral stationary phases for chromatographic chiral separation and as chiral adsorbents for enantioselective adsorption/crystallization, resulting in good enantioseparation efficiency. In summary, the present review will focus on recent progress of the polymers with optical activity for chiral resolution, especially the literature published in the past 10 years. In addition, development prospects and future challenges of optically active helical polymers will be discussed in detail.

19 F-Labeled Probes for Recognition-Enabled Chromatographic 19 F NMR

The NMR technique is among the most powerful analytical methods for molecular structural elucidation, process monitoring, and mechanistic investigations; however, the direct analysis of complex real-world samples is often hampered by crowded NMR spectra that are difficult to interpret. The combination of fluorine chemistry and supramolecular interactions leads to a unique detection method named recognition-enabled chromatographic (REC) 19 F NMR, where interactions between analytes and 19 F-labeled probes are transduced into chromatogram-like 19 F NMR signals of discrete chemical shifts. In this account, we summarize our endeavor to develop novel 19 F-labeled probes tailored for separation-free multicomponent analysis. The strategies to achieve chiral discrimination, sensitivity enhancement, and automated analyte identification will be covered. The account will also provide a detailed discussion of the underlying principles for the design of molecular probes for REC 19 F NMR where appropriate.

Exploitation of the enantioselectivity space of coated amylose tris(3,5-dimethylphenylcarbamate) in mixtures of 2-propanol and acetonitrile

Chiral stationary phases (CSPs) with coated amylose tris(3,5-dimethylphenylcarbamate) (ADMPC) selector have long been recognized for their excellent chiral recognition ability in liquid chromatography. The conformational versatility behind this feature is the source of their known hysteretic behavior, which has been previously observed in polar organic (PO) mode eluents containing 2-propanol (IPA). Mixtures of IPA and acetonitrile (MeCN), a typical PO mode eluent system, have not been examined in this aspect yet, even though hysteresis is promising for finding unique unexplored enantioselectivities. Not only was the hysteresis detectable on ADMPC using mixtures of IPA and MeCN, but it was the typical behavior in a diverse set of test compounds. The difference in the retention time of the same analyte under conditions which only differed in the eluent history on the column can go up to 20-fold. The assumed hindered conformational changes of the selector were reflected in retention drift at certain eluent compositions. On the two sides of the transitions, distinct, useful states of the selector were detected. A series of IPA - MeCN compositions with defined pretreatment was selected and recommended as an extension of the preliminary, first choice method screening set that used only alcohols. The incorporation of a solvent possessing substantially different characteristics enhances the potential in practical applications, while keeping the technical simplicity. Stability and robustness of the additional states of the CSP were characterized. The examined columns of different brands shared the observed behavior. Kinetic stability of a column state is adequate for successful application. The evaluated states of ADMPC provide multiple enantiorecognition potential by using mixtures of IPA and MeCN also considering the pretreatment of the column. Unprecedented double and triple elution order reversals along the composition range supported the versatility of the available states. Our findings further enhance the usefulness of ADMPC-containing CSPs. We provide instructions for the application of the widespread chiral selector in common eluent mixtures to avoid pitfalls regarding reproducibility and robustness.

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.

as a Versatile and Predictable Chiral Stationary Phase for the Resolution of Racemic Mixtures

Metal-organic frameworks (MOFs) have become promising materials for multiple applications due to their controlled dimensionality and tunable properties. The incorporation of chirality into their frameworks opens new strategies for chiral separation, a key technology in the pharmaceutical industry as each enantiomer of a racemic drug must be isolated. Here, we describe the use of a combination of computational modeling and experiments to demonstrate that high-performance liquid chromatography (HPLC) columns packed with TAMOF-1 as the chiral stationary phase are efficient, versatile, robust, and reusable with a wide array of mobile phases (polar and non-polar). As proof of concept, in this article, we report the resolution with TAMOF-1 HPLC columns of nine racemic mixtures with different molecular sizes, geometries, and functional groups. Initial in silico studies allowed us to predict plausible separations in chiral compounds from different families, including terpenes, calcium channel blockers, or P-stereogenic compounds. The experimental data confirmed the validity of the models and the robust performance of TAMOF-1 columns. The added value of in silico screening is an unprecedented achievement in chiral chromatography.

Beyond Stereoisomeric Effects: Exploring the Importance of Intermolecular Electron Spin Interactions in Biorecognition

This work shows that electron spin polarization and stereoisomeric effects make comparable contributions to the enantioselective binding of amino acids. Magneto-electrochemical quartz crystal microbalance methods are used to study the adsorption of chiral amino acids onto a monolayer film of chiral molecules that are spin polarized by an underlying ferromagnetic substrate. The direction of the electron spin polarization affects both the kinetics and thermodynamics of the enantiospecific adsorption of the amino acids. Comparison of these data with the circular dichroism (CD) spectra of the amino acid adsorbates shows that the CD spectrum of the interacting group provides a good figure-of-merit for predicting the contributions of electron spin to the intermolecular interaction. These findings demonstrate the importance of electron spin in enantioselective intermolecular interactions between chiral amino acids and represent a paradigm shift for how selectivity should be viewed in biorecognition.

Chiral Discrimination of Nitrile Compounds Using a 19F-Labeled Palladium Probe

This study presents a ¹⁹F-labeled cyclopalladium probe for the rapid discrimination of chiral nitriles in pharmaceuticals, natural products, and agrochemicals. The probe binds reversibly to chiral nitriles, generating distinct ¹⁹F nuclear magnetic resonance signals for each enantiomer and enabling quick determination of enantiocomposition. The method allows for simultaneous detection of seven pairs of enantiomeric nitriles and application in assessing the enantiomeric excess of an asymmetric C-H cyanation reaction.

Chiral Recognition by Supramolecular Porphyrin-Hemicucurbit[8]uril-Functionalized Gravimetric Sensors

Enantiorecognition of a chiral analyte usually requires the ability to respond with high specificity to one of the two enantiomers of a chiral compound. However, in most cases, chiral sensors have chemical sensitivity toward both enantiomers, showing differences only in the intensity of responses. Furthermore, specific chiral receptors are obtained with high synthetic efforts and have limited structural versatility. These facts hinder the implementation of chiral sensors in many potential applications. Here, we utilize the presence of both enantiomers of each receptor to introduce a novel normalization that allows the enantio-recognition of compounds even when single sensors are not specific for one enantiomer of a target analyte. For this purpose, a novel protocol that permits the fabrication of a large set of enantiomeric receptor pairs with low synthetic efforts by combining metalloporphyrins with (R,R)- and (S,S)-cyclohexanohemicucurbit[8]uril is developed. The potentialities of this approach are investigated by an array of four pairs of enantiomeric sensors fabricated using quartz microbalances since gravimetric sensors are intrinsically non-selective toward the mechanism of interaction of analytes and receptors. Albeit the weak enantioselectivity of single sensors toward limonene and 1-phenylethylamine, the normalization allows the correct identification of these enantiomers in the vapor phase indifferent to their concentration. Remarkably, the achiral metalloporphyrin choice influences the enantioselective properties, opening the way to easily obtain a large library of chiral receptors that can be implemented in actual sensor arrays. These enantioselective electronic noses and tongues may have a potential striking impact in many medical, agrochemical, and environmental fields.

Retention time prediction for chromatographic enantioseparation by quantile geometry-enhanced graph neural network

The enantioseparation of chiral molecules is a crucial and challenging task in the field of experimental chemistry, often requiring extensive trial and error with different experimental settings. To overcome this challenge, here we show a research framework that employs machine learning techniques to predict retention times of enantiomers and facilitate chromatographic enantioseparation. A documentary dataset of chiral molecular retention times in high-performance liquid chromatography (CMRT dataset) is established to handle the challenge of data acquisition. A quantile geometry-enhanced graph neural network is proposed to learn the molecular structure-retention time relationship, which shows a satisfactory predictive ability for enantiomers. The domain knowledge of chromatography is incorporated into the machine learning model to achieve multi-column prediction, which paves the way for chromatographic enantioseparation prediction by calculating the separation probability. The proposed research framework works well in retention time prediction and chromatographic enantioseparation facilitation, which sheds light on the application of machine learning techniques to the experimental scene and improves the efficiency of experimenters to speed up scientific discovery.

Chirality Sensing of N-Heterocycles via 19F NMR

Methods to rapidly detect and differentiate chiral N-heterocyclic compounds become increasingly important owing to the widespread application of N-heterocycles in drug discovery and materials science. We herein report a ¹⁹F NMR-based chemosensing approach for the prompt enantioanalysis of various N-heterocycles, where the dynamic binding between the analytes and a chiral ¹⁹F-labeled palladium probe create characteristic ¹⁹F NMR signals assignable to each enantiomer. The open binding site of the probe allows the effective recognition of bulky analytes that are otherwise difficult to detect. The chirality center distal to the binding site is found sufficient for the probe to discriminate the stereoconfiguration of the analyte. The utility of the method in the screening of reaction conditions for the asymmetric synthesis of lansoprazole is demonstrated.

Postmodification of an Amine-Functionalized Covalent Organic Framework for Enantioselective Adsorption of Tyrosine

The development of chiral covalent organic frameworks (COFs) by postsynthetic modification is challenging due to the common occurrences of racemization and crystallinity decrement under harsh modification conditions. Herein, we employ an effective site-selective synthetic strategy for the fabrication of an amine-functionalized hydrazone-linked COF, NH₂-Th-Tz COF, by the Schiff-base condensation between aminoterephthalohydrazide (NH₂-Th) and 4,4',4″-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde (Tz). The resulting NH₂-Th-Tz COF with free amine groups on the pore walls provides an appealing platform to install desired chiral moieties through postsynthetic modification. Three chiral moieties including tartaric acid, camphor-10-sulfonyl chloride, and diacetyl-tartaric anhydride were postsynthetically integrated into NH₂-Th-Tz COF by reacting amine groups with acid, acyl chloride, and anhydride, giving rise to a series of chiral COFs with distinctive chiral pore surfaces. Moreover, the crystallinity, porosity, and chirality of chiral COFs were retained after modification. Remarkably, the chiral COFs exhibited an exceptional enantioselective adsorption capability toward tyrosine with a maximum enantiomeric excess (ee) value of up to 25.20%. Molecular docking simulations along with experimental results underscored the pivotal role of hydrogen bonds between chiral COFs and tyrosine in enantioselective adsorption. This work highlights the potential of site-selective synthesis as an effective tool for the preparation of highly crystalline and robust amine-decorated COFs, which offer an auspicious platform for the facile synthesis of tailor-made chiral COFs for enantioselective adsorption and beyond.

Engineering UiO-68-Typed Homochiral Metal-Organic Frameworks for the Enantiomeric Separation of Fmoc-AAs and Mechanism Study

Homochiral metal-organic frameworks (HMOFs) have been widely investigated in the application of enantiomeric separation. Nonetheless, it remains a significant challenge to explore the effect of multiple weak interactions between HMOF adsorbents and chiral adsorbates on enantiomeric separation performance still. In this work, robust chiral amine-alcohol-functionalized UiO-68-typed Zr-HMOFs 1-3 with the same hydrogen-bonding sites but slightly different π-binding sites were prepared for the enantioseparation of amino acid derivatives (Fmoc-AAs) with large π-binding groups. As a consequence of multiple host-guest interactions, these Zr-HMOFs exhibit speedy adsorption and high adsorption capacity for Fmoc-L/D-AAs and dissimilar enantioselectivity for the adsorption of their enantiomers. Materials 1 and 2 exhibit excellent enantioselective separation performance for Fmoc-valine with a single terminal π-binding group, while material 3 displays excellent enantioselective separation performance for Fmoc-phenylalanine and Fmoc-tryptophan with π-binding groups at both ends. As evidently demonstrated by our experimental and density functional theory (DFT) computational results, when the number of π-binding groups preset in the confined chiral space of adsorbents matches the number of π-binding groups of chiral adsorbates, the synergism of π-π or σ-π interactions will increase enantioselectivity; otherwise, the competition interactions from redundant identical binding sites will weaken enantioselectivity. Our case not only provides a tremendously typical system for investigating the collaborative discrimination of multiple weak interactions and exploring the impact of relatively excessive binding sites of HMOF adsorbents or chiral adsorbates on the enantioselective separation performance but also provides guidance for targeted functional modifications of high-performance chiral porous materials.

Planar Chirality for Acid/Base Responsive Macrocyclic Pillararenes Induced by Amino Acid Derivatives: Molecular Dynamics Simulations and Machine Learning

Chirality is ubiquitous in nature, ranging from a DNA helix to a biological macromolecule, snail's shell, and even a galaxy. However, the precise control of chirality at the nanoscale is a challenge due to the structure complexity of supramolecular assemblies, the small energy differences between different enantiomers, and the difficulty in obtaining polymorphic crystals. The planar chirality of water-soluble pillar[5]arenes (called WP5-Na with Na ions in the side chain) host triggered by the addition of chiral L-amino acid hydrochloride (L-AA-OEt) guests and acid/base is rationalized by the relative stability of different chiral isomers, being estimated by molecular dynamics (MD) simulations and quantum chemical calculations. As an increase in the pH value, the change from a positive to a negative value of the free energy difference (ΔG) between two conformations, pR-WP5-Na⊃L-AA-OEt and pS-WP5-Na⊃L-AA-OEt, suggests an inversed preference of the pS-WP5-Na conformer induced by the deprotonated L-arginine ethyl ester (L-Arg-OEt) at pH = 14, which is supported by the circular dichroism (CD) experiments. On the basis of 2256 WP5-Na⊃L-Ala-OEt and 3299 WP5-Na⊃L-Arg-OEt conformers sampled from MD, the gradient boosting regression (GBR) model exhibits a satisfactory performance (R² = 0.91) in predicting the chirality of WP5-Na complexations using host-guest binding descriptors, including the geometry matching and binding sites and modes (electrostatics and hydrogen bonding). The machine learning model also performs well on external tests of different hosts (using different side chains and cavity sizes) with the addition of 22 other different guests, with the average chirality prediction accuracy of ML versus experimental CD determinations of 92.8%. The easily accessible host-guest features, binding position coordination and size matching between the cavity and guest, exhibit a close correlation to the chirality of different macrocyclic molecules, water-soluble pillar[6]arenes (WP6) versus WP5, in complexation with different amino acid guests. The exploration of efficient host-guest features in ML displays the great potential of building a large space of various assembled systems and accelerating the on-demand design of chiral supramolecular systems at the nanoscale.

Tailoring the Recognition Property of a 19F-Labeled Gallium-Based NMR Probe: The Influence of the Metal Center

Chirality is a fundamental property of nature and an essential element of the life process. As the biological activities, metabolic pathways, and toxicity of individual enantiomers are often varied, methods to rapidly and accurately discriminate chiral analytes are in great demand. Here, we report a ¹⁹F-labeled gallium-based probe for the enantiodifferentiation of chiral monoamines, diamines, amino alcohols, amino acids, and N-heterocycles. A comparison between the new gallium-based probe and the previously developed aluminum aminotrisphenolate complex was performed. It was revealed that the gallium metal center displays a much stronger affinity toward the amino group compared to the hydroxy group, thereby producing simplified ¹⁹F NMR signals for analytes with multiple Lewis basic sites. For sterically bulky analyte, the replacement of the aluminum with gallium is envisioned to expand the binding pocket of the probe to allow different binding models to interconvert rapidly. This feature is important to the creation of easily interpretable ¹⁹F signals corresponding to each enantiomer. It is further demonstrated that the gallium-based probe is suitable for the assessment of the enantiomeric excess values of the crude products obtained in asymmetric reactions without the need for purification.

Genetically engineered bacterium-modified magnetic particles assisted chiral recognition and colorimetric determination of D/L-tryptophan in millets

Chiral recognition of enantiomers has always been a thorny issue since they exhibit the same properties under an achiral environment. Herein, polydopamine-functionalized magnetic particles (MP@PDA) were synthesized to immobilize the genetically engineered bacterium Escherichia coli DH5α (MP@PDA-E. coli). L-tryptophan (Trp) instead of D-Trp can be stereo-specifically degraded by tryptophanase in E. coli. The degradation product indole reacts with 4-dimethylaminobenzaldehyde to generate a rose-red adduct. Thus, MP@PDA-E. coli was employed to fabricate a chiral colorimetric method for chiral recognition and determination of L-Trp. The method averts the purification of tryptophanase. More importantly, tryptophanase demonstrates excellent enantioselective ability for L-Trp. The method can not only quantitatively detect L-Trp but also realize the measurement of the enantiomer percentage in the enantiomeric mixture. The feasibility was verified by detecting L-Trp in millet samples from different origins. Furthermore, a portable device was fabricated to make the method more convenient.

Preparation of Chiral Porous Organic Cage Clicked Chiral Stationary Phase for HPLC Enantioseparation

Porous organic cages (POCs) are a new subclass of porous materials, which are constructed from discrete cage molecules with permanent cavities via weak intermolecular forces. In this study, a novel chiral stationary phase (CSP) has been prepared by chemically binding a [4 + 6]-type chiral POC (C120H96N12O4) with thiol-functionalized silica gel using a thiol-ene click reaction and applied to HPLC separations. The column packed with this CSP presented good separation capability for chiral compounds and positional isomers. Thirteen racemates have been enantioseparated on this column, including alcohols, diols, ketones, amines, epoxides, and organic acids. Upon comparison with a previously reported chiral POC NC1-R-based column, commercial Chiralpak AD-H, and Chiralcel OD-H columns, this column is complementary to these three columns in terms of its enantiomeric separation; and can also separate some racemic compounds that cannot be separated by the three columns. In addition, eight positional isomers (iodoaniline, bromoaniline, chloroaniline, dibromobenzene, dichlorobenzene, toluidine, nitrobromobenzene, and nitroaniline) have also been separated. The influences of the injection weight and column temperature on separation have been explored. After the column has undergone multiple injections, the relative standard deviations (RSDs) for the retention time and selectivity were below 1.0 and 1.5%, respectively, indicating the good reproducibility and stability of the column for separation. This work demonstrates that POCs are promising materials for HPLC separation.

Chiral covalent organic framework-based open tubular capillary electrochromatography column for enantioseparation of selected amino acids and pesticides

A novel chiral covalent organic framework (CCOF) was synthesized with an imine covalent organic framework TpBD (synthesized via Schiff-base reaction between phloroglucinol (Tp) and benzidine (BD)) modified using (1S)-(+)-10-camphorsulfonyl chloride as chiral ligand by chemical bonding method for the first time, and was characterized by X-ray diffraction, Fourier-transform infrared spectra, X-ray photoelectron spectroscopy, nitrogen adsorption/desorption, thermogravimetry analysis, and zeta-potential. The results revealed that the CCOF had good crystallinity, high specific surface area and good thermal stability. Then, the CCOF was employed as stationary phase in open-tubular capillary electrochromatography (OT-CEC) column (the CCOF-bonded OT-CEC column) for enantioseparation of 21 single chiral compounds (12 natural amino acids including acidic, neutral and basic, 9 pesticides including herbicides, insecticides and fungicides) and simultaneous enantioseparation of mixture amino acids and pesticides with similar structures or properties. Under the optimized CEC conditions, all the analytes reached the baseline separation with high resolutions of 1.67-25.93 and selectivity factors of 1.06-3.49 in 8 min. Finally, the reproducibility and stability of the CCOF-bonded OT-CEC column were measured. Relative standard deviations (RSDs) of retention time and separation efficiency were 0.58-4.57% and 1.85-4.98%, and not obviously changed after 150 runs. These results demonstrate that COFs-modified OT-CEC explore a promising method to separate chiral compounds.