Chiral Recognition Properties Research Articles

Recent progress for chiral stationary phases based on chiral porous materials in high-performance liquid chromatography and gas chromatography separation.

Chirality is a fundamental property of nature. Separation and analysis of racemates are of great importance in the fields of medicine and the production of chiral biopharmaceutical intermediates. Chiral chromatography has the characteristics of a wide separation range, fast separation speed, and high efficiency. The development and preparation of novel chiral stationary phases with good chiral recognition and separation capacity is the core and key of chiral chromatographic separation and analysis. In this work, the representative research progress of novel chiral porous crystal materials including chiral covalent organic frameworks, chiral porous organic cages, chiral metal-organic frameworks, and chiral metal-organic cages used as chiral stationary phases of capillary gas chromatography and high-performance liquid chromatography over the last 4 years is reviewed in detail. The chiral recognition and separation properties of the representative studies in this review are also introduced and discussed.

Dissecting the chiral recognition of TLR4/MD2 with Neoseptin-3 enantiomers by molecular dynamics simulations.

Toll-like receptor 4 (TLR4) is a pivotal innate immune recognition receptor that regulates intricate signaling pathways within the immune system. Neoseptin-3 (Neo-3), a recently identified small-molecule agonist for mouse TLR4/MD2, exhibits chiral recognition properties. Specifically, the L-enantiomer of Neo-3 (L-Neo-3) effectively activates the TLR4 signaling pathway, while D-Neo-3 fails to induce TLR4 activation. However, the underlying mechanism by which TLR4 enantioselectively recognizes Neo-3 enantiomers remains poorly understood. In this study, in silico simulations were performed to investigate the mechanism of chiral recognition of Neo-3 enantiomers by TLR4/MD2. Two L-Neo-3 molecules stably resided within the cavity of MD2 as a dimer, and the L-Neo-3 binding stabilized the (TLR4/MD2)2 dimerization state. However, the strong electrostatic repulsion between the hydrogen atoms on the chiral carbon of D-Neo-3 molecules caused the relative positions of two D-Neo-3 molecules to continuously shift during the simulation process, thus preventing the formation of D-Neo-3 dimer as well as their stable interactions with the surrounding residues in (TLR4/MD2)2. Considering that L-Neo-3 could not sustain a stable dimeric state in the bulk aqueous environment, it is unlikely that L-Neo-3 entered the cavity of MD2 as a dimeric unit. Umbrella sampling simulations revealed that the second L-Neo-3 molecule entering the cavity of MD2 exhibited a lower binding energy (-25.75 kcal mol-1) than that of the first L-Neo-3 molecule (-14.31 kcal mol-1). These results imply that two L-Neo-3 molecules enter the cavity of MD2 sequentially, with the binding of the first L-Neo-3 molecule facilitating the entry of the second one. This study dissects the binding process of Neo-3 enantiomers, offering a comprehensive understanding of the atomic-level mechanism underlying TLR4's chiral recognition of Neo-3 molecules.

Novel supramolecular deep eutectic solvent (SUPRADES) as a sole chiral selector in capillary electrophoresis for the enantiomeric separation of fluorine-substituted amphetamine analogs

In this study, a novel supramolecular deep eutectic solvent consisting of sulfated-β-CD and citric acid (S-β-CD-CA) is reported for the first time. This innovative system was evaluated as a sole chiral selector in capillary electrophoresis for the enantioseparation of six fluorine-substituted amphetamine analogs, yielding remarkable outcomes. Baseline separations of all amphetamine analogs under study were achieved in less than 21.00 min using the S-β-CD-CA as the chiral selector. It was observed that the addition of 0.050 % v/v S-β-CD-CA into the background electrolyte resulted in the baseline separation of five out of the six fluorine-substituted amphetamine analogs, while in the case of the para-substituted amphetamine analog, 4-fluoramphetamine (4-FA), a higher percentage (0.15 % v/v) was required to achieve baseline enantioseparation. These findings emphasized the potential of this new supramolecular system in providing a class of solvents with promising chiral recognition properties.

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.

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).

Impact of cyclofructan derivatives as efficient chiral selector in chiral analysis: An overview.

The development of chiral selectors for the separation and analysis of chiral molecules has been an evolving process happening over three decades, since the introduction of the first chiral stationary phase (CSP) in 1938. The main impetus for designing new chiral selectors is to get to most promising one which has a broad chiral recognition property, separation capability for a wide range of chiral analytes, and the cost-effective CSP, which is also a major concern. Today, we have more than 100 commercially available CSPs, and these are prepared by coating or immobilizing the classical chiral selectors on to the chromatographic support, normally, silica gel. The purpose of this review is to look at progress and the impact of cyclofructan derivatives, a novel chiral selector introduced recently, for performing chiral analysis.

Deep-red circularly polarised luminescent C70 derivatives

Optically active fullerenes, including C60 and C70 derivatives carrying organic substituents, are used in a range of applications because of their unique spectroscopic, catalytic, and chiral recognition properties. However, their inherent photoexcited chirality is yet to be elucidated because of their very poor fluorescence quantum yield (Φf). We synthesised a new chiral C70 derivative, X70A, with 20% yield, by reacting bis-borylated xanthene with C70 in a one-step double addition reaction, followed by a successful optical resolution. The isolation of two separate X70A enantiomers was confirmed by mirror-image circular dichroism spectroscopy in the range of 300–750 nm. In toluene, the enantiomeric pair of X70A clearly revealed mirror-image circularly polarised luminescence (CPL) spectra with a high |glum| value of 7.0 × 10−3 at 690 nm. The first fullerene-based deep-red CPL of X70A should provide a new guideline for the design of chiral nanocarbon materials.

Neutral Chiral Tetrakis-Iodo-Triazole Halogen-Bond Donor for Chiral Recognition and Enantioselective Catalysis.

Halogen bonding represents a powerful tool in the field of noncovalent interactions. However, applications in enantioselective recognition and catalysis remain almost nonexistent, due in part to the distinct features of halogen bonds, including long covalent and noncovalent bond distances and high directionality. Herein, this work presents a novel chiral tetrakis‐iodo‐triazole structure as a neutral halogen bond donor for both chiral anion‐recognition and enantioinduction in ion‐pair organocatalysis. NMR‐titration studies revealed significant differences in anion affinity between the halogen bonding receptor and its hydrogen bonding parent. Selective recognition of chiral dicarboxylates and asymmetric induction in a benchmark organocatalytic reaction were demonstrated using the halogen bond donor. Inversions in the absolute sense of chiral recognition, enantioselectivity, and chiroptical properties relative to the related hydrogen donor were observed. Computational modeling suggested that these effects were the result of distinct anion‐binding modes for the halogen‐ versus hydrogen‐bond donors.

Cellulose phenylcarbamate-derived hybrid bead-type chiral packing materials for efficient chiral recognition

For efficient enantioseparation, eight organic–inorganic hybrid bead-type chiral packing materials (CPMs) with high organic contents have been developed using cellulose tris(3,5-dimethylphenylcarbamates) bearing small amount of 3-(triethoxysilyl)propyl groups and tetraethyl orthosilicate by a modified Stober process in the basic condition. The hybrid beads could be formed more efficiently in the basic condition without using surfactant, compared to that in the acidic condition. The preparation conditions, including type and amount of catalyst, amount of water and temperature, significantly affected the organic contents and morphology of the hybrid CPMs. Their chiral recognition properties were then examined by high-performance liquid chromatography. The obtained hybrid CPMs with a higher organic content possessed better enantioseparation ability than the traditional CPMs and previous analogue derived from the acidic condition. For some racemates, the hybrid CPMs exhibited even higher enantioselectivity than the commercial Chiralpak IB N, which is one of the most powerful immobilized-type CPMs and prepared from cellulose tris(3,5-dimethylphenylcarbamate).

Regioselectively substituted cellulose mixed esters synthesized by two-steps route to understand chiral recognition mechanism and fabricate high-performance chiral stationary phases

It is challenging to design and fabricate new and high-performance cellulose-based chiral stationary phases (CSPs), due to the indistinct chiral recognition mechanism and the inherent difficulty to control the structure of cellulose derivatives. Herein, taking advantage of the high regioselective benzoylation of cellulose in 1-allyl-3-methylimidazolium chloride, a series of regioselectively substituted cellulose mixed esters, cellulose 6-benzoate-2,3-phenylcarbamate, are directly obtained by a facile two-steps route without protecting and deprotecting process. The resultant cellulose mixed esters exhibit high chiral recognition capability. In particular, when the benzoate group has an electron-donating substituent on phenyl ring, such as 4-tert-butyl group, the corresponding regioselectively substituted cellulose mixed esters have much better enantioseparation capability than cellulose tri(3,5-dimethylphenylcarbamate), which is commercially available as Chiralcel OD column, one of the most powerful CSPs. More importantly, via adjusting the chemical structure of cellulose derivatives and adding a post-treatment process to optimize their chiral recognition properties, the chiral recognition mechanism is clearly revealed. The synergy of the hydrophobic helical conformation, weak hydrogen-bond donating ability and appropriate distribution of substituents of cellulose derivatives is essential to fabricate high-performance CSPs.

Multiple Hydrogen Bonds Promoted ESIPT and AIE‐active Chiral Salicylaldehyde Hydrazide

AbstractThe simpler, the better! A series of simple and highly fluorescent salicylaldehyde hydrazide molecules (41 samples) have been designed and prepared. Even though these soft materials contain a very small π‐conjugated system, they can go through multiple intramolecular and intermolecular hydrogen bonds promoted excited‐state intramolecular proton‐transfer (ESIPT) to display strong blue, green, yellow, and orange aggregation‐induced emission (AIE) with large Stokes shifts (up to 184 nm) and high fluorescence quantum yields (Ф up to 0.20). Unusual mechanochromic fluorescence enhancements are also found in some solid samples. Through coordination, hydrogen and halogen bonds, these flexible molecules can be used as Mg2+ (Ф up to 0.46) probes, universal anion (Ф up to 0.14) and unprotected amino acids (Ф up to 0.16) probes, and chiral diamine (enantiomeric selectivity and Ф up to 0.36 and 0.062, respectively) receptors. Combining their advantages of AIE and biocompatibility, these low cytotoxic dyes have potential application in living cell imaging. Furthermore, the effects of different functional groups on the molecule arrangement, ESIPT, AIE, probe, and chiral recognition properties are also examined, which provide a simple and bright paradigm for the design of multiple‐stimuli‐responsive smart materials.

Effect of different immobilization strategies on chiral recognition properties of Cinchona-based anion exchangers.

In the enantiomeric separation of highly polar compounds, a traditionally challenging task for high-performance liquid chromatography, ion-exchange chiral stationary phases have found the main field of application. In this contribution, we present a series of novel anion-exchange-type chiral stationary phases for enantiomer separation of protected amino phosphonates and N-protected amino acids. Two of the prepared selectors possessed a double and triple bond within a single molecule. Thus, they were immobilized onto silica support employing either a thiol-ene (radical) or an azide-yne (copper(I)-catalyzed) click reaction. We evaluated the selectivity and the effect of immobilization proceeding either by the double bond of the Cinchona alkaloid or a triple bond of the carbamoyl moiety on the chromatographic performance of the chiral stationary phases using analytes with protecting groups of different size, flexibility, and π-acidity. The previously observed preference toward protecting groups possessing π-acidic units, which is a typical feature of Cinchona-based chiral stationary phases, was preserved. In addition, increasing the bulkiness of the selectors' carbamoyl units leads to significantly reduced retention times, while very high selectivity toward the tested analytes is retained.

Differentiation of enantiomeric anions by NMR spectroscopy with chiral bisurea receptors.

Chiral anionic species are ubiquitous and play important roles in biological systems. Despite the recent advancements in synthetic anion receptors bearing urea functionalities, urea-based chiral solvating agents (CSAs) that can separate the NMR signals of racemic anions remain limited. Herein, three dibenzofuran-based C2-symmetric chiral bisureas were synthesized from the reaction of (R,R)-4,6-bis(1-aminopropyl)dibenzo[b,d]furan with phenyl isocyanate, phenyl thioisocyanate, or tosyl isocyanate. The chiral anion recognition properties of these bisureas were examined by 1H NMR spectroscopy using dl-tetrabutylammonium mandelate (TBAM) as a model substrate. A clear baseline separation of the enantiomeric signals of the benzylic proton of TBAM was achieved upon mixing with 0.5 equivalents of bis(phenylurea). In contrast to previous urea-based chiral anion receptors that differentiate the enantiomers of chiral anions by forming 1 : 1 host-guest complexes, a high chiral recognition ability of chiral bis(phenylurea) was achieved owing to the generation of an equilibrium between free guests, 1 : 1 host-guest complexes, and 1 : 2 host-guest complexes. Chiral bis(phenylurea) was also successfully employed in the separation of the enantiomeric 1H NMR signals of various racemic anions.

Surface molecularly imprinted material for enantiomeric resolution of ibuprofen: Preparation and study on chiral recognition and resolution property

ABSTRACTThe surface imprinted material of one single enantiomer of ibuprofen ((S)-ibuprofen) was successfully prepared using the novel surface imprinting technique of synchronously graft/crosslinking-polymerizing and molecule imprinting. Micron sized-silica gel particles as matrix were first modified with coupling agent γ-mercaptopropyl trimethoxysilane (MPMS), obtaining MPMS-SiO2 particles, on which there was a great deal of mercapto group (‒SH). Hydroxyethyl methylacrylate (HEMA) as functional monomer and dibenzoyl peroxide (BPO) as initiator were dissolved into 1,2-dichloroethane (DCE), and HEMA molecules are automatically combined around (S)-ibuprofen molecules by right of multi-site hydrogen bonds, forming self-assembly complex. A surface-initiating system of ‒SH/BPO was constituted at the interface between MPMS-SiO2 particles and DCE solution. The free radicals produced on the surfaces of MPMS-SiO2 particles initiated HEMA molecules around (S)-ibuprofen molecules to produce graft/crosslinking-polymerization on the surfaces of SiO2 particles. At the same time, (S)-ibuprofen molecules were wrapped in the cross-linked networks so that the surface-imprinting of (S)-ibuprofen was realized, forming (S)-ibuprofen surface imprinted particles ((S)-IBU-SIP). The enantiomeric recognition and resolution properties of (S)-IBU-SIP particles were investigated. The experimental results show that (S)-IBU-SIP particles have excellent enantiodiscrimination property and enantioseparation ability. The binding capacity of (S)-IBU-SIP particles for (S)-ibuprofen gets up to 177 mg g−1, and the selectivity coefficient of (S)-IBU-SIP particles for (S)-ibuprofen relative to (R)-ibuprofen is 5.03. For the ibuprofen raceme solution, after solid phase extraction by (S)-IBU-SIP particles, the optical purity (ee value of (S)-ibuprofen) of the eluent reaches up to 82.5%, whereas the optical purity (ee value of (R)-ibuprofen) of the supernatant is 44.2%.

Fluorescent Phthalocyanine Assembly Distinguishes Chiral Isomers of Different Types of Amino Acids and Sugars.

The functions of some natural supramolecular architectures, such as ribosomes, are dependent on the recognition of different types of chiral biomolecules. However, the recognition of different types of chiral molecules (multiobject chiral recognition), such as amino acids and sugars, by independent and identically artificial supramolecular assembly, was rarely achieved. In this article, simple amphiphilic achiral phthalocyanine was found to form supramolecular chiral assemblies with charged water-soluble polymers upon host-guest interactions at the air/water interface. Among these systems, one identical phthalocyanine/poly(l-lysine) assembly not only can distinguish enantiomers of different amino acids but also can recognize several epimers of monose. The chiral recognitions were achieved by comparing either the steady-state fluorescence intensity or fluorescence quenching rate of phthalocyanine/poly(l-lysine) assemblies, before and after interaction with different small chiral molecules. It was demonstrated that the interactions between poly(l-lysine) and different small chiral molecules could change the aggregation of phthalocyanines. And the sensitivity of fluorescence and the excellent multiobject chiral recognition properties of the phthalocyanine/poly(l-lysine) assembly are dependent on the subtle molecular packing mode and the cooperation of different noncovalent interactions.

Preparation of molecule imprinted membrane of single enantiomer of amino acid with an innovative strategy and study on its chiral recognition and resolution properties

BACKGROUND The molecularly imprinted membranes (MIMs) with high performance are very valuable in the chiral resolution of amino acids. In this work, a novel molecularly imprinted membrane of the single enantiomer of amino acids with graft type was successfully prepared by using an advanced surface imprinting technique. The aminated microfiltration membrane of polysulfone, AMPSF membrane, was first prepared, and so a surface-initiating system of -NH2/S2O8- was constituted. In an aqueous solution, the molecules of dimethylaminoethyl methacrylate (DMAEMA) as functional monomer were combined around L-glutamic acid (L-Glu) as template by right of electrostatic interaction and hydrogen bonding. By initiating of -NH2/S2O8- system, DMAEMA and the crosslinking agent N,N′-Methylenebisacrylamide (MBA) produced graft/crosslinking-polymerization on the surface of AMPSF membrane, whereas L-Glue molecules were wrapped within the grafted polymer layer, obtaining grafting type L-Glue molecule-imprinted membrane (L-Glue-MIM). The ability of the chiral recognition and enantioseparation of L-Glue-MIM were examined. RESULTS The experimental results show that L-Glue-MIM possesses specific chiral recognition selectivity and excellent enantioseparation ability. In the permeation experiment, L-Glue molecules in the raceme solution can smoothly pass across L-Glue-MIM, but D-Glue molecules are seriously blocked. As a result, Glu raceme solution is well separated, and the optical purity (ee value) of the penetrating fluid gets up to 82%. CONCLUSION Grafting type molecularly imprinted membrane of the single enantiomer of amino acids was successfully prepared by using an innovative and advanced surface-imprinting technique of “synchronously graft-polymerizing and molecule imprinting”, and it supplies a newer route to prepare imprinted membrane with high performance for chiral resolution of amino acids and other chiral drugs. Such study is reported for the first time. © 2017 Society of Chemical Industry

Recent progress of chiral stationary phases for separation of enantiomers in gas chromatography.

Chromatography techniques based on chiral stationary phases are widely used for the separation of enantiomers. In particular, gas chromatography has developed rapidly in recent years due to its merits such as fast analysis speed, lower consumption of stationary phases and analytes, higher column efficiency, making it a better choice for chiral separation in diverse industries. This article summarizes recent progress of novel chiral stationary phases based on cyclofructan derivatives and chiral porous materials including chiral metal-organic frameworks, chiral porous organic frameworks, chiral inorganic mesoporous materials, and chiral porous organic cages in gas chromatography, covering original research papers published since 2010. The chiral recognition properties and mechanisms of separation toward enantiomers are also introduced.

Exploring the Chiral Recognition of Carboxylates by C2-Symmetric Receptors Bearing Glucosamine Pendant Arms.

Two urea-based receptors containing a glucosamine derivative were synthesized and investigated in terms of their ability to recognize chiral and achiral anions. Both receptors demonstrated a high affinity toward carboxylates in very competitive DMSO/water mixtures. The chiral recognition properties of these compounds were studied using structurally differentiated guests derived from mandelic acid and α-amino acids. We found that receptor 1 exhibits significantly higher enantioselectivities than compound 2 for all anions investigated, with a KS/KR ratio of up to 2. This low enantiodiscrimination in the case of receptor 2 is attributed to a lack of interactions between its sugar moieties and the side chain of chiral anions, due to their inadequate spatial arrangement.

Preparation of surface imprinted material of single enantiomer of mandelic acid with a new surface imprinting technique and study on its chiral recognition and resolution properties

A surface imprinted material of the single enantiomer of mandelic acid with high performance was successfully prepared with a new surface imprinting technique of synchronously graft-polymerizing and molecule imprinting, and its enantiomeric recognition and resolution properties were investigated. Micro-sized silica gel particles were first modified with coupling agent γ-mercaptopropyl trimethoxysilane (MPMS), obtaining the modified particles MPMS-SiO2 on which mercapto groups were introduced. A surface initiating system of −SH/BPO was constituted with the mercapto group (-SH) on MPMS-SiO2 particles and dibenzoyl peroxide (BPO) in N,N-dimethyl formamide (DMF) solution. In DMF solution, (R)-mandelic acid molecule was used as the template and the functional monomer hydroxyethyl methylacrylate (HEMA) were combined together by right of multi-site hydrogen bonds. The free radicals produced on MPMS-SiO2 particles initiate HEMA molecules around (R)-mandelic acid molecules and the crosslinking agent N,N'-Methylenebisacrylamide (MBA) to produce graft/crosslinking-polymerization. At the same time, the template (R)-mandelic acid molecules were enveloped within the thin grafted polymer layer on the surfaces of SiO2 particles, obtaining (R)-mandelic acid surface imprinted material MIP-PHEMA/SiO2. The experimental results show that MIP-PHEMA/SiO2 particles have excellent enantiomeric recognition and resolution ability. The binding capacity of MIP-PHEMA/SiO2 particles for (R)-mandelic acid reaches up to 278mg/g. As the resolution experiment of a racemic mixture was carried out with MIP-PHEMA/SiO2 particles as solid adsorbent, relative another enantiomer, (S)-mandelic acid, the selectivity coefficient of the imprinted particles for (R)-mandelic acid is 5.02. As a consequence, the two enantiomers were well separated, and the optical purities (ee values) of the supernatant and eluant get up to 44% (corresponding to (S)-mandelic acid excess) and 85% (corresponding to (R)-mandelic acid excess), respectively.

Progress in Chiral Recognition Based on Molecular Tweezers

Chirality is a common phenomenon in nature. Designing and synthesis of novel artificial receptors to emulate the molecular recognition in living organism has become the challenging forefront of bioorganic chemistry and supramolecular chemistry. Molecular tweezers have been widely applied in chiral molecular recognition and detection for its easily modified structure. Molecular tweezers and their chiral recognition properties detected by spectrometry, chromatography and mass spec- trometry are summarized in this paper, and the prospect of molecular tweezers is also discussed. Keywords molecular tweezers artificial receptor; chiral recognition; spectrometry; chromatography; mass spectrometry