Chiral Transfer and Evolution in Cysteine Induced Cobalt Superstructures.

Chiral organic additives have unveiled the extraordinary capacity to form chiral inorganic superstructures, however, complex hierarchical structures have hindered the understanding of chiral transfer and growth mechanisms. This study introduces a simple hydrothermal synthesis method for constructing chiral cobalt superstructures with cysteine, demonstrating specific recognition of chiral molecules and outstanding electrocatalytic activity. The mild preparation conditions allow in situ tracking of chirality evolution in the chiral cobalt superstructure, offering unprecedented insights into the chiral transfer and amplification mechanism. The resulting superstructures exhibit a universal formation process applicable to other metal oxides, extending the understanding of chiral superstructure evolution. This work contributes not only to the fundamental understanding of chirality in self-assembled structures but also provides a versatile method for designing chiral inorganic nanomaterials with remarkable molecular recognition and electrocatalytic capabilities.

Dynamics of Molecular Self-Assembly of Short Peptides at Liquid-Solid Interfaces - Effect of Charged Amino Acid Point Mutations.

Self-organizing solid-binding peptides on atomically flat solid surfaces offer a unique bio/nano hybrid platform, useful for understanding the basic nature of biology/solid coupling and their practical applications. The surface behavior of peptides is determined by their molecular folding, which is influenced by various factors and is challenging to study. Here, the effect of charged amino acids is studied on the self-assembly behavior of a directed evolution selected graphite-binding dodecapeptide on graphite surface. Two mutations, M6 and M8, are designed to introduce negatively and positively charged moieties, respectively, at the anchoring domain of the wild-type (WT) peptide, affecting both binding and assembly. The questions addressed here are whether mutant peptides exhibit molecular crystal formation and demonstrate molecular recognition on the solid surface based on the specific mutations. Frequency-modulated atomic force microscopy is used for observations of the surface processes dynamically in water at molecular resolution over several hours at the ambient. The results indicate that while the mutants display distinct folding and surface behavior, each homogeneously nucleates and forms 2D self-organized patterns, akin to the WT peptide. However, their growth dynamics, domain formation, and crystalline lattice structures differ significantly. The results represent a significant step toward the rational design of bio/solid interfaces, potent facilitators of a variety of future implementations.

Synthesis Strategies, Preparation Methods, and Applications of Chiral Metal-Organic Frameworks.

Chiral Metal-Organic Frameworks (CMOFs) is a kind of material with great application value in recent years. Formed by the coordination of metal ions or metal clusters with organic ligands. It has ordered and adjustable pores, multi-dimensional network structure, large specific surface area and excellent adsorption properties. This material structure combines the properties of metal-organic frameworks (MOFs) with the chiral properties of chiral molecules. It has great advantages in catalysis, adsorption, separation and other fields. Therefore, it has a wide range of applications in chemistry, biology, medicine and materials science. In this paper, various synthesis strategies and preparation methods of chiral metal-organic frameworks are reviewed from different perspectives, and the advantages of each method are analyzed. In addition, the applications of chiral metal-organic framework materials in enantiomer recognition and separation, circular polarization luminescence and asymmetric catalysis are systematically summarized, and the corresponding mechanisms are discussed. Finally, the challenges and prospects of the development of chiral metal-organic frame materials are analyzed in detail.

Chiral Self-Sorting Process With C2 Symmetric Bisimidazoline Ligands.

We have studied the coordination chemistry of chiral imidazoline-based C2-symmetric ligands with zinc (II) and copper (II). Two types of bisimidazoline ligands were studied, one with the free amine (BIM-H) and the other with the amine protected by a toluene sulfonyl group in position 6 (BIM-Ts). The complexes formed were isolated, purified, and characterized, in particular by X-ray diffraction studies and CD in the case of the enantiopure complexes. By playing with the choice of ligand system (enantiopure or racemate), we were able to show that the selective formation of homoleptic and heteroleptic metal complexes can be controlled by means of the chiral molecular instruction of bisimidazoline ligands.

Chiral recognition mechanism studies of Tyr-Arg-Phe-Lys-NH2 tetrapeptide on crown ether-based chiral stationary phase.

Even though chiral recognition for crown-ether CSPs is generally understood, on a molecular level, exact mechanisms for the resolution are still unclear. Furthermore, short peptide analytes often contain multiple amino moieties capable of binding to the crown ether selector. In order to extend the understanding in chiral recognition mechanisms, polar organic mode separation of Tyr-Arg-Phe-Lys-NH2 tetrapeptide llll/dddd enantiomers on S- and R-(3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6 stationary phases was studied with 50-mM perchloric acid in methanol as mobile phase. Deviation from the generally acceptable 1:1 stoichiometry was supported by mass spectroscopy analysis of the formed complexes between tetrapeptide enantiomer and crown ether selectors, which revealed adducts possessing 1:1, 1:2, and 1:3 stoichiometry. Further investigation of complexation induced shifts by NMR indicated on different binding mechanisms between llll/dddd enantiomers of Tyr-Arg-Phe-Lys-NH2 and crown ether selectors. Enantioselective proton shifts were observed in studied tetrapeptide tyrosine and phenylalanine residues exclusively for llll enantiomer upon binding with S-(3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6 selector (and dddd enantiomer with R-(3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6 selector), indicating that these two amino acid residues contribute to chiral recognition. The obtained results were in agreement with the LC data.

Inherently Chiral Oligomers Based on Indole-Benzothiophene Core.

In recent years, transductors of chiral information based on conducting polymers have gained considerable attention. In particular, inherently chiral materials, which allow differentiation between the antipodes of a chiral analyte in terms of energetic variations, are highly desired. In this work, we successfully synthesized a novel inherently chiral oligomer based on an indole-benzothiophene core, namely, 2-([2,2'-bithiophen]-5-yl)-3-(2-([2,2'-bithiophen]-5-yl)benzo[b]thiophen-3-yl)-N-methylindole (BTIndT4). The electrochemical characterization evidences a stabilization of electrogenerated radical cations due to the presence of the indole group, which guides the oligomerization, producing well-ordered polymeric matrices. Furthermore, the in situ electrochemical conductance analysis demonstrates a simultaneous intrachain and interchain transfer of charge carriers. Finally, the highly efficient enantiorecognition capabilities of the antipodes of the oligo-BTIndT4 films toward the enantiomers of tryptophan and 3,4-dihydroxyphenylalanine (DOPA), as model chiral analytes, were demonstrated.

Preparation of amino acid chiral ionic liquid and visual chiral recognition of glutamine and phenylalanine enantiomers.

In this paper, the amino acid chiral ionic liquid (AACIL) was prepared with L-phenylalanine and imidazole. It was characterized by CD, FT-IR, 1H NMR, and 13C NMR spectrum. The chiral recognition sensor was constructed with AACIL and Cu(II), which exhibited different chiral visual responses (solubility or color difference) to the enantiomers of glutamine (Gln) and phenylalanine (Phe). The effects of solvent, pH, time, temperature, metal ions, and other amino acids on visual chiral recognition were optimized. The minimum concentrations of Gln and Phe for visual chiral recognition were 0.20 mg/ml and 0.28 mg/ml, respectively. The mechanism of chiral recognition was investigated by FT-IR, TEM, SEM, TG, XPS, and CD. The location of the host-guest inclusion or molecular placement has been conformationally searched based on Gaussian 09 software.

Update on chiral recognition mechanisms in separation science.

The stereospecific analysis of chiral molecules is an important issue in many scientific fields. In separation sciences, this is achieved via the formation of transient diastereomeric complexes between a chiral selector and the selectand enantiomers driven by molecular interactions including electrostatic, ion-dipole, dipole-dipole, van der Waals or π-π interactions as well as hydrogen or halogen bonds depending on the nature of selector and selectand. Nuclear magnetic resonance spectroscopy and molecular modeling methods are currently the most frequently applied techniques to understand the selector-selectand interactions at a molecular level and to draw conclusions on the chiral separation mechanism. The present short review summarizes some of the recent achievements for the understanding of the chiral recognition of the most important chiral selectors combining separation techniques with molecular modeling and/or spectroscopic techniques dating between 2020 and early 2024. The selectors include polysaccharide derivatives, cyclodextrins, macrocyclic glycopeptides, proteins, donor-acceptor type selectors, ion-exchangers, crown ethers, and molecular micelles. The application of chiral ionic liquids and chiral deep eutectic solvents, as well as further selectors, are also briefly addressed. A compilation of all published literature on chiral selectors has not been attempted.

Investigation on improvement of enantioseparation based on clindamycin phosphate by chiral deep eutectic solvents in capillary electrophoresis.

In this work, the potential synergetic effect between deep eutectic solvents and an antibiotic chiral selector (clindamycin phosphate) for enantioseparation was investigated in capillary electrophoresis. We synthesized a series of deep eutectic solvents with choline chloride as hydrogen bond acceptor and three α-hydroxyl acids (l-lactic acid, l-malic acid, and l-tartaric acid) as hydrogen bond donors. Compared to the single clindamycin phosphate separation system, significantly improved separations of model drugs were observed in several synergetic systems. Compared to deep eutectic solvents with a single hydrogen bond donor, deep eutectic solvents with mixed-type hydrogen bond donors were superior. The influences of several key parameters including the type and proportion of organic modifier, clindamycin phosphate concentrations, deep eutectic solvents concentrations, and buffer pH were investigated in detail. The mechanism of the enhanced separations in deep eutectic solvents systems was investigated by means of electroosmotic flow analysis, nuclear magnetic resonance analysis, and molecular modeling. It was the first time that the synergetic systems between deep eutectic solvents and antibiotic chiral selector were established in capillary electrophoresis, and these deep eutectic solvents were demonstrated to have a good synergetic effect with clindamycin phosphate for enantioseparation.

Covalent organic frameworks and related innovative materials in chiral separation and recognition.

Chiral recognition and enantioseparation are of paramount importance in various fields, including pharmaceuticals, agrochemicals, and material science. Covalent organic frameworks (COFs) have emerged as promising materials for chiral separation due to their unique structural features and tunable properties. This review provided a comprehensive overview of recent progress in the application of COFs and related innovative materials for chiral separation and recognition. Various strategies were analyzed for the design and synthesis of chiral COFs, including the incorporation of chiral building blocks, post-synthetic modification, and the integration of chiral selectors. The applications of chiral COFs in chromatographic techniques, membrane separations, and other emerging methods were critically evaluated with the emphasis on their advantages and limitations. Additionally, the review summarized the potential of combining COFs with other nanomaterials, such as metal-organic frameworks (MOFs) and nanoparticles, to enhance chiral recognition and separation performance. The fundamental principles and mechanisms of chiral recognition were discussed, highlighting the role of chiral selectors and their interactions with enantiomers. Finally, current challenges and future perspectives in this field were discussed, providing insights into the development of more efficient and versatile chiral separation systems based on COFs and related materials.

Enantiomeric separation of flavanone on Chiralpak® IA column and determination of the chiral mechanism.

Thirteen flavanone racemates were successfully separated using a Chiralpak® IA column and isopropanol-hexane (50:50, v/v). The mobile phase flow rate and detection wavelength were 0.5 mL/min and 254 nm. The retention times values ranged from 5.50 and 56.45 min. The values of the retention, separation, and resolution factors ranged from 0.63 to 21.67, 1.12 to 2.45, and 0.13 to 11.94. The docking binding energies ranged from -6.2 to -8.2 kcal/mol, showing enthalpy-determined host-guest complex formation. The molecular docking results and the experimental data were agreed well. The results showed that S-enantiomers had stronger bindings with chiral selectors compared to R-enantiomers. Consequently, the R-enantiomers eluted first followed by S-enantiomers. The reported method is highly useful to determine the enantiomeric composition of the reported flavanone in any sample.

Fabrication strategies for chiral self-assembly surface.

Chiral self-assembly is the spontaneous organization of individual building blocks from chiral (bio)molecules to macroscopic objects into ordered superstructures. Chiral self-assembly is ubiquitous in nature, such as DNA and proteins, which formed the foundation of biological structures. In addition to chiral (bio) molecules, chiral ordered superstructures constructed by self-assembly have also attracted much attention. Chiral self-assembly usually refers to the process of forming chiral aggregates in an ordered arrangement under various non-covalent bonding such as H-bond, π-π interactions, van der Waals forces (dipole-dipole, electrostatic effects, etc.), and hydrophobic interactions. Chiral assembly involves the spontaneous process, which followed the minimum energy rule. It is essentially an intermolecular interaction force. Self-assembled chiral materials based on chiral recognition in electrochemistry, chiral catalysis, optical sensing, chiral separation, etc. have a broad application potential with the research development of chiral materials in recent years.

Amino-Ligand-Coordinated Dicopper Active Sites Enable Catechol Oxidase-Like Activity for Chiral Recognition and Catalysis.

Developing highly active and selective advanced nanozymes for enzyme-mimicking catalysis remains a long-standing challenge for basic research and practical applications. Herein, we grafted a chiral histidine- (His-) coordinated copper core onto Zr-based metal-organic framework (MOF) basic backbones to structurally mirror the bimetal active site of natural catechol oxidase. Such a biomimetic fabricated process affords MOF-His-Cu with catechol oxidase-like activity, which can catalyze dehydrogenation and oxidation of o-diphenols and then transfer electrons to O2 to generate H2O2 by the cyclic conversion of Cu(II) and Cu(I). Specifically, the elaborate incorporation of chiral His arms results in higher catalytic selectivity over the chiral catechol substrates than natural enzyme. Density functional theory calculations reveal that the binding energy and potential steric effect in active site-substrate interactions account for the high stereoselectivity. This work demonstrates efficient and selective enzyme-mimicking catalytic processes and deepens the understanding of the catalytic mechanism of nanozymes.

Dynamic Stereochemistry of M8 Pd6 Supramolecular Cages Based on Metal-Center Lability for Differential Chiral Induction, Resolution, and Recognition.

A series of isostructural supramolecular cages with a rhombic dodecahedron shape have been assembled with distinct metal-coordination lability (M8 Pd6 -MOC-16, M=Ru2+ , Fe2+ , Ni2+ , Zn2+ ). The chirality transfer between metal centers generally imposes homochirality on individual cages to enable solvent-dependent spontaneous resolution of Δ8 /Λ8 -M8 Pd6 enantiomers; however, their distinguishable stereochemical dynamics manifests differential chiral phenomena governed by the cage stability following the order Ru8 Pd6 >Ni8 Pd6 >Fe8 Pd6 >Zn8 Pd6 . The highly labile Zn centers endow the Zn8 Pd6 cage with conformational flexibility and deformation, enabling intrigue chiral-Δ8 /Λ8 -Zn8 Pd6 to meso-Δ4 Λ4 -Zn8 Pd6 transition induced by anions. The cage stabilization effect differs from inert Ru2+ , metastable Fe2+ /Ni2+ , and labile Zn2+ , resulting in different chiral-guest induction. Strikingly, solvent-mediated host-guest interactions have been revealed for Δ8 /Λ8 -(Ru/Ni/Fe)8 Pd6 cages to discriminate the chiral recognition of the guests with opposite chirality. These results demonstrate a versatile procedure to control the stereochemistry of metal-organic cages based on the dynamic metal centers, thus providing guidance to maneuver cage chirality at a supramolecular level by virtue of the solvent, anion, and guest to benefit practical applications.

Homochiral Zeolitic Imidazolate Framework with Defined Chiral Microenvironment for Electrochemical Enantioselective Recognition.

The recognition and separation of chiral molecules with similar structure are of great industrial and biological importance. Development of highly efficient chiral recognition systems is crucial for the precise application of these chiral molecules. Herein, a homochiral zeolitic imidazolate frameworks (c-ZIF) functionalized nanochannel device that exhibits an ideal platform for electrochemical enantioselective recognition is reported. Its distinct chiral binding cavity enables more sensitive discrimination of tryptophan (Trp) enantiomer pairs than other smaller chiral amino acids owing to its size matching to the target molecule. It is found that introducing neighboring aldehyde groups into the chiral cavity will result in an inferior chiral Trp recognition due to the decreased adsorption-energy difference of D- and L-Trp on the chiral sites. This study may provide an alternative strategy for designing efficient chiral recognition devices by utilizing the homochiral reticular materials and tailoring their chiral environments.

From Short- to Long-Range Chiral Recognition on Surfaces: Chiral Assembly and Synthesis.

Research on chiral behaviors of small organic molecules at solid surfaces, including chiral assembly and synthesis, can not only help unravel the origin of the chiral phenomenon in biological/chemical systems but also provide promising strategies to build up unprecedented chiral surfaces or nanoarchitectures with advanced applications in novel nanomaterials/nanodevices. Understanding how molecular chirality is recognized is considered to be a mandatory basis for such studies. In this review, a series of recent studies in chiral assembly and synthesis at well-defined metal surfaces under ultra-high vacuum conditions are outlined. More importantly, the intrinsic mechanisms of chiral recognition are highlighted, including short/long-range chiral recognition in chiral assembly and two main strategies to steer the reaction pathways and modulate selective synthesis of specific chiral products on surfaces.

Molecular Assembly and Gelating Behavior of (l)-Alanine Derivatives.

In this study, a low-molecular-weight organogelator derived from (l)-amino acids was designed and synthesized. Gelation assays using (l)-amino acid derivatives were performed to confirm the gelation ability, which was found to be high in several compounds. The (l)-alanine derivatives were determined to be excellent gelators, forming good gels even when smaller amounts were added. These results led to a library of amino acid-derived organogelators. In addition, the thermal properties of the (l)-alanine derivatives with high gelation performance were measured. Differential scanning calorimetry measurements revealed that the thermal stability of the gels could be controlled by changing the gelator concentration. The surface states of the obtained gels were observed by field-emission scanning electron microscopy and atomic force microscopy measurements, which confirmed the structure of the self-molecular aggregates. Self-molecular aggregates were observed to be helical or sheet-like, and the gels were constructed by forming aggregates by self-molecular recognition.

Enantioselective Fluorescence Recognition of Free α-Amino Acids by Ion-Type Ammonium Salt-Based Sensors.

Optically pure amino acids have extensive applications in pharmaceuticals, pesticides, food, materials, and other fields. Enantiomers recognition of chiral amino acids using optical methods with synthetic chiral sensors has attracted extensive attention. Most reported sensors typically identify guests by covalent or hydrogen bonding or hydrophobic interaction with amino acids and their derivatives. In this paper, a series of ion-type quaternary ammonium salt-based enantioselective fluorescent sensors were synthesized for chiral recognition of free α-amino acids via electrostatic interaction. The fluorescence intensity ratios ID/IL (ID, IL, fluorescence intensity of sensor when treated with D- or L-amino acid) were up to 2.1 and enantioselective fluorescence enhancement ratios ef (ef=[IL-I0]/[ID-I0] or [ID-I0]/[IL-I0]. (I0, fluorescence intensity of the sensor)) were up to 5.0. Among them, sensor 3 showed best enantioselective recognition performance toward tryptophan (Trp), and L-Trp significantly quenched the fluorescence of sensor 3, but D-Trp greatly enhanced the fluorescence of sensor 3, its ID/IL was 2.11 and ef was 1.8. The mechanistic investigation by NMR spectrum revealed that a tight three-point interaction, including electrostatic interaction, hydrogen bond, and π-π stacking, between sensor 3 and D-Trp was formed.

Chiral recognition of tryptophan enantiomer based on the electrode modified by polyaniline adsorption bovine serum albumin complex.

A chiral sensing platform was constructed via adsorptive functionalization of ammonium persulfate doped polyaniline (APS-DPANI) with bovine serum albumin (BSA). The novelty of this work is the construction of such chiral interface with adsorption principle. The material has been characterized by scanning electron microscope, Fourier transform infrared and X-ray photoelectron spectroscopy, and thermogravimetric and water contact angle analyses. It displayed considerable stability in multi-run cyclic voltammetric scanning. Moreover, the superior conductivity of APS-DPANI and the decent binding ability of BSA endowed this sensing platform with an excellent recognition effect for tryptophan (Trp) enantiomers in the differential pulse voltammetry (DPV) test. The recognition was highly reproducible, and the detection limits for L- and D-isomer were 0.071 and 0.0478 mM, respectively.

Wireless electromechanical enantio-responsive valves.

Microfluidic valves based on chemically responsive materials have gained considerable attention in recent years. Herein, a wireless enantio-responsive valve triggered by bipolar electrochemistry combined with chiral recognition is reported. A conducting polymer actuator functionalized with the enantiomers of an inherently chiral oligomer was used as bipolar valve to cover a tube loaded with a dye and immersed in a solution containing chiral analytes. When an electric field is applied, the designed actuator shows a reversible cantilever-type deflection, allowing the release of the dye from the reservoir. The tube can be opened and closed by simply switching the polarity of the system. Qualitative results show the successful release of the colorant, driven by chirality and redox reactions occurring at the bipolar valve. The device works well even in the presence of chemically different chiral analytes in the same solution. These systems open up new possibilities in the field of microfluidics, including also controlled drug delivery applications.