Cyclodextrin Porous Liquid Materials for Efficient Chiral Recognition and Separation of Nucleosides

Stereoselective transport of 2-aryl propionic acid enantiomers in porous media subjected to chiral organic acids

The study examined the transport behavior of the 2-aryl propionic acid (2-APA) chiral pharmaceutical enantiomers by means of a laboratory-scale saturated quartz sand column experiment. Four typical of 2-APA and their enantiomers were selected for the study under different types of chiral organic acids (COAs)-mediated effects. Differences in the transport of the 2-APA enantiomeric pairs have been identified in response to various pH, types of COAs, and enantiomeric structures of COAs. Redundancy analysis identified the factors responsible for the largest differences in transport of 2-APA enantiomeric pairs, while spectroscopic characterization and density function theory (DFT) studies elucidated the underlying mechanisms contributing to the differences in transport of enantiomeric pairs. Obvious correlations among homochirality or heterochirality between COAs and 2-APA enantiomeric pairs were observed for changes in the mobility of 2-APA. The results indicate widespread COAs significantly affect the transport behavior of chiral man-made chemicals, suggesting more attention is needed to fill the gap in the perception of the transport behavior of chiral compounds.

Incorporating Photoresponses into Porous Liquids

Porous liquids combine the properties of a porous solid with those of a liquid, creating a porous flowable media. Since their discovery, these materials have gathered widespread interest within the scientific community, with substantial numbers of new systems being discovered, often with a focus on increasing the pore volume and gas capacity. Which begs the question, what does the future hold for porous liquids? Recently, the first examples of photoresponsive porous liquids have emerged, allowing changes in porosity to be observed under UV irradiation. Here, we expand on our previous report of photoresponsive porous liquids and explore the conceptualisation of responsive porous liquids and how these materials could be developed with the ability to respond to light, thereby offering a potential mechanism of controllable uptake and release in these systems. This concept article summarises different approaches that could be used to incorporate a photoresponse in a porous liquid before discussing recently reported systems, alongside important factors to consider in their design. Finally, by taking inspiration from the methods used to translate porous solids into the liquid state, combined with the field of photoresponsive materials, we discuss potential strategies that could be employed to realise further examples of photoresponsive porous liquids.

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

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

Recognition of chiral propranolol by fluorescent aptamerlight switch based on GO

In this work, specific aptamers with affinity for S-propranolol were screened by SELEX technology based on the graphene oxide (GO) adsorption platform, and a GO-FAM labeled aptamer-propranolol fluorescent optical switch system was constructed for the recognition of chiral propranolol. It was found that the fluorescence quenching of FAM labeled aptamer could be caused by the adsorption of GO. However, when S-propranolol was introduced, S-propranolol could pull out the aptamer adsorbed by GO, and the fluorescence of the system could be restored. But, R-propranolol could not be realized. Therefore, a simple and sensitive fluorescent optical switch system was established to identify chiral propranolol and perform highly sensitive detection of S-propranolol.

Determination of trace fluoroquinolones in honey and milk based on cyclodextrin modified magnetic metal-organic frameworks solid phase extraction coupled with ultra-high performance liquid chromatography

Long-term intake of animal-derived foods with excessive fluoroquinolones (FQs) will cause damage to human health, so it is critical to establish a feasible approach for sensitive and rapid monitoring of FQs residues. In this study, a new cyclodextrin modified magnetic metal-organic frameworks (Fe3O4@UiO-66-CD) was successfully synthesized by amidation reaction and applied to magnetic solid phase extraction (MSPE) for FQs analysis. The adsorption behavior of Fe3O4@UiO-66-CD was consistent with the pseudo-second-order kinetics and Freundlich isothermal adsorption model, which indicated that the designed material had various interactions on FQs, such as host-guest interaction and π-π interaction. The parameters of MSPE were optimized and the determination method of norfloxacin, enrofloxacin, lomefloxacin and gatifloxacin was established by using MSPE combined with ultra-high performance liquid chromatography (UHPLC) and fluorescence detector (FLD). The method validation results displayed that the detection limits were 0.02-0.09 ng/mL, and the RSDs of intra-day and inter-day precision were less than 4.1 and 6.4 %, respectively. In the target FQs analysis of real honey and milk samples, the recoveries at different fortified concentrations were in the ranges of 88.4 % to 108.6 % with RSD ≤ 5.7 %. The results showed that the proposed method was sensitive, accurate and reliable for the determination of trace FQs in animal-derived foods.

Fabrication of zwitterionic magnetic microporous organic network for efficient extraction of fluoroquinolone antibiotics from meat samples

A zwitterionic magnetic microporous organic network (MMON-SO3H-NH2) with numerous amino and sulfonic acid ion-pare binding sites was designed and synthesized for efficient magnetic solid-phase extraction (MSPE) of fluoroquinolones (FQs) from meat samples. The core-shell MMON-SO3H-NH2 offered large specific surface area, rapid magnetic responsiveness, good stability, and multiple binding sites for FQs. The density functional theory and independent gradient model evaluations confirmed hydrogen bonding, π-π and ion-pair interactions between MMON-SO3H-NH2 and FQs. Under the optimal conditions, the established MMON-SO3H-NH2-MSPE-HPLC-UV method gave wide linear range (0.15-1000 μg L-1), low limits of detection (0.05-4.5 μg L-1) and limits of quantitation (0.15-13 μg L-1), and high enrichment factors (82.1-99.6) using 3 mg of adsorbent. This work demonstrates that the preparation of zwitterionic MONs is an efficient way to promote the extraction performance of MONs for zwitterionic targets and provides an effective sample pretreatment method for enriching and monitoring FQs in complex food matrices.

L-ribose specific recognition surface constructed by pillar[5]arene-based host-guest interaction

In living organisms, chiral molecules have specific chiral conformations that produce different physiological effects. Ribose is one of the components of RNA, which mainly plays a role in regulating biological activity. Inspired by the biological recognition of sugars, functional chiral surfaces for recognizing L-ribose through non-covalent interactions were constructed. In the strategy of this study, a functional chiral gold surface based on host-guest interactions was constructed through the assembly of the host molecule single-function alynyl pillar[5]arene(SAP5) and the guest molecule (S) -mandelate-violet (SMV). The association constant of SMV and SAP5 was calculated to be 2.95×104 M-1, with a binding ratio of 1:1. By impedance and contact angle detection, the constructed functional interface has good detection effect on L-ribose in the range of 1×10-7 M to1× 10-2 M. In addition, CV was disassembled from the aromatic cavity of pillar[5]arene after adding zinc powder and it can repeat five times with good recyclability, thus achieving the organic combination of interface recognition and intelligence.

New opportunities for cyclodextrins in supramolecular assembly: metal organic frameworks, crystalline self-assembly, and catalyzed assembly

Cyclodextrins (CDs) are widely used macrocycles in supramolecular assembly due to their easy availability, versatile functionality and excellent biocompatibility. Although they are well-known for forming host-guest complexes with a wide range of guests and this host-guest chemistry has long been utilized in industry and academia, new opportunities have arisen in recent years, particularly in supramolecular assembly. In the present review, we will first provide a basic introduction to CDs and then summarize their emerging roles in the fields of supramolecular chemistry and materials. This includes their involvement in hybrid frameworks with inorganic components such as metal ions and polyoxometalates, crystalline self-assembly with amphiphilic molecules, and their new possibility of "catassembly" and induced chiral supramolecular structures that have previously been overlooked. Finally, we will comment on the future perspectives of CDs to inspire more ideas and efforts, with the aim of promoting diverse applications of CDs in supramolecular materials.

Rational Design of Porous Ionic Liquids for Coupling Natural Gas Purification with Waste Gas Conversion

Removal of trace impurities for natural gas purification coupled with waste gas conversion is highly desired in industry. We here report a type of porous ionic liquids (PILs) that can realize the continuous flow separation of CH4 /CO2 /H2 S and the conversion of the captured H2 S to useful products. The PILs are synthesized through a step-by-step surface modification of ionic liquids (ILs) onto UiO-66-OH nanocrystals. The introduction of free tertiary amine groups on the nanocrystal surface endows these PILs with an exceptional ability to enrich H2 S from CO2 and CH4 with impressive selectivity, while the permanent pores of UiO-66-OH act as containers to store an exceptionally higher amount of the selectively captured H2 S than the corresponding nonporous ILs. Simultaneously, the tertiary amines as dual functional moieties offer effective catalytic sites for the conversion of the H2 S stored in PILs into 3-mercaptoisobutyric acid, a key intermediate required for the synthesis of Captopril (an antihypertensive drug). Molecular dynamics, density functional theory calculations and Grand Canonical Monte Carlo simulations help understand both the mechanisms of separation and catalysis performance, confirming that the tertiary amines as well as the permanent pores in UiO-66-OH play vital roles in the whole procedure.

Interfacial Polymerization of Highly Active Thiolated Cyclodextrin for the Fabrication of a Loose Nanofiltration Membrane with a Chlorine-Resistant Poly(thioester) Linkage

Cyclodextrins have been frequently used to fabricate membranes via interfacial polymerization (IP). However, the relatively low reactivity of pristine cyclodextrins often induces a lower cross-linking density and unsatisfactory separation performance. In this work, to introduce a highly active thiolated β-cyclodextrin (CD-SH) monomer into IP progress, we constructed a dense and porous poly(thioester) linkage on a commercial membrane surface with loose nanofiltration by IP of CD-SH and trimesoyl trichloride (TMC) as the monomer in an aqueous phase and organic phase separately for the first time. Furthermore, the reactivity of CD-SH has been fully demonstrated by the two-phase IP aiming at unmodified β-CD, a CD-SH/TMC freestanding membrane with a thicker interfacial layer and a smoother surface, and a PAN/CD-SH membrane with a narrow porous distribution. The composite membrane possessed superior separation performance for a high rejection (83.1-99.6%) of different anionic dyes and a low rejection (<20%) of salts, as well as a high-efficiency sieving ability of dye/dye and dye/salt mixtures. The membrane with a poly(thioester) selective layer could steadily operate in a long-term filtration test and exhibit great stability, chloride-resistance performance, and recyclability.

Chiral discrimination of cyclodecapeptide to anti-COVID-19 clinical candidates: a theoretical study

Various undesirable side effects are frequently associated with isomers of chiral clinical agents. The separation of chiral medicines remains a challenging issue in the medicines research. In this work, we employed cyclic decapeptide as the host molecule and the M06-2X theoretical computational method for chiral recognition of four clinical candidate guests and their isomers, including bucillamine, molnupiravir, azvudine, and VV116, which are relevant for the treatment of COVID-19. The obtained results indicated that bucillamine and molnupiravir and their respective isomers may be distinguished by cyclic decapeptide and that some of the isomers of Azvudine and VV116 may be discriminated by cyclic decapeptide. The inclusion conformation, deformation analysis, and electrostatic potential analysis also visualized the binding modes and binding sites between cyclic peptides and medicine candidates. A series of weak interaction analyses suggest that hydrogen bonding and dispersion interactions may be the primary factors for the recognition and separation of the clinical candidates by cyclic decapeptides. Visualized analyses of noncovalent interaction, hydrogen bond interaction, and NBO, AIM topological demonstrated that the difference of dispersion interaction is not obvious between the complexes, while the type and number of hydrogen bonds are very different, hinting that hydrogen bonds might be crucial for the differentiation of molnupiravir and its isomers. These findings might provide a theoretical reference for the identification and separation of chiral compounds in host-guest interaction.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11224-023-02149-5.

Highly Efficient versus Null Electrochemical Enantioselective Recognition Controlled by Achiral Colinkers in Homochiral Metal-Organic Frameworks

Chiral materials capable of electrochemical enantiomeric recognition are highly desirable for many applications, but it is still very challenging to achieve high recognition efficiency for lack of the knowledge of structure-property relationships. Here, we report the completely distinct enantiomeric recognition related to slightly different achiral colinkers in isomorphic homochiral metal-organic frameworks with the same chiral linker. Cu-TBPBe, for which the achiral colinker has two pyridyl rings connected by ─CH═CH─, shows excellent enantioselectivity and sensitivity for electrochemical recognition of l-tryptophan (Trp) with a detection limit of 3.16 nM. The l-to-d ratio of differential pulse voltammetric (DPV) currents reaches 53, which is much higher than the values (2-14) reported for previous electrochemical sensors. By contrast, Cu-TBPBa, in which the achiral colinker has -CH₂-CH₂- between pyridyl rings, is incapable of discrimination between l-Trp and d-Trp. Structural and spectral analyses suggest that the achiral conjugated colinker and the chiral moieties around it cooperate to produce a chiral pocket in favor of enantioselective adsorption through multiple hydrogen-bonding and π-π stacking interactions. The work demonstrated that Cu-TBPBe can be used to fabricate reliable electrochemical sensors for ultrasensitive quantification of Trp enantiomers in racemic mixtures and in complex biological systems such as urine. The work also highlights that an achiral coligand can be of vital importance in determining enantiomeric discrimination, opening up a new avenue for the design of chiral sensing materials.

Synthesis of crown ether-based microporous organic networks: A new type of efficient adsorbents for chlorophenols

Microporous organic networks (MONs) are a booming class of functional materials in elimination of environmental pollutants. However, the limit varieties of MONs still restrict their broad applications. Here we report the synthesis of a novel type of crown ether (CE)-based MONs via the coupling between brominated 18-crown-6 ether and different aromatic alkynyls. The constructed CE-based MONs integrates the good conjugation property of MONs and the inherent host-guest binding sites of CE, allowing the ultrafast and efficient adsorption and removal of a typical environmental priority pollutant 2,4,6-trichlorophenol (2,4,6-TCP). The hydrophobic CE-based MONs can also address the recovery challenge of unstable discrete CE in most organic and inorganic solvents. All CE-based MONs displayed fast adsorption kinetics (< 3 min) and large adsorption capacities (229.1-341.7 mg g^-1) for 2,4,6-TCP. The CE-based MONs also gave stable adsorption capacities for 2,4,6-TCP in pH range of 4.0-6.0, NaCl concentration of 0-40 mg L^-1, HA concentration of 0-30 mg L^-1, or H₂O₂ ratio of < 5 %. Density functional theory calculation, Fourier transform infrared and X-ray photoelectron spectra evaluation revealed adsorption process involved hydrophobic, π-π and hydrogen bonding interactions. The CE-based MONs also showed favorable reusability and good adsorption for other toxic chlorophenols. This work highlights the potential of CE-based MONs in contaminants elimination.

Mechanistic understanding and the rational design of a SiO2@CD catalyst for selective protection of L-lysine

The mechanism of the selective protection of L-lysine mediated by β-cyclodextrin (β-CD) was investigated by preliminary experiments, including the reaction efficiency influenced by different reaction conditions, and the existence of (1a·CD)' and 1a·CD·2a was evidenced by ESI-MS and 2D Rotating Frame Overhauser Effect Spectroscopy (ROESY) analysis. The results indicated that the formation of (1a·CD)' is critical for the product selectivity and the further formation of the ternary complex 1·CD·2 is responsible for the reaction efficiency. Thus, the yields and selectivity were significantly influenced by the structure, size and reactivity of the reactants. During the mechanistic investigations, we realized that the formation of the product and the β-CD complex at the final stage of the reaction would cause difficulty in product purification by a previously reported homogeneous method. In light of this understanding, an efficient and practical protocol for selective protection of L-lys based on a heterogeneous catalyst SiO₂@CD was developed. The use of the SiO₂ immobilized β-CD catalyst prevented the formation of the "capped" products by controlling the spatial rearrangement of β-CDs on solid supports, which represents a considerable synthetic improvement over the tedious and wasteful organic solvent extraction for product purification.

Practical considerations in the design and use of porous liquids

The possibility of creating well-controlled empty space within liquids is conceptually intriguing, and from an application perspective, full of potential. Since the concept of porous liquids (PLs) arose several years ago, research efforts in this field have intensified. This review highlights the design, synthesis, and applicability of PLs through a thorough examination of the current state-of-the-art. Following a detailed examination of the fundamentals of PLs, we examine the different synthetic approaches proposed to date, discuss the nature of PLs, and their pathway from the laboratory to practical application. Finally, possible challenges and opportunities are outlined.

Type II porous ionic liquid based on metal-organic cages that enables L-tryptophan identification

Porous liquids with chemical separation properties are quite well-studied in general, but there is only a handful of reports in the context of identification and separation of non-gaseous molecules. Herein, we report a Type II porous ionic liquid composed of coordination cages that exhibits exceptional selectivity towards l-tryptophan (l-Trp) over other aromatic amino acids. A previously known class of anionic organic–inorganic hybrid doughnut-like cage (HD) is dissolved in trihexyltetradecylphosphonium chloride (THTP_Cl). The resulting liquid, HD/THTP_Cl, is thereby composed of common components, facile to prepare, and exhibit room temperature fluidity. The permanent porosity is manifested by the high-pressure isotherm for CH₄ and modeling studies. With evidence from time-dependent amino acid uptake, competitive extraction studies and molecular dynamic simulations, HD/THTP_Cl exhibit better selectivity towards l-Trp than other solid state sorbents, and we attribute it to not only the intrinsic porosity of HD but also the host-guest interactions between HD and l-Trp. Specifically, each HD unit is filled with nearly 5 l-Trp molecules, which is higher than the l-Trp occupation in the structure unit of other benchmark metal-organic frameworks.

Porous liquids are potentially useful materials for the identification and separation of non-gaseous compounds. Herein, the authors report a type II porous ionic liquid with permanent porosity and high selectivity towards l-tryptophan (l-Trp) over other aromatic amino acids.

Chiral induction in a novel self-assembled supramolecular system composed of α-cyclodextrin porous liquids, chiral silver nanoparticles and planar conjugated molecules

The newly developed porous liquids known as liquids with permanent microporosity, have considerable application potential in many unknown areas. Herein, a supramolecular system composed of α-cyclodextrin porous liquid, chiral silver nanoparticles and planar conjugated molecules (methylene blue and indigo carmine) was designed and the induced chirality of the system was observed. It was found that the induced chirality can be easily tuned by changing the pH value of the mixture solution. The induced chiral signal of methylene blue in the developed self-assembled supramolecular system occurred when the pH was between 8 and 10, and furthermore the induced chirality of indigo carmine was found when the pH was between 6.5 and 7.5. The intensity of induced chirality decreases upon increasing temperatures and ionic strength. This study may offer a new approach for the creation of a chiral supramolecular system based on host-guest and electrostatic interaction and make cyclodextrin porous liquids promising candidates for applications in chiral induction.

Preparation and chiral resolution properties of bridged bis(cyclodextrin)s hybrid spheres for high performance liquid chromatography

Selenium-bridged bis(β-cyclodextrin)s organic-inorganic hybrid silica material with regular spherical shape as new type of chiral stationary phase was directly synthesized under the one-pot hydrothermal synthesis method, and the chiral stationary phase was further characterized by infrared spectroscopy, scanning electron microscopy, thermogravimetry, and elemental analysis. The results of chiral separation showed that eight chiral compounds including various types of chiral alcohols and flavanone were successfully separated in the reversed-phase separation mode by high performance liquid chromatography, which showed the better chiral resolution effect than that on the C2 position of single β-cyclodextrin. The mechanism of chiral separation was likely due to multiple interactions such as inclusion, hydrogen bonding, electrostatic interaction, dipole-dipole interaction, and the synergistic effect of two cyclodextrins during the chiral resolution process. The synergy of the two cyclodextrins has great potential for development in chiral resolution.

Enantioselective Recognition and Separation of C2 Symmetric Substances via Chiral Metal-Organic Frameworks

A promising route toward the enantioselective recognition and separation of racemic molecules is the design of chiral metal-organic frameworks (CMOFs) with high enantioselectivity and stability. Herein, we report porous CMOFs Δ- and Λ-RuEu-MOFs constructed from the D₃-symmetry helical chiral Ru(phen)₃-derived tricarboxylate ligand and Eu₂ units, which can be utilized as adsorbents for the enantioselective recognition and separation of 1,1'-bi-2-naphthol (BINOL) derivatives. Investigation of the circular dichroism enantiodifferentiation between the host and guest suggested that Δ- and Λ-RuEu-MOFs can be employed as chiral sensors to discriminate axial enantiomers due to their diastereomeric host-guest relationship. Density functional theory calculations reveal that chiral recognition is attributed to the distinguishing binding affinities stemming from N···H-O hydrogen bonds and π-π stacking between the host and guest. Moreover, the reticulate structure of Δ- and Λ-RuEu-MOFs can be readily recycled and reused for the successive enantioselective separation of BINOL up to 80% ee.

Recent advances in chiral discrimination on host-guest functionalized interfaces

Chiral discrimination has gained much focus in supramolecular chemistry, since it is one of the fundamental processes in biological systems, enantiomeric separation and biochemical sensors. Though most of the biochemical processes can routinely recognize biological enantiomers, enantioselective identification of chiral molecules in artificial systems is currently one of the challenging topics in the field of chiral discrimination. Inaccuracy, low separation efficiency and expensive instrumentation were considered typical problems in artificial systems. Recently, chiral recognition on the interfaces has been widely used in the fields of electrochemical detection and biochemical sensing. For the moment, a series of macrocyclic host functionalized interfaces have been developed for use as chiral catalysts or for enantiomeric separation. Here, we have briefly exposited the most recent advances in the fabrication of supramolecular functionalized interfaces and their application for chiral recognition.

Fabrication and evaluation of tetraazahexaphenylmacrocycle-bonded stationary phase with multiple retention mechanisms

A 34-membered tetraazahexaphenylmacrocycle (N₄Ph₆) with a rigid π-conjugated moiety was chemically bonded to silica gel with 3-chloropropyltrimethoxysilane as the coupling agent to prepare a novel SiO₂@N₄Ph₆ stationary phase. Several common organic analytes, including alkylbenzenes, polycyclic aromatic hydrocarbons, anilines, phenols, phthalates, and folic acid, were selected as probes to investigate its chromatographic performance. The as-developed SiO₂@N₄Ph₆ stationary phase showed superiority retention and high selectivity for probe molecules through multiple interactions, including hydrophobic, π-π, hydrogen-bonding, and steric interactions. Density functional theory calculation results using folic acid as model solute provided an intuitive and a quantitative description of the multiple retention mechanisms.