A chiral metal-organic framework {(HQA)(ZnCl2)(2.5H2O)}n for the enantioseparation of chiral amino acids and drugs

Chiral ligand-exchange capillary electrochromatography with thermo-responsive poly(N,N-dimethylacrylamide) as coating for efficient separation of D,L-amino acids

Herein, for addressing the issue in enantioseparation and being inspired by regulating hydrophobic/hydrophilic interactions in smart polymer through varying external environmental conditions, we demonstrated a chiral ligand exchange-capillary electrochromatography-UV (CLE-CEC-UV) system by introducing a thermo-responsive poly(styrene-maleic anhydride-N,N-dimethylacrylamide) (PSMDMA) to the coating for enantioseparation of dansylated D,L-amino acids (Dns-D,L-AAs). As metal ions-chiral ligands complexes in running buffer played the role of chiral selectors, the proposed CLE-CEC-UV system exhibited significantly enhanced enantioseparation efficiency for the first time, especially, 15-pairs were baseline separated at 40 °C, compared to 4-pairs baseline separated at 20 °C among 16-pairs of Dns-D,L-AAs. It was attributed to the high hydrophobic interactions between Dns-D,L-AAs and PSMDMA coating at high temperature. Additionally, the formation of strong coordinated complexes between metal ions, Dns-D,L-AAs and chiral ligands was also a key to achieve high CLE-CEC-UV performance. The proposed method showed an excellent linear dependence relation between concentration of L-alanine and UV absorbance intensity, ranging from 0.1 to 3.0 mmol L-1, with limit of detection (LOD) of 10.0 μmol L-1. Further evaluation of alanine aminotransferase activity in various mice organs with L-alanine as the substrate confirmed the potential application of the CLE-CEC-UV technique. Moreover, the coated capillary exhibited good repeatability with relative standard deviations less than 1.86 % for migration time and less than 3.94 % for resolution. This work highlights and integrates the advantages of smart polymer coatings and metal ion-chiral ligand complexes, encouraging development of more unique CLE-CEC systems for efficient enantioseparation and practical application in living bio-systems.

Recent Advances in the Construction and Applications of Monolithic and Open-tubular Capillary Electrochromatography (2022-2024)

Capillary electrochromatography (CEC) has attracted significant attention and gained considerable recognition in the field of separation science owing to its excellent separation efficiency. While numerous reviews on CEC have been published in recent years, a comprehensive and systematic summary of the typical synthesis strategies for electrochromatographic stationary phases and their state-of-the-art applications in CEC remains lacking. This review highlights recent advances (over the past 3 years) and representative applications (including chiral separation, microextraction-coupled analysis, metabolomics, enzyme analysis, and food analysis) of monolithic and open-tubular stationary phases in CEC. The advantages and limitations of each methodology are critically analyzed to present a balanced evaluation. Additionally, this work outlines future prospects regarding the development trends in electrochromatographic stationary phase preparation methods and the evolving applications of CEC.

Recent developments and applications of solid membrane in chiral separation

Chirality is a fundamental property in nature, and chiral molecules are closely related to human health and the origin of life. Therefore, the exploration and preparation of optically active compounds of paramount importance. Membrane separation is a large-scale and continuous separation technique that has been developing quickly in recent years. It has many potential applications, particularly in chiral membrane separation technology, which is currently a hotspot for study. Depending on the types of membranes, chiral membranes can be divided into two categories: chiral solid membranes and chiral liquid membranes. Solid membranes outperform the others in terms of better mechanical performance and separation efficiency. This review presents in-depth summaries of chiral solid membranes made of different materials, and their applications in drug separation. It also providing insights into the potential for the future development of chiral solid membranes.

Capillary electrophoresis separations with Betaine:Urea, a deep eutectic solvent as the separation medium

BACKGROUND

Capillary electrophoresis (CE) is a highly versatile separation technique widely used in analytical chemistry. Traditionally, CE can be categorized as either aqueous or non-aqueous systems based on the buffer solvents employed. For decades, non-aqueous CE has been predominantly associated with the use of organic solvents, a perception deeply ingrained in the scientific community. However, growing concerns about the health and environmental impacts of these solvents, driven by the principles of green chemistry, have prompted a reevaluation of their use. In response to these concerns, our group recently introduced a deep eutectic solvent (DES), specifically Proline:Urea, as an innovative and eco-friendly separation medium for CE. This approach not only enhances the sustainability of CE separations but also offers a new perspective for the development of innovative CE separation media.

RESULTS

Building on our previous work, here we report the use of the second DES, Betaine:Urea (BU), as a new separation medium that offers further improved performance for CE applications. The DES was systematically characterized, with key physical properties relevant to CE separations, such as thermal properties, viscosity, dielectric constant, Joule heating effect, and UV transmittance, being thoroughly examined. Using a complex sample of 10 structurally similar naphthalene derivatives, we demonstrated the efficiency of BU in capillary zone electrophoresis (CZE) for separating analytes with varying charges (including cations, neutrals, and anions) and sizes. Additionally, we established the first micellar electrokinetic chromatography (MEKC) system in this DES using sodium dodecyl sulfate (SDS) as the surfactant. This system successfully resolved 6 structurally similar neutrals that could not be separated by conventional aqueous SDS-MEKC, highlighting the versatility of this DES-type separation medium. Furthermore, BU showed several advantages over the previously reported DES, Proline:Urea, particularly in terms of stability, viscosity, and Joule heating effects.

SIGNIFICANCE

This study holds the potential to challenge the traditional notion that "CE separation media are merely categorized into aqueous and organic solvents". Given that DESs are "designer" solvents with highly tunable properties and environmentally friendly characteristics, the introduction of BU as a viable alternative to traditional solvents not only expands the media available for CE separations, but also offers a more efficient and potentially more sustainable option for specific analyses.

Capillary electrochromatography synergistic enantioseparation system for racemate malic acid based on a novel nanomaterial synthesized by chiral molecularly imprinted polymer and chiral metal-organic framework

BACKGROUND

Chirality is one of the most fundamental features of nature. In terms of biological activities, pharmacological effects, etc., enantiomers often show great differences among each other. Therefore, it is important to develop highly efficient enantioseparation and analysis methods. Capillary electrochromatography (CEC) is one of the most popular methods in the field of enantioseparation. In the chiral stationary phase of CEC, chiral molecularly imprinted polymers (CMIPs) and chiral metal-organic frameworks (CMOFs) have shown great potential of enantioseparation. However, the enantioseparation performance of CMOFs and CMIPs alone as chiral separation media is less satisfactory.

RESULTS

In this work, a novel nanomaterial synthesized by CMOFs and CMIPs was used as stationary phase in CEC synergistic enantioseparation system and the relevant reports have not been internationally found by authors. As a proof-of-concept demonstration, a coated capillary column was prepared by a one-step method using l-malic acid (template), [Cu2(D-Cam)2Dabco] (Cu-MOF) and dopamine (functional monomer/cross-linking agent), which greatly simplified the modification process of the capillary columns. Compared with Cu-MOF and CMIP alone, the CEC synergistic enantioseparation system based on Cu-MOF@MIP has significantly better enantioseparation performance of malic acid enantiomers (resolution: 1.03/0.58 → 4.22), and there is also a satisfactory performance in the quantitative analysis in real samples. Finally, through molecular docking and adsorption experiments, it was systematically proved that Cu-MOF@MIP had a significantly stronger binding ability for l-malic acid than d-malic acid.

SIGNIFICANCE

Cu-MOF with chiral recognition ability have synergize with CMIPs to greatly improve the chiral selectivity of Cu-MOF@MIP, which is firstly used for the construction of the CEC chiral separation system. This pioneering synergistic chiral separation system creates a potential direction for efficient enantioseparation. Considering the diversity of CMOFs and CMIPs, the stationary phases hold great promise in chiral separation science.

Recent progress of chiral metal-organic frameworks in enantioselective separation and detection

Chiral compounds are abundantly distributed in both the natural world and biological systems. It is crucial to identify and detect chiral compounds in living systems or to separate and determine them in the natural environment. Many researchers have developed a range of chiral materials with different functionalizations to separate and detect chiral substances. Chiral metal-organic frameworks (CMOFs) have the potential to be used in enantioselective separation and detection due to their large surface areas, regulated framework topologies, particular substrate interactions, and accessible chiral sites. CMOFs contribute significantly to the development of enantiomer separation and detection in medicine, agriculture, food, environment, and other fields. This review focuses on four synthesis methods of CMOFs and their applications in chiral separation and chiral sensing in the past five years, mainly including chromatographic separation, membrane separation, optical sensing, electrochemical sensing, and other sensing methods. Finally, the challenges and potential growth direction of CMOFs in enantiomer separation and detection are discussed and prospected.

A comprehensive review on the enantiomeric separation of chiral drugs using metal-organic frameworks

Chiral drugs play an important role in modern medicine, but obtaining pure enantiomers from racemic mixtures can pose challenges. When a drug is chiral, only one enantiomer (eutomer) typically exhibits the desired pharmacological activity, while the other (distomer) may be biologically inactive or even toxic. Racemic drug formulations introduce additional health risks, as the body must still process the inactive or detrimental enantiomer. Some distomers have also been linked to teratogenic effects and unwanted side effects. Therefore, developing efficient and scalable methods for separating chiral drugs into their pure enantiomers is critically important for improving patient safety and outcomes. Metal-organic frameworks (MOFs) show promise as novel materials for chiral separation due to their highly tunable structures and interactions. This review summarizes recent advancements in using MOFs for chromatographic and spectroscopic resolution of drug enantiomers. Both the opportunities and limitations of MOF-based separation techniques are discussed. A thorough understanding of these methods could aid the continued development of pure enantiomer formulations and help reduce health risks posed by racemic drug mixtures.

Chiral metal-organic frameworks grown in situ for monolithic capillary electrochromatographic enantioseparation

Chiral metal-organic frameworks (CMOFs) with chiral selectivity are one of the high-quality stationary phases for capillary electrochromatography (CEC). However, there is a problem of unsatisfactory enantioseparation performance of capillary columns due to insufficient loading. In this work, a lamellar CMOF (Cu-TC) was grown in situ on the surface of the monolith in a capillary monolithic column to obtain a Cu-TC@monolithic column. The CEC system constructed based on the Cu-TC@monolithic column shows a satisfactory chiral separation performance. Compared with the Cu-TC-based coated column (Cu-TC@coated column), the enantioseparation performance of the CEC system based on the Cu-TC@monolithic column was greatly improved, and the resolutions (Rs) of the model analytes were increased by 80-500%. In addition, the effects of experimental conditions such as the number of cycles of Cu-TC in situ growth, buffer concentration, buffer pH, organic solvent addition and applied voltage on the performance of CEC were also investigated. Finally, the chiral selection mechanism of the stationary phase was explored by selective adsorption experiments. The present work provides a new idea for the development of capillary stationary phases, which has great potential considering the diversity of CMOFs.

Deep eutectic solvents as a green alternative to organic solvents for β-cyclodextrin pseudo-stationary phase in capillary electrophoresis

β-cyclodextrin (β-CD), as an important pseudo-stationary phase (PSP) in capillary electrophoresis (CE), frequently confronts challenges stemming from its limited water solubility, particularly when high concentrations are required for resolving complex analytes. Traditionally, researchers often resort to the use of (toxic) organic solvents to enhance the solubility of β-CD, establishing non-aqueous capillary electrophoresis (NACE) for specific separations. However, such practices are hazardous to health and run counter to the principles of green analytical chemistry. In this study, we demonstrate a deep eutectic solvent (DES), Proline:Urea (PU), as a promising alternative to conventional organic solvents for β-CD-based CE separations. The DES exhibits a solubility of up to 30% for β-CD, a significant improvement compared to the 1.8% solubility in the aqueous phase. Utilizing this DES-type separation medium, we achieved simultaneous baseline separation of a complex analyte composed of eight structurally similar naphthoic acid derivatives. Furthermore, we conducted a systematic comparison of β-CD's performance in aqueous CE buffers, organic solvents, and DESs, highlighting the superiority of this novel and environmentally friendly CE separation medium.

Itaconic Acid-Based Organic-Polymer Monolithic Column for Hydrophilic Capillary Electrochromatography and Its Application in Pharmaceutical Analysis

Itaconic acid is an excellent hydrophilic monomer owing to the dicarboxylic group possessing strong polarity. This study reports on the preparation of a new organic-polymer monolithic column poly(itaconic acid-co-3-(acryloyloxy)-2-hydroxypropyl methacrylate) (poly(IA-co-AHM)) featuring excellent hydrophilic chromatography ability and its application in pharmaceutical analysis. The monolithic column was successfully synthesized by using the monomer itaconic acid and the cross-linker AHM through an in situ copolymerization method. Optical microscopy, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) were employed for the characterization of the poly(IA-co-AHM) monolithic column, and all of these demonstrated that the prepared itaconic acid-based monolithic column exhibited satisfactory permeability and a homogeneous porous structure. Owing to the carboxylic groups of itaconic acid, a cathodic electroosmotic flow (EOF) was generated on the itaconic acid-based monolithic column among the pH ranges of the mobile phase from 4.0 to 9.0. Depending on the powerful hydrophilic interactions, different kinds of polar substances, including thioureas, nucleoside drugs, sulfonamides, and polypeptides, were separated efficiently by the itaconic acid-based monoliths poly(IA-co-AHM). The separations of polar compounds were successfully realized, even at a lower level of 50% acetonitrile content on this monolithic column. The highest column efficiencies corresponding to N,N'-dimethylthiourea and idoxuridine were 102 720 and 124 267 N/m, respectively. The poly(IA-co-AHM) monolithic column displayed excellent repeatability, whose relative standard deviations (RSDs) of the retention time and peak area were both lower than 5.0%. All experimental results demonstrated that the new itaconic acid-functionalized monolithic column was greatly appropriate to separate the polar compounds under the HILIC mode.

Strategies for chiral separation: from racemate to enantiomer

Chiral separation has become a crucial topic for effectively utilizing superfluous racemates synthesized by chemical means and satisfying the growing requirements for producing enantiopure chiral compounds. However, the remarkably close physical and chemical properties of enantiomers present significant obstacles, making it necessary to develop novel enantioseparation methods. This review comprehensively summaries the latest developments in the main enantioseparation methods, including preparative-scale chromatography, enantioselective liquid-liquid extraction, crystallization-based methods for chiral separation, deracemization process coupling racemization and crystallization, porous material method and membrane resolution method, focusing on significant cases involving crystallization, deracemization and membranes. Notably, potential trends and future directions are suggested based on the state-of-art "coupling" strategy, which may greatly reinvigorate the existing individual methods and facilitate the emergence of cross-cutting ideas among researchers from different enantioseparation domains.