Chiral separations

Influence of theoretical and semi-empirical peak models on the efficiency calculation in chiral chromatography

Height equivalent to theoretical plate (H) equations, such as the van Deemter or Knox-Saleem equations, and other efficiency vs. linear velocity equations (u), provide kinetic insights into chromatographic separations phenomena and column performance. In enantioselective separations, the peak shape of the two enantiomers can differ significantly and are often asymmetric. The peak efficiency calculations heavily impact these efficiency-flow profiles, leading to erroneous estimations of eddy diffusion, longitudinal diffusion, and mass transfer terms. In this work, new asymmetric peak functions are employed for modeling enantiomer peaks based on the Haarhoff-Van der Linde function, its generalized variant (GHVL), once Generalized Asymmetric Gaussian (AGN), and Twice Generalized Gaussian (TGN). The new models (AGN, TGN, and GHVL) incorporate higher statistical moments besides the zeroth, first, and second moments to account for two-sided asymmetry (fronting or tailing). The fit results are compared with the traditional efficiency calculation methods endorsed by official pharmacopeia and numerical estimation of moments from the raw data. Enantiomeric separations of ibuprofen and dl-homophenylalanine were chosen as probe molecules. The results demonstrate that non-linear least squares fitted functions provide better estimations of peak efficiency data even in the presence of high noise. In particular, the generalized models consistently offered the best quality fits for various peak shapes in chiral separations. Conversely, the half-height Gaussian method greatly overpredicted skewed peak efficiencies. This investigation reveals that the commonly held assumptions of peak shape and numerical integration of raw data are highly insufficient for chiral chromatography. The impact of asymmetry on plate height should not be overlooked when accurate data from efficiency-flow rate curves is derived. We advocate for the broader adoption of these new generalized peak (AGN, TGN, GHVL) models because they provide robustness at various SNRs that account for right or left asymmetry while accurately representing peak geometry.

Enantiomeric Separation of Triacylglycerols Consisting of Three Different Fatty Acyls and Their Chiral Chromatographic Elution Behavior

Chromatographic separation of triacylglycerol (TG) enantiomers is a highly challenging task of analytical chemistry because of the similar physicochemical properties. The analysis of chiral TGs is crucial for improving the knowledge of lipid biochemistry and for understanding the nutritional properties of fats and oils. Thus, this study aimed to systematically investigate the chiral resolution of TGs consisting of three different fatty acyls (FAs). Thirty-three asymmetric TG enantiopairs, including 49 synthesized enantiopure TGs and racemic TGs, were analyzed with a recycling chiral HPLC system. Twenty-six enantiopairs were successfully separated. Overall, having both unsaturated and saturated FAs in the outer positions or a difference in carbon chain length between two saturated FAs at the outer positions favored the separation of enantiomers. The retention time at separation correlated negatively with the sn-3 carbon number of the early eluting enantiomer and positively with the carbon number difference between sn-1 and sn-3. When the samples were studied in separate groups based on unsaturation and regioisomers, both the acyl carbon number and the degree of unsaturation of FAs in all three positions influenced the separation and elution behavior of chiral TGs, indicating an active role of both intermolecular interactions and steric hindrances. This is the first systematic study of the chiral separation of TGs consisting of three different FAs using a large number of enantiopairs. The novel findings on the behavior of TG enantiomers in a chiral environment provide important guidance and reference for a stereospecific study of the chemistry and biochemistry of natural lipids.

Enantioselective Optical Trapping of Multiple Pairs of Enantiomers by Focused Hybrid Polarized Beams

Enantiomers (opposite chiral molecules) usually exhibit different effects when interacting with chiral agents, thus the identification and separation of enantiomers are of importance in pharmaceuticals and agrochemicals. Here an optical approach is proposed to enantioselective trapping of multiple pairs of enantiomers by a focused hybrid polarized beam. Numerical results indicate that such a focused beam shows multiple local optical chirality of opposite signs in the focal plane, and can trap the corresponding enantiomers near the extreme value of optical chirality density according to the handedness of enantiomers. The number and positions of trapped enantiomers can be changed by altering the value and sign of polarization orders of hybrid polarized beams, respectively. The key to realizing enantioselective optical trapping of enantiomers is that the chiral optical force exerted on enantiomers in this focused field is stronger than the achiral optical force. The results provide insight into the optical identification and separation of multiple pairs of enantiomers and will find applications in chiral detection and sensing.

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.

On chip all-optical distinguishing of independently placed distinct types of single Rayleigh particle

In order to determine whether a particle is plasmonic, dielectric, or chiral, different complex processes and chemicals are applied in lab setups and pharmaceutical industries. Sorting or categorizing a particle based on distinct optical forces can be a novel technique. When a beam of light interacts with a particle, it usually pushes the particle in the direction of the light's propagation. Counterintuitively, it can also pull the particle toward the light beam or move it toward a lateral direction. As far as we know, to date, no comprehensive report exists regarding a single optical arrangement capable of inducing entirely distinct behaviors of force for three disparate types of independently placed single Rayleigh particle. This study introduces an all-optical technique aimed at effectively sorting nanoscale Rayleigh-sized objects employing a plasmonic substrate, when each distinct type of single particle is placed over the substrate independently. Unfortunately, this proposed technique does not work for the cluster or mixture of distinct particles. In our proposed configuration, a simple linearly polarized plane wave is incident onto the plasmonic substrate, thereby engendering completely different responses from three different types of nanoparticles: Gold (plasmonic), SiO2 (dielectric), and Chiral particles. We conducted individual tests for our setup using linearly polarized plane waves at angles of 30-degree, 45-degree, and 60-degree individually. Consistent results were obtained across all angles. In each of the three distinct setups involving the aforementioned particle, a dielectric Rayleigh particle experiences an optical pulling force, a plasmonic Rayleigh particle experiences an optical pushing force, and a chiral Rayleigh particle encounters an optical lateral force. These distinctive force behaviors manifest as a result of the intricate interplay between the material properties of the nanoparticles and the characteristics of the plane-polarized beam, encompassing aspects such as plasmonic response, chirality, and refractive index. Moreover, this technique presents an environmentally sustainable and economically viable alternative to the utilization of expensive and potentially hazardous chemicals in nanoparticle sorting processes within industrial domains.

Occurrence of D-amino acids in natural products

Since the identified standard genetic code contains 61 triplet codons of three bases for the 20 L-proteinogenic amino acids (AAs), no D-AA should be found in natural products. This is not what is observed in the living world. D-AAs are found in numerous natural compounds produced by bacteria, algae, fungi, or marine animals, and even vertebrates. A review of the literature indicated the existence of at least 132 peptide natural compounds in which D-AAs are an essential part of their structure. All compounds are listed, numbered and described herein. The two biosynthetic routes leading to the presence of D-AA in natural products are: non-ribosomal peptide synthesis (NRPS), and ribosomally synthesized and post-translationally modified peptide (RiPP) synthesis which are described. The methods used to identify the AA chirality within naturally occurring peptides are briefly discussed. The biological activity of an all-L synthetic peptide is most often completely different from that of the D-containing natural compounds. Analyzing the selected natural compounds showed that D-Ala, D-Val, D-Leu and D-Ser are the most commonly encountered D-AAs closely followed by the non-proteinogenic D-allo-Thr. D-Lys and D-Met were the least prevalent D-AAs in naturally occurring compounds.

The progress on porous organic materials for chiral separation

Chiral compounds have similar structures and properties, but their pharmacological action is very different or even opposite. Therefore, the separation of chiral compounds has great significance in pharmaceutical and agriculture. Porous organic materials are novel crystalline porous materials, which possess high surface area, controllable pore size, and favorable functionalization. Therefore, porous organic materials are considered to be an ideal material for chiral separation. In this review, we summarized the progress of chiral porous organic materials for chiral separation in recent years. Furthermore, the applications of chiral porous organic materials as chiral separation medias (chromatography stationary phases and membrane materials) in enantioseparation were highlighted. Finally, the remaining challenges and future directions for porous organic materials in chiral separation were also briefly outlined further to promote the development of porous organic materials in chiral separation.

Identification and separation of chiral particles by focused circularly polarized vortex beams

The identification and separation of chiral substances are of importance in the biological, chemical, and pharmaceutical industries. Here, we demonstrate that a focused circularly polarized vortex beam can, in the focal plane, selectively trap and rotate chiral dipolar particles via radial and azimuthal optical forces. The handedness and topological charge of the incident beam have strong influence on identifying and separating behavior: left- and right-handed circular polarizations lead to opposite effects on the particle of trapping and rotating, while the sign of topological charge will change the particle's rotation direction. Such effects are a direct result of the handedness and topological charge manifesting themselves in the directions of the spin angular momentum (SAM) and Poynting vector. The research provides insight into the chiral light-matter interaction and may find potential application in the identification and separation of chiral nanoparticles.

Chiral Metal-Organic Frameworks

In the past two decades, metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) assembled from metal ions or clusters and organic linkers via metal-ligand coordination bonds have captivated significant scientific interest on account of their high crystallinity, exceptional porosity, and tunable pore size, high modularity, and diverse functionality. The opportunity to achieve functional porous materials by design with promising properties, unattainable for solid-state materials in general, distinguishes MOFs from other classes of materials, in particular, traditional porous materials such as activated carbon, silica, and zeolites, thereby leading to complementary properties. Scientists have conducted intense research in the production of chiral MOF (CMOF) materials for specific applications including but not limited to chiral recognition, separation, and catalysis since the discovery of the first functional CMOF (i.e., d- or l-POST-1). At present, CMOFs have become interdisciplinary between chirality chemistry, coordination chemistry, and material chemistry, which involve in many subjects including chemistry, physics, optics, medicine, pharmacology, biology, crystal engineering, environmental science, etc. In this review, we will systematically summarize the recent progress of CMOFs regarding design strategies, synthetic approaches, and cutting-edge applications. In particular, we will highlight the successful implementation of CMOFs in asymmetric catalysis, enantioselective separation, enantioselective recognition, and sensing. We envision that this review will provide readers a good understanding of CMOF chemistry and, more importantly, facilitate research endeavors for the rational design of multifunctional CMOFs and their industrial implementation.

Impact of chirality on the amorphous state of conglomerate forming systems: a case study of N-acetyl-α-methylbenzylamine

The present work aims at addressing the issue of molecular handedness in glassy and liquid states and its impact on heterogeneous equilibrium. For this purpose, we evaluated the glass forming ability (GFA), crystallization propensity, molecular mobility and hydrogen bonding structure of a chiral conglomerate forming system, N-acetyl-α-methylbenzylamine (Nac-MBA), at various enantiomeric excesses (ees) using experimental and computational techniques. We revealed that the rich relaxational landscape (Debye (D), α, β_JG and ϒ) and the temperature dependence of the time scale of each process were insensitive to chirality. The most remarkable impact of chirality was expressed on the GFA and the recrystallization of heterochiral arrangements. In fact the GFA increases with decreasing ee, while the crystallization propensity increases with increasing ee. The counter enantiomer acted as a disruptor of crystallization and favored the glass formation upon cooling. The molecular dynamics simulation (MDS) results on the architecture of chiral sequences showed that homochiral sequences were more favorable when compared to heterochiral ones in the liquid state. However, this predisposition to form homochiral sequences in the liquid state was not the precursor of the future crystalline structure, since the liquid or the glassy system recrystallizes as heterochiral sequences. As per our understanding the crystallization was mostly controlled by the mean free migration path of an enantiomer to build homochiral or heterochiral sequences. In the present case, it seems that the mean free migration path achieved by an enantiomer for heterochiral sequences is shorter compared to homochiral arrangements in such a way that the crystallization of the metastable racemic compound is kinetically more favorable.

Effect of configuration-dependent multi-body forces on interconversion kinetics of a chiral tetramer model

We describe a reformulation of the four-site molecular model for chiral phenomena introduced by Latinwo et al. ["Molecular model for chirality phenomena," J. Chem. Phys. 145, 154503 (2016)]. The reformulation includes an additional eight-body force that arises from an explicit configuration-dependent term in the potential energy function, resulting in a coarse-grained energy-conserving force field for molecular dynamics simulations of chirality phenomena. In this model, the coarse-grained interaction energy between two tetramers depends on their respective chiralities and is controlled by a parameter λ, where λ < 0 favors local configurations involving tetramers of opposite chirality and λ > 0 gives energetic preference to configurations involving tetramers of the same chirality. We compute the autocorrelation function for a quantitative chirality metric and demonstrate that the multi-body force modifies the interconversion kinetics such that λ ≠ 0 increases the effective barrier for enantiomer inversion. Our simulations reveal that for λ > 0 and temperatures below a sharply defined threshold value, this effect is dramatic, giving rise to spontaneous chiral symmetry breaking and locking molecules into their chiral identity.

Immobilization of cellulase on monolith supported with Zr(IV)-based metal-organic framework as chiral stationary phase for enantioseparation of five basic drugs in capillary electrochromatography

Metal-organic framework (UiO-66-NH₂)-incorporated organic polymer monolith was prepared by thermal polymerization. By virtue of the superior physical and chemical properties, the UiO-66-NH₂-modified organic monolith was then functionalized by chiral selector cellulase via the condensation reaction between the primary amino groups and aldehyde groups. The synthesized materials were characterized by Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectrometry, thermogravimetric analysis, and nitrogen sorption isotherm. The cellulase@poly(glycidyl methacrylate-UiO-66-NH₂-ethylene glycol dimethacrylate) (cellulase@poly(GMA-UiO-66-NH₂-EDMA)) monolith was applied to enantiomerically separate the basic racemic forms of metoprolol, atenolol, esmolol, bisoprolol, and propranolol. In contrast to the cellulase@poly(GMA-co-EDMA) monolith without UiO-66-NH₂, the cellulase@poly(GMA-UiO-66-NH₂-EDMA) monolith reveals significantly improved enantiodiscrimination performance for metoprolol (Rs: 0 → 1.67), atenolol (Rs: 0 → 1.50), esmolol (Rs: 0 → 1.52), bisoprolol (Rs: 0 → 0.36), and propranolol (Rs: 0 → 0.44). The immobilization pH of cellulase, buffer pH, UiO-66-NH₂ concentration, and the proportion of organic modifier were evaluated in detail with enantiomerically separating chiral molecules. The intra-day, inter-day, column-to-column, and inter-batch precision have been discussed, the result was preferable, and the relative standard deviation (RSD) of separation parameters was <4.3%. Schematic representation of the preparation of a UiO-66-NH₂-modified organic polymer monolith for enantioseparating five racemic β-blockers. UiO-66-NH₂ was synthesized and converted into a monolith as the stationary phase. Then, the modified monolith containing cellulase as the chiral selector was applied in a capillary electrochromatography system for enantioseparating chiral drugs.

Evaluation of a polysaccharide-based chiral reversed-phase liquid chromatography screen strategy in pharmaceutical analysis

Chirality control plays a critical role in developing stereoisomeric drugs. Due to the complexity and lack of predictability in chiral separations, column screening remains the gold standard to initiate chiral method development for active pharmaceutical ingredients (APIs) and synthetic intermediates. Chiral reversed-phase (RP) liquid chromatography (LC) has gained favor over other modes due to its versatility and compatibility in analyzing a wide range of chiral compounds in various matrices. Herein, we established a tier-based chiral RPLC screen strategy by constructing and analyzing a database of 101 chiral screens with a total of 3,401 entries (unique LC runs) for proprietary APIs or intermediates at Bristol Myers Squibb. Up to 17 polysaccharide-based chiral stationary phases (CSPs) and four mobile phases (MPs) have been screened with gradient elution. A selection of ten CSPs with two MPs was found sufficient to achieve successful separation for 82% of the total screens. Two RPLC screen tiers (Tier 1: AZ, OD, ID, and IG) and (Tier 2: AY, OJ, OZ, IA, IC, and IH) were proposed along with two MPs (acidic and neutral) to target ~70% hit rate for Tier 1, and ~80% for the combined set. We also implemented a user-friendly workflow to enable walk-up chiral RPLC screening with automated reports and system suitability tests.

Current analytical methods for determination of glucosinolates in vegetables and human tissues

Numerous epidemiological studies have indicated the potential effects of glucosinolates and their metabolites against cancer as well as other non-communicable diseases, such as cardiovascular disease and neurodegenerative disorders. However, information on the presence and quantity of glucosinolates in commonly consumed vegetables and in human fluids is sparse, largely because well-standardised methods for glucosinolate determination are not available, resulting in published data being inconsistent and conflicting. Thus, studies published since 2002 on the most recent developments of glucosinolate extraction and identification have been collected and reviewed with emphasis on determination of the intact glucosinolates by LC-MS and LC-MS/MS. This overview highlights the glucosinolate extraction methods used, the stability of glucosinolates during extraction, the availability of stable isotope labelled internal standards and the use of NMR for purity analysis, as well as the current analytical techniques that have been applied for glucosinolate analysis, e.g. liquid chromatography with mass spectrometric detection (LC-MS). It aims to interpret the findings with a focus on the development of a validated method, which will help to determine the glucosinolate content of vegetative plants and human tissues, and the identification and determination of selected glucosinolate metabolites.

A sensitive HPLC-MS/MS method for the detection, resolution and quantitation of cathinone enantiomers in horse blood plasma and urine

Resolution of cathinone enantiomers in equine anti-doping analysis is becoming more important to distinguish the inadvertent ingestion of plant-based products from those of deliberate administration of designer synthetic analogs. With this in mind, a rapid and sensitive method was developed and validated for the detection, resolution and quantitative determination of cathinone enantiomers in horse blood plasma and urine. The analytes were recovered from the blood plasma and urine matrices by using a liquid-liquid extraction after adjusting the pH to 9. The recovered analytes were derivatized with N_α-(2,4-dinitro-5-fluorophenyl)-L-valinamide, a chiral derivatizing agent analogous to Marfey's reagent. The resulting diastereoisomers were baseline resolved under a reversed-phase liquid chromatographic condition. Derivatization of the analytes not only allowed the separation of the enantiomers using cost-effective traditional liquid chromatography conditions and reversed-phase columns but also increased the sensitivity, at least to an order of magnitude, when tandem mass spectrometry is used for the detection. A limit of detection of 0.05 ng/mL was achieved for cathinone enantiomers for both matrices. Acceptable intraday and interday precision and accuracy along with satisfactory dilution accuracy and precision were observed during the method validation. The method suitability was tested using the post administration urine samples collected after single doses of cathinone and ephedrine as single-enantiomeric form and methcathinone as racemic form. Finally, a proof of concept of the isomeric ratio in urine samples to distinguish the presence of cathinone as a result of accidental ingestion of plant-based product from that of an illicit use of a designer product is demonstrated. To the best of our knowledge, this is the first such work where cathinone enantiomers were resolved and quantified in horse blood plasma and urine at sub nanogram levels.

Equilibrium and Kinetic Study of l- and d-Valine Adsorption in Supramolecular-Templated Chiral Mesoporous Materials

Adsorption kinetic studies are conducted to investigate the potential to use chiral mesoporous materials nanoporous guanosine monophosphate material-1 (NGM-1) and nanoporous folic acid material-1 (NFM-1) for the enantiomeric separation of l- and d-valine. A pseudo-second-order (PSO) kinetic model is applied to test the experimental adsorption equilibrium isotherms, according to both the Langmuir and Freundlich models and the characteristic parameters for each model are determined. The calcined versions of both NGM-1 and NFM-1 fit the Langmuir model with maximum sorption capacities of 0.36 and 0.26 g/g for the preferred adsorption enantiomers, d-valine and l-valine, respectively. Experimental results and the analysis of adsorption models suggest a strong adsorbate–adsorbent interaction, and the formation of a monolayer of tightly packed amino acid on the internal mesopore surface for the preferred enantiomers.

Structural Elucidation of the Mechanism of Molecular Recognition in Chiral Crystalline Sponges

To gain insight into chiral recognition in porous materials we have prepared a family of fourth generation chiral metal-organic frameworks (MOFs) that have rigid frameworks and adaptable (flexible) pores. The previously reported parent material, [Co2 (S-mandelate)2 (4,4'-bipyridine)3 ](NO3 )2 , CMOM-1S, is a modular MOF; five new variants in which counterions (BF4- , CMOM-2S) or mandelate ligands are substituted (2-Cl, CMOM-11R; 3-Cl, CMOM-21R; 4-Cl, CMOM-31R; 4-CH3 , CMOM-41R) and the existing CF3 SO3- variant CMOM-3S are studied herein. Fine-tuning of pore size, shape, and chemistry afforded a series of distinct host-guest binding sites with variable chiral separation properties with respect to three structural isomers of phenylpropanol. Structural analysis of the resulting crystalline sponge phases revealed that host-guest interactions, guest-guest interactions, and pore adaptability collectively determine chiral discrimination.

The Application of Covalent Organic Frameworks for Chiral Chemistry

Covalent organic frameworks (COFs) made their debut in 2005 and caused enthusiastic attention because of their ordered, crystalline structure. They are constructed with pure organic building blocks that are linked together by robust covalent linkages. COFs are applied in numerous fields due to their large surface area, architecture and chemistry stabilities, functional pore walls, and tunable frameworks. Incorporating COFs with chiral compounds can build chiral COFs (CCOFs), which have exhibited significant advantages in the chiral chemistry field. This review focuses on the applications of COFs for chiral catalysis, chiral separation, and chiral sensoring up to now. Furthermore, the synthesis and design strategies of CCOFs are also discussed in this article, since the COFs used in chiral chemistry are generally CCOFs. There also sums up the benefits and defects of COFs used in the chiral field and outlines future opportunities. The studies described in this review demonstrate not only the advantages of COFs in practical use but also novel solutions for the problems in the chirality area.

Tailoring the gradient and scattering forces for longitudinal sorting of generic-size chiral particles

Based on the concepts of conservative and non-conservative optical forces (COF and NCOF), we analyze the physical mechanism of longitudinal chirality sorting along the direction of light propagation in some simple optical fields. It is demonstrated, both numerically and analytically for particle of arbitrary size, that the sorting relies solely on the NCOF, which switches its direction when particle chirality is reversed. For particles larger than half of the optical wavelength λ, the NCOF far surpasses its counterpart COF, enabling the longitudinal chirality sorting. When the particle is much smaller than λ, however, the COF outweighs the NCOF, destroying the sorting mechanism. A scenario is thus proposed that totally eliminates the COF while leaving the sorting NCOF unchanged, extending the applicability of longitudinal chirality sorting to small particles.

Covalent Organic Frameworks in Separation

In the wake of sustainable development, materials research is going through a green revolution that is putting energy-efficient and environmentally friendly materials and methods in the limelight. In this quest for greener alternatives, covalent organic frameworks (COFs) have emerged as a new generation of designable crystalline porous polymers for a wide array of clean-energy and environmental applications. In this contribution, we categorically review the merits and shortcomings of COF bulk powders, nanosheets, freestanding thin films/membranes, and membranes on porous supports in various separation processes, including separation of gases, pervaporation, organic solvent nanofiltration, water purification, radionuclide sequestration, and chiral separations, with particular reference to COF material pore size, host–guest interactions, stability, selectivity, and permeability. This review covers the fabrication strategies of nanosheets, films, and membranes, as well as performance parameters, and provides an overview of the separation landscape with COFs in relation to other porous polymers, while seeking to interpret the future research opportunities in this field.

Thin Films of Homochiral Metal–Organic Frameworks for Chiroptical Spectroscopy and Enantiomer Separation

Chiral nanoporous solids are a fascinating class of materials, allowing efficient enantiomer separation. Here, we review the status, applications, and potential of thin films of homochiral metal–organic frameworks (MOFs). Combining the advantages of MOFs, whose well-defined, crystalline structures can be rationally tuned, with the benefits of thin films enables new opportunities for the characterization of the enantioselectivity, e.g., via chiroptical spectroscopy and straightforward molecular uptake quantifications. By incorporating photoresponsive molecules in the chiral MOF films, the enantioselectivity of the material can be dynamically remote-controlled. The most promising application of MOF films is their use as membranes, where the enantioselective separation of chiral molecules is demonstrated and parameters for further improvements are discussed.

Preparation and evaluation of biselector bonded-type multifunctional chiral stationary phase based on dialdehyde cellulose and 6-monodeoxy-6-monoamino-β-cyclodextrine derivatives

A novel biselector bonded-type multifunctional chiral stationary phase (MCSP) was prepared by covalently crosslinking dialdehyde cellulose (DAC) with 6-monodeoxy-6-monoamino-β-cyclodextrine (CD) via Schiff base reaction. The biselector bonded-type MCSP had good chiral and achiral chromatographic performance in normal phase (NP) and reversed phase (RP) modes. Seven and eight enantiomers were successfully separated on the prepared biselector bonded-type MCSP in NP and RP modes, respectively. The biselector bonded-type MCSP showed enhanced chiral resolution ability compared with single selector chiral stationary phases due to the simultaneous introduction of DAC and 6-monodeoxy-6-monoamino-β-CD on the surface of silica gel. Aromatic compounds including polycyclic aromatic hydrocarbons, anilines, phenols, phenylates, and aromatic acids were choosed as analytes to investigate the achiral chromatographic performance of the biselector bonded-type MCSP in NP and RP modes. Chromatographic evaluation results showed that the above aromatic compounds were essentially capable of achieving baseline separation by hydrophobic interaction, π-π interaction, and π-π electron-donor-acceptor interaction. Moreover, the host-guest inclusion effect of 6-monodeoxy-6-monoamino-β-CD and the multiple interactions made the biselector bonded-type MCSP have good steric selectivity. The preparation method of the biselector bonded-type MCSP was simple and provided a new idea and strategy for the preparation of the subsequent novel biselector MCSP.

Evolution of multi-functional capillary electrophoresis for high-efficiency selection of aptamers

Aptamers have drawn considerable attention as newly emerging molecular recognition elements in clinical diagnostics, drug delivery, therapeutics, environmental monitoring, and food safety analyses. As the in vitro screening antibody analogs, aptamers are enabled to recognize various types of targets with high affinity and specificity like or even superior to antibodies. However, the restrictions and inefficiency of selection have been hampering their wider application. Among various modified systematic evolution of ligands by exponential enrichment (SELEX) methods, capillary electrophoresis (CE)-SELEX holds multiple functions and advantages with the powerful qualitative and quantitative analysis capabilities, less consumption of sample and analytical reagent, natural binding environment, higher screening efficiency, and availability in multiple modes. This review summarizes the key developments in the area of CE-SELEX by leading research groups, including our teams' ten years of research and experience to help researchers fully understand and utilize CE-SELEX. Aptamers' history, applications, as well as the SELEX developments, have been briefly described; the advantages of CE-SELEX are highlighted compared with the conventional SELEX methods. Further, we describe some essential CE-SELEX models and provide an overview of the CE-SELEX, including the targets and ssDNA library, every technical point in the selection process, and post-SELEX protocol. We expect this review will inspire more researchers to have insight into the screening problems from CE-SELEX viewpoint and will help to improve the selection efficiency and probability of success to meet the growing needs of aptamers' discovery in bioanalytical and medical fields.

Preparation and chromatographic performance of a multifunctional immobilized chiral stationary phase based on dialdehyde microcrystalline cellulose derivatives

A novel high-performance liquid chromatography (HPLC) multifunctional immobilized chiral stationary phase was prepared by bonding dialdehyde microcrystalline cellulose to aminosilica via Schiff base reaction and then derivatized with 3,5-dimethylphenyl isocyanate. The HPLC multifunctional immobilized chiral stationary phase could not only achieve chiral separation but also achieve achiral separation. Chiral separation evaluation showed that 1-(1-naphthyl)ethanol and mandelonitrile got separation in normal phase (NP) mode. Ranolazine, benzoin ethyl ether, metalaxyl, and diclofop were successfully separated in reversed phase (RP) mode. Aromatic compounds such as polycyclic aromatic hydrocarbons (PAHs), anilines, and aromatic acids were selected as analytes to investigate the achiral separation performance of the multifunctional immobilized chiral stationary phase in NP and RP modes. The achiral separation evaluation showed that six PAHs could get good separation within 10 minutes in NP mode. Four aromatic acids were well separated in RP mode. The retention mechanism of aromatic compounds on the stationary phase was discussed, founding that π-π interaction, π-π electron-donor-acceptor (EDA) interaction, and hydrogen bonding interaction played important roles during the achiral separation process. This multifunctional immobilized chiral stationary phase had the advantages of simple bonding steps, short reaction time, and no need for space arm.

Functional Cyclodextrin-Clicked Chiral Stationary Phases for Versatile Enantioseparations by HPLC

The urgent demand for pure biological and pharmaceutical enantiomers has brought together great efforts in developing chiral techniques. High-performance liquid chromatography employing chiral stationary phases (CSPs) has evolved as a powerful tool for both chiral analysis and manufacture of pure enantiomers. Herein, we describe a facile method to prepare a phenylcarbamate cyclodextrin (CD)-based CSPs via azide/alkyne click chemistry. The functionalities of CD rims are altered to mediate the enantioseparation performance in multimode high-performance liquid chromatography.

Enantiomeric Recognition and Separation by Chiral Nanoparticles

Chiral molecules are stereoselective with regard to specific biological functions. Enantiomers differ considerably in their physiological reactions with the human body. Safeguarding the quality and safety of drugs requires an efficient analytical platform by which to selectively probe chiral compounds to ensure the extraction of single enantiomers. Asymmetric synthesis is a mature approach to the production of single enantiomers; however, it is poorly suited to mass production and allows for only specific enantioselective reactions. Furthermore, it is too expensive and time-consuming for the evaluation of therapeutic drugs in the early stages of development. These limitations have prompted the development of surface-modified nanoparticles using amino acids, chiral organic ligands, or functional groups as chiral selectors applicable to a racemic mixture of chiral molecules. The fact that these combinations can be optimized in terms of sensitivity, specificity, and enantioselectivity makes them ideal for enantiomeric recognition and separation. In chiral resolution, molecules bond selectively to particle surfaces according to homochiral interactions, whereupon an enantiopure compound is extracted from the solution through a simple filtration process. In this review article, we discuss the fabrication of chiral nanoparticles and look at the ways their distinctive surface properties have been adopted in enantiomeric recognition and separation.

HPLC Enantioseparation on Cyclodextrin-Based Chiral Stationary Phases

As one of the commonly used chiral separation materials, cyclodextrin-based chiral stationary phases (CD-CSP) have been developed rapidly in the past 30 years. A large number of CD-CSPs have been designed and applied for enantioseparation in high-performance liquid chromatography (HPLC). The development of novel CD-CSPs focuses on two aspects: the immobilization chemistry and the functionalization of the CD skeleton. Although such studies are not regarded as the prime research topic in analytical chemistry, there are still many recent works pushing this research forward tardily. In this chapter, the fabrication procedure of a triazole-bridged duplex CD-CSP and its application to HPLC enantioseparations is described.

Enantioselective complexation of 1-phenylethanol with chiral compounds bearing urea moiety

A detailed study of diastereomeric complexes of chiral ureido-1,1'-binaphthalene derivatives with chiral 1-phenylethanol showed that a derivative bearing only one urea unit makes five times more stable complex with (S)-enantiomer than with (R)-enantiomer of the alcohol. This phenomenon could be used in chiral discrimination processes. The influence of individual parts of the structure on the complexation properties is shown. The probable structure of diastereomeric complexes based on experimental results and computational methods is proposed.

Organocatalytic synthesis of axially chiral atropisomers

Axially chiral atropisomers are privileged frameworks present in ligands, catalysts and natural products. Organocatalytic mediated reactions offer an interesting and unique alternative for their synthesis.