Ligand chromatography as a novel method for the investigation of mixed complexes: stereoselective effects in -amino acid copper (II) complexes

Sustainable solutions for direct TLC enantioseparation with in-home thought-out, prepared/modified chiral stationary phases

TLC is used globally, yet less attention has been paid to TLC (in enantioseparation) despite its advantages. The present paper describes/reviews successfully practiced direct approaches of 'chiral additive in achiral stationary phase' (as an application of in-home thought out, prepared, tested, and modified chiral stationary phase), 'pre-mixing of chiral reagent with the enantiomeric mixture' (an approach using both achiral phases during chromatographic separation) and 'chiral additive in mobile phase', and chiral ligand exchange for enantioseparation of DL-amino acids, their derivatives, and some active pharmaceutical ingredients. It provided efficient enantioseparation, quantitative determination, and isolation of native forms via in-situ formation of non-covalent diastereomeric pair. The mechanism of enantioseparation in these approaches has been discussed along with the isolation and establishment of the structure of diastereomers. This may help chemists gain useful insights into fields outside their specialization and the experts get brief accounts of recent key developments, providing solutions for sustainable development of less expensive methods for control of enantiomeric purity and isolation of native enantiomers.

Functionalized Chiral Materials for Use in Chiral Sensors

Chirality represents a fundamental attribute within living systems and is a pervasive phenomenon in the natural world. The identification and analysis of chiral materials within natural environments and biological systems hold paramount importance in clinical, chemical, and biological sciences. Within chiral analysis, there is a burgeoning focus on developing chiral sensors exhibiting exceptional selectivity, sensitivity, and stability, marking it as a forefront area of research. In the past decade (2013-2023), approximately 1990 papers concerning the application of various chiral materials in chiral sensors have been published. Biological materials and nanomaterials have important applications in the development of chiral sensors, which accounting for 26.67% and 45.24% of the material-related applications in these sensors, respectively; moreover, the development of chiral nanomaterials is closely related to the development of portable and stable chiral sensors. Natural chiral materials, utilized as selective recognition units, are combined with carriers characterized by good physical and chemical properties through functionalization to form various functional chiral materials, which improve the recognition efficiency of chiral sensors. In this article, from the perspective of biological materials, polymer materials, nanomaterials, and other functional chiral materials, the applications of chiral sensors are summarized and the research prospects of chiral sensors are discussed.

Chiral secondary amino acids, their importance, and methods of analysis

Naturally occurring secondary amino acids, with proline as the main representative, contain an alpha-imino group in a cycle that is typically four-, five-, and six-membered. The unique ring structure exhibits exceptional properties-conformational rigidity, chemical stability, and specific roles in protein structure and folding. Many proline analogues have been used as valuable compounds for the study of metabolism of both prokaryotic and eukaryotic cells and for the synthesis of compounds with desired biological, pharmaceutical, or industrial properties. The D-forms of secondary amino acids play different roles in living organisms than the L-forms. They have different metabolic pathways, biological, physiological, and pharmacological effects, they can be indicators of changes and also serve as biomarkers of diseases. In the scientific literature, the number of articles examining D-amino acids in biological samples is increasing. The review summarises information on the occurrence and importance of D- and L-secondary amino acids-azetidic acid, proline, hydroxyprolines, pipecolic, nipecotic, hydroxypipecolic acids and related peptides containing these D-AAs, as well as the main analytical methods (mostly chromatographic) used for their enantiomeric determination in different matrices (biological samples, plants, food, water, and soil).

'Ab Ovo' Chiral Phases and Chiral Reagents for Liquid Chromatographic Separation and Isolation of Enantiomers

The de-novo approach of mixing chirally pure reagents or Cu(II)-L-amino acid complexes in the slurry of silica gel for preparing TLC plates was reported from author's laboratory and was successful for separation and isolation of enantiomers. Using high molar absorptivity molecules, e. g., 1,5-difluoro-2,4-dinitrobenzene and cyanuric chloride, more than 38 new chiral derivatizing reagents were synthesized in our laboratory by straightforward nucleophilic substitution with simple chiral auxiliaries. Besides, (S)-naproxen, (S)-ketoprofen, and (S)-levofloxacin were used as chiral platforms. A conceptual approach using both achiral phases in chromatography for enantioseparation was also adopted. ¹ H NMR and DFT based software were used to explain structures of non-covalent and covalent diastereomeric pairs and determination of configuration and separation mechanism. The methods can be easily used to determine and control enantiomeric purity with advantages over a variety of commercial chiral phases.

Simultaneous separation of atenolol enantiomers and its acid/alkaline degradation impurities on mixed-mode chiral ligand exchange stationary phases

Simultaneous separation of the enantiomer and impurities is a huge challenge for the quality control of the chiral drug. In this work, mixed-mode chiral ligand exchange stationary phases (CSPs) modified by octyl and sulfhydryl ligands were prepared by vapor deposition and click chemistry methods. Qualitative and quantitative determination of the prepared CSPs were achieved by Fourier transform infrared spectroscopy, solid-state ¹³ C CP/MAS NMR, and elemental analysis. The chiral resolution of CSPs was investigated through a comprehensively chromatographic evaluation of various racemates. Besides, the thermodynamic experiment was carried out to elucidate the contribution of hydrophobic ligand to the improvement of chiral recognition and selectivity. Atenolol and its degradation products were analyzed on the synthesized CSPs and compared with the commercial chiral column. A good separation of atenolol enantiomers from its acid and alkaline degradation impurities was simultaneously achieved on the C₈ /L-Hypro CSP. This new CSP is expected to have more applications in the quality control of other chiral drugs.

Liquid Chromatographic Enantioseparations Utilizing Chiral Stationary Phases Based on Crown Ethers and Cyclofructans

Natural compounds can exist in different forms, where molecules possessing chirality play an essential role in living organisms. Currently, one of the most important tasks of modern analytical chemistry is the enantioseparation of chiral compounds, in particular, the enantiomers of compounds having biological and/or pharmaceutical activity. Whether the task is to analyze environmental or food samples or to develop an assay for drug control, well-reproducible, highly sensitive, stereoselective, and robust methods are required. High-performance liquid chromatography best meets these conditions. Nevertheless, in many cases, gas chromatography, supercritical fluid chromatography, or capillary electrophoresis can also offer a suitable solution. Amino acids, proteins, cyclodextrins, derivatized polysaccharides, macrocyclic glycopeptides, and ion exchangers can serve as efficient selectors in liquid chromatography, and they are quite frequently applied and reviewed. Crown ethers and cyclofructans possessing similar structural characteristics and selectivity in the enantiodiscrimination of different amine compounds are discussed less frequently. This review collects information on enantioseparations achieved recently with the use of chiral stationary phases based on crown ethers or cyclofructans, focusing on liquid chromatographic applications.

Direct enantioseparation of mandelic acid by high-performance liquid chromatography using a phenyl column precoated with a small amount of cyclodextrin additive in a mobile phase

Direct enantioseparation of mandelic acid by high-performance liquid chromatography (HPLC) with a reversed phase column and a mobile phase containing a small amount of hydroxylpropyl-β-cyclodextrin (HP-β-CD) was studied as an efficient method for saving consumption of the CD additive. As a result, it was proposed that racemic mandelic acid can be analyzed with a phenyl column by using a mobile phase composed of 10 mM ammonium acetate buffer (pH 4.2) and 0.02% (w/v) HP-β-CD at a flow rate of 1.0 mL/min at 40°C after the passage of 10 mM ammonium acetate buffer (pH 4.2) containing 0.1% (w/v) HP-β-CD as a precoating mobile phase for 60 min. It is suggested that HP-β-CD is bound with a phenyl group on the surface of the stationary phase to allow a phenyl column to act as a transient chiral column, and injected mandelic acid can form the ternary complex with the adsorbed HP-β-CD. The longer retention time of D-mandelic acid than the L-isomer for HPLC can be explained from the higher stability of the HP-β-CD complex with D-mandelic acid, which was confirmed by CE experiment with HP-β-CD as a selector. The efficiency of a phenyl column compared with other stationary phases was also discussed.

Chiral Enantioselective Assemblies Induced from Achiral Porphyrin by l- and d-Lysine

π-Conjugated porphyrins have aroused particular attention for nanofabrication and biomimics; however, little attention has been paid to porphyrins-based chiral analysis owing to the achiral feature of porphyrins. Here, we demonstrated a chiral self-assembly of achiral porphyrin induced by l- and d-lysine (l- and d-Lys), and the resultant porphyrin self-assembly exhibited alterable morphologies depending on the inducer used (l- or d-Lys). The supramolecular chirality of the self-assembly was characterized by circular dichroism (CD) spectra, confirming successful transfer of molecular chirality from l- and d-Lys to the self-assembly. The enantioselective property of the chiral self-assembly was also investigated by using tryptophan (Trp) isomers as the model, and the results indicated that the developed chiral self-assembly showed significantly higher affinity toward l-Trp than d-Trp. Also in this work, the l-/d-Lys-induced chiral self-assembly of porphyrin and the supramolecular interaction between the self-assembly and l-/d-Trp were also studied by density functional theory (DFT).

Liquid chromatographic methods for separation, determination, and bioassay of enantiomers of etodolac: A review

The control of enantiomeric purity and determination of individual enantiomeric drug molecules remains the subject of importance for clinical, analytical, and regulatory purposes and to facilitate an accurate evaluation of the risks posed by them to human health. A large number of pharmaceuticals are marketed and administered as racemates. Etodolac is among such nonsteroidal anti-inflammatory drugs. Overall literature reports on its enantioseparation are scanty. Liquid chromatography (LC) methods of enantioseparation of (±)-etodolac, including certain unconventional ones, are well covered and discussed in this paper. Methods of direct approach without using chiral columns or chiral thin-layer chromatography plate and of indirect approach using certain chiral derivatizing agents such as (S)-naproxen and (S)-levofloxacin are described. Most interesting aspects include establishment of structure and molecular asymmetry of chemically different types of diastereomeric derivatives using liquid chromatography with mass spectrometry (LC-MS), ¹ H NMR spectroscopy and by drawing conformations in three dimensional views by using certain software. The methods provide chirality recognition even in the absence of pure enantiomers. Besides, recovery of pure enantiomers by detagging or via solubility difference of chiral inducing reagent and the analyte, without racemization at any stage, has been achieved. The limits of detection and quantification are much lower than the industry benchmarks.

Chiral ligand exchange capillary electrochromatography with dual ligands for enantioseparation of D,L-amino acids

Utilizing block copolymers as coatings, a protocol of chiral ligand exchange capillary electrochromatography (CLE-CEC) protocol was designed and developed with dual ligands for D,L-amino acids enantioseparation. Four block copolymers including poly maleic anhydride-co-styrene-co-N-methacryloyl-L-histidine methyl ester [P(MAn-St-MAH)], poly maleic anhydride-co-styrene-co-N-methacryloyl-L-lysine methyl ester [P(MAn-St-MAL)], poly maleic anhydride-co-styrene-co-N-methacryloyl-L-phenylalanine methyl ester [P(MAn-St-MAP)] and poly maleic anhydride-co-styrene-co-N-methacryloyl-L-threonine methyl ester [P(MAn-St-MAT)] were synthesized by reversible addition fragmentation chain transfer polymerization reaction. Key factors affecting the enantioresolution were optimized, including the concentration of Zn (II) central ion, pH value of buffer solution and monomers of the block copolymers. The enantioresolution of the proposed CLE-CEC system could be enhanced dramatically by employing P(MAn-St-MAH) as the immobilized chiral ligand and by coordinating the synergistic effect of free ligand in buffer solution. The principle of improved enantioresolution of the CLE-CEC system with dual ligands was discussed. Well enantioseparation was successfully realized with 7 pairs of D,L-amino acids enantiomers baseline separation and 5 pairs part separation. For quantitative analysis of D,L-alanine, a good linearity was established in the range of 9.4 μM to 1.5 mM (r² = 0.997) with the limits of detection (LODs) 3.7 μM of D-alanine, 2.0 μM for L-alanine, and limits of quantification (LOQs) 9.0 μM for D-alanine and 6.0 μM for L-alanine. The peak area and migration time reproducibility (n = 6) were 4.1% and 3.5% for D-alanine, 3.7% and 3.1% for L-alanine. Further, the enzyme kinetics study of alanine aminotransferase was investigated with the constructed CLE-CEC system.

Pretreatment of corn stover by solid acid for d-lactic acid fermentation

Solid acid is a new acid that is safe and green, which has been widely used in the fields of acid pickling. In this study, we adopted solid acid to pretreat corn stover and used the pretreated corn stover in the fermentation of d-lactic acid. Finally, we obtained optimal conditions for the pretreatment of corn stover by solid acid: digestion temperature of 120°C, digestion time of 80min, and solid acid concentration of 1.5%. Then adding cellulase of 30FPU/g, the conversion rate of glucose reached 71.06% after enzymatic hydrolysis for 72h. In addition, the changes of corn stover structure after pretreatment were further represented by using scanning electron microscope (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). At the same time, we used the pretreated corn stover as fermentation substrate and Lactobacillus. delbrueckii sp. bulgaricus as the starting strain to produce d-lactic acid. The yield reached 18g/L, with the optical purity being 99%e.e. This research has provided a new way to comprehensively utilizae corn stover.

Analytical Chemistry in Russia

Research in Russian analytical chemistry (AC) is carried out on a significant scale, and the analytical service solves practical tasks of geological survey, environmental protection, medicine, industry, agriculture, etc. The education system trains highly skilled professionals in AC. The development and especially manufacturing of analytical instruments should be improved; in spite of this, there are several good domestic instruments and other satisfy some requirements. Russian AC has rather good historical roots.

Resolution of Enantiomers of (RS)-Baclofen by Ligand-Exchange Thin-Layer Chromatography

A new chromatographic method has been developed for direct enantioresolution of (RS)-baclofen by ligand-exchange thin-layer chromatography (TLC) adopting two different approaches; (A) TLC plates were prepared by mixing the ligand exchange reagents (LER) with silica gel slurry and the chromatograms were developed with different achiral solvents or solvents having no chiral additive, and (B) the LER consisting of Cu(II)-L-amino acid complex was used as chiral mobile phase additive and the plain plates of silica gel having no chiral selector were used. Cu(II) acetate and four L-amino acids (namely, L-tryptophan, L-histidine, L-proline and L-phenylalanine) were used for the preparation of LERs. Spots were located by the use of iodine vapor. Effect of temperature and the mole ratio of Cu(II)-to-amino acid on enantioresolution were also studied. The results for the two methods have been compared, and the issue of involvement of the Cu(II) cation for the best performance of the two methods has been discussed with respect to the same mobile phase. L-Trp proved to be a good ligand using a common mobile phase in each case.

Gone in seconds: praxis, performance, and peculiarities of ultrafast chiral liquid chromatography with superficially porous particles

A variety of brush-type chiral stationary phases (CSPs) were developed using superficially porous particles (SPPs). Given their high efficiencies and relatively low back pressures, columns containing these particles were particularly advantageous for ultrafast "chiral" separations in the 4-40 s range. Further, they were used in all mobile phase modes and with high flow rates and pressures to separate over 60 pairs of enantiomers. When operating under these conditions, both instrumentation and column packing must be modified or optimized so as not to limit separation performance and quality. Further, frictional heating results in axial thermal gradients of up to 16 °C and radial temperature gradients up to 8 °C, which can produce interesting secondary effects in enantiomeric separations. It is shown that the kinetic behavior of various CSPs can differ from one another as much as they differ from the well-studied C18 reversed phase media. Three additional interesting aspects of this work are (a) the first kinetic evidence of two different chiral recognition mechanisms, (b) a demonstration of increased efficiencies at higher flow rates for specific separations, and

A simple and sensitive detection of glutamic-pyruvic transaminase activity based on fluorescence quenching of bovine serum albumin

It is well known that Cu(ii) can coordinate withl-alanine (Cu–Ala), which can be destroyed through the addition of glutamic-pyruvic transaminase (GPT) since GPT can effectively catalyze the conversion ofl-alanine into keto-acetic acid.

Chiral ligand exchange high-speed countercurrent chromatography: mechanism and application in enantioseparation of aromatic α-hydroxyl acids

This work concentrates on the separation mechanism and application of chiral ligand exchange high-speed countercurrent chromatography in enantioseparation of ten racemic aromatic α-hydroxyl acids, including mandelic acid, 2-chloromandelic acid, 4-methoxymandelic acid, 4-hydroxymandelic acid, α-methylmandelic acid, 4-hydroxy-3-methoxy-mandelic acid, 3-chloromandelic acid, 4-bromomandelic acid, α-cyclopentylmandelic acid and α-cyclohexylmandelic acid, in which five of the racemates were successfully enantioseparated by analytical apparatus with an optimized solvent system. The two-phase solvent system was composed of butanol-water (1:1, v/v) or hexane-n-butanol-water (0.5:0.5:1, v/v), to which N-n-dodecyl-l-proline was added in the organic phase as chiral ligand and cupric acetate was added in the aqueous phase as a transition metal ion. Various influence factors in high-speed countercurrent chromatography were optimized by enantioselective liquid-liquid extraction. The separation mechanism for chiral ligand exchange high-speed countercurrent chromatography was proposed based on the results of present studies. Successful enantioseparations of 72mg of mandelic acid, 76mg of 2-chloromandelic acid and 74mg of 4-methoxymandelic acid were achieved individually with high resolution by preparative high-speed countercurrent chromatography. The HPLC purity of all enantiomers was over 96% with the recovery in the range of 82-90% from the collected fractions.