Enantiomeric Selectivity Research Articles

Enantioselective analysis and degradation of isofenphos-methyl in vegetables by liquid chromatography-tandem mass spectrometry.

The enantioselective degradation of isofenphos-methyl in cowpea, cucumber, and pepper under field conditions was investigated to elucidate the enantioselective environmental behaviors of this pesticide. The concentrations of the enantiomers were determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The degradation rates of isofenphos-methyl enantiomers were the fastest in cowpea, followed by cucumber and pepper, with half-lives ranging from 1.48 to 8.06days. The enantioselective degradation of isofenphos-methyl was characterized by calculating and comparing the values of enantiomer fraction (EF) and enantiomeric selectivity (ES). The degradation rates and enantioselectivities of isofenphos-methyl were different for the three vegetables. (R)-(-)-isofenphos-methyl was degraded faster than (S)-(+)-isofenphos-methyl in cowpea and cucumber, whereas (S)-(+)-isofenphos-methyl underwent preferential degradation in pepper. These results could serve as a reference for the study of enantioselective behavior of isofenphos-methyl in plants and further food safety evaluation, where the enantiomeric differences should be considered in the risk assessment.

Enantioselective effects of chiral amide herbicides napropamide, acetochlor and propisochlor: The more efficient R-enantiomer and its environmental friendly

Amide herbicides, which are used extensively worldwide, are often chiral. Enantiomeric selectivity comes from the different effects of the enantiomers on target and non-target organisms. In this study, the enantiomers of three amide herbicides were purified by the semi-preparative column and were used to investigate the enantioselective effects on target Echinochloa crusgalli (lowland rice weeds), and non-target Microcystis aeruginosa, and the yeast transformed with the human TRβ plasmid organisms. The results showed that (i) the R-enantiomers of the three amide herbicides exhibited the strongest activity toward weed inhibition and the lowest toxicity toward non-target organisms; (ii) napropamide was better suited for controlling root growth, while acetochlor and propisochlor were better for leaves control; (iii) herbicides at certain low concentrations (0.01 mg L−1 for acetochlor and propisochlor) could be utilized to promote plant growth. These findings encourage the use of R-amide herbicides instead of their racemates to increase the efficiency of weed control and reduce the risk to non-target organisms. On the other hand, the adverse effects are caused mostly by S-enantiomer, using R-enantiomer-enriched products may offer great environmental/ecological benefits.

Effect of Binding on Enantioselectivity of Epoxide Hydrolase.

Molecular dynamics simulations and free energy calculations have been used to investigate the effect of ligand binding on the enantioselectivity of an epoxide hydrolase (EH) from Aspergillus niger. Despite sharing a common mechanism, a wide range of alternative mechanisms have been proposed to explain the origin of enantiomeric selectivity in EHs. By comparing the interactions of ( R)- and ( S)-glycidyl phenyl ether (GPE) with both the wild type (WT, E = 3) and a mutant showing enhanced enantioselectivity to GPE (LW202, E = 193), we have examined whether enantioselectivity is due to differences in the binding pose, the affinity for the ( R)- or ( S)- enantiomers, or a kinetic effect. The two enantiomers were easily accommodated within the binding pockets of the WT enzyme and LW202. Free energy calculations suggested that neither enzyme had a preference for a given enantiomer. The two substrates sampled a wide variety of conformations in the simulations with the sterically hindered and unhindered carbon atoms of the GPE epoxide ring both coming in close proximity to the nucleophilic aspartic acid residue. This suggests that alternative pathways could lead to the formation of a ( S)- and ( R)-diol product. Together, the calculations suggest that the enantioselectivity is due to kinetic rather than thermodynamic effects and that the assumption that one substrate results in one product when interpreting the available experimental data and deriving E-values may be inappropriate in the case of EHs.

Preparation of macroporous silica and its application on polysaccharide derivative based chiral stationary phase

In order to prepare 100-nm macroporous silica, spherical silica (5 μm in particle size, 10 nm in pore diameter) was treated by hydrothermal or baking methods. During hydrothermal treatment, 22 g/L NaF was added, which efficiently promoted the enlargement of pore diameters. By this method, the average pore diameter of silica reached 100 nm after heating for 48 h at 160℃ in an autoclave, but showed a poor size distribution. In the baking method, pore diameter enlargement was controlled by modifying the baking temperature, time, and the amount of double salt LiCl-NaCl added. The addition of 1.125 g LiCl·H2O and 0.75 g NaCl per 10 g silica and baking at 500℃ for 3-5 h yielded silica with 100-nm pore diameters. This method was more efficient, easier, and better in the product pore diameter distribution than hydrothermal treatment. The obtained silica was very similar to commercial Fuji-1000 gel. The macroporous silicas obtained from the hydrothermal and baking treatments were both modified with aminopropyl silane and coated by cellulose tri(3,5-dimethylphenyl carbamate) to prepare chiral stationary phases (CSPs). The CSP based on the baking-treated silica exhibited much better selectivity and resolution for enantiomers.

A validated chiral LC method for the enantiomeric separation of AT13148 on polysaccharide-based chiral stationary phase

A sensitive and accurate LC method for the determination of AT13148 enantiomeric purity has been developed and validated. Baseline separation with a resolution higher than 1.8 was accomplished within 15 min using a Chiralpak AD-H column (250 × 4.6 mm; particle size 5 μm) and n-hexane: 2-propanol: diethylamine (85:15:0.1, v/v) as mobile phase at a flow rate of 1 mL min−1. Eluted analytes were monitored by UV absorption at 254 nm. The effects of mobile phase components, temperature and flow rate on enantiomeric selectivity and resolution of enantiomers were investigated. Calibration curves were plotted within the concentration range between 7 and 500 μg mL−1 (n = 11), and the recoveries between 98.24 and 100.99% were obtained, with relative standard deviation lower than 1.32%. LOD and LOQ for AT13148 were 2.46 and 7.38 μg mL−1 and for its enantiomer were 2.54 and 7.49 μg mL−1, respectively. It was demonstrated that the developed method was accurate, robust and sensitive for the determination of enantiomeric purity of AT13148, especially for the analysis of bulk samples.

Preparation of polysaccharide-based chiral stationary phases on SiO2@Ag core-shell particles by means of coating and intermolecular polycondensation and comparative liquid chromatography enantioseparations.

Material with a core-shell structure based on a porous silica core and Ag nanoparticles (AgNPs) as shell (SiO2@Ag) was prepared with the seed growth method. Cellulose derivatives as chiral selectors were coated on the surfaces of SiO2@Ag particles by means of coating and intermolecular polycondensation as chiral stationary phases (CSPs), respectively. Fifteen racemates were used to evaluate their chiral recognition ability by high-performance liquid chromatography (HPLC). The results implied that the CSPs based on SiO2@Ag particles obtained stronger interaction with the analytes leading to better enantioseparation due to the existence of AgNPs on the surfaces of silica gel, compared with a cellulose derivative-modified silica column. In addition, the new CSPs showed special selectivity for ketone enantiomers, and AgNPs played a synergistic effect with cellulose derivatives on enantioseparation. Graphical abstract Schematic of SiO2@Ag stationary phase.

Chiral Resolution and Enantioselectivity of Synthetic Cathinones: A Brief Review.

Chiral discrimination has become one of the most important fields in analytical and medicinal chemistry, and forensic toxicology. The enantiomers may have different binding to proteins that may lead to many pharmacological and toxicological differences between them, including kinetic (at the absorption, distribution, metabolism and excretion level) or dynamic (level of potency and efficacy or even differences in mechanism of action) variations. Cathinone derivatives are chiral compounds, sold via the internet for recreational use, and little is known about their enantiomeric selectivity. Consequently, it is of crucial importance for the development of resolution methods to obtain pure enantiomers to study their biological effects. In the last few years, techniques for chiral drug analysis, as chromatography, have been developed and some works related to the analytical enantiomeric resolution of synthetic cathinones were described. However, information about synthetic cathinones in the literature is scarce specially concerning single enantiomers. In this mini-review, analytical chiral resolution and biological differences between enantiomers of cathinone derivatives will be addressed.

Determination of Enantiomeric Purity of GSK962040 Based on Liquid Chromatography Using Chiral Stationary Phase Combined with a Pre-column Derivatization Procedure

A sensitive and accurate LC method was developed and further validated for the determination of enantiomeric purity of GSK962040. Before separation, a pre-column derivatization procedure was performed. Baseline separation with a resolution higher than 1.9 was accomplished within 15 min using a Chiralpak AD-H (250 × 4.6 mm; particle size 5 μm) column, with n-hexane: 2-propanol (85:15 v/v) as mobile phase at a flow rate of 1 mL min−1. The eluted analytes were subsequently detected with a UV detector at 260 nm. The effects of mobile phase components and temperature on enantiomeric selectivity as well as resolution of enantiomers were thoroughly investigated. The calibration curves were plotted within the concentration range between 4 and 200 μg mL−1 (n = 8), and recoveries between 98.15 and 101.48% were obtained, with relative standard deviation (RSD) lower than 1.42%. The LOD and LOQ for the Boc-GSK962040 were 1.23 and 4.15 μg mL−1 and for its enantiomer were 1.38 and 4.76 μg mL−1, respectively. The developed method was also evaluated and validated by analyzing bulk samples with different enantiomeric ratios of GSK962040. It was demonstrated that the method was accurate, robust and sensitive, and also had practical utilities for real analysis.

Determination of AR-42 enantiomeric purity by HPLC on chiral stationary phase

A sensitive and accurate liquid chromatographic method for the determination of AR-42 enantiomeric purity has been developed and validated. Baseline separation with a resolution higher than 1.9 was accomplished within 10 min using a CHIRALPAK AD column (250 mm × 4.6 mm; particle size 5 μm) and n-hexane/2-propanol/diethylamine (75:25:0.1 v/v/v) as mobile phase at a flow rate of 1 mL min−1. Eluted analytes were monitored by UV absorption at 260 nm. The effects of mobile phase components, temperature and flow rate on enantiomeric selectivity and resolution of enantiomers were investigated. Calibration curves were plotted within the concentration range between 0.001 and 0.5 mg mL−1 (n = 10), and the recoveries between 98.23 and 101.87% were obtained, with relative standard deviation lower than 1.31%. Limit of detection and limit of quantitation for AR-42 were 0.39 and 1.28 μg mL−1 and for its enantiomer were 0.36 and 1.19 μg mL−1, respectively. It was demonstrated that the developed method was accurate, robust and sensitive for the determination of enantiomeric purity of AR-42, especially for the analysis of bulk samples.

Amperometric sensing of L-phenylalanine using a gold electrode modified with a metal organic framework, a molecularly imprinted polymer, and β-cyclodextrin-functionalized gold nanoparticles

The authors describe the construction of a molecularly imprinted electrochemical sensor for enantiomeric recognition of L-phenylalanine (L-Phe). Firstly, thiolated β-cyclodextrin (β-CD) and L-cysteine were used to functionalize gold nanoparticles via gold-thiol chemistry and to act as a basis for the formation of a microporous metal-organic framework (MOF). A MOF imprinted polymer was deposited on the functionalized gold nanoparticles by electropolymerization in the presence of the template L-Phe and the functional monomer 4-aminothiophenol. The modified electrode was characterized by cyclic voltammetry and electrochemical impedance spectroscopy in the presence of hexacyanoferrate acting as the redox probe. The electrode, operated best at 0.2 V vs. Ag/AgCl, exhibits excellent selectivity for L-Phe, with an enantioselectivity coefficient of 2.12 over D-Phe. In addition to its good enantiomeric selectivity, the sensor has a 0.33 pM detection limit which is much lower than that reported for other electrodes. The sensor has been successfully applied to the analysis of urine spiked with L-Phe.

Functional and Structural Characterization of a (+)-Limonene Synthase from Citrus sinensis.

Terpenes make up the largest and most diverse class of natural compounds and have important commercial and medical applications. Limonene is a cyclic monoterpene (C10) present in nature as two enantiomers, (+) and (-), which are produced by different enzymes. The mechanism of production of the (-)-enantiomer has been studied in great detail, but to understand how enantiomeric selectivity is achieved in this class of enzymes, it is important to develop a thorough biochemical description of enzymes that generate (+)-limonene, as well. Here we report the first cloning and biochemical characterization of a (+)-limonene synthase from navel orange (Citrus sinensis). The enzyme obeys classical Michaelis-Menten kinetics and produces exclusively the (+)-enantiomer. We have determined the crystal structure of the apoprotein in an "open" conformation at 2.3 Å resolution. Comparison with the structure of (-)-limonene synthase (Mentha spicata), which is representative of a fully closed conformation (Protein Data Bank entry 2ONG ), reveals that the short H-α1 helix moves nearly 5 Å inward upon substrate binding, and a conserved Tyr flips to point its hydroxyl group into the active site.

Distribution, isomerization and enantiomer selectivity of hexabromocyclododecane (HBCD) diastereoisomers in different tissue and subcellular fractions of earthworms

In this study, earthworms Eisenia fetida (E. fetida) were exposed to a soil artificially contaminated with individual hexabromocyclododecane (HBCD) diastereoisomers (α-, β- and γ-HBCDs) to investigate the distribution, isomerization and enantiomer selectivity of HBCDs at tissue and subcellular levels. At the tissue level, the concentrations of HBCDs all followed the order of gut>bodyfluid>body wall, which suggested that earthworms accumulated HBCDs mainly via ingesting soil particles. At the subcellular level, the concentrations of HBCDs in an extracellular fraction consisting of granules, tissue fragment, cell membrane and intact cells (fraction A) were higher than those in an intracellular fractions consisting of the microsomal and cytosol (fraction B+C). This confirmed the passive diffusion during the distribution of HBCDs into the intracellular compartment. The distribution proportions of HBCDs varied among different tissue and subcellular fractions, and all changed over time within 14 days. The variable distributions of HBCDs in different fractions were a result of the comprehensive effects of dynamics and thermodynamics processes. The β- and γ-HBCDs were isomerized to α-HBCD in all tissue and subcellular fractions except for fraction C, and the isomerization ratios varied a lot, which seemed to be related to HBCDs residence time. The selective enrichment of (−) α-, (−) β and (−) γ-HBCDs was found in all fractions and this is consistent with that in the whole earthworm. Besides, the extents of enantio-selectivity did not change significantly among different tissue and subcellular fractions.

DNA Duplex Engineering for Enantioselective Fluorescent Sensor.

The rapid identification of biomacromolecule structure that has a specific association with chiral enantiomers especially from natural sources will be helpful in developing enantioselective sensor and in speeding up drug exploitation. Herein, owing to its existence also in living cells, apurinic/apyrimidinic site (AP site) was first engineered into ds-DNA duplex to explore its competence in enantiomer selectivity. An AP site-specific fluorophore was utilized as an enantioselective discrimination probe to develop a straightforward chiral sensor using natural tetrahydropalmatine (L- and D-THP) as enantiomer representatives. We found that only L-THP can efficiently replace the prebound fluorophore to cause a significant fluorescence increase due to its specific binding with the AP site (two orders magnitude higher in affinity than binding with D-THP). The AP site binding specificity of L-THP over D-THP was assessed via intrinsic fluorescence, isothermal titration calorimetry, and DNA stability. The enantioselective performance can be easily tuned by the sequences near the AP site and the number of AP sites. A single AP site provides a perfect binding pocket to differentiate the chiral atom-induced structure discrepancy. We expect that our work will inspire interest in engineering local structures into a ds-DNA duplex for developing novel enantioselective sensors.

Sensing tyrosine enantiomers by using chiral CdSe/CdS quantum dots capped with N-acetyl-l-cysteine

Despite of the importance of enantiomers, the fluorescence sensing of enantiomers and the interpretation of the “preferential interaction” still remain insufficiently explored. In this study, we report the recognition of tyrosine (Tyr) enantiomers by chiral N-acetyl-L-cysteine (L-NAC) capped CdSe/CdS quantum dots (QDs) under alkaline experimental condition. L-Tyr could greatly quench the fluorescence of CdSe/CdS QDs, while D-Tyr displayed no effect on the fluorescence. The one-step synthesized chiral L-NAC-CdSe/CdS QDs demonstrated high selectivity for Tyr enantiomers. In particular, the mechanism of chiral recognition has been studied by UV/vis absorption spectra and circular dichroism (CD) spectra. The changes of intensity and sign of CD spectra corroborated the attachment of L-Tyr to the surface of QDs, which may be valuable aids in obtaining a better understanding of the possible mechanism of enantioselective recognition.

Thermodynamic study of HPLC enantioseparations of some sulfur-containing amino acids on teicoplanin columns in ion-pairing reversed-phase mode

ABSTRACTHomocysteine is an essential amino acid produced in the metabolism of methionine. It can be regulated through two metabolic routes either to cysteine or back to methionine. The imbalance in the methionine or homocysteine metabolism is connected with many disorders. In this work, HPLC enantioseparations of cysteine, homocysteine, and methionine were performed. Enantiomers were separated on natural teicoplanin and teicoplanin aglycone chiral stationary phases when buffered mobile phase with the presence of an ion-interaction reagent was used. The column temperature effect on the enantiomeric retention, selectivity, and resolution was studied in the range of 5–45°C at 5°C increment. Thermal stability of homocysteine standard was investigated by X-ray powder-diffraction measurements at elevated temperature in the range of 25–100°C at 5°C increment in air. Moreover, the thermodynamic analysis was used to quantify contributions of entropic and enthalpic terms on separations of investigated amino acid enantiomers. The elution order of enantiomers was determined in all cases.

Prenucleation Self-Assembly and Chiral Discrimination Mechanisms during Solution Crystallisation of Racemic Diprophylline.

The crystallisation behaviour of (RS)-diprophylline (DPL) in two different solvents is investigated to assess the incidence of solvated pre-associations on nucleation, crystal growth and chiral discrimination. In the solvated state, Raman spectroscopy shows that dimeric associations similar to those depicted in the crystalline solid solution (ssRII) predominate in isopropanol (IPA), which may account for the systematic spontaneous nucleation of this crystal form from this solvent. By contrast, spontaneous nucleation in DMF yields the stable racemic compound RI, consistently with the distinct features of the Raman spectrum collected in this solvent. A crystal growth study of ssRII in IPA reveals that the crystal habitus is impacted by the solution enantiomeric excess; this is explained by increased competition between homo- and heterochiral pre-associations. This is supported by a molecular modelling study on the enantiomeric selectivity of the DPL crystal lattices. The combination of assessment methods on solution chemistry, nucleation and chiral discrimination provides methodological tools from which the occurrence of solid solutions can be rationalised.

Efficient kinetic resolution of 1,5-dihydroxy-1,2,3,4-tetrahydronaphthalene catalyzed by immobilized Burkholderia cepacia lipase in batch and continuous-flow system

Abstract Sol-gel immobilized lipase from Burkholderia cepacia (Amano PS) was utilized for the first time as biocatalyst for kinetic resolution of 1,5-dihydroxy-1,2,3,4-tetrahydronaphthalene, by transesterification in batch and continuous-flow mode. The ternary silane precursor mixture of vinyltrimethoxysilane, phenyltrimethoxysilane and tetramethoxysilane, in equimolar ratio, yielded the best immobilized enzyme in terms of catalytic activity and enantiomer selectivity. Vinyl acetate was the most efficient acyl donor and proved to be the best reaction medium as well, but needing a certain amount (about 43%) of tetrahydrofuran as co-solvent to increase the solubility of the substrate. The optimal conditions determined for the kinetic resolution in the batch system were utilized for the continuous-flow process in the packed-bed reactor, too. The immobilized lipase demonstrated excellent operational stability during a continuous 4-day experiment, preserving the productivity (rflow) value above 180 μmol h−1 g catalyst−1 and the enantiomer selectivity (E) above 200. The (R)-selectivity of the Burkholderia cepacia in relation with this substrate was evidenced by NMR analysis.

Improved racemate resolution of pentan-2-ol and trans-(Z)-cyclooct-5-ene-1,2-diol by lipase catalysis

Lipases are important catalysts in chiral synthesis due to their wide substrate recognition combined with a high stereoselectivity. We demonstrate here that the state, free or immobilized, of Candida antarctica lipase B (CaLB) affects enantioselectivity and also alters the temperature dependancy of the enzyme. This indicates that CaLB undergoes various conformations induced by its interaction with the different immobilization supports studied. Molecular imprinting experiments, using immobilized enzyme co-dried with mimic substrate molecules, enhanced the enantiomeric ratio two-fold or three-fold, depending on the immobilization support. The structure of the acyl donor has a pronounced effect on CaLB catalyzed resolution, due to the proximity of the acyl and alcohol moieties during catalysis. When the acylation of pentan-2-ol was examined, we found that the 3C methyl propanoate donor afforded the highest resolution. Trans-(Z)-cyclooct-5-en-1,2-diol was used as a model racemic substrate to study the ability of lipase to catalyze the resolution of difunctionalized compounds. There was a clear enhancement in the enantiomer selectivity of the biotransformation of the diol when vinyl butanoate is used as the acyl donor. The conversion and enantiomeric excess of (1R,2R)-monoacetates were enhanced, using immobilized CaLB, when the chain length of the donors increased from C2 to C4.

Enantioselective hydrolysis of amino acid esters by non-chiral copper complexes equipped with bis (β-cyclodextrin)s

Two non-chiral copper(II) complexes equipped with bis(β-cyclodextrin)s (bisCDs) were explored as hydrolase models for the enantioselective hydrolysis of two pairs of alkyl chain-possessing amino acid ester enantiomers. The two bisCD complexes are pyridine-linked with different CD cavity orientations, denoted as CuL1 (L1=2,6-bis(6-mono-amino-β-cyclodextrin-methyl)-pyridine) and CuL2 (L2=2,6-bis(3-mono-amino-β-cyclodextrin-methyl)-pyridine). Kinetic studies indicated that the “back-to-back” bisCD complex CuL1 showed higher catalytic efficiency and more pronounced enantioselectivity for all substrates than the “face-to-face” bisCD complex CuL2. Overall preference of l-isomers was observed for both complexes. In the presence of CuL1, the formation of catalyst-substrate Michaelis complexes during the hydrolysis was demonstrated by saturation kinetic study and Electrospray ionization mass spectrometry (ESI–MS) analysis. Enantiomer selectivity (vmaxL/vmaxD) value for N-Boc-N'-Boc-Lysine 4-nitrophenyl esters (Boc-Lys(Boc)-ONp), the longer alkyl-chain analogs, is twice of that for N-Boc-Alanine 4-nitrophenyl esters (Boc-Ala-ONp). The enantioselective hydrolysis of Boc-Lys(Boc)-ONp promoted by CuL1 was confirmed by chiral high-performance liquid chromatography (HPLC) analysis. The participation of CD cavities during enantioselective hydrolysis was investigated through inhibition assay. The enantioselectivity in hydrolyzing different amino acid esters promoted by CuL1 was compared. The mechanism involved in the cooperation of two adjacent CD cavities of bisCD was proposed.

In silico analysis of enantioselective binding of immunomodulatory imide drugs to cereblon.

BackgroundThalidomide and its analogs, lenalidomide and pomalidomide (referred to as immunomodulatory imide drugs or IMiDs) have been known to treat multiple myeloma and other hematologic malignancies as well as to cause teratogenicity. Recently the protein cereblon was identified as the primary target of IMiDs, and crystallographic studies of the cereblon–IMiDs complex showed strong enantioselective binding for the (S)-enantiomer of IMiDs.ResultsUsing the structures of cereblon and IMiDs [both (S)-enantiomers and (R)-enantiomers] we performed docking simulations in order to replicate this enantiomeric selectivity and to identify the region(s) contributing to this selectivity. We confirmed the enantioselective binding of IMiDs to cereblon with high accuracy, and propose that the hairpin connecting the β10–β11 region of cereblon (residues 351–355) contributes to this selectivity and to the increased affinity with IMiDs.ConclusionsOur docking results provide novel insights into the binding mode of IMiD-like molecules and contribute to a deeper understanding of cereblon-related biology.Electronic supplementary materialThe online version of this article (doi:10.1186/s40064-016-2761-9) contains supplementary material, which is available to authorized users.