Enantiomeric Discrimination Research Articles

Enantioselective in-vitro elimination kinetics of nerve agents in blood monitored by derivatization and LC-MS/MS analysis.

We present a simple method for chiral separation and analysis of organophosphorus nerve agents and apply it to monitor the enantioselective blood elimination kinetics of sarin in-vitro. The method is implemented in standard reverse phase LC-MS operating conditions, relieving the user of the dedicated operating conditions frequently demanded in chiral LC-MS analysis. The method consists of formation of diastereomers by a rapid derivatization with (R)-2-(1 aminoethyl) phenol, followed by LC-MS/MS analysis. Derivatization enantioselectivity was studied by comparing the reaction of optically pure sarin and racemic sarin, proving no substantial enantiomeric preference in the reaction and demonstrating the enantiomeric discrimination abilities of the technique. Enantioselective sarin elimination pathways were probed in-vitro by following the fast elimination kinetics of the two sarin enantiomers as well as its hydrolysis metabolite (isopropyl methyl-phosphonic acid, IMPA) in whole blood and plasma compared to water. Sarin enantiomers showed the known marked differences in elimination kinetics with rapid elimination of the (+) enantiomer and slower elimination of the (-) enantiomer in whole blood and plasma as well as dose-dependent kinetics (faster elimination at lower concentrations). We found that small amounts of acetonitrile in plasma prevent the rapid elimination of the (+) enantiomer, resulting in similar, slower elimination kinetics for both enantiomers.

Chiral discrimination of amino acids using phosphorene assisted graphene-enhanced Raman spectroscopy

Discrimination of enantiomers poses a scientific challenge as the chemical and physical properties of enantiomers are nearly identical. The chiral analysis is usually performed by separation techniques, including chromatography, electrophoresis, or optical instrumentation based on an interaction of the analyzed sample with a polarized beam of light. Here we present a novel method for a chiral screening based on a combination of the black phosphorus@Graphene nanocomposite and Raman spectroscopy. The nanocomposite allows to enhance the Raman signal with factors higher than 100 asymmetrically and provide altered signals for mixtures containing varying enantiomeric ratios of target compounds. Tryptophan, Phenylalanine, DOPA, Isoleucine, and Leucine were selected as model compounds; the method allows us to discriminate between mixtures with 10, 25, 50, 75, and 100% enantiomeric purity.

Isoleucine Residues Determine Chiral Discrimination of Odorant-Binding Protein.

Enzymes, receptors, and carrier proteins discriminate between enantiomers of natural and synthetic chemicals. Whereas the structural details of this phenomenon have been investigated in enzymes and receptors, much less is known for carrier proteins of hydrophobic ligands, particularly concerning the contribution of asymmetric centers in the side chains of amino acids to chirally selective binding. Working with a pig odorant-binding protein, we have found that the replacement of either one or both isoleucine residues in the binding pocket by leucines abolishes discrimination of menthol and carvone enantiomers. The results indicate that isoleucines are crucial for chiral discrimination of hydrophobic ligands, and that asymmetry in the side chain may be as important as the overall asymmetry of the protein. The results provide suggestions and guidelines for improving chiral selectivity of binding proteins and enzymes, with consequent applications in the production of enantiomerically pure drugs.

Ionic Current Rectification Triggered Photoelectrochemical Chiral Sensing Platform for Recognition of Amino Acid Enantiomers on Self-Standing Nanochannel Arrays.

Chirality is an intrinsic and essential property of nature, and the enantiomeric discrimination of chiral molecules can provide important information leading to a better understanding of chiral recognition in biological systems and furthering the development of useful molecular devices in biochemical and pharmaceutical studies. Therefore, the exploration of new detection techniques with high accuracy and reliability for high-throughput enantioselective detection is still highly desirable. Herein, a chiral enantiomers selective recognition platform based on self-standing titanium dioxide nanochannel arrays (TiO2 NCAs) is proposed to implement the ionic current rectification triggered photoelectrochemical (PEC) detection, which provides a novel sensing platform to discriminate chiral amino acid through synchronous output dual response signals of ionic current and photocurrent. The utilization of nanochannel arrays guaranteed the high-throughput detection, and the dual signal model avoided a "false positive" detection. In addition, based on the fabricated detection platform, various chiral substances can be facilely detected through integrating different chiral regulation units, which shed light on the construction of reliable chiral sensors for practical applications in a biological environment.

Enantiomeric Discrimination by Surface-Enhanced Raman Scattering-Chiral Anisotropy of Chiral Nanostructured Gold Films.

A surface-enhanced Raman scattering-chiral anisotropy (SERS-ChA) effect is reported that combines chiral discrimination and surface Raman scattering enhancement on chiral nanostructured Au films (CNAFs) equipped in the normal Raman scattering Spectrometer. The CNAFs provided remarkably higher enhancement factors of Raman scattering (EFs) for particular enantiomers, and the SERS intensity was proportional to the enantiomeric excesses (ee) values. Except for molecules with mesomeric species, all of the tested enantiomers exhibited high SERS-ChA asymmetry factors (g), ranging between 1.34 and 1.99 regardless of polarities, sizes, chromophores, concentrations and ee. The effect might be attributed to selective resonance coupling between the induced electric and magnetic dipoles associated with enantiomers and chiral plasmonic modes of CNAFs.

Enantiomeric Discrimination by Surface‐Enhanced Raman Scattering–Chiral Anisotropy of Chiral Nanostructured Gold Films

AbstractA surface‐enhanced Raman scattering‐chiral anisotropy (SERS‐ChA) effect is reported that combines chiral discrimination and surface Raman scattering enhancement on chiral nanostructured Au films (CNAFs) equipped in the normal Raman scattering Spectrometer. The CNAFs provided remarkably higher enhancement factors of Raman scattering (EFs) for particular enantiomers, and the SERS intensity was proportional to the enantiomeric excesses (ee) values. Except for molecules with mesomeric species, all of the tested enantiomers exhibited high SERS‐ChA asymmetry factors (g), ranging between 1.34 and 1.99 regardless of polarities, sizes, chromophores, concentrations and ee. The effect might be attributed to selective resonance coupling between the induced electric and magnetic dipoles associated with enantiomers and chiral plasmonic modes of CNAFs.

The Stereocontrolled Biosynthesis of Mirror-Symmetric 2,4-Diaminobutyric Acid Homopolymers Is Critically Governed by Adenylation Activations.

Among the four bioactive cationic homo-poly(amino acids) discovered in nature, two are mirror-image isomers of poly(2,4-diaminobutyric acid) (poly-Dab) whose biosynthesis has long been unexplained. Their structural analogy plausibly suggested that they could share a common biosynthetic pathway utilizing ε-poly(l-lysine) synthetase-like enzymology but with an unprecedented process for enantiomeric inversion of polymer building blocks. To investigate this possibility, we comparatively explored the biosynthesis of poly-l-Dab and its mirror-image isomer poly-d-Dab in Streptomyces celluloflavus USE31 and Streptoalloteichus hindustanus NBRC15115, respectively, through genome mining, genetic inactivation, and heterologous expression combined with biochemical assays. While they shared the same biosynthetic pathway, the poly-d-Dab biosynthetic gene cluster additionally harbored the racemase gene. The critical finding that poly-d-Dab synthetase, in contrast to the synthetase generating the l-isomer, selectively activated d-Dab through adenylation conclusively demonstrated that free diffusible d-Dab preactivationally generated by the racemase is directly activated to be incorporated into the polymer. Our study thus represents the first demonstration of the stereoselective biosynthesis of a nonribosomal peptide governed by adenylation activity for a d-amino acid other than alanine. In silico sequence comparison between poly-Dab synthetases allowed us to identify amino acid residues potentially responsible for the discrimination of Dab enantiomers. Our results will provide significant insight not only for the future discovery of novel bioactive cationic poly(amino acids) but also for the creation of designer nonribosomal peptides with d-configuration.

Cathinones - Routine NMR methodology for enantiomer discrimination and their absolute stereochemistry assignment, using R-BINOL

The demand for narcotics is still rising in societies worldwide and designer drugs, new psychoactive substances, (NPS) such as chiral cathinones, play an increasing role in a market. Since the psychoactive properties of their enantiomers may differ substantially in biological activity, there is an urgent need for simple unambiguous method of assessment of their enantiomer composition, which is not yet published. Here we propose a simple NMR methodology of chiral discrimination for several cathinones using R-BINOL as CSA (chiral solvating agent). The distinct differences are observed in 1H, 13C and NOESY spectra of solvation complexes of both enantiomers of mephedrone with R-BINOL. Due to these large differences in chemical shifts, if unequal population of the enantiomers are found, the limit of detection can be on the level of 13C satellite lines (0.55% of a major enantiomer) and their relative intensity can be validated by the integral comparison of a minor form with that of a satellite line of a major form. The methodology of the absolute stereochemistry of enantiomers is also proposed basing on observed differences in kinetics of solvation shifts and NOESY effects in both diastereromers. The computed structures of both solvation complexes are discussed. It helps validation of the assessment of the absolute configuration of both enantiomers.

Visible Enantiomer Discrimination via Diphenylalanine-Based Chiral Supramolecular Self-Assembly on Multiple Platforms.

The development of enantioselective recognition is of great significance in medical science and pharmaceutical industry, which associates with the molecular recognition phenomenon widely observed in biological systems. In particular, the facile and straight achievement of visual enantioselective recognition has been drawing increasing consideration, but it is still a challenge. Herein, a heterochiral diphenylalanine-based gelator (LFDF) is synthesized, presenting left-handed nanofibers during self-assembly in ethanol, which accomplishes the phenylalaninol enantiomer recognition on multiple platforms. When adding l- or d-phenylalaninol into LFDF supramolecular solution followed by ultrasonic treatment, precipitate and gel are formed, respectively. Meanwhile, LFDF supramolecular gel completely collapses in a minute after dropping l-phenylalaninol, while the gel almost remains when d-type is employed. Moreover, a fluorescent supramolecular xerogel (ThT-LFDF) is fabricated by combining the LFDF gelator with thioflavine T (ThT), which could detect l-phenylalaninol accompanying with fluorescence quenching while d-type with barely decreasing. And the ThT-LFDF xerogel system shows a good sensitivity (reaches to ppm) for the detection of l-phenylalaninol. It is found that the chirality of the assembled nanofibers, as well as amino and carboxyl of phenylalaninol, plays a critical role on the discrimination process. The multiple and visible enantioselective recognition of phenylalaninol through chiral supramolecular self-assemblies shows potential applications in the fields of medical science and pharmaceutical industry.

R-VAPOL-phosphoric acid based 1H and 13C-NMR for sensing of chiral amines and acids.

Enantiomers have significant importance in pharmaceuticals, biology and modern chemistry and therefore distinguishing and quantifying the enantiomeric forms is of utmost importance. Herein, we propose diphenyl-3,3′-biphenanthryl-4,4′-diyl phosphate (R-VAPOL-PA) as a promising chiral solvating agent to discriminate amines and acids of poly-functional groups such as chiral amines, amino alcohols and hydroxy acids. The methodological approach involves using the nature of hydrogen bonds and ion pairs as a mode of weak interactions to form diastereomers where the probe is associated with enantiomers. The resulting diastereomer difference in the NMR spectrum enables the chiral discrimination with a complete baseline peak separation and an accurate enantiomeric excess (ee) analysis. We also carried out density functional theory (DFT) calculations to understand the complex formation to explain enantiodiscrimination by analysing the formation and stability of different chiral complexes. The binding energy differences between enantiomeric forms revealed by DFT calculations are qualitatively in agreement with the diastereomer difference in the NMR spectrum and unequivocally establishes the suggested experimental protocol of R-VAPOL-PA-based enantiomeric discrimination.

A single-component yet multifunctional tongue-mimicking sensor array for upconversion fluorescence biosensing.

In this work, we prepared a type of multiplexing upconversion nanoparticle (UCNP). There are three fluorescence emission peaks when our UCNPs are excited with 980 and 808 nm lasers. These fluorescence peaks of UCNPs can be quenched ("turn off") to varying degrees via fluorescence resonance energy transfer (FRET) when the UCNPs are coated with a polydopamine (PDA) layer, which is a universal quencher self-polymerized from dopamine (DA). Here, we create a novel single-component nanoprobe that can be used for the pattern recognition of antioxidants in a "turn on" manner by integrating with the prevention of PDA formation with an antioxidant. Our sensing strategy is based on the recovery of the fluorescence intensity of three emission peaks to different degrees due to different antioxidants with differential inhibition of PDA formation. Then, these three fluorescence emission peaks of UCNPs are innovatively selected as the sensor array, which enables us to discriminate multiple antioxidants and their mixtures. Simultaneously, the sensor array shows excellent performance in the chiral discrimination of cysteine enantiomers. This is a novel, innovative sensor array that requires only a single component to achieve the upconversion fluorescence pattern and recognize chiral molecules, and it elucidates a more innovative concept towards widespread applications.

PEGylated NALC-functionalized gold nanoparticles for colorimetric discrimination of chiral tyrosine.

In this work, acid and matrix-tolerant multifunctionalized gold nanoparticles (AuNPs) with an integrated chiral selector towards tyrosine (Tyr) and polyethylenglycol (PEG) chains were developed for visual chiral discrimination of Tyr in biological samples under acid conditions. In brief, AuNPs multifunctionalized with N-acetyl-l-cysteine (NALC) and PEG (PEG/NALC-AuNPs) were prepared via a simple strategy. In the presence of l-Tyr, the color of PEG/NALC-AuNP solution changed from red to gray, while no obvious color change was observed with the introduction of d-Tyr, which indicated that the introduction of PEG onto the surface of AuNPs has no effect on the chiral recognition between l-Tyr and NALC. A computer-aided molecular model was used to clarify the chiral recognition mechanism between NALC and Tyr enantiomers and to further guide the optimization of sensitivity. The resultant PEG/NALC-AuNP sensor presented a significantly improved stability under acid and alkali conditions compared with conventional NALC-AuNPs, resulting in a wider dynamic range (500 nM-100 μM) and a 50 times reduced detection limit by simply adjusting the pH of the sensor system under acid conditions (pH 2-2.5). More importantly, the PEG/NALC-AuNPs can realize the visual chiral discrimination of Tyr enantiomers in biological samples due to their significantly improved long-term stability and reduced interaction towards non-target species.

Colorimetric chiral discrimination of lysine enantiomers and configurable logic gate operation based on fluorescein-functionalized polydiacetylene vesicles

This study describes a sensitive sensor for based on fluorescein-functionalized diacetylene (F-DA) chromatic conjugated vesicle which allows colorimetric visualization for the chiral recognition of lysine by the naked-eye.

Chiral discrimination of amlodipine from pharmaceutical products using capillary electrophoresis

The present work describes the development of a capillary electrophoresis (CE) method for the chiral discrimination of amlodipine (AML) enantiomers using cyclodextrine (CD) derivatives as chiral selectors. A large number of native and derivatized, neutral and ionized CD derivatives were screened to find the optimal chiral selector; and carboximethyl-β-CD (CM-β-CD) was selected for the enantiomeric discrimination. A factorial analysis study was performed by orthogonal experimental design in which several factors were varied at the same time to optimize the separation method. The optimized method (25 mM phosphate buffer, pH = 9.0, 15 mM CM-β-CD, 15 °C, + 25 kV, 30 mbar/1 second, detection wavelength 230 nm) was successfully applied for the baseline separation of AML enantiomers within 5 minutes. Successful validation and application of the proposed CE method suggest its routine use in enantioselective control of AML in pharmaceutical preparations.

Enantiomeric Recognition of α-Aminoacids by a Uranyl Salen-Bis-Porphyrin Complex.

A novel uranyl salen-bis-porphyrin complex, in which two porphyrin subunits and salen moiety were directly linked, was synthesized for the recognition of tetrabutylammonium (TBA) amino acids. This uranyl salen complex, due to the presence of porphyrins with their fluorescence properties, represents the first example of a luminescence of uranyl salen complexes. UV/Vis measurements indicate the formation of 1:1 host–guest complexes, whereas UV-vis and fluorescence studies revealed that this complex acts as a receptor for the enantiomeric recognition of α-aminoacids derivatives, with high association constants and an excellent enantiomeric discrimination between the two enantiomers of phenylalanine–TBA.

Robust and highly efficient discrimination of chiral molecules through three-mode parallel paths

We propose to discriminate chiral molecules by combining one- and two-photon processes in a closed-loop configuration. The one-photon-coupling intrinsic {\pi}-phase difference between two enantiomers leads to their different superposition states, which is then followed by a two-photon process through three-mode parallel paths (3MPPs), enabling the discrimination of enantiomers by inducing their entirely-different population distributions. The 3MPPs are constructed by "chosen paths", a method of shortcuts to adiabaticity (STA), exhibiting a fast two-photon process. As an example, we propose to perform the scheme in 1, 2-propanediol molecules, which shows relatively robust and highly-efficient results under considering the experimental issues concerning unwanted transitions, imperfect initial state, pulse shaping, control errors and the effect of energy relaxations. The present work may provide help for laboratory researchers in a robust separation of chiral molecules.

Synthesis of Isotopically Labelled Disparlure Enantiomers and Application to the Study of Enantiomer Discrimination in Gypsy Moth Pheromone‐Binding Proteins

To study the binding mechanism of disparlure (7,8)‐epoxy‐2‐methyloctadecane enantiomers with pheromone‐binding proteins (PBPs) of the gypsy moth, oxygen‐17 or 18 and 5,5,6,6‐deuterium labelled disparlure enantiomers were prepared in an efficient, enantioselective route. Key steps involve the asymmetric α‐chlorination of dodecanal by SOMO catalysis and Mitsunobu inversion of a 1,2‐chlorohydrin. The pheromone, (+)‐disparlure (7R, 8S), was tested in two infested zones, demonstrating that it is very attractive towards male gypsy moths. Studies of the binding of (+)‐disparlure and its antipode to gypsy moth PBPs by 2H &17O NMR at 600 MHz are reported. Chemical shifts, spin‐lattice relaxation decay times (T1) and transverse relaxation decay times (T2) of deuterium atoms of disparlure enantiomers in 2H NMR show that the binding of disparlure enantiomers to PBP1 differs from binding to PBP2, as expected from their opposite binding preferences (PBP1 binds (–)‐disparlure, and PBP2 binds (+)‐disparlure more strongly). Models of the disparlure enantiomers bound to one internal binding site and two external binding sites of both PBPs were constructed. The observed chemical shift changes of deuterated ligand signals, from non‐bound to bound, T1 and T2 values are correlated with results from the simulations. Together these results suggest that the disparlure enantiomers adopt distinct conformations within the binding sites of the two PBPs and interact with residues that line the sites.

Synthesis, Fluorescence and NMR Spectroscopic Studies of a Novel Phosphinoxido-18-crown-6 Ether Containing an Anthracene Fluorophore Unit

The synthesis of the (R,R) and (S,S) enantiomers of a new enantiopure monophospha-18-crown-6 ether (1), which contains an anthracene fluorophore unit and methyl groups at its stereogenic centers, was accomplished. The structure of one enantiomer ((S,S)-1) was studied using one-dimensional (1H, 13C{1H}, and 31P{1H}) and two-dimensional NMR spectra. Because (R,R)-1 and (S,S)-1 can act as new fluorescent chemosensors, we examined their enantiomeric differentiation abilities toward the enantiomers of protonated chiral primary amines and amino acid esters (PEA, 1-NEA, PGME, PAME) using UV-Vis and fluorescence spectroscopies. These monophospha-crown ethers showed moderate enantiomeric discrimination abilities.

QCM based enantioselective discrimination of enantiomers by a pair of serine derived homochiral coordination polymers

Decoding enantioselective molecular interactions between sensors and guests into readable signal represents a great challenge in developing selective sensing technology. In this work, a pair of serine derivatives based homochiral coordination polymer (HCP) enantiomers, (L)-SA-Cd and (D)-SA-Cd, were synthesized and explored as enantioselective sensors towards guest enantiomers. Quartz crystal microbalance (QCM) technology was employed to indicate the gravimetric change of (L)- and (D)-SA-Cd towards variable chiral guests, and an enantioselective factor of 1.72 ± 0.15, 1.81 ± 0.08, 1.37 ± 0.03 and 2.89 ± 0.09 were achieved for lactic acid, menthol, valinol and 1-phenylethylamine (PEA), respectively. PEA was further selected to comprehensively study the enantioselectivity via electrochemical tests, HPLC analysis and theoretical calculations. By comparison with state-of-art works, the enantioselective discrimination for PEA enantiomers is better than a vast majority of similar reports. (L)- and (D)-form of SA-Cd exhibited mirror behaviors towards guest enantiomers, and control experiments indicated the role of HCP construction in enhancing enantioselectivity. H-bonding effect was found to be the binding force between SA-Cd and PEA, as verified by FT-IR and UV–Vis titration studies. Further DFT calculations revealed the existence of conformation oriented H-bonding between the chiral –OH groups of serine fragment and –NH2 group of PEA. The findings indicate that HCP construction represents an effective strategy for promoting enantioselectivity, and monitoring gravimetric change could be a promising general method in decoding most of the enantioselective recognition process.

An electrochemical chiral sensor based on electrochemically modified electrode for the enantioselective discrimination of D-/L-tryptophan

Chirality is a universal characteristic of natural systems and discrimination of enantiomers of a chiral molecule plays a major role particularly in chemical biology and in pharmacology. In this study, a novel electrochemical chiral sensor was developed for direct discrimination of D- and L-tryptophan (Trp) in an aqueous medium. The chiral sensor was produced by hierarchical modification of reduced graphene oxide, gold nanoparticles, poly-L-cysteine, and poly-L-phenylalanine methyl ester on the glassy carbon electrode. Each of the layers was produced by electrochemical techniques, such as electrochemical reduction and polymerization. After structural and morphological characterizations, the electrochemical behaviors of the enantiomeric pairs of Trp at the modified electrode were investigated by cyclic voltammetry and square wave voltammetry. A distinctive separation between the oxidation peak potentials of D- and L-Trp was observed at 0.73 and 0.83 V, respectively. In order to investigate the amperometric response towards D- and L-Trp, chronoamperometry technique was also used in the concentration range of 0.1–0.8 mM. Furthermore, the electrochemical performance of the modified electrodes was investigated in a mixed solution of D- and L-Trp. The results showed that the prepared electrode could be used as an electrochemical chiral sensor for Trp enantiomers.