Chiral Discrimination Mechanism Research Articles

Preparation of D-NCCDs and its application in fluorescent/colorimetric dual-mode discrimination of glutamine enantiomers.

Anew type of carbon dots (D-NCCDs) was synthesized by 3, 5-diaminobenzoic acid, N,N-dimethyl-o-phenylenediamine, and D-cysteine. The morphology and structure of D-NCCDs were investigated by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and FT-IR spectra, and the chirality was characterized by circular dichroism. In the presence of hydrogen peroxide, the fluorescence of D-NCCDs at 487nm (λex = 410nm) showed great discrimination ability towards glutamine enantiomers. The ratio of fluorescence intensity (F/F0) to the concentration of D-Gln showed good linearity in the range 0.5-10mM, with a detection limits of 0.11mM. Meanwhile, the color of the solution gradually changed from light yellow to yellow-brown. The UV-Vis absorption ratio (A/A0) at 410nm showed good linearity with the concentration of D-Gln in the range 0.5 to 20mM; the detection limit is 7.7μM. But the fluorescence and absorbance of D-NCCDs showed no significant change after the addition of L-glutamine. Thus, fluorescence and colorimetry dual-mode discrimination of glutamine enantiomers was achieved. The fluorescence enantioselectivity of Gln (FL-Gln/FD-Gln) is 1.62, and the colorimetric enantioselectivity of Gln (AD-Gln/AL-Gln) is 2.14. The chiral discrimination mechanism of D-NCCDs to Gln enantiomers was also investigated systematically. This work not only can discriminate glutamine enantiomers with high sensitivity and convenience, but also offers a new strategy for preparing new dual mode chiral nanoprobes.

Chiral Discrimination Mechanisms by Silylated-Acetylated Cyclodextrins: Superficial Interactions vs. Inclusion.

Cyclodextrin derivatives constitute a powerful class of auxiliary agents for the discrimination of apolar chiral substrates. Both host-guest inclusion phenomena and interactions with the derivatizing groups located on the surface of the macrocycle could drive the enantiodiscrimination; thus, it is important to understand the role that these processes play in the rational design of new chiral selectors. The purpose of this study is to compare via nuclear magnetic resonance (NMR) spectroscopy the efficiency of silylated-acetylated α-, β-, and γ-cyclodextrins in the chiral discrimination of 1,1,1,3,3-pentafluoro-2-(fluoromethoxy)-3-methoxypropane (compound B) and methyl 2-chloropropionate (MCP). NMR DOSY (Diffusion Ordered SpectroscopY) experiments were conducted for the determination of the bound molar fractions and the association constants, whereas ROESY (Rotating-frame Overhauser Enhancement SpectroscopY) measurements provided information on the hosts' conformation and on the interaction phenomena with the guests. Compound B, endowed with fluorinated moieties, is not deeply included due to attractive Si-F interactions occurring at the external surface of the cyclodextrins. Therefore, a low selectivity toward the size of cyclodextrin cavity is found. By contrast, enantiodiscrimination of MCP relies on the optimal fitting between the size of the guest and that of the cyclodextrin cavity.

Clay Column Chromatography for Optical Resolution: A Series of Derivatized Amino Acids

Abstract Chromatographic resolution of a series of derivatized amino acids was attempted on a column packed with an ion-exchange adduct of Δ-[Ru(phen)3]2+ (phen = 1,10-phenanthroline) and synthetic hectorite. An amino acid was modified to N-3, 5-dinitrobenzoyl amino acid methyl ester (denoted by DNB-aa-me). For aa = Ala, Phe, Leu, Ile, Ser, Val, Thr, Tyr, Asp and Glu, racemic DNB-aa-me was resolved nearly to baseline separation on being eluted with methanol. For aa = Trp and His, racemic DNB-aa-me was resolved partially. No resolution was achieved for Pro and Lys. The mechanism of chiral discrimination was investigated by means of solid-state vibrational circular dichroism spectroscopy.

A Dimeric Thiourea CSA for the Enantiodiscrimination of Amino Acid Derivatives by NMR Spectroscopy.

The reaction of benzoyl isothiocyanate with (1R,2R)-1,2-bis(2-hydroxyphenyl)ethylenediamine afforded a new thiourea chiral solvating agent (CSA) with a very high ability to differentiate 1H and 13C NMR signals of simple amino acid derivatives, even at low concentrations. The enantiodiscrimination efficiency was higher with respect to that of the parent monomer, a thiourea derivative of 2-((1R)-1-aminoethyl)phenol, thus putting into light the relevance of the cooperativity between the two molecular portions of the dimer in a cleft conformation stabilized by interchain hydrogen bond interactions. An achiral base additive (DABCO or DMAP) played an active role in the chiral discrimination processes, mediating the interaction between the CSA and the enantiomeric mixtures. The chiral discrimination mechanism was investigated by NMR spectroscopy through the determination of complexation stoichiometries, association constants, and the stereochemistry of the diastereomeric solvates.

Synthesis and Chiral Separation of Some 4-thioflavones.

A thionation reaction was performed on some chiral flavanones using Lawesson's reagent (LR) and leads to the formation of new chiral thiocarbonyl flavanes. LR in this thionation reaction with Hesperetin and Naringenin gives new flavan-4-thiones with yields ranged between 41 and 52%. Based on the Wittig reaction principle, LR is currently the most widely used reagent for this type of reaction. Enantiomeric separation by high-performance liquid chromatography methods was then set-up using three different polysaccharide-based chiral stationary phases (CSPs). Chiral separations were successfully accomplished with high resolution (1.22≤Rs≤5.23). The chiral discrimination mechanism(s) between the analytes under study, mobile phase, and the CSPs were discussed.

L-Histidinium Chiral Ionic Liquid Functionalized β-Cyclodextrin as Chiral Selector in Capillary Electrophoresis.

Nowadays, ionic liquids (ILs) functionalized cyclodextrins (CDs) have drawn increasing attention in chiral separation. Herein, a novel β-CD derivative functionalized by L-histidinium IL, mono-6-deoxy-6-L-histidinium-β-cyclodextrin chloride (L-HMCDCl), was synthesized for the first time and utilized for enantioseparation of nefopam and chlorphenamine in capillary electrophoresis. The L-HMCDCl exhibited superior enantioselectivity compared with native β-CD. The effect of some key parameters such as chiral selector concentration, buffer pH and applied voltage on the enantioseparation was investigated in detail. In the interest of the chiral discrimination mechanism and the enhanced enantioselectivity of L-HMCDCl, molecular modeling with AutoDock was employed to study the interaction, which was in good agreement with experimental results.

Enantiomeric resolution of quinolones on crown ether CSP: Thermodynamics, chiral discrimination mechanism and application in biological samples

The enantiomers of quinolone racemates were resolved using chiral crown ether within 8 min. Thermodynamics data and modeling results were used to determine chiral recognition mechanism. The column used was (+)-Crownpack column (250 mm × 4.6 mm, 5 µm) with three mobile phases I: ACN:Water (80:20) + 10 mM H2SO4 and 10 mM CH3COONH4, II: ACN:Water (80:20) + 20 mM perchloric acid and III: EtOH:Water (80:20) + 20 mM perchloric acid. The flow rate of the mobile phases was 1.0 mL/min with UV detection at different wavelengths. The ranges of retention (k), separation (α), and resolution (Rs) factors were 1.00–5.40, 1.37–2.00 and 1.50–3.30. The tailing factor was 1.o for all peaks with 900–2325 as the number of theoretical plates were 8.0–10.0 and 32.4–22.1 µg. The difference in enthalpy, entropy and free energy varied in the range of −0.350 to −0.024, 18.74 × 10−4 to 3.94 × 10−4 and −0.918 to −0.143, respectively. The thermodynamic and docking results showed chiral discrimination due to physical forces of amnio group cations penetration into the chiral cavity of the chiral selector following hydrogen bindings. The binding energy of S-enantiomers was higher than R-enantiomers; confirming stronger binding of S-enantiomers with CSP than R-enantiomers. The described chiral-HPLC method was used for the analysis of the quinolone enantiomers in urine samples and the results were quite satisfactory. Therefore, the reported method may be used for the enantiomeric separation of quinolone enantiomers in urine samples.

Thiourea Derivative of 2-[(1R)-1-Aminoethyl]phenol: A Flexible Pocket-like Chiral Solvating Agent (CSA) for the Enantiodifferentiation of Amino Acid Derivatives by NMR Spectroscopy.

Thiourea derivatives of 2-[(1R)-1-aminoethyl]phenol, (1S,2R)-1-amino-2,3-dihydro-1H-inden-2-ol, (1R,2R)-(1S,2R)-1-amino-2,3-dihydro-1H-inden-2-ol, and (R)-1-phenylethanamine have been compared as chiral solvating agents (CSAs) for the enantiodiscrimination of derivatized amino acids using nuclear magnetic resonance (NMR) spectroscopy. Thiourea derivative, prepared by reacting 2-[(1R)-1-aminoethyl]phenol with benzoyl isothiocyanate, constitutes an effective CSA for the enantiodiscrimination of N-3,5-dinitrobenzoyl (DNB) derivatives of amino acids with free or derivatized carboxyl functions. A base additive 1,4-diazabicyclo[2.2.2]octane(DABCO)/N,N-dimethylpyridin-4-amine (DMAP)/NBu4OH) is required both to solubilize amino acid derivatives with free carboxyl groups in CDCl3 and to mediate their interaction with the chiral auxiliary to attain efficient differentiation of the NMR signals of enantiomeric substrates. For ternary systems CSA/substrate/DABCO, the chiral discrimination mechanism has been ascertained through the NMR determination of complexation stoichiometry, association constants, and stereochemical features of the diastereomeric solvates.

Simultaneous enantioseparation of nonsteroidal anti-inflammatory drugs by a one-dimensional liquid chromatography technique using a dynamically coated chiral porous silicon pillar array column

The preparation of a highly efficient chiral liquid chromatography (LC) column is explored by dynamically coating a reversed-phase porous silicon pillar array column with hydroxypropyl-β-cyclodextrin (Hp-β-CD) as the chiral selector. Analyte mixtures composed of non-steroidal anti-inflammatory drugs were tested to reveal the enantioseparation potential of the column. The mechanism of chiral discrimination was investigated. The adsorbed Hp-β-CDs on the column surface experience different interaction with enantiomers. The chiral stationary phase showed satisfying stability and could be easily restored by recovering the selector with sufficient flushing and repeating the loading procedure. The peak capacity of the column was evaluated, and it was found high enough to separate three enantiomer couples using a one-dimensional LC technique.

Efficient 1H NMR chiral discrimination of sulfoxides caused by the dynamic nature of (R,R)-3′,3″-biBINOL

A chiral BINOL dimer, (R,R)-3′,3″-BiBINOL, which possesses both rigid (atropos) and dynamic (tropos) axial chiralities, was found to work as an effective NMR chiral solvating reagent for the determination of the enantiomeric purities of various chiral sulfoxides. The unique chiral discrimination mechanism was also revealed by using DFT calculations and X-ray crystallographic analysis.

Structural Aspects of Solid Solutions of Enantiomers.

A mixture of two enantiomers can crystallize according to three types of heterogeneous equilibria: a racemic compound (a 1:1 stoichiometric compound), a conglomerate (a physical mixture of particles with opposite chirality) or, more rarely, as a solid solution (a crystalline architecture exhibiting a lack of chiral discrimination with respect to the two enantiomers). Due to the scarce occurrence of solid solutions, only a few examples of such behavior are known, and even fewer systems have been investigated by means of single crystal X-ray diffraction. Yet, preliminary work performed in the 1970s by several research teams revealed that structural investigations of solid solutions could provide valuable insights into chiral discrimination mechanisms at the crystal lattice scale. In the present paper, our aim is to review published cases of enantiomeric solid solutions for which both melting phase diagrams and crystal structures are available in order to analyze the lack of chiral discrimination associated to these phases. Our methodology consists in considering both the molecular and crystallographic aspects of stereoselectivity with the final aim of identifying structural criteria responsible for the occurrence of solid solutions. The experimental conditions allowing access to solid solutions will also be considered in light of these structural criteria.

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.

Synthesis and characterization of D-/L-methionine grafted PEDOTs for selective recognition of 3,4-dihydroxyphenylalanine enantiomers

Two pairs of amino-acid functionalized poly(3,4-ethylenedioxythiophene) (PEDOT) derivatives, namely, poly(N-(tert-butoxycarbonyl)-L-methionyl (3,4-ethylenedioxythiophene-2’-yl)methylamide) (L-PEDOT-Boc-Met) and poly(N-(tert-butoxycarbonyl)-D-methionyl (3,4-ethylenedioxythiophene-2’-yl)methylamide) (D-PEDOT-Boc-Met); poly(L-methionyl (3,4-ethylenedioxythiophene-2’-yl)methylamide) (L-PEDOT-Met) and poly(D-methionyl (3,4-ethylenedioxythiophene-2’-yl)methylamide) (D-PEDOT-Met) were synthesized via chemical oxidative polymerization of corresponding monomers. The structural characterization, spectroscopic properties and thermal stability of these monomers and polymers were systematically explored by FTIR spectra, Raman spectra, XRD spectra, UV-Vis spectra and thermogravimetric analysis. As chiral electrode materials, these polymers were employed to successfully recognize 3,4-dihydroxyphenylalanine (DOPA) enantiomers by cyclic voltammetry (CV) in sulphuric acid solution. The measurement results reveal that the tendency was hetero-chiral interaction between L-PEDOT-Met/PVA/GCE and D-DOPA, D-PEDOT-Met/PVA/GCE and L-DOPA, respectively. Also, the mechanism of chiral discrimination was discussed. All the results implied that the combination of electrochemical molecular recognition technology and chiral PEDOT materials can be a promising approach for chiral recognition and may open new opportunities for facile, biocompatible, sensitive and robust chiral assays in biochemical applications.

Outwitting EF-Tu and the ribosome: translation with d-amino acids.

Key components of the translational apparatus, i.e. ribosomes, elongation factor EF-Tu and most aminoacyl-tRNA synthetases, are stereoselective and prevent incorporation of d-amino acids (d-aa) into polypeptides. The rare appearance of d-aa in natural polypeptides arises from post-translational modifications or non-ribosomal synthesis. We introduce an in vitro translation system that enables single incorporation of 17 out of 18 tested d-aa into a polypeptide; incorporation of two or three successive d-aa was also observed in several cases. The system consists of wild-type components and d-aa are introduced via artificially charged, unmodified tRNAGly that was selected according to the rules of ‘thermodynamic compensation’. The results reveal an unexpected plasticity of the ribosomal peptidyltransferase center and thus shed new light on the mechanism of chiral discrimination during translation. Furthermore, ribosomal incorporation of d-aa into polypeptides may greatly expand the armamentarium of in vitro translation towards the identification of peptides and proteins with new properties and functions.

Resolutions of sibutramine with enantiopure tartaric acid derivatives: chiral discrimination mechanism

The resolution of sibutramine 1 was investigated with enantiopure tartaric acid derivatives. Based on the resolving efficiency assay using a ‘Dutch resolution’, O,O′-di-p-anisoyl-(R,R)-tartaric acid (R,R)-DMTA was identified as an effective resolving agent, which is easily obtained from natural and inexpensive (R,R)-tartaric acid. Compound (R)-1 was obtained with high enantiomeric purity and yield. The chiral discrimination mechanism and resolving effect in the process were explained with X-ray crystallographic studies. The crystal structures of the conglomerate salts revealed that the more soluble diastereomer (S)-1·(R,R)-DMTA formed a parallel ribbon supramolecular structure while the less soluble diastereomer (R)-1·(R,R)-DMTA formed a spiral net packing structure by enantio-differentiation self-assembly. The effect of the side substituent of the resolving agent on the enantioseparation of 1 via a supramolecular chiral host consisting of tartaric acid analogues is discussed.

A comprehensive study of the enantioseparation of chiral drugs by cyclodextrin using capillary electrophoresis combined with theoretical approaches

Four chiral drugs were enantioseparated by native beta-cyclodextrin (β-CD) and negatively charged carboxymethyl-beta-cyclodextrin (CM-β-CD) using capillary electrophoresis coupled with electrochemiluminescence detection (CE-ECL). Using 50mM pH 5.5 Tris–H3PO4 with 10mM CM-β-CD as a running buffer, high resolution efficiency could be obtained. With the help of isothermal titration calorimetry (ITC), nuclear magnetic resonance (NMR) and molecular modeling, the chiral recognition mechanism was comprehensively investigated. Thermodynamic parameters data from ITC revealed that CM-β-CD exhibited stronger binding affinity with analytes than β-CD, and that the driving forces of CM-β-CD responsible for chiral recognition were mainly electrostatic interactions between negatively charged CM-β-CD and positively charged analytes. In addition, from both a macroscopic and microscopic point of view, the results of NMR and molecular modeling investigation adequately confirm the conclusion by comparing the stereochemical structures of complexes. Combination of ITC, NMR and molecular modeling techniques not only can assist CE to investigate the chiral discrimination mechanism, but also can predict and guide CE enantioseparation efficiency conversely.

Enantiomeric separation of biaryl atropisomers using cyclofructan based chiral stationary phases

Normal phase chiral HPLC methods are presented for the enantiomeric separation of 30 biaryl atropisomers including 18 new compounds recently produced via a novel synthetic approach. Three new cyclofructan based chiral stationary phases were evaluated. Separations were achieved for all but six analytes and the LARIHC™ CF6-P alone provided 15 baseline separations. Effects of polar modifiers and temperature effects also were studied. Apparent thermodynamic parameters were determined by van’t Hoff plots. Preparative scale methods were developed and employed resulting in the first ever isolation of these novel atropisomers in their pure enantiomeric form. Insights into the mechanism of retention and chiral discrimination are presented.

ChemInform Abstract: Chirality and Protein Biosynthesis

AbstractReview: 247 refs.

Non-superimposable mirror image crystals of enantiomers by spontaneous resolution and the chiral discrimination mechanism

Abstract Non-superimposable mirror image crystals of both enantiomers ( S / R ) of cyclic γ -alkenyl alcohol ( 2 ) have been recognized and remarkably identified by the naked eye. More interestingly, both crystals are an outcome of most astonishingly H-bond and intermolecular σ/π–π interactions. They accounted for the relatively rare and less predictable spontaneous resolution with optical purity >99% ee from the racemic mixture. The chiral discrimination mechanism of this spontaneous resolution has also been proposed.

Chiral Discrimination and Writhe-dependent Relaxation Mechanism of Human Topoisomerase IIα

Human topoisomerase IIα unlinks catenated chromosomes and preferentially relaxes positive supercoils. Supercoil chirality, twist density, and tension determine topoisomerase IIα relaxation rate and processivity. Strand passage rate is determined by the efficiency of transfer segment capture that is modulated by the topoisomerase C-terminal domains. Single-molecule measurements reveal the mechanism of chiral discrimination and tension dependence of supercoil relaxation by human topoisomerase IIα. Type IIA topoisomerases (Topo IIA) are essential enzymes that relax DNA supercoils and remove links joining replicated chromosomes. Human topoisomerase IIα (htopo IIα), one of two human isoforms, preferentially relaxes positive supercoils, a feature shared with Escherichia coli topoisomerase IV (Topo IV). The mechanistic basis of this chiral discrimination remains unresolved. To address this important issue, we measured the relaxation of individual supercoiled and "braided" DNA molecules by htopo IIα using a magnetic tweezers-based single-molecule assay. Our study confirmed the chiral discrimination activity of htopo IIα and revealed that the strand passage rate depends on DNA twist, tension on the DNA, and the C-terminal domain (CTD). Similar to Topo IV, chiral discrimination by htopo IIα results from chiral interactions of the CTDs with DNA writhe. In contrast to Topo IV, however, these interactions lead to chiral differences in relaxation rate rather than processivity. Increasing tension or twist disrupts the CTD-DNA interactions with a subsequent loss of chiral discrimination. Together, these results suggest that transfer segment (T-segment) capture is the rate-limiting step in the strand passage cycle. We propose a model for T-segment capture that provides a mechanistic basis for chiral discrimination and provides a coherent explanation for the effects of DNA twist and tension on eukaryotic type IIA topoisomerases.