Chiral Recognition Mechanism Research Articles

Wavelength dependence of chiral recognition using ions between photoexcited tryptophan and sugars

The reactivity of cold gas-phase clusters was investigated as a model for chemical evolution in interstellar molecular clouds. Chiral recognition using ions between amino acids and sugars was examined by mass spectrometry and ultraviolet spectroscopy measurements of gas-phase tryptophan (Trp) enantiomers hydrogen bonded with methyl-α-d-glucoside in Na+-containing or protonated forms at 8 K. The chiral recognition mechanism between Trp and the glucoside changed depending on the photoexcitation wavelength: the enantiomer-selective reactions at 265–280 nm and the electronic structure of Trp at 280–290 nm. The attachment of Na+ to d-Trp and the Cα–Cβ bond cleavage of d-Trp were the enantiomer-selective reactions in the Na+-containing and protonated clusters, respectively. The wavelength dependences of the enantiomer-selective reactions were similar between the Na+-containing and protonated clusters. This is because the enantiomer-selective reactions were triggered by photoinduced electron transfer from the indole ring to the carboxyl group of Trp in the clusters.

Determination of Kynurenine Enantiomers by Alpha-Cyclodextrin, Cationic-βeta-Cyclodextrin and Their Synergy Complemented with Stacking Enrichment in Capillary Electrophoresis

We present high resolution fast, cost-effective and sensitive Capillary zone electrophoresis (CZE) methods for determination of enantiomeric compounds of Kynurenine pathway, i.e. D, L-Kynurenine (KYN), in human serum and urine samples by cationic-β-CD and its synergistic dual chiral selector system (SD-CSs) with α-CD in 50 mM borax borate buffer (pH 9.0) as BGE. Acid-mediated stacking enrichment by HCl delivered 15 nM limit of detection (LOD) and 50 nM limit of quantification (LOQ). The methods gave advantages of linearity in the concentration range of 50 nM-1000 nM, reproducibility (RSD ≤ 3.35), selectivity against interfering amino acids, and remarkable recoveries. SD-CSs delivered resolution of D, L-KYN twice that of individual chiral selectors (CSs) under similar conditions. The binding constants (Kb) and electrophoretic mobilities (µeff) of D, L-KYN with different concentrations of CSs were calculated to find the migration order of enantiomers. The chiral recognition mechanism was investigated by molecular docking and molecular mechanics, which revealed strong hydrogen bonding between Kynurenine enantiomers and the SD-CSs as compared to individual CS as the key player in binding, formation of stable complexes which led to the ultimate separation.

Nafion-stabilized black phosphorus nanosheets-maltosyl-β-cyclodextrin as a chiral sensor for tryptophan enantiomers.

In this work, we have fabricated nafion (NF) stabilized black phosphorus nanosheets (BPNSs) and 6-O-α-maltosyl-β-cyclodextrin (G2-β-CD) composite (BPNSs-G2-β-CD) as novel electrochemical sensoring platform for chiral recognition of tryptophan (Trp) enantiomers. BPNSs-G2-β-CD composite modified glassy carbon electrode (BPNSs-G2-β-CD/GCE) was further coated with NF which served as a protective film to immobilize BPNSs-G2-β-CD on the electrode surface to achieve high stability. Under the optimum conditions, the oxidation peak current ratio of L-Trp to D-Trp (IL/ID) and the difference between the peak potential (ΔEp=ED - EL) were observed to be 1.49 and 20mV at NF/BPNSs-G2-β-CD/GCE by square wave voltammetry (SWV). In addition, a linear calibration curve could be obtained for peak current versus Trp enantiomers in the concentration range 0.01-1.00mM with detection limits of 1.07μM and 1.71μM for L-Trp and D-Trp (signal-to-noise ratio of 3, S/N=3), respectively. The chiral recognition mechanism was also proposed, and the intermolecular hydrogen bonding interactions as well as the hydrophobic-cavity-triggered embedding effect dominated the effective chiral recognition. Moreover, the proposed NF/BPNSs-G2-β-CD/GCE showed excellent stability, good reproducibility and anti-interference capability. Therefore, the designed chiral sensor is expected to be practically applied for the sensitive recognition of Trp enantiomers in real samples.

Separation of terbutaline enantiomers in capillary electrophoresis with neutral cyclodextrin-type chiral selectors and investigation of the structure of selector-selectand complexes using nuclear magnetic resonance spectroscopy.

The major goal of this study was to determine the affinity pattern of the terbutaline (TB) enantiomers toward α-, β-, γ-, and heptakis(2,3-di-O-acetyl)-β-cyclodextrins and using NMR spectroscopy for the understanding of the fine mechanisms of interaction between the cyclodextrins (CD) and TB enantiomers. It was shown once again that CE in combination with NMR spectroscopy represents a sensitive tool to study the affinity patterns and structure of CD complexes with chiral guests. Opposite affinity patterns of TB enantiomers toward native α- and β-CDs were associated with significant differences between the structure of the related complexes in solution. In particular, the complex between TB enantiomers and α-CD was of the external type, whereas an inclusion complex was formed between TB enantiomers and β-CD. One of the possible structures of the complex between TB and heptakis(2,3-di-O-acetyl)-β-CD (HDA-β-CD) was quite similar to that of TB and β-CD, although the chiral recognition pattern and enantioselectivity of TB complexation with these two CDs were very different.

Enantioseparation of 4-Nitrophenylalanine using (S)-SDP-metal complex as chiral extractant

The development of new and efficient chiral extractant is of great concern in the field of chiral extraction. Axial chiral diphosphine, such as (S)-BINAP, has been proved to be effective chiral extractant. However, the stereoselectivity of chiral diphosphine with novel structure has been rarely investigated in chiral extraction. (S)-SDP, a famous spiro diphosphine used in asymmetric synthesis, was firstly employed to separate 4-Nitrophenylalanine enantiomers in this study. The influences of metal precursors, pH, extractant concentration and temperature on extraction were investigated. Separation factor (α), enantiomeric excess (ee) and performance factor (pf) were adopted to evaluate the efficiency of enantioseparation. (S)-SDP-Pd was suitable for separation of 4-Nitrophenylalanine enantiomers with the highest α of 3.32. L-4-Nitrophenylalanine was preferentially recognized by (S)-SDP-Pd in extraction. The extraction process was systematically optimized by response surface method. The optimum extraction condition was pH of 7.1, (S)-SDP-Pd concentration of 2.2 mmol/L and temperature of 14.0 °C. At this optimum condition, the maximum pf was 0.08536. In addition, the possible chiral recognition mechanism was discussed.

Revelation of the chiral recognition of alanine and leucine in an l-phenylalanine-based metal-organic framework.

As a proof-of-concept, an l-phenylalanine-based metal-organic framework Zn-MOF, namely [Zn2(l-Phe)2(bpe)2]n, was designed for experimentally revealing the chiral recognition mechanism of alanine and leucine by means of 13C CP MAS NMR spectroscopy, X-ray photoelectron spectroscopy and control experiments.

Synthesis and application of tetramethylammonium-carboxymethylated-β-cyclodextrin: A novel ionic liquid in capillary electrophoresis enantioseparation

In this study, a carboxymethylated-β-cyclodextrin-based chiral ionic liquid (CIL), tetramethylammonium-carboxymethylated-β-cyclodextrin (TMA-CM-β-CD) was successfully designed and synthesized. This cyclodextrin-based ionic liquids (ILs) was used as the sole chiral selector in capillary electrophoresis, and it is very interesting to find that the chiral separation capability can be remarkably improved when a conventional cyclodextrin chiral selector evolved into an IL chiral selector. The ionic liquid showed satisfactory separation performance towards twelve tested drugs. A series of parameters affecting the enantioseparation, such as the type and proportion of organic modifier, buffer pH, chiral selector concentration, as well as applied voltage were systematically investigated. Additionally, the molecular docking program Autodock was applied to further demonstrate the mechanism of chiral recognition and the enhanced enantioselectivity of TMA-CM-β-CD, which kept in agreement with our experimental results.

Synthesis of a chiral ionic liquid, cholinium-clindamycin phosphate, as sole chiral selector in capillary electrophoresis

Recently, in separation science, ionic liquids (ILs) have been commonly used as modifiers for buffer solutions, dynamic coating solutions, or coating solutions on carriers in capillary electrophoresis. However, only several papers have reported the use of chiral ILs as the sole chiral selector. In this paper, a chiral ionic liquid, cholinium-clindamycin phosphate (Ch-CP), was synthesized and employed as a sole chiral selector in capillary electrophoresis (CE). A series of parameters affecting the separation were optimized, including chiral selector concentration, buffer pH, proportion of organic modifier, as well as the applied voltage. Under the optimal conditions, compared to clindamycin phosphate (CP), the IL selector showed better enantioseparation capability and improved peak shapes for five racemic drugs. In addition, Molecular docking program Autodock was employed to elucidate the chiral recognition mechanism of Ch-CP, the computing results conformed to the experimental results.

New chiral stationary phases for liquid chromatography based on small molecules: Development, enantioresolution evaluation and chiral recognition mechanisms.

Recently, we reported the development of new chiral stationary phases (CSPs) for liquid chromatography (LC) based on chiral derivatives of xanthones (CDXs). Based on the most promising CDX selectors, 12 new CSPs were successfully prepared starting from suitable functionalized small molecules including xanthone and benzophenone derivatives. The chiral selectors comprising one, two, three, or four chiral moieties were covalently bonded to a chromatographic support and further packed into LC stainless-steel columns (150×2.1mm I.D.). The enantioselective performance of the new CSPs was evaluated by LC using different classes of chiral compounds. Specificity for enantioseparation of some CDXs was observed in the evaluation of the new CSPs. Besides, assessment of chiral recognition mechanisms was performed by computational studies using molecular docking approach, which are in accordance with the chromatographic parameters. X-Ray analysis was used to establish a chiral selector 3D structure.

Prediction on separation factor of chiral arylhydantoin compounds and recognition mechanism between chiral stationary phase and the enantiomers

Chiral hydantoins display potential antidepressant and anticancer activities. The quantitative structure property relationship (QSPR) study on chiral compounds plays a key role in predicting the retention factor and the separation factor even the elution order of the enantiomers. In this study, the structures of chiral arylhydantoin compounds have been built to compute molecular structural parameters using VolSurf program. The satisfying models were established between the parameters and the retention factor as well as the separation factor. Analysis on the variables shows that the large difference values of volume, hydrophilic regions and hydrogen bond acceptor and donor regions at some energy levels will result in the large separation factor. The differences of hydrophobic regions and the unbalance between the centre of mass and the barycentre of the hydrophobic region at high energy levels are also favorable to the separation. The differences of surface area, hydrophilic regions and hydrogen bond acceptor and donor regions at other energy levels as well as hydrophobic regions and the unbalance at low energy levels between the enantiomers are disadvantageous to the separation. The analysis on the chiral recognition mechanism demonstrates that the differences of the interaction energies between the enantiomers are mainly affected by the hydrogen bond, π-π interaction and CH3-π interaction. The retention factor and the separation factor, especially the elution order of the enantiomers can be easily predicted using the models. The study provides a helpful guidance for studying other chiral compounds.

Molecular dynamics simulations of the chiral recognition mechanism for a polysaccharide chiral stationary phase in enantiomeric chromatographic separations

We use explicit-solvent fully atomistic molecular dynamics (MD) simulations, permitting all the interactions between the atoms constituting the polymeric chiral stationary phase (CSP), the solvent molecules and the drug molecule enantiomers, to better understand the chiral recognition mechanism that makes chromatographic separation possible. Using amylose tris(3,5-dimethylphenyl carbamate) (ADMPC) as prototype, three solvent systems, and ten racemates as solutes, we seek a molecular dynamics average quantity that could serve as a metric that predicts which of the two enantiomers will elute first and also correlates with the ratio of retention times for enantiomers. To better understand the molecular dynamic chiral recognition that provides the discrimination which results in the separation of enantiomers by high performance liquid chromatography, we examine the differences in hydrogen bonding lifetimes in various donor–acceptor pairs between the drugs and the ADMPC, and map out the differences in ring-ring interactions between the drugs and the ADMPC. Several MD average quantities related to hydrogen-bonding lifetimes correlate with the ratio of retention times for the enantiomers. One of these quantities provides a prediction of the correct elution order 90% of the time, and the ratios of these quantities for the enantiomers provide linear correlation (0.85 coefficient) with experimental separation factors.

Evaluation of an ionic liquid chiral selector based on clindamycin phosphate in capillary electrophoresis.

Recently, increasing attention has been given to the research on chiral ionic liquids (CILs) in chiral separation field; however, only a few literatures focus on the exploration of CILs as the sole chiral selector. In this study, an ionic liquid chiral selector based on antibiotic, namely tetramethylammonium clindamycin phosphate (TMA-CP), was originally synthesized and subsequently utilized for enantioseparation in capillary electrophoresis (CE). Remarkably improved separations of eight racemic analytes were achieved in TMA-CP system in contrast to the clindamycin phosphate (CP) system. The optimal separation conditions were determinated by systematic experiments on several crucial parameters including the type and proportion of organic modifier, CIL concentration, buffer pH, and applied voltage. Additionally, molecular modeling with AutoDock was applied to probe into the chiral recognition mechanism of the ionic liquid chiral selectors, which well corresponded with the experimental results. It is the first time that antibiotic-based ionic liquid was exploited as favorable sole chiral selector in CE, and this strategy has paved a new way for development of novel ionic liquids chiral selectors based on antibiotics. Graphical abstract.

Enantioseparation, recognition mechanisms and binding of xanthones on human serum albumin by liquid chromatography.

Aim: To develop a method for enantioseparation of several chiral derivatives of xanthones (CDXs) by LC using a human serum albumin-chiral stationary phase (HSA-CSP) and screening CDX-HSA affinity. Additionally, recognition mechanisms were investigated. Materials & methods: The influence of organic modifier, buffer type, pH and ionic strength of mobile phase, and temperature were explored. The affinity was determined by measuring the retention times and further calculation of bound percentage. Chiral recognition mechanisms were investigated by docking. Results: Enantioselectivity and resolution values ranged from 1.40 to 9.16 and 1.51 to 4.97. Bound percentages ranged from 79.02 to 99.99%. Conclusion: LC systematic study and binding affinity of CDXs on HSA-CSP are presented here for the first time, expanding the applications of HSA-CSP for this class of compounds.

Visual chiral recognition of D/L-leucine using cube-shaped gold nanoparticles as colorimetric probes.

Chiral recognition of enantiomers is fundamentally important. In this work, an effective and simple chiral sensing strategy for discrimination of leucine (Leu) enantiomers was constructed. In this chiral sensing, cube-shaped gold nanoparticles (AuNCs) as colorimetric probes are used to recognize Leu enantiomers. L-Leu can induce rapidly the aggregation of AuNCs, leading to change of AuNCs solution from red to colorless, while D-Leu cannot induce the color change. This distinct color changes allow naked-eye to distinguish chiral isomers of Leu. The chiral sensing was applied to measure the enantiometric excess of L-Leu in the whole range (from -100% to 100%). This chiral analysis can be performed by naked eye or simple ultraviolet-visible spectrometer. In addition, the mechanism of chiral recognition has been studied with circular dichroism (CD) spectra, UV-vis absorption spectra and zeta potential. In this method, the chiral recognition is due to the intrinsic chirality of AuNCs, and the AuNCs don't need any chiral labeling or modification. This chiral sensing method is simple, cheap, rapid and easy to operate. Furthermore, this work provides one experimental evidence for intrinsic chirality of nanoparticles, and helps people understand the chirality of nanostructures.

Enantioseparation and molecular modeling study of five β-adrenergic blockers on Chiralpak IC column.

A new high-performance liquid chromatography (HPLC) method was developed for the enantiomeric resolution of five β-adrenergic blockers on a Chiralpak IC column (250mm×4.6mm, 5.0μm particle size) in normal phase mode. The mobile phase used was n-hexane-ethanol-diethylamine in different proportions at the flow rate of 1.0mL/min with the column temperature of 25°C using a UV detector at 230nm. The influences of base additives and alcohol modifiers were evaluated and optimized. The maximum resolution values for bevantolol, propranolol carteolol, esmolol, and metoprolol were 4.80, 2.77, 2.09, 2.30, and 1.11, respectively. To gain a better understanding of the interaction between chiral stationary phase and analyte enantiomers, the molecular docking of chiral stationary phase with five pairs of enantiomer was carried out using AutoDock molecular docking technique. By simulation studies, the mechanism of chiral recognition was determined. According to the results, hydrogen bond interactions and π-π interactions were the chief interactions for the chiral recognition.

Regioselectively substituted cellulose mixed esters synthesized by two-steps route to understand chiral recognition mechanism and fabricate high-performance chiral stationary phases

It is challenging to design and fabricate new and high-performance cellulose-based chiral stationary phases (CSPs), due to the indistinct chiral recognition mechanism and the inherent difficulty to control the structure of cellulose derivatives. Herein, taking advantage of the high regioselective benzoylation of cellulose in 1-allyl-3-methylimidazolium chloride, a series of regioselectively substituted cellulose mixed esters, cellulose 6-benzoate-2,3-phenylcarbamate, are directly obtained by a facile two-steps route without protecting and deprotecting process. The resultant cellulose mixed esters exhibit high chiral recognition capability. In particular, when the benzoate group has an electron-donating substituent on phenyl ring, such as 4-tert-butyl group, the corresponding regioselectively substituted cellulose mixed esters have much better enantioseparation capability than cellulose tri(3,5-dimethylphenylcarbamate), which is commercially available as Chiralcel OD column, one of the most powerful CSPs. More importantly, via adjusting the chemical structure of cellulose derivatives and adding a post-treatment process to optimize their chiral recognition properties, the chiral recognition mechanism is clearly revealed. The synergy of the hydrophobic helical conformation, weak hydrogen-bond donating ability and appropriate distribution of substituents of cellulose derivatives is essential to fabricate high-performance CSPs.

Investigation of the enantioselectivity of tetramethylammonium-lactobionate chiral ionic liquid based dual selector systems toward basic drugs in capillary electrophoresis.

Recently, chiral ionic liquids (CILs) have received increasing interest in chiral separation by CE. Nevertheless, the lack of deep perception of the specific mechanism about CILs in CE for enantioseparation still perplexes a legion of researchers, despite the strenuous efforts. In this paper, a lactobionic acid based ionic liquid, tetramethylammonium-lactobionate (TMA-LA) was applied for the first time in CE to establish dual selector system with clindamycin phosphate (CP) for enantiomeric separation. Compared to single CP system or single TMA-LA system significantly improved separations of seven tested rameric drugs (propranolol, nefopam, citalopram, chlorphenamine, metoprolol, bisoprolol, and esmolol) were observed in the dual selector systems. Several crucial parameters such as type and proportion of organic modifier, buffer composition and pH, and concentration of TMA-LA/CP were systematically investigated to achieve satisfied enantioseparation. Meanwhile, molecular modeling was applied to demonstrate the chiral recognition mechanism of the TMA-LA/CP dual-selector separation system using the molecular docking software Autodock, which well supported the experimental results. The existence of TMA-LA/CP complex may give rise to a higher discriminatory ability against the enantiomers, indicating the reason of improved separation in TMA-LA/CP system. All the influence factors evaluated by means of Statistical Product and Service Solutions (SPSS) to research the influences on the chiral separation system.

Synthesis and application of amino triazolium-modified lactobionic acid as chiral selector in capillary electrophoresis

In this paper, a chiral selector, N-(4-H-1, 2, 4-triazolium)-lactobionamides (LA-ATM), was synthesized and applied for enantioseparation in capillary electrophoresis (CE) for the first time. Compared with lactobionic acid (LA) separation system, enhanced enantioseparation of five tested drugs was achieved in this modification system. In order to achieve good chiral separation, several parameters such as chiral selector concentration, buffer pH, applied voltage as well as the type and proportion of organic modifier were systematically investigated. Molecular modeling was applied to demonstrate the chiral recognition mechanism of the LA-ATM, which well supported the experimental results. Furthermore, a mathematical equation built up based on the molecular mechanics calculations was used in predicting resolution of tested drugs using LA or LA-ATM mediated CE, the predicted result was well correlated with the experimental result.

Enantioseparation of chiral pharmaceuticals by vancomycin-bonded stationary phase and analysis of chiral recognition mechanism.

The drug chirality is attracting increasing attention because of different biological activities, metabolic pathways, and toxicities of chiral enantiomers. The chiral separation has been a great challenge. Optimized high-performance liquid chromatography (HPLC) methods based on vancomycin chiral stationary phase (CSP) were developed for the enantioseparation of propranolol, atenolol, metoprolol, venlafaxine, fluoxetine, and amlodipine. The retention and enantioseparation properties of these analytes were investigated in the variety of mobile phase additives, flow rate, and column temperature. As a result, the optimal chromatographic condition was achieved using methanol as a main mobile phase with triethylamine (TEA) and glacial acetic acid (HOAc) added as modifiers in a volume ratio of 0.01% at a flow rate of 0.3mL/minute and at a column temperature of 5°C. The thermodynamic parameters (eg, ΔH, ΔΔH, and ΔΔS) from linear van 't Hoff plots revealed that the retention of investigated pharmaceuticals on vancomycin CSP was an exothermic process. The nonlinear behavior of lnk' against 1/T for propranolol, atenolol, and metoprolol suggested the presence of multiple binding mechanisms for these analytes on CSP with variation of temperature. The simulated interaction processes between vancomycin and pharmaceutical enantiomers using molecular docking technique and binding energy calculations indicated that the calculated magnitudes of steady combination energy (ΔG) coincided with experimental elution order for most of these enantiomers.

Voltammetric enantiomeric differentiation of tryptophan by using multiwalled carbon nanotubes functionalized with ferrocene and β-cyclodextrin

Two types of ferrocene-functionalized multiwalled carbon nanotubes (MWCNTs) are described. The first was obtained by adsorption of ferrocene (Fc) on MWCNTs via π-π stacking and is referred to as Fc/MWCNTs. The second was prepared via a conjugation reaction between ferrocenecarboxylic acid and aminated MWCNTs to result on covalently modified MWCNTs referred to as MWCNT-Fc. MWCNT/Fc has a better conductivity than MWCNT-Fc. But electrochemical recognition results clearly showed the MWCNTs-Fc/β-CD/GCE can exhibit better chiral recognition ability to tryptophan enantiomers than β-CD/Fc/MWCNTs/GCE. By combining the merits of β-cyclodextrin (β-CD) and MWCNTs-Fc, an electrochemical sensor for l-tryptophan was obtained. Its characterizations by SEM, TEM, TGA, Raman spectroscopy and XPS showed that the Fc/MWCNTs and MWCNTs-Fc have successfully been synthesized. Enantioselective recognition was accomplished by differential pulse voltammetry. Tryptophan enantiomers can be differentiated with an enantioselectivity coefficient (ID-Trp/IL-Trp) of 5.78. Moreover, the chiral recognition mechanism of β-CD recognition tryptophan enantiomers was proposed.