Chiral Recognition of Peptide Enantiomers by Cinchona Alkaloid Derived Chiral Selectors: Mechanistic Investigations by Liquid Chromatography, NMR Spectroscopy, and Molecular Modeling

Chiral recognition mechanism studies of Tyr-Arg-Phe-Lys-NH2 tetrapeptide on crown ether-based chiral stationary phase

Even though chiral recognition for crown-ether CSPs is generally understood, on a molecular level, exact mechanisms for the resolution are still unclear. Furthermore, short peptide analytes often contain multiple amino moieties capable of binding to the crown ether selector. In order to extend the understanding in chiral recognition mechanisms, polar organic mode separation of Tyr-Arg-Phe-Lys-NH2 tetrapeptide llll/dddd enantiomers on S- and R-(3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6 stationary phases was studied with 50-mM perchloric acid in methanol as mobile phase. Deviation from the generally acceptable 1:1 stoichiometry was supported by mass spectroscopy analysis of the formed complexes between tetrapeptide enantiomer and crown ether selectors, which revealed adducts possessing 1:1, 1:2, and 1:3 stoichiometry. Further investigation of complexation induced shifts by NMR indicated on different binding mechanisms between llll/dddd enantiomers of Tyr-Arg-Phe-Lys-NH2 and crown ether selectors. Enantioselective proton shifts were observed in studied tetrapeptide tyrosine and phenylalanine residues exclusively for llll enantiomer upon binding with S-(3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6 selector (and dddd enantiomer with R-(3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6 selector), indicating that these two amino acid residues contribute to chiral recognition. The obtained results were in agreement with the LC data.

Effect of Various Parameters and Mechanism of Reversal Order of Elution in Chiral HPLC

Background:

Chiral separation involves many phenomena in which the elution order of the enantiomers has its unique position. The phenomenon of elution order of the enantiomers has also been used in the determination of optical purity which is favorable to elute the major component after minor enantiomeric impurity but the main problem is that, this phenomenon is rare.

Results:

This review rumors the reversal order of elution of many chiral molecules in HPLC. Besides, this review pronounces the effects of pH, derivatisation of drugs, the composition of the mobile phase, and temperature on the reversal order of elution of chiral drugs. The efforts are also made to discuss the possible future perspectives of reversal order of elution.

Conclusion:

Various parameters such as pH, mobile phase composition, temperature, and chemical structure of the analytes play a role in the phenomena of the reversal order of elution of many chiral molecules which are discussed in the article.

Supramolecular complexations of natural products

Complexations of natural products with synthetic receptors as well as the use of natural products as host compounds are reviewed, with an emphasis on possible practical uses or on biomedical significance. Applications such as separation, sensing, enzyme monitoring, and protection of natural drugs are first outlined. We then discuss examples of complexes with all important classes of natural compounds, such as amino acids, peptides, nucleosides/nucleotides, carbohydrates, catecholamines, flavonoids, terpenoids/steroids, alkaloids, antibiotics and toxins.

Host-Guest Chemistry of Alkaloids

Binding of alkaloids by different hosts (native and modified cyclodextrins, cucurbiturils, calixarenes, and metal complexes of porphyrin and Salphen-type ligands), as well as receptor properties of alkaloid based hosts are reviewed. With alkaloids as guests, the largest binding constants and most significant spectral changes, in particular strong fluorescence enhancements induced by complexation with isoquinoline alkaloids, are observed with cucurbituril hosts. Cyclodextrins are successfully employed for improvement of solubility and for chiral separation of alkaloids of different types. Receptor properties of native and modified cinchona and bisbenzylisoquinoline alkaloids have attracted considerable attention for development of chiral selectors for analysis and separation.

Computational study of enantioseparation by amylose tris(3,5-dimethylphenylcarbamate)-based chiral stationary phase

The mechanism of chiral separation on amylose tris(3,5-dimethylphenylcarbamate) is studied with docking simulations of enantiomers by molecular dynamics. All-atom models of amylose tris(3,5-dimethylphenylcarbamate) on the modified silica gel surface were constructed for the docking simulations of metalaxyl and benalaxyl. The elution orders and energetic differences were also predicted based on the intermolecular interactions, which were in agreement with the experimental results. The radial distribution function was employed to analyze the structural features of the enantiomer-chiral stationary phase complex and used to elucidate the mechanism of chiral separation. The separation of metalaxyl and benalaxyl is mainly controlled by the hydrogen bond. And the binding sites had slight differences for the pair of enantiomers, but obvious differences between different chemicals.

Chiral separation by enantioselective liquid-liquid extraction

The literature on enantioselective liquid-liquid extraction (ELLE) spans more than half a century of research. Nonetheless, a comprehensive overview has not appeared during the past few decades. Enantioselective liquid-liquid extraction is a technology of interest for a wide range of chemists and chemical engineers in the fields of fine chemicals, pharmaceuticals, agrochemicals, fragrances and foods. In this review the principles and advances of resolution through enantioselective liquid-liquid extraction are discussed, starting with an introduction on the principles of enantioselective liquid-liquid extraction including host-guest chemistry, extraction and phase transfer mechanisms, and multistage liquid-liquid extraction processing. Then the literature on enantioselective liquid-liquid extraction systems is reviewed, structured on extractant classes. The following extractant classes are considered: crown ether based extractants, metal complexes and metalloids, extractants based on tartrates, and a final section with all other types of chiral extractants.

Self-association promoted conformational transition of (3R,4S,8R,9R)-9-[(3,5-bis(trifluoromethyl)phenyl))-thiourea](9-deoxy)-epi-cinchonine

The conformational diversity of the (3R,4S,8R,9R)-9-[(3,5-bis(trifluoromethyl)phenyl))-thiourea](9-deoxy)-epi-cinchonine organocatalyst is discussed. Low-temperature NMR experiments confirmed a self-association process, which promotes the quinoline rotation between two intramolecularly hydrogen-bonded monomeric conformers of the catalyst. The balanced population of the coexisting monomeric and dimeric species allowed us to conduct a structural study of a rather complex conformational dynamics of the pure catalyst. The study is extended by a comparison with other members of the bifunctional amine-thiourea organocatalyst family. Changes in the molecular structure of the catalysts influence the interplay between intra- and intermolecular hydrogen bonding, and yield different extent of catalyst self-association. By assessing the conformation of the individual states, we established the thermodynamic model of a self-association promoted conformational transition.

Channel-like crystal structure of cinchoninium L-O-phosphoserine salt dihydrate

Studies on the interactions between L-O- phosphoserine, as one of the simplest fragments of membrane components, and the Cinchona alkaloid cinchonine, in the crystalline state were performed. Cinchoninium L-O-phosposerine salt dihydrate (PhSerCin) crystallizes in a monoclinic crystal system, space group P2(1), with unit cell parameters: a = 8.45400(10) A, b = 7.17100(10) A, c = 20.7760(4) A, alpha = 90 degrees , beta = 98.7830(10) degrees , gamma = 90 degrees , Z = 2. The asymmetric unit consists of the cinchoninium cation linked by hydrogen bonds to a phosphoserine anion and two water molecules. Intermolecular hydrogen bonds connecting phosphoserine anions via water molecules form chains extended along the b axis. Two such chains symmetrically related by twofold screw axis create a "channel." On both sides of this channel cinchonine cations are attached by hydrogen bonds in which the atoms N1, O12, and water molecules participate. This arrangement mimics the system of bilayer biological membrane.

Chiral recognition by enantioselective liquid chromatography: mechanisms and modern chiral stationary phases

An overview of the state-of-the-art in LC enantiomer separation is presented. This tutorial review is mainly focused on mechanisms of chiral recognition and enantiomer distinction of popular chiral selectors and corresponding chiral stationary phases including discussions of thermodynamics, additivity principle of binding increments, site-selective thermodynamics, extrathermodynamic approaches, methods employed for the investigation of dominating intermolecular interactions and complex structures such as spectroscopic methods (IR, NMR), X-ray diffraction and computational methods. Modern chiral stationary phases are discussed with particular focus on those that are commercially available and broadly used. It is attempted to provide the reader with vivid images of molecular recognition mechanisms of selected chiral selector-selectand pairs on basis of solid-state X-ray crystal structures and simulated computer models, respectively. Such snapshot images illustrated in this communication unfortunately cannot account for the molecular dynamics of the real world, but are supposed to be helpful for the understanding. The exploding number of papers about applications of various chiral stationary phases in numerous fields of enantiomer separations is not covered systematically.

Investigation of monovalent and bivalent enantioselective molecular recognition by electrospray ionization-mass spectrometry and tandem mass spectrometry

In this work is described the investigation of bivalent versus monovalent enantioselective molecular recognition in the context of enantioselective separations. Electrospray ionization-mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS) are used for evaluating enantioselective systems through the measurement of (1) relative solution-phase binding constants via titration and (2) relative gas-phase binding via collision threshold dissociation. In HPLC, a cinchonane-type chiral stationary phase (CSP) based on tert.-butylcarbamoylquinine provides vastly increased retention and enantioselectivity for separation of bivalent versus monovalent alkoxy-benzoyl-N-blocked leucine enantiomers. The bivalent enantiomers are able to span and simultaneously interact with multiple interaction sites on the CSP surface, leading to enhanced separation. ESI-MS titration measurements also show an increased avidity for binding between bivalent selector and bivalent selectand, compared with the monovalent system. However, enhanced enantioselectivities measured in HPLC for the bivalent system cannot be reproduced by MS due to inherent mechanistic differences. Assumed discrepancies in relative response factors also give rise to systematic errors which are discussed. The results of MS/MS gas-phase experiments show that enantioselectivity is essentially lost in the absence of solvation, but that dissociation thresholds can provide a measure of relative dissociation energy in the bivalent interaction system compared to the monovalent counterpart. Such measurements may prove useful and efficient in better understanding multivalent interactions, in line with current theoretical considerations of effective concentrations and ion trap effects. This is the first application of mass spectrometric methods for assessing increased avidity of binding in multivalent enantioselective molecular recognition.

Edge-to-face CH/pi aromatic interaction and molecular self-recognition in epi-cinchona-based bifunctional thiourea organocatalysis

The impact of cooperativity between intermolecular interactions is demonstrated by the molecular self-recognition properties of highly enantioselective epi-cinchona bifunctional thiourea organocatalysts. Low-temperature NMR experiments in inert solvents have revealed two sets of nonequivalent resonances in equal population for thiourea-modified members of the epi-quinine and epi-quinidine families. In solution, the predominance of an asymmetric (C1) dimeric self-assembly with noteworthy structural motifs became evident: simultaneous intra- and intermolecular thiourea hydrogen bonding and a CH/pi interaction were observed. Both the stereochemical and the diverse conformational features of the system favor the observed quinoline T-shaped aromatic pi-pi stacking interaction. The structure findings are supported by quantitative proton-proton distance data that were available from NOE buildup curves. The 3D structure of the dimeric assembly has been modeled in agreement with the H-H distance restraints. Owing to the geometrical preference associated with the dimerization process, the self-assembled bifunctional system is interpreted as a charge-transfer complex with the potential for catalyst self-activation.

Enantioseparation of phenylalanine analogs on a quinine-based anion-exchanger chiral stationary phase: Structure and temperature effects

Application of a cinchona alkaloid-based chiral anion-exchanger stationary phase for the direct high-performance liquid chromatographic enantioseparation of N-protected unusual phenylalanine analogs is reported. The N-benzyloxycarbonyl, N-3,5-dinitrobenzyloxycarbonyl, N-benzoyl and N-3,5-dinitrobenzoyl derivatives were well separable with high resolution. To achieve optimal separation of the enantiomers, the chromatographic conditions and temperature were varied. Linear van't Hoff plots were observed in the studied temperature range, 278-343 K, and the apparent changes in enthalpy, delta(deltaH degrees), entropy, delta(delta S degrees), and Gibbs free energy, delta(delta G degrees), were calculated. The values of the thermodynamic parameters depended on the nature of the N-acyl groups, on the structures of the compounds, and especially on the nature of the substituent on C3 of phenylalanine.

Quinine carbamate chiral stationary phases: Systematic optimization of steric selector-selectand binding increments and enantioselectivity by quantitative structure-enantioselectivity relationship studies

A series of quinine carbamate-based chiral stationary phases (CSPs) differing solely in the carbamate residue have been devised and a congeneric set of N-3,5-dinitrobenzoyl (DNB) amino acids (AAs) was separated into enantiomers on these CSPs by HPLC using a buffered hydro-organic mobile phase. Some details on retention and chiral recognition mechanisms have been investigated by application of quantitative structure-property relationship (QSPR) studies using the linear free energy relationship methodology, i.e., the extrathermodynamic approach. Retention factors of the high affinity enantiomer (log k2) and enantioselectivities (log alpha) were correlated with Taft's steric parameter as structural descriptor for the variability in the carbamate and AA residues, and statistically significant QSPR models could be obtained. They confirmed that the variance in the dependent variable (log k2, log alpha) is mainly associated with the steric bulkiness of the selectors' carbamate and of the AA residues. The retention factor of the second eluted enantiomers and the enantioselectivity first increased with steric bulkiness of the carbamate and AA residues, which may be explained by stronger dispersive interactions. After reaching an optimum, the dependent variable, however, declined with further increase of the steric bulkiness of the substituents, probably because of steric hindrance. The variability of the retention factors of the first eluted enantiomer could not be explained by steric descriptors. Instead, it has become obvious that the retention arises mainly from interactions of the DNB-AA and the quinine carbamate backbone, as it turned out to be more or less constant and solely to a minute amount modulated by the carbamate residue and the AA side chain. The QSPR models were fully in agreement with an earlier postulated chromatographically and spectroscopically derived hypothetical selector-selectand binding model.

Studies of the resolution of racemic 1,1'-bi-2-naphthol with a dipeptide chiral selector identified from a small library

Several new stationary phases were prepared to study the structure-activity relationship of the chiral resolution of racemic 1,1'-bi-2-naphthol with a modified dipeptide Asn-Asn selector. The number of amino acid, the side chain protecting groups of the amino acid, and the Fmoc end-capping group all proved important for enantioselectivity. The linker also influenced enantioselectivity. Influence of the length of the linker appears to be related to the accessibility of chiral selectors. The bond through which the selector is attached to the linker proved important. Based on these results, it is postulated that hydrogen bonding interactions between one side chain amide group of one Asn and the oxygen on the backbone of another Asn with the two hydroxyl groups of the analyte play an important role in the resolution of racemic 1,1'-bi-2-naphthol with the modified dipeptide Asn-Asn selector.

Adsorption of the enantiomers of 3-chloro-1-phenyl-propanol on silica-bonded chiral quinidine carbamate

The interactions of 3-chloro-1-phenyl-propanol with a quinidine carbamate-bonded chiral stationary phase under NPLC conditions were studied by measuring the adsorption isotherm data of its enantiomers by frontal analysis, modeling these data with a suitable isotherm model, and comparing the experimental overloaded elution band profiles with those calculated with this isotherm and the equilibrium dispersive model of liquid chromatography. The affinity energy distribution was calculated from the adsorption isotherm data. The results show that the surface of the adsorbent is heterogeneous and exhibits a bimodal adsorption energy distribution. This fact is interpreted in terms of the presence of two different types of adsorption sites on the stationary phase, nonselective and enantioselective sites. Albeit the bi-Langmuir isotherm model successfully accounts for the single-component data corresponding to both enantiomers, the competitive bi-Langmuir isotherm model does not allow an accurate prediction of the overloaded band profiles of the racemic mixture. Thermodynamic data are drawn for explanation. Some aspects of the retention mechanism are discussed in the light of the data obtained.

Peptide enantiomer separations: Influence of sequential isomerism and the introduction of achiral glycine moieties on chiral recognition

The influence of sequential isomerism and the introduction of achiral, conformationally flexible glycine moieties into a peptide chain on the chiral recognition mechanism of a cinchona alkaloid based chiral selector has been evaluated. For this purpose, enantiomers of N-terminally protected alanine-glycine di- and tripeptides were separated by liquid chromatography-mass spectrometry on a corresponding chiral stationary phase (CSP). To obtain complementary information, the reversed phase retention behaviour of the various peptides was also evaluated and subsequently used to further elucidate the chromatographic characteristics of the CSP. For peptides that contained glycines in the N-terminal region chiral recognition was compromised, while glycines located at the C-terminus had no or little negative effect.

Chromatographic separation of phenylpropanol enantiomers on a quinidine carbamate-type chiral stationary phase

The retention and the separation of the enantiomers of 1-phenylpropanol (1PP), 2-phenylpropanol (2PP), and 3-chloro-1-phenylpropanol (3CPP) on silica-bonded quinidine carbamate under normal phase HPLC conditions were investigated. A relatively high selectivity of the stationary phase for 3CPP and 1PP (alpha approximately 1.07-1.09) was achieved with eluents containing ethyl acetate as the polar modifier. These mobile phases were examined in detail. Based on the set of chromatographic and thermodynamic data collected, conclusions regarding the mechanism of enantioselectivity and the structure of the selector chiral center are made.

Analyte Templating: Enhancing the Enantioselectivity of Chiral Selectors upon Incorporation into Organic Polymer Environments

A simple strategy for preserving and enhancing the chiral recognition capacity of polymer-embedded chiral selectors is proposed, capitalizing on a temporary blockage of the receptor binding site with tightly binding analytes during the polymerization process. We demonstrate that the copolymerization of a quinine tert-butylcarbamate selector monomer with chiral (and achiral) 3,5-dichlorobenzoyl amino acids allows one to control to a certain extent the binding characteristics of the resultant polymeric chiral stationary phases. The structural and stereochemical requirements of the templating analytes for maximizing the chiral recognition capacity of the polymer-embedded selectors are probed. The chromatographic chiral recognition characteristics of the analyte-templated polymeric chiral stationary phases are analyzed with respect to binding capacities and affinities and compared to those obtained with a conventional silica-based surface-grafted reference material. Changes in substrate-specific enantioselectivity originating from analyte templating are also addressed.

Stereoselective peptide analysis

The stereochemistry of a peptide determines its spatial features and can profoundly influence its chemical properties and biological activity. Thus, the analysis of the stereochemical properties of a peptide is an important aspect of its characterisation. For such investigations a "selector" that engages in stereoselective interactions with the peptide analytes is often used. A substantiated knowledge of the underlying molecular recognition mechanism will therefore be helpful in understanding existing and developing new stereoselective analysis systems. After a short introduction concerning the fundamentals of peptide stereoisomers and their biological implications, the stereoselective peptide analysis methods described in the literature are comprehensively reviewed. The characteristics and applications of the employed methods based on various techniques including chromatography (pressure- and electrokinetically driven), capillary electrophoresis, nuclear magnetic resonance spectroscopy and mass spectrometry are discussed. The various selectors that have been utilised to discriminate peptide enantiomers and/or diastereomers are described concurrently. The review concludes with an overview of combinations and comparisons of techniques that have been applied to the analysis of peptide stereoisomers and constitute a trend for further developments.

Measurement of Solution-Phase Chiral Molecular Recognition in the Gas Phase Using Electrospray Ionization-Mass Spectrometry

Development of chiral selectors (SOs) is important both for understanding chiral molecular recognition processes and for their use in the separation of chiral species (selectands). Their evaluation by chromatographic procedures (e.g., as chiral stationary phase) can, however, be time-consuming. In this respect, electrospray ionization-MS (ESI-MS) is tested here as a possible alternative for screening enantioselective binding by SOs. The set of well-characterized cinchona alkaloid SOs are investigated with respect to their enantioselective binding to a set of model enantiomers, dinitrobenzoyl-(R)- and dinitrobenzoyl-(S)-leucine. MS-based enantioselectivity values from normalized gas-phase ion abundances for the diastereomeric complexes are compared empirically to chromatographic (HPLC) enantioselectivity results and shown to be consistent. Investigations into the fundamentals of measuring unbiased enantioselectivity values in the limit of dilute solution by correlation between experimental and modeled theoretical data are shown. Titration experiments are used to extract binding constants and are compared with published calorimetric (ITC) data. Results show that while the magnitude of binding affinities determined for various diastereomeric complexes is attenuated, the relative ranking and stereochemical preference in binding are consistently reproduced. This work represents a fundamental study of solution- versus gas-phase correlation for enantioselective systems by ESI-MS and indicates that, although not all questions and assumptions can be clearly engaged, for these enthalpically driven binding systems, the relative degree of binding affinity and selectivity is preserved.

Investigations of molecular recognition aspects related to the enantiomer separation of 2-methoxy-2-(1-naphthyl)propionic acid using quinine carbamate as chiral selector: An NMR and FT-IR spectroscopic as well as X-ray crystallographic study

The chiral recognition mechanism of a cinchona alkaloid-based chiral stationary phase (CSP) showing high enantiomer discrimination potential for 2-methoxy-2-(1-naphthyl)propionic acid (MalphaNP acid) was investigated. Conformational and structural analyses of the 1:1 complexes of 9-O-(tert-butylcarbamoyl) quinine selector (SO) and MalphaNP acid (selectand, SA) were carried out employing NMR spectroscopy in solution, Fourier-transform infrared (FT-IR) spectroscopy, and solid-state X-ray diffraction analysis. Intramolecular NOEs of a soluble analogue of the CSP afforded the conformational states of the free and complexed form of the selector. The (1)H-NMR spectra revealed that the free form of the SO constitutes anti-open as well as anti-closed and/or syn-closed conformers. Upon complexation with the (S)-MalphaNP acid enantiomer to form the more stable diastereomeric associate, a conformational transition of the selector takes place, resulting in the synthesis of the anti-open conformer nearly exclusively. FT-IR spectra reveal that, besides the primary ion-pairing interaction, stereoselective hydrogen bonding stabilizes the more stable complex via the amide hydrogen of the SO. X-ray diffraction analysis of 9-O-(tert-butylcarbamoyl)quinine and (S)-MalphaNP acid complex further revealed the occurrence of a bidentate H-bond-mediated ionic interaction between SO and SA as well as the lack of pi-pi interaction in the 1:1 complex, and corroborated the conclusions derived from spectroscopic and chromatographic studies.

Liquid chromatographic–mass spectrometric separation of oligoalanine peptide stereoisomers: influence of absolute configuration on enantioselectivity and two-dimensional separation of diastereomers and enantiomers

This contribution describes the chromatographic separation of peptide stereoisomers. Thereby, one focus is laid on the influence of the absolute configurations of peptide enantiomer pairs on their enantioselective separation. Three different N-terminal protecting groups and three different chiral stationary phases (CSPs) based on cinchona alkaloid derivatives were employed and oligoalanine di-, tri- and tetra-peptides were used as model set. The absolute configurations of the individual enantiomeric pairs were found to profoundly influence both the elution order and the enantioselectivity. The stereoselective molecular recognition mechanism was observed to be dependent on the combination of configuration and the chosen protecting group and CSP. As the CSPs on their own exhibited insufficient diastereoselectivity, a two-dimensional liquid chromatography-mass spectrometry (LC-MS) system was developed for the separation of both diastereomers and enantiomers of peptides in the second part of this study. Diastereomers were separated by reversed phase (RP) and the resulting enantiomeric pair fractions were transferred to a CSP for enantioseparation. All eight stereoisomers of a tripeptide (Ala-Ala-Ala) and 9 out of 10 stereoisomers of a tetrapeptide (Ala-Ala-Ala-Ala) could be successfully resolved.