Evaluation of the macrocyclic antibiotic avoparcin as a new chiral selector for HPLC

Enantiomeric Separation of New Chiral Azole Compounds

Twelve new azole compounds were synthesized through an ene reaction involving methylidene heterocycles and phenylmaleimide, producing four oxazoles, five thiazoles, and one pyridine derivative, and ethyl glyoxylate for an oxazole and a thiazole compound. The twelve azoles have a stereogenic center in their structure. Hence, a method to separate the enantiomeric pairs, must be provided if any further study of chemical and pharmacological importance of these compounds is to be accomplished. Six chiral stationary phases were assayed: four were based on macrocyclic glycopeptide selectors and two on linear carbohydrates, i.e., derivatized maltodextrin and amylose. The enantiomers of the entire set of new chiral azole compounds were separated using three different mobile phase elution modes: normal phase, polar organic, and reversed phase. The most effective chiral stationary phase was the MaltoShell column, which was able to separate ten of the twelve compounds in one elution mode or another. Structural similarities in the newly synthesized oxazoles provided some insights into possible chiral recognition mechanisms.

Mass spectrometry detection of basic drugs in fast chiral analyses with vancomycin stationary phases

Current trends in chiral analysis of pharmaceutical drugs are focused on faster separations and higher separation efficiencies. Core-shell or superficially porous particles (SPP) based chiral stationary phases (CSPs) provide reduced analysis times while maintaining high column efficiencies and sensitivity. In this study, mobile phase conditions suitable for chiral analyses with electrospray ionization LC-MS were systematically investigated using vancomycin as a representative CSP. The performance of a 2.7 µm SPP based vancomycin CSP (SPP-V) 10 cm × 0.21 cm column was compared to that of a corresponding 5 µm fully porous particles based analogue column. The results demonstrated that the SPP-V column provides higher efficiencies, 2-5 time greater sensitivity and shorter analysis time for a set of 22 basic pharmaceutical drugs. The SPP-V was successfully applied for the analysis of the degradation products of racemic citalopram whose enantiomers could be selectively identified by MS.

Evaluation of the Edman degradation product of vancomycin bonded to core-shell particles as a new HPLC chiral stationary phase

A modified macrocyclic glycopeptide-based chiral stationary phase (CSP), prepared via Edman degradation of vancomycin, was evaluated as a chiral selector for the first time. Its applicability was compared with other macrocyclic glycopeptide-based CSPs: TeicoShell and VancoShell. In addition, another modified macrocyclic glycopeptide-based CSP, NicoShell, was further examined. Initial evaluation was focused on the complementary behavior with these glycopeptides. A screening procedure was used based on previous work for the enantiomeric separation of 50 chiral compounds including amino acids, pesticides, stimulants, and a variety of pharmaceuticals. Fast and efficient chiral separations resulted by using superficially porous (core-shell) particle supports. Overall, the vancomycin Edman degradation product (EDP) resembled TeicoShell with high enantioselectivity for acidic compounds in the polar ionic mode. The simultaneous enantiomeric separation of 5 racemic profens using liquid chromatography-mass spectrometry with EDP was performed in approximately 3 minutes. Other highlights include simultaneous liquid chromatography separations of rac-amphetamine and rac-methamphetamine with VancoShell, rac-pseudoephedrine and rac-ephedrine with NicoShell, and rac-dichlorprop and rac-haloxyfop with TeicoShell.

A comprehensive methodology for the chiral separation of 40 tobacco alkaloids and their carcinogenic E/Z-(R,S)-tobacco-specific nitrosamine metabolites

The predominant enantiomer of nicotine found in nature is (S)-nicotine and its pharmacology has been widely established. However, pharmacologic information concerning individual enantiomers of nicotine-related compounds is limited. Recently, a modified macrocyclic glycopeptide chiral selector was found to be highly stereoselective for most tobacco alkaloids and metabolites. This study examines the semi-synthetic and native known macrocyclic glycopeptides for chiral recognition, separation, and characterization of the largest group of nicotine-related compounds ever reported (tobacco alkaloids, nicotine metabolites and derivatives, and tobacco-specific nitrosamines). The enantioseparation of nicotine is accomplished in less than 20s for example. All liquid chromatography separations are mass spectrometry compatible for the tobacco alkaloids, as well as their metabolites. Ring-closed, cyclized structures were identified and separated from their ring-open, straight chain equilibrium structures. Also, E/Z-tobacco-specific nitrosamines and their enantiomers were directly separated. E/Z isomers also are known to have different physical and chemical properties and biological activities. This study provides optimal separation conditions for the analysis of nicotine-related isomers, which in the past have been reported to be ineffectively separated which can result in inaccurate results. The methodology of this study could be applied to cancer studies, and lead to more information about the role of these isomers in other diseases and as treatment for diseases.

Effective methodologies for enantiomeric separations of 150 pharmacology and toxicology related 1°, 2°, and 3° amines with core-shell chiral stationary phases

Core-shell particles (superficially porous particles, SPPs) have been proven to provide high-throughput and effective separations of a variety of chiral molecules. However, due to their limited commercialization, many separations have not been reported with these stationary phases. In this study, four SPP chiral stationary phases (CSPs) were utilized for the enantiomeric separation of 150 chiral amines. These amines encompass a variety of structural and drug classes, which are particularly important to the pharmaceutical industry and in forensics. This comprehensive evaluation demonstrates the power of these CSPs and the ease of method development and optimization. The CSPs used in this study included the macrocyclic glycopeptide-based CSPs (VancoShell and NicoShell), the cyclodextrin-based CSP (CDShell-RSP), and the cyclofructan-based CSP (LarihcShell-P). These CSPs offered versatility for a variety of applications and worked in a complementary fashion to baseline separate all 150 amines. The LarihcShell-P was highly effective for separating primary amines. VancoShell, NicoShell, and CDShell-RSP were useful for separating all types of amines. These CSPs are multi-modal and can be utilized with mass spectrometry compatible solvents. Eighteen racemic controlled substances were simultaneously baseline separated in a single liquid chromatography-mass spectrometry (LC-MS) analysis. Details in high-performance liquid chromatography (HPLC) parameters will be discussed as well as the improved chromatographic performance afforded by the SPP CSPs.

Solvent-Free Lipase-Catalyzed Synthesis: Unique Properties of Enantiopure D- and L- Polyaspartates and Their Complexation

Amino acids are attractive monomers for the large-scale preparation of chiral polyamides. For enzymatic polymerization of amino acids using protease in aqueous environment as the catalysis system, one main restriction is oligomer formation, usually along with other displayed advantages. Herein we developed an efficient solvent-free lipase-catalyzed polymerization of diethyl D- or L-aspartate, providing chiral D- and L-polyaspartates with an average degree of polymerization (DPavg) up to 60 and having about 96% β-linkages. Additionally, their distinct chemical and physical properties were characterized by circular dichroism (CD) spectra, X-ray powder diffraction (XRD), microscopic observation, and thermal analysis. Poly(β-D-AspEt) and Poly(β-L-AspEt) showed vertically mirrored negative and positive CD signals, high crystallinity, and entirely different microscopic morphology. They are thermal stable while having different decomposition (Td), melting (Tm), and cold crystallization temperatures (Tcc), respectively. Our results also showed that the complexation of enantiopure D- and L-polyaspartates was not stereocomplex but homocomplex.

Analysis of ketoprofen enantiomers in human and rat plasma by hollow-fiber-based liquid-phase microextraction and chiral mobile-phase additive HPLC

A simple, inexpensive, and efficient preconcentration and cleanup three-phase hollow fiber liquid-phase microextraction method (HF-LPME) was developed for the extraction of the anti-inflammatory ketoprofen (KTP) from human and rat plasma and HPLC enantioseparation of its enantiomers using vancomycin as a chiral mobile-phase additive with an achiral C8 column. The effects of different parameters influencing the efficiency of extraction were optimized for aqueous samples. Under optimized conditions, KTP enantiomers were extracted from 0.5 mL of plasma diluted to 5 mL with salinated and acidified deionized water (pH = 2) with 25 μL of alkalinized acceptor phase (pH = 11) during 30 min at room temperature. The mean recoveries of (−)-(R)- and (+)-(S)-KTP were 72.8% and 70.9%, respectively. The quantification limit was 20 ng/mL with linear response over the 20–2000 ng/mL concentration range for both enantiomers. Assay precision was studied within-day and between-day using 100 ng/mL KTP solutions. For both KTP enantiomers, relative standard deviations were lower than 12%. The proposed microextraction method was applied for the extraction of KTP enantiomers from human and rat plasma samples after oral administration of pure (±)-KTP in 4 mg/kg dosage for rats and 50 mg dosage for humans to assess the enantiospecific bioavailability of KTP enantiomers in their plasma. In vivo inversion studies revealed that the bioavailability of S-KTP is higher than that of the R enantiomers in rat, but they are similar in human plasma. The developed method showed that HF-LPME is a promising technique for sample preparation for the analyses of chiral drugs in biological samples.

Enantiomeric separation of 2-arylpropionic acid nonsteroidal anti-inflammatory drugs by HPLC with hydroxypropyl-beta-cyclodextrin as chiral mobile phase additive

The enantio-separations of eight 2-arylpropionic acid nonsteroidal anti-inflammatory drugs (2-APA NSAIDs) were established using reversed-phase high-performance liquid chromatography with hydroxypropyl-beta-cyclodextrin (HP-beta-CD) as chiral mobile phase additive for studying the stereoselective skin permeation of suprofen, ketoprofen, naproxen, indoprofen, fenoprofen, furbiprofen, ibuprofen and carprofen. The effects of the mobile phase composition, concentration of HP-beta-CD and column temperature on retention and enantioselective separation were investigated. With 2-APA NSAIDs as acidic analytes, the retention times and resolutions of the enantiomers were strongly related to the pH of the mobile phase. In addition, both the concentration of HP-beta-CD and temperature had a great effect on retention time, but only a slight or almost no effect on resolutions of the analytes. Enantioseparations were achieved on a Shimpack CLC-ODS (150 x 4.6 mm i.d., 5 microm) column. The mobile phase was a mixture of methanol and phosphate buffer (pH 4.0-5.5, 20 mM) containing 25 mM HP-beta-CD. This method was flexible, simple and economically advantageous over the use of chiral stationary phase, and was successfully applied to the enantioselective determination of the racemic 2-APA NSAIDs in an enantioselective skin permeation study.

Chiral analysis of pharmaceuticals by capillary electrophoresis using antibiotics as chiral selectors

The review summarizes the use of the chiral capillary electrophoresis (CE) with different class of antibiotics as chiral selectors in the pharmaceutical field. Basic factors influencing the enantioseparation are shortly discussed. Non-aqueous capillary electrophoresis is also included as well as the coupling of CE to MS. The selection of a chiral selector according the ionic state and structure of the analyte is described. Summary of pharmaceutical applications of chiral CE is given.

Evaluation of dalbavancin as chiral selector for HPLC and comparison with teicoplanin-based chiral stationary phases

Dalbavancin is a new compound of the macrocyclic glycopeptide family. It was covalently linked to 5 microm silica particles using two different binding chemistries. Approximately 250 racemates including (a) heterocyclic compounds, (b) chiral acids, (c) chiral amines, (d) chiral alcohols, (e) chiral sulfoxides and sulfilimines, (f) amino acids and amino acid derivatives, and (g) other chiral compounds were tested on the two new chiral stationary phases (CSPs) using three different mobile phases. As dalbavancin is structurally related to teicoplanin, the same set of chiral compounds was screened on two commercially available teicoplanin CSPs for comparison. The dalbavancin CSPs were able to separate some enantiomers that were not separated by the teicoplanin CSPs and also showed improved separations for many racemates. However, there were other compounds only separated or better separated on teicoplanin CSPs. Therefore, the dalbavancin CSPs are complementary to the teicoplanin CSPs.

Development of new HPLC chiral stationary phases based on native and derivatized cyclofructans

An unusual class of chiral selectors, cyclofructans, is introduced for the first time as bonded chiral stationary phases. Compared to native cyclofructans (CFs), which have rather limited capabilities as chiral selectors, aliphatic- and aromatic-functionalized CF6s possess unique and very different enantiomeric selectivities. Indeed, they are shown to separate a very broad range of racemic compounds. In particular, aliphatic-derivatized CF6s with a low substitution degree baseline separate all tested chiral primary amines. It appears that partial derivatization on the CF6 molecule disrupts the molecular internal hydrogen bonding, thereby making the core of the molecule more accessible. In contrast, highly aromatic-functionalized CF6 stationary phases lose most of the enantioselective capabilities toward primary amines, however they gain broad selectivity for most other types of analytes. This class of stationary phases also demonstrates high "loadability" and therefore has great potential for preparative separations. The variations in enantiomeric selectivity often can be correlated with distinct structural features of the selector. The separations occur predominantly in the presence of organic solvents.

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.

Retention mechanism of high-performance liquid chromatographic enantioseparation on macrocyclic glycopeptide-based chiral stationary phases

The development of methods for the separation of enantiomers has attracted great interest in the past 20 years, since it became evident that the potential biological or pharmacological applications are mostly restricted to one of the enantiomers. In the past decade, macrocyclic antibiotics have proved to be an exceptionally useful class of chiral selectors for the separation of enantiomers of biological and pharmacological importance by means of high-performance liquid chromatography (HPLC), thin-layer chromatography and electrophoresis. The glycopeptides avoparcin, teicoplanin, ristocetin A and vancomycin have been extensively used as chiral selectors in the form of chiral bonded phases in HPLC, and HPLC stationary phases based on these glycopeptides have been commercialized. In fact, the macrocyclic glycopeptides are to some extent complementary to one another: where partial enantioresolution is obtained with one glycopeptide, there is a high probability that baseline or better separation can be obtained with another. This review sets out to characterize the physicochemical properties of these macrocyclic glycopeptide antibiotics and, through their application, endeavors to demonstrate the mechanism of separation on macrocyclic glycopeptides. The sequence of elution of the stereoisomers and the relation to the absolute configuration are also discussed.

HPLC separation of amino acid enantiomers and small peptides on macrocyclic antibiotic-based chiral stationary phases: a review

The search for new and effective chiral selectors capable of separating a wide variety of enantiomeric compounds is an ongoing process. In the past decade, macrocyclic antibiotics have proved to be an exceptionally useful class of chiral selectors for the separation of enantiomers of biological and pharmacological importance by means of HPLC, TLC and electrophoresis. More chiral analytes have been resolved through the use of glycopeptides than with all the other macrocyclic antibiotics combined (ansamycins, thiostrepton, aminoglycosides, etc.). The glycopeptides avoparcin, teicoplanin, ristocetin A and vancomycin have been extensively used as chiral selectors in the form of chiral bonded phases in HPLC, and HPLC stationary phases based on these glycopeptides have been commercialized. Teicoplanin, vancomycin, their analogs and ristocetin A seem to be the most useful glycopeptide HPLC bonded phases for the enantioseparation of proteins and unusal native and derivatized amino acids. In fact, the macrocyclic glycopeptides are to some extent complementary to one another: where partial enantioresolution is obtained with one glycopeptide, there is a high probability that baseline or better separation can be obtained with another. This review sets out to characterize the physicochemical properties of these antibiotics and their application in the enantioseparations of amino acids. The mechanism of separation, the sequence of elution of the stereoisomers and the relation to the absolute configuration are also discussed.

Adsorption behavior of a teicoplanin aglycone bonded stationary phase under harsh overload conditions

Silica-bonded teicoplanin aglycone allows enantioseparation of amino acids by reversed-phase liquid chromatography with a low organic solvent content. However, a reversible change in the adsorption behavior leading to a retention time shift (RTS) was observed when a preparative scale column was treated with harsh preparative chromatography-like conditions between finite-injection HPLC runs conducted under exactly the same conditions. This behavior was observed for all five investigated aliphatic and aromatic amino acids. In all cases, the retention times were prolonged after the overload conditions and the RTS was more pronounced for the later eluting d-enantiomer. We defined a standardized method for measuring the RTS and performed a systematic investigation on the influence of experimental conditions (type and concentration of pH modifier and organic modifier, temperature, pH) on the RTS. In this way a solvent composition--90/10 50 mM NH4Ac pH 5.8/MeOH--was identified that yielded no observable shift in retention time after overload conditions for both enantiomers. In order to treat the observed phenomenon on a mechanistic level, we applied band profile analysis based on the stochastic theory of chromatography and identified two different enantioselective sites. When the band profile analysis was performed on elution profiles obtained from runs with prolonged retention time after harsh overload conditions, the retention time shift could be attributed to both differentiable types of adsorption sites. One site was found to make both, enantioselective and non-selective contributions.

Enantioselective resolution of thyroxine hormone by high-performance liquid chromatography utilizing a highly fluorescent chiral tagging reagent

A highly fluorescent chiral tagging reagent, 4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N,N-dimethylaminosulfonyl-2,1,3-benzoxadiazole, [R(-)-DBD-PyNCS], was employed to develop an indirect resolution method for efficient separation of thyroxine enantiomers,D-T(4) and L-T(4). The reaction of R(-)-DBD-PyNCS with the thyroxine enantiomers proceeds effectively at 40 degrees C for 20 min in the presence of basic medium to produce the corresponding pair of diastereomers. No racemization occurs during the tagging reaction under the optimized conditions. Various experimental parameters for derivatization reaction including the species of catalyst, the concentration of tagging reagent and reaction temperatures, have been examined to get a highest yield for T(4) derivatives. The structure of T(4) derivatives was identified based on ESI-MS/MS measurements in negative mode. The efficient separation of D-, L-T(4) derivatives was achieved by isocratic elution with water-acetonitrile mobile phase containing 1% AcOH on a reversed phase column utilizing a conventional fluorescence detector. The resolution (Rs) value of the diastereomers derived from thyroxine was 5.1. The calibration curves of both the D-T(4) and L-T(4) were linear over the concentration range of 0.1-20 microg/ml. The limits of detection (S/N = 3) for both D-T(4) and L-T(4) were 0.2 ng per injection. The proposed method was applied to the determination of D-T(4) and L-T(4) in pharmaceutical formulations and human serum samples.

Chiral separation of T3 enantiomers using stereoselective antibodies as a selector in micro-HPLC

This work deals with the application of stereoselective antibodies against L-T3 as a tailor-made chiral selector in micro-HPLC. The separations were performed in microbore columns using commercially available anti-L-T3 antibodies chemically bonded to 5 microm silica gel. The enantiomers of T3 were baseline separated under mild continuous isocratic elution conditions using 10 mM phosphate buffer, pH 7.4. The D-enantiomer eluted with the void volume, while the L-enantiomer was retained by the antibody phase and eluted second. An indirect competitive and non-competitive enzyme linked immunosorbent assay (ELISA) was used for testing the stereoselectivity of anti-L-T3 antibodies.

Elucidation of chiral recognition processes of macrocyclic antibiotic vancomycin

A theoretical investigation was carried out on the retention and separation of enantiomeric molecules including nonsteroidal anti-inflammatory drugs, anti-neoplastic compounds and N-derivatized amino acids by capillary electrophoresis using macrocyclic antibiotics, a new class of chiral selectors, as stationary phase. Firstly docking methods were used to study the enantiorecognition in chiral electrophoresis. The molecular dynamics simulations of the two diastereoisomer complexes were then performed in order to understand how these antibiotics recognize the enantiomers. Another approach was applied in this study to establish a quantitative structure-enantioselectivity relationship (QSER) model, able to describe the resolution of a series of chiral compounds in capillary electrophoresis using vancomycin as the resolving agent.

New chiral stationary phase with macrocyclic glycopeptide antibiotic eremomycin chemically bonded to silica

A new chiral stationary phase (CSP) was prepared by attachment of macrocyclic glycopeptide antibiotic eremomycin to the epoxy-activated silica under mild conditions. In contrast to CSP with immobilized vancomycin, which is a close structural analogue of eremomycin, the prepared CSP reveals high enantioselectivity for separation of amino acids enantiomers. It was demonstrated by the example of ristocetin A CSP that method of the immobilization of macrocyclic glycopeptide antibiotics affects remarkably the resulting enantioselectivity.

Vancomycin as chiral selector for enantioselective separation of selected profen nonsteroidal anti-inflammatory drugs incapillary liquid chromatography

The chiral selector vancomycin was used either as mobile phase additive or bound as a chiral stationary phase (CSP) for the stereoselective separation of seven racemic nonsteroidal anti-inflammatory drugs (NSAIDs), fenoprofen, carprofen, flurbiprofen, indoprofen, flobufen, ketoprofen, and suprofen, by capillary liquid chromatography. The effect of the type of stationary phase, the chiral column Chirobiotic V or the achiral stationary phases Nucleosil 100 C8 HD and Nucleosil 100 C18 HD, and the concentration of vancomycin in the mobile phase on separation of the drug enantiomers were evaluated. All the drugs, except flobufen, were successfully enantioseparated on Nucleosil 100 C8 HD with 4 mM vancomycin present in the mobile phase (composed of methanol and buffer) in the reversed phase mode. On the vancomycin-bonded chiral stationary phase, it was difficult to get enantioseparations of the profen NSAIDs. However, flobufen gave better enantioseparation on the vancomycin CSP. The better enantioresolution of the majority of profen derivatives on the achiral columns with vancomycin added to the mobile phase can be attributed in particular to the higher separation efficiency of this capillary chromatographic system. In addition, vancomycin dimers, formed in the mobile phase, seem to offer a better steric arrangement for stereoselective interaction to these analytes than the vancomycin bonded on the CSP. These substantial differences in the CS structure significantly influence the chiral discrimination mechanism.

The enantioseparation of amino acids on a teicoplanin chiral stationary phase using non-aqueous mobile phases after pre-column derivatization with sulfur-containing reagents: the considerations of mobile phase composition and analyte structure variation on resolution enhancement

Amino acids were derivatized with sulfur-containing reagents in alkaline medium and enantioresolved by HPLC on a teicoplanin chiral stationary phase. Much better resolution was achieved using methanol-based mobile phase compared with elution with acetonitrile-based solvent mixture. The value of selectivity factor for many derivatives examined in the study can be easily several tenths of 100 or even exceed 100 in some cases. This magnitude of resolution is suitable for the preparative-scale application of isolating alpha-amino acid enantiomers in the derivatized form using a short column. The resolution is believed to be a result of the hydrogen bonding and steric hindrance by the nitrogen and sulfur atoms from the isothiocyanatyl fragment of the reagent, respectively, and is enhanced as the fragment is structurally phenylated (e.g. 2,4-difluorophenyl isothiocyanate and others examined in this study). The enhancement is even more significant if the aromatic moiety of the reagent becomes more acidic through halogenation with chlorine or fluorine for the stronger pi-pi interaction. However, this type of enhancement is offset to some extent and sometimes obscured by a chlorinated reagent that is relatively large in size as compared with the fluorinated one. Judging from the chromatographic data and the elution profile, the mechanisms leading to the resolutions are believed to be different under the elution of polar-organic mobile phases (i.e. methanol- and acetonitrile-based mobile phases).

Comparison of vancomycin-based stationary phases with different chiral selector coverage for enantioselective separation of selected drugs in high-performance liquid chromatography

Two vancomycin-based chiral stationary phases (CSPs) with different coverage of the chiral selector vancomycin (Chirobiotic V and Chirobiotic V2) were compared. beta-Blockers and profens, as structurally diverse groups of drugs, were chosen as analytes. Retention and enantioseparation of beta-blockers were studied in reversed-phase (RP) and polar-organic (PO) separation modes. Higher retention and better enantioresolution were obtained on the CSP with higher coverage of vancomycin in the both separation modes. Baseline separation of four beta-blockers (eight enantiomers) in the PO mode was achieved on the Chirobiotic V2 column within 15 min. The enantioseparation of profens did not bring so excellent and easy to interpret results. Higher retention of profens on the Chirobiotic V2 column was not always accompanied by an improvement of their chiral separation in the RP mode. The polar-organic mode was not suitable for these derivatives at all. The most interesting result was obtained with flobufen; its chiral center is further away from the rigid part of the molecule, which mostly causes difficulties in enantioselective recognition. Nevertheless, the enantiomers of flobufen were shown to be much better (baseline) resolved on the CSP with lower coverage of the chiral selector (Chirobiotic V).

Enantiomeric separation of dansyl amino acids using macrocyclic antibiotics as chiral mobile phase additives by narrow-bore high-performance liquid chromatography

Seven macrocyclic antibiotics were evaluated as chiral selectors for the enantiomeric separation of 11 dansyl amino acids using narrow-bore high-performance liquid chromatography (HPLC). The macrocyclic antibiotics were incorporated as mobile phase additives to determine the enantioselective effects on the chiral analytes. The resolution and capacity factor (k') of each analyte were assessed while varying the structure of macrocyclic antibiotic and the mobile phase buffer pH. The selectivity of the chiral selectors was measured as a function of changes in these parameters. All 11 dansyl amino acids were separated by at least one of the chiral selectors. Three-dimensional computer modeling of the more effective chiral selectors illustrated the importance of macrocyclic antibiotic structure concerning stereospecific analyte interaction.