Facile liquid chromatographic enantioresolution of native amino acids and peptides using a teicoplanin chiral stationary phase

Convenient Synthesis of Acetonide Protected 3,4-Dihydroxyphenylalanine (DOPA) for Fmoc Solid-Phase Peptide Synthesis

We report a facile approach to the synthesis of acetonide and Fmoc protected 3,4-dihydroxyphenylalanine (DOPA), Fmoc-DOPA(acetonide)-OH. By protecting the amino group of DOPA with a phthaloyl group and the carboxyl group as a methyl ester, acetonide protection of the catechol of DOPA derivative was realized in the presence of p-toluenesulfonic acid. Following removal of protecting groups, the intermediate was converted to Fmoc-DOPA(acetonide)-OH, which was successfully incorporated into a short DOPA-containing peptide, derived from marine tubeworm cement proteins Pc1 and Pc2.

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.

Considerations on HILIC and polar organic solvent-based separations: use of cyclodextrin and macrocyclic glycopetide stationary phases

There is a natural tendency in science to prefer straightforward, logical classification systems. The use of mobile phase-stationary phase combinations that do not fit neatly into the standard "normal phase" or "reversed-phase" categories has been going on for over 50 years. The term "hydrophilic interaction chromatography" (HILIC) is sometimes being used as a general category for these "other" separations. In some cases, it may be appropriate and in others, not. Indeed the mechanistic constrains used to define the method seem to be varying with time. Given the name HILIC, it is assumed that water is not only present in the mobile phase, but also plays an essential role in the retention mechanism. However, there is residual water present in all organic solvents. Regardless, the number of reported separations in this alternative mode has increased tremendously in the last two decades. This is due to the advent of new stationary phases and an emphasis on polar, biologically important molecules. We discuss the relationships between HILIC and other chromatographic modes. We then examine two classes of stationary phases that have played a major role in these separations. These particular stationary phases can be used to provide appreciable mechanistic information as well.

Determination of D/L-amino acids by zero needle voltage electrospray ionisation

Ion formation may be made more efficient than in normal electrospray ionization (ESI) for certain classes of compounds, such as the polar amino acids Glu, Asn, His, Ser, Asp, Arg, Tyr and Lys, by adjusting the voltage of a normal ESI interface needle to zero voltage. For aspartic acid (Asp) the gain in signal-to-noise (S/N) ratio of the liquid chromatography/mass spectrometry (LC/MS) chromatograms obtained in the selective ion monitoring (SIM) mode (m/z 134) with zero needle potential was 40-50 times higher than detection at 4 kV. Ion formation at zero potential is likely to follow a mechanism related to sonic spray ionization. The utility of the zero needle voltage ESI was illustrated by determining the age of a human tooth by the aspartic acid epimerization method. The procedure involved separating the D- and L-aspartic acid of a tooth extract on a chiral HPLC column and detection by zero voltage ESI-MS3.

Chromatographic enantioseparation of amino acids using a new chiral stationary phase based on a macrocyclic glycopeptide antibiotic

The separation of the enantiomers of several a-amino acids was studied on a new chiral stationary phase (CSP) which is based on the macrocyclic glycopeptide antibiotic eremomycin attached to silica particles. Retention and separation factors were determined under analytical conditions at ambient temperature for different mobile phase compositions. In order to evaluate the potential with respect to preparative separations the adsorption isotherms of D- and L-methionine were determined for one mobile phase composition applying the elution by characteristic point method. The isotherms were validated by comparing experimentally determined elution profiles with predictions based on the equilibrium dispersive model. Finally, the performance of the eremomycin CSP was compared with a commercially available CSP based on the macrocyclic antibiotic teicoplanin. After determining the isotherms of D- and L-methionine also for the teicoplanin phase, the equilibrium dispersive model was used for both CSP to identify optimal operating conditions. For the separation and conditions considered the new eremomycin CSP revealed a better performance compared to the teicoplanin CSP.

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.

Could linear solvation energy relationships give insights into chiral recognition mechanisms? 2. Characterization of macrocyclic glycopeptide stationary phases

Five parameter linear solvation energy relationships (LSER) are known to have little or no shape recognition ability. However, it is proposed to use LSER studies to get insights into chiral recognition mechanisms. Since the two enantiomers have exactly the same five A-V solute descriptors being still separated by chiral stationary phases (CSPs), it can be considered that they form two different transient diastereoisomers with the CSP. It is then possible to perform LSER studies on the enantioselectivity factors taken as the two enantiomer retention factor ratios. In a first step, the five a-v system parameters of four CSPs of the macrocyclic glycopeptide types were determined using a set of test solutes with known A-V descriptors, both in the reversed phase and the normal phase modes. In a second step, the A-V descriptors of 18 enantiomeric pairs were tentatively established using five achiral columns with known a-v parameters. This was successful for the five molecular enantiomers only. It was found that the predicted retention factor for the molecular enantiomers separated on a given CSP corresponded either to retention factor of the first experimentally eluted enantiomer or to the second one or to none of them. Using the enantioselectivity factors it was possible to obtain the Deltaa-Deltav parameters corresponding to the difference in CSP properties seen by the two enantiomers. For the five molecular enantiomeric pairs in the reversed phase mode with a teicoplanin CSP, it was found that there was an elevated contribution by the e coefficient that we interpret as a possible interaction between surface charges on the teicoplanin CSP and solute induced dipoles. Steric effects, seen on the v parameter, are second in magnitude followed by H-bond and polar interactions. Only one solute could be studied in the normal phase mode showing a different mechanism with polar and steric major interactions.

Tuneable Peak Deformations in Chiral Liquid Chromatography

Modern chiral stationary phases are often combined with eluents comprising a mixture of organic solvents and polar additives. The latter may cause extreme deformations of the eluted enantiomer bands in both analytical and preparative separations. In this work, we give a theoretical background for these deformations. As an experimental verification, we separate the enantiomers of different beta-blockers on a teicoplanin stationary phase (Chirobiotic T) in the presence of triethylamine/acetic acid. We show that it is possible to tune the peak shapes of the two enantiomers by varying the organic solvent composition. An advantageous situation occurs when the first eluted peak is transformed to an anti-Langmuirian shape while keeping the second enantiomer in a normal Langmuirian shape. In this situation, the two peaks tail in opposite directions with their sharp sides pointing closely to each other. It is then possible to obtain baseline resolution at higher load than when both enantiomer peaks tail in the same direction. Adsorption isotherm parameters were determined using the inverse method; no other method could be used due to the system complexity. Computer simulations, based on these parameters, agreed very well with the observed deformations, thus confirming our hypothesis of their origin.

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.

Quantification of D-amino acids in the central nervous system of Aplysia californica by liquid chromatography/tandem mass spectrometry

A sensitive, specific and reliable liquid chromatography/tandem mass spectrometry (LC/MS/MS) method has been developed for simultaneous determination of D-amino acids in the central nervous system (CNS) of Aplysia californica. In order to correct for any potential matrix effects on measured signals, deuterium-labeled L-Asp-d3 was used as an internal standard. Pre-column derivatization of the sample with 7-fluoro-4-nitrobenzoxadiazole (NBD-F) allowed both effective in-line pre-concentration and sensitive MS/MS detection of the analytes. An extraction column (50x0.25 mm, 5 microm C18 silica particles) was used to pre-concentrate/stack samples. Enantiomeric separation of amino acid enantiomers was achieved on a chiral column packed with teicoplanin aglycone bonded silica particles (170x0.25 mm, 5 microm) with an MS-friendly mobile phase. The characteristic precursor to product ion transitions, m/z 297-->279 (for NBD-Asp), m/z 269-->223 (For NBD-Ser), m/z 311-->293 (for NBD-Glu) and m/z 300-->282 (for NBD-L-Asp-d3) were monitored for the quantification. Samples from the CNS of A. californica and heart tissues were analyzed. D-Asp was detected at high levels in all the ganglia and nerve tissues, but not in the heart tissue. Further, neither D-Ser nor D-Glu was detected in Aplysia, a widely used neuronal model.

Linear free energy relationship as a tool for characterization of three teicoplanin-based chiral stationary phases under various mobile phase compositions

Teicoplanin, teicoplanin aglycon, and methylated teicoplanin aglycon chiral stationary phases (CSPs) have been compared on the basis of the regression coefficients calculated from the linear free energy relationship (LFER) equation. The parameters have been obtained from the measurements of a set of 34 structurally diverse solutes. Influence of mobile phase composition - variation of methanol (MeOH) content - on the participation of different interactions types in the retention mechanism has been evaluated. Retention of the various interaction forces in analytes differs with both the CSP and the mobile phase composition. Hydrophobic interactions play a major role in mobile phases for high buffer contents. The more hydrophobic the CSP, the more important are they in the retention mechanism. With increase of MeOH contents in the mobile phase the major role in the interaction mechanism is shifted to more polar forces in which basicity and dipolarity/polarizability dominate. Although the LFER model does not address chiral aspects, we have attempted to explore the importance of the individual interactions in chiral discrimination of amino acids and their N-tert-butyloxycarbonyl derivatives.

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.

Integrated operation of continuous chromatography and biotransformations for the generic high yield production of fine chemicals

The rapid progress in biocatalysis in the identification and development of enzymes over the last decade has enormously enlarged the chemical reaction space that can be addressed not only in research applications, but also on industrial scale. This enables us to consider even those groups of reactions that are very promising from a synthetic point of view, but suffer from drawbacks on process level, such as an unfavourable position of the reaction equilibrium. Prominent examples stem from the aldolase-catalyzed enantioselective carbon-carbon bond forming reactions, reactions catalyzed by isomerising enzymes, and reactions that are kinetically controlled. On the other hand, continuous chromatography concepts such as the simulating moving bed technology have matured and are increasingly realized on industrial scale for the efficient separation of difficult compound mixtures - including enantiomers - with unprecedented efficiency. We propose that coupling of enzyme reactor and continuous chromatography is a very suitable and potentially generic process concept to address the thermodynamic limitations of a host of promising biotransformations. This way, it should be possible to establish novel in situ product recovery processes of unprecedented efficiency and selectivity that represent a feasible way to recruit novel biocatalysts to the industrial portfolio.

Evaluation and comparison of a methylated teicoplanin aglycone to teicoplanin aglycone and natural teicoplanin chiral stationary phases

HPLC enantiomeric separations of a wide variety of racemic analytes was evaluated using chiral stationary phases (CSPs) based on the macrocyclic glycopeptides teicoplanin (T), teicoplanin aglycone (TAG), and methylated teicoplanin aglycone (Me-TAG) in two different mobile phase modes, i.e., the RP mode and the polar organic (PO) mode. Comparison of the enantiomeric separations using Chirobiotic T, Chirobiotic TAG, and the methylated form of TAG were conducted in order to gain a better understanding of the roles of the polar functional groups on the CSP. Substantial effects due to the cleavage of saccharides and/or methylation on chiral separations were observed in both separation modes. Improved separation efficiencies for many acidic analytes were obtained by methylating the H-bonding groups of TAG. These groups were believed to be a contributing factor to band broadening on TAG due to their negative effect on mass transfer between the stationary phase and mobile phase. Ionic/dipolar interactions between the carboxylate group of the analytes and the amine groups on T, TAG, or Me-TAG are important for chiral discrimination. Therefore, analytes possessing a carboxyl group are good candidates for successful separations on these CSPs. Hydrophobic interactions are important for enantiomeric separations in the RP mode where the H-bonding interactions between analytes and the chiral selectors are relatively weak. Me-TAG offers higher hydrophobicity, which can accentuate the interactions of analytes with hydrophobic moieties, but these interactions are not necessarily stereoselective. In the PO mobile phase, electrostatic/dipolar interactions between polar functional groups are the dominating interactions in chiral recognition. Another important factor is steric fit, which could be changed with every modification of the T structure. Therefore, substantial changes of enantioseparations were obtained within this studied group of CSPs. The PO mode was shown to be the most powerful mobile phase mode for enantiomeric separations on T-based stationary phases, mainly due to the improved efficiency. Methylation of the TAG proved to be a very useful tool for investigating the chiral recognition mechanism for this group of chiral selectors.

Twenty years of research on silica-based chiral stationary phases

This review provides an overview of twenty years of pioneering work (from 1985 to 2005) of our research group in the preparation and application of enantioselective packing materials for HPLC. After a brief introduction to the rational design of a new chiral stationary phase, a detailed presentation in chronological order of appearance in the literature is given of the currently developed repertoire of chiral stationary phases and their typical applications. Emphasis is placed on the different synthetic strategies exploited to obtain highly efficient, stable, and versatile chiral stationary phases.

Suitability of teicoplanin–aglycone bonded stationary phase for simulated moving bed enantioseparation of racemic amino acids employing composition-constrained eluents

The suitability of a teicoplanin-aglycone based chiral stationary phase for the simulated moving bed (SMB) enantioseparation of amino acids under enzyme-compatible conditions was shown following a procedure that is based solely on model-based simulations and HPLC experiments. A set of eight amino acids could be separated employing aqueous solvent containing only 10% (v/v) methanol, five of them with baseline resolution. The impact of type and concentration of organic modifier and pH modifier and pH on the separation characteristics of racemic methionine was investigated. Invariant elution profiles of repetitive adsorption/desorption of large amounts of methionine representing SMB-like conditions suggest stable adsorption behavior. Competitive loading capacity (20 mg of methionine per g of chiral stationary phase (CSP)) and SMB productivity (1 g of D-methionine per g of CSP per day) were predicted. The applied transport-dispersive model based on a competitive Bi-Langmuir isotherm was validated and its parameter estimated by model-based experimental analysis.

Enantiomeric separation of amino acids derivatized with 7-fluoro-4-nitrobenzoxadiazole by capillary liquid chromatography/tandem mass spectrometry

Pre-column derivatization allowed stacking amino acid enantiomers on C18 reversed-phase micro extraction columns, thus facilitating sample loading in capillary HPLC/tandem mass spectrometry. Two tagging reagents, i.e. 7-fluoro-4-nitrobenzoxadiazole (NBD-F) and 1-fluoro-2,4-dinitrobenzene (DNB-F) were evaluated. Both of them reacted readily with amino acids at an elevated temperature, resulting in derivatives that were effectively stacked and suitable for a sensitive MS/MS detection as well. Separation of the tagged enantiomers on a teicoplanin chiral stationary phase (CSP) with mobile phases compatible with MS detection was investigated. NBD-amino acid enantiomers (12 pairs) tested were all base-line resolved. However, the efforts to separate DNB-F tagged amino acid enantiomers on this CSP were not successful. Separation conditions including pH, organic modifiers, and column dimension were studied. All the NBD-amino acids studied could be sensitively detected by MS/MS detection set in the negative ion mode, but only a few including NBD-Asp, BND-Glu, NBD-Ser, and NBD-Thr were detected in the positive ion mode. Thus, the selectivity for enantiomeric determination of excitatory amino acids (e.g. Asp and Glu) was further improved by choosing MS/MS detection in the positive ion mode.

Gas-chromatographic separation of stereoisomers of dipeptides

Synthetic dipeptides comprising mixtures of enantiomers, diastereomers, or sequential isomers were converted into their N-perfluoroacetyl dipeptide esters (perfluoroacetyl: trifluoroacetyl, pentafluoroacetyl, heptafluorobutyryl; ester: methyl, 1-propyl, 2-propyl, 2,2,2-trifluoroethyl) and analyzed by GC-MS on the chiral stationary phases Chirasil-L-Val and Lipodex-E using helium as carrier gas. Further, dipeptides were converted into their N-trifluoroacetyl dipeptide S-(+)-2-butyl esters and separated on achiral phenylmethyl polysiloxane column (HP-5 MS). Derivatization of dipeptides was performed at ambient temperature in order to avoid formation of the corresponding diketopiperazines. The best separation of stereoisomers was achieved with TFA and PFP methyl esters on Chirasil-L-Val.

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).

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.

Positional and geometrical anionic isomer separations by capillary electrophoresis-electrospray ionization-mass spectrometry

Capillary electrophoresis-electrospray ionization-mass spectrometry (CE-ESI-MS) was applied to the analysis of polar positional and geometrical anionic isomers. Since the investigated positional and geometrical anionic isomers have different pK(a) values, they could be separated by CE-ESI-MS under simple analytical conditions using a bare fused-silica capillary and volatile ammonium acetate buffer after optimizing buffer pH. Ortho-, meta-, para-hydroxybenzoate positional isomers were completely separated on a fused-silica capillary with 20 mM ammonium acetate buffer at pH 10.0, and cis-, trans-cyclohexane dicarboxylate geometrical isomers could be also separated with 20 mM ammonium acetate buffer at pH 4.0. Several analytical parameters affecting ESI-MS sensitivity were also investigated. It was found that both running buffer pH and sheath liquid pH had significant effects on the selectivity and the sensitivity on CE-ESI-MS analysis while sheath flow rate and other parameters had little influence. Under optimized conditions, linearity, detection limit, and repeatability of the analysis of hydroxybenzoate isomers were examined, and good results were obtained. It was found that the method presented in this paper is a simple, robust, and cost-effective method for simultaneous analysis of positional and geometrical anionic isomers as well as of other small anionic compounds.

Chiral separation of beta-adrenergic antagonists, profen non-steroidal anti-inflammatory drugs and chlorophenoxypropionic acid herbicides using teicoplanin as the chiral selector in capillary liquid chromatography

Three groups of structurally diverse chiral compounds were used to study the interaction mechanism responsible for stereoselective recognition with teicoplanin as chiral selector in capillary liquid chromatography. Teicoplanin-based chiral stationary phase (CSP) was used. The effect of the variation of mobile phase composition on retention and enantioselective separation was studied. The mobile phase composition suitable for enantioresolution of the various chiral compounds differed according to the interaction forces needed for chiral recognition. Mobile phases with high buffer portion (70-90 vol.%) were preferred for separation of enantiomers of profen non-steroidal anti-inflammatory drugs and chlorophenoxypropionic acid herbicides that require hydrophobic interactions, inclusion and pi-pi interactions for stereoselective recognition with teicoplanin. Higher concentration triethylamine in the buffer (0.5-1.0%) increased resolution of these acids. On the other hand, H-bonding and electrostatic interactions are important in stereoselective interaction mechanism of beta-adrenergic antagonists with teicoplanin. These interaction types predominate in the reversed phase separation mode with high organic modifier content (95% methanol) and in polar organic mobile phases. For this reason beta-adrenergic antagonists were best enantioresolved in the polar organic mode. The mobile phase composed of methanol/acetic acid/triethylamine, 100/0.01/0.01 (v/v/v), provided enantioresolution values of all the studied beta-adrenergic antagonists in the range 1.1-1.9. Addition of teicoplanin to the mobile phase, which was suitable for enantioseparation of certain compounds on the CSP, was also investigated. This system was used to dispose of nonstereoselective interactions of analytes with silica gel support that often participate in the interaction with CSPs. Very low concentration of teicoplanin in the mobile phase (0.1 mM) resulted in enantioselective separation of 2,2- and 2,4-chlorophenoxypropionic acids.

Comparative study of three teicoplanin-based chiral stationary phases using the linear free energy relationship model

Teicoplanin (T) is a macrocyclic glycopeptide that is highly effective as a chiral selector for enantiomeric separations. In this study, we used three teicoplanin-based chiral stationary phases (CSPs) - native teicoplanin, teicoplanin aglycon (TAG) and recently synthesized methylated teicoplanin aglycon (MTAG). In order to examine the importance of various interaction types in the chiral recognition mechanism the three related CSPs were evaluated and compared using a linear free energy relationship (LFER). The capacity factors of 19 widely different solutes, with known solvation parameters, were determined on each of the columns under the same mobile phase conditions used for the chiral separations. The regression coefficients obtained revealed the magnitude of the contribution of individual interaction types to the retention on the compared columns under those specific experimental conditions. Statistically derived standardized regression coefficients were used to evaluate the contribution of individual molecular interactions within one stationary phase. It has been concluded that intermolecular interactions of the hydrophobic type significantly contribute to retention on all the CSPs studied here. Other retention increasing factors are n- and pi-electron interactions and dipole-dipole or dipole-induced dipole ones, while hydrogen donating or accepting interactions are more predominant with the mobile phase than with the stationary phases. However, these types of interactions are not equally significant for all the CSPs studied.

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.

Adsorption Equilibria of Benzodiazepines on a Hybrid Polymeric Chiral Stationary Phase

The chromatographic behavior of a series of racemic benzodiazepines was evaluated under linear and nonlinear conditions on a new hybrid polymeric (DACH-ACR) chiral stationary phase (CSP). Differently substituted benzodiazepines were employed as probes to make hypotheses concerning possible molecular interaction mechanisms originating between target compounds and active sites on the CSP. Hydrogen bonds were found to be pivotal for chromatographic retention and chiral selectivity. The competitive effect from a mobile-phase (MP) modifier able to interact with the CSP through H-bonds was investigated. The performance of the polymeric DACH-ACR CSP for preparative purposes was also evaluated. The competitive adsorption isotherms of two benzodiazepines, lorazepam and temazepam, were measured at different MP compositions through the so-called inverse method. The adsorption data were fitted with a competitive bi-Langmuir adsorption isotherm. Enantiomeric separations under nonlinear conditions were modeled by using the equilibrium dispersive (ED) model of chromatography. Theoretical overloaded band profiles (obtained by solving the system of partial differential equations described by the ED model) matched, in a significantly accurate way, the profiles experimentally measured.

Enantioselective chromatography in drug discovery

Molecular chirality is a fundamental consideration in drug discovery, one necessary to understand and describe biological targets as well as to design effective pharmaceutical agents. Enantioselective chromatography has played an increasing role not only as an analytical tool for chiral analyses, but also as a preparative technique to obtain pure enantiomers from racemates quickly from a wide diversity of chemical structures. Different enantioselective chromatography techniques are reviewed here, with particular emphasis on the most widespread high performance liquid chromatography (HPLC) and the rapidly emerging supercritical fluid chromatography (SFC) techniques. This review focuses on the dramatic advances in the chiral stationary phases (CSPs) that have made HPLC and SFC indispensable techniques for drug discovery today. In addition, screening strategies for rapid method development and considerations for laboratory-scale preparative separation are discussed and recent achievements are highlighted.

Thermodynamic origin of the chiral recognition of tryptophan on teicoplanin and teicoplanin aglycone stationary phases

The D-, L-tryptophan binding and the chiral recognition properties of the teicoplanin and teicoplanin aglycone (TAG) chiral stationary phase (CSPs) were compared at various column temperatures. The solute adsorption isotherms (bi-Langmuir model) were determined for both the two CSPs using the perturbation method. It was demonstrated that the sugar units were involved in the reduction of the apparent enantioselectivity through two phenomena: (i) the inhibition of some enantioselective contacts with low-affinity binding regions of the aglycone and (ii) a decrease in the stereoselective properties of the aglycone high-affinity binding pocket. The phenomenon (ii) was governed by both a decrease in the ratio of the enantiomer adsorption constant and a strong reduction of the site accessibility for D- and L-tryptophan. In addition, a temperature effect study was performed to investigate the chiral recognition mechanism at the aglycone high-affinity pocket. An enthalpy-entropy compensation analysis derived from the Grunwald model as well as the comparison with the literature data demonstrated that the enantioselective binding mode was dependent on an interface dehydration process. The change in the enantioselective process observed between the TAG and teicoplanin CSP was characterized by a difference of ca. 2-3 ordered water molecules released from the species interface.