Enantiomeric and diastereomeric high-performance liquid chromatographic separation of cyclic beta-substituted alpha-amino acids on a teicoplanin chiral stationary phase

Direct HPLC separation of carnosine enantiomers with two chiral stationary phases based on penicillamine and teicoplanin derivatives

Carnosine is present in high concentrations in specific human tissues such as the skeletal muscle, and among its biological functions, the remarkable scavenging activity toward reactive carbonyl species is noteworthy. Although the two enantiomers show almost identical scavenging reactivity toward reactive carbonyl species, only d-carnosine is poorly adsorbed at the gastrointestinal level and is stable in human plasma. Direct methods for the enantioselective analysis of carnosine are still missing even though they could find more effective applications in the analysis of complex matrices. In the present study, the use of two different chiral stationary phases is presented. A chiral ligand-exchange chromatography stationary phase based on N,S-dioctyl-d-penicillamine resulted in the direct enantioseparation of carnosine. Indeed, running the analysis at 25°C and 1.0 mL/min with a 1.5 mM copper(II) sulfate concentration allowed us to obtain separation and resolution factors of 3.37 and 12.34, respectively. However, the use of a copper(II)-containing eluent renders it hardly compatible with mass spectrometry detectors. With the teicoplanin-based stationary phase, a mass spectrometry compatible method was successfully developed. Indeed, a water/methanol 60:40 v/v pH 3.1 eluent flowed at 1.0 mL/min and with a 25°C column temperature produced separation and resolution factors of 2.60 and 4.16, respectively.

Enantioselective determination of protein amino acids in fertilizers by liquid chromatography-tandem mass spectrometry on chiral teicoplanin stationary phase

High-performance liquid chromatography on a glycopeptide antibiotic teicoplanin-based chiral stationary phase coupled with tandem mass spectrometry was developed for fast and reliable enantioseparation and determination of protein amino acids in hydrolyzed fertilizer samples. The effect of the mobile phase parameters (type and content of organic modifier and pH) and the column temperature on the enantioselectivity was investigated. Under optimized conditions, the majority (15 of 19) of d/l-amino acid pairs were resolved with a resolution factor (Rs) higher than 1.5 with a run time of 15 min. A triple quadrupole tandem mass spectrometer operating in multiple reaction monitoring mode with an electrospray ionization (ESI) ion source was employed for detection. The method was validated in terms of linearity, limits of detection, limits of quantitation, precision, and accuracy. Linear responses were obtained with determination coefficients higher than 0.998 for all analytes, and limits of detection were from 0.04 to 0.24 μg/mL. Sample spike/recovery experiments gave recovery values ranging from 73% for d-threonine to 116% for L-tryptophan. Relative standard deviations for inter- and intraday precision experiments were lower than 21.7%. The developed method was successfully applied for determination of the free amino acid enantiomers in five commercially available hydrolyzed protein fertilizer samples.

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.

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.

Study of mechanisms of chiral discrimination of amino acids and their derivatives on a teicoplanin-based chiral stationary phase

The behavior of a series of amino acids and some of their methyl ester hydrochloride, N-acetyl and N-tert-butyloxycarbonyl derivatives has been investigated on a teicoplanin-based chiral stationary phase by changing the chromatographic conditions, namely, the type and amount of mobile phase organic modifier and the ionic strength of the solutions. By using species with significantly different characteristics and chemical reactivity, some general conclusions regarding the chiral recognition process on this kind of stationary phase have been formulated. The importance of the carboxylic moiety for the formation of the complex between enantiomers and the aglycone basket of teicoplanin has been demonstrated via chromatography. Additionally, the increased possibility to make an hydrogen bond between the amidic hydrogen of the acetylated compounds and an amidic group on the stationary phase has been proposed to be pivotal for the stability of the complex aglycone D-enantiomer. Phenomena leading to the exclusion from the chiral stationary phase of one or both enantiomers have been rationalized by considering the ionic interactions between stationary phase, molecules to be separated and the surrounding medium and/or steric hindrance effects. The understanding of some of the observed phenomena may be important for optimizing the performance of the separation on aglycone-based media.

Enantioselective synthesis of (S)-2-amino-4-phenylbutanoic acid by the hydantoinase method

Biosynthesis of (S)-(+)-2-amino-4-phenylbutanoic acid (1) was performed by nonenantioselective hydantoinase and L-N-carbamoylase using racemic 5-[2-phenylethyl]-imidazolidine-2,4-dione (rac-2) as a substrate. The compounds involved in this biocatalysis process could be simultaneously resolved by high-performance liquid chromatography using Chirobiotic T column with a mobile phase of EtOH/H(2)O = 10/90 at pH 4.2-4.5. To our knowledge, this is the first report of the successful production of 1 by the combination of recombinant hydantoinase and L-N-carbamoylase.

Study of tryptophan enantiomer binding to a teicoplanin-based stationary phase using the perturbation technique. Investigation of the role of sodium perchlorate in solute retention and enantioselectivity

The retention of D,L-tryptophan enantiomers on an immobilized teicoplanin column was investigated in relation to the mobile phase sodium perchlorate concentration using the perturbation method to determine the solute distribution isotherms. From the experimental data, it appeared that the bi-Langmuir model was able to describe D- and L-enantiomer retention on the immobilized selector over the salt concentration range. An increase in the apparent enantioselectivity with an increase in sodium perchlorate concentration was observed. The chiral recognition enhancement was governed by (i) an increase in the difference of the adsorption constants for binding to the high-affinity site (aglycone pocket) between the two enantiomers and (ii) enhancement of the number of aglycone chiral regions interacting with D-tryptophan. It is suggested that an ion-pair formation mechanism between perchlorate and solute and/or selector is responsible for this behavior. In addition, this work shows that additional secondary sites on the teicoplanin surface are involved in the apparent enantioselectivity at low sodium perchlorate concentrations.

Effects of temperature on retention of chiral compounds on a ristocetin A chiral stationary phase

The isocratic retention of enantiomers of chiral analytes, i.e. tryptophan, 1,2,3,4-tetrahydroisoquinoline and gamma-butyrolac tone analogs, was studied on a ristocetin A chiral stationary phase at different temperatures and with different mobile phase compositions, using the reversed-phase, polar-organic and normal-phase modes. By variation of the both mobile phase composition and the temperature, baseline separations could be achieved for these enantiomers. The retention factors and selectivity factors for the enantiomers of all investigated compounds decreased with increasing temperature. The natural logarithms of the retention factors (ln k) of the investigated compounds depended linearly on the inverse of temperature (1/T). van't Hoff plots afforded thermodynamic parameters, such as the apparent change in enthalpy (deltaH(o)), the apparent change in entropy (deltaS(o)) and the apparent change in Gibbs free energy (deltaG(o) ) for the transfer of analyte from the mobile to the stationary phase. The thermodynamic parameters (deltaH(o), deltaS(o) and deltaG(o)) were calculated in order to promote an understanding of the thermodynamic driving forces for retention in this chromatographic system.

Direct chiral separation of unnatural amino acids by high-performance liquid chromatography on a ristocetin a-bonded stationary phase

Direct high-performance liquid chromatographic chiral separation of numerous underivatized unnatural amino acids on a ristocetin A-bonded chiral stationary phase used in the reversed-phase and in the polar organic chromatographic modes is reported. The effects of different parameters such as mobile phase composition, temperature, and the structure of the analytes on the selectivity in both chromatographic modes are discussed. By variation of the parameters, the separation of the stereoisomers was optimized and, as a result, baseline resolution was achieved in most cases.

Indirect high-performance liquid chromatographic separation of stereoisomers of beta-alkyl-substituted amino acids by the application of (S)-N-(4-nitrophenoxycarbonyl)phenylalanine methoxyethyl ester as chiral derivatizing agent

The indirect high-performance liquid chromatographic enantioresolution of beta-alkyl-substituted analogues of tyrosine, phenylalanine, 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid and tryptophan is reported. (S)-N-(4-Nitrophenoxycarbonyl)phenylalanine methoxyethyl ester, a recently developed chiral derivatizing agent, was used for pre-column derivatization of the investigated analytes. The diastereoisomers formed were analysed under reversed-phase conditions. The effects of parameters such as the amount and type of the organic modifier and the type of the stationary phase on the resolution and retention of the derivatives were investigated. Chromatographic conditions were found for the separation of all four stereoisomers of each analyte.

Comparison of three chiral stationary phases with respect to their enantio- and diastereoselectivity for cyclic beta-substituted alpha-amino acids

Three chiral stationary phases were examined for the enantio- and diastereoseparation of cycloaliphatic beta-substituted alpha-quaternary alpha-amino acids. Resolution of diastereomeric analytes is feasible with a chiral crown ether based column, whereas the separation of enantiomers, except for one pair of amino acids, could not be achieved. The two chiral stationary phases with the glycopeptide antibiotic teicoplanin and with the copper(II)-D-penicillamine complex, respectively, are, however, both very potent in the separation of the enantiomeric, as well as of the diastereomeric amino acids. A baseline separation of all four stereoisomeric forms in one chromatographic run was possible with the exception of one type of amino acid. The results of the method development are presented in this paper.

Dansyl amino acid enantiomer separation on a teicoplanin chiral stationary phase: effect of eluent pH

The retention and separation of a series of D,L dansyl amino acids (used as test solutes) on a teicoplanin stationary phase were investigated over a wide range of mobile phase (citrate buffer-methanol, 90:10, v/v) pH. An approach based on the development of various equilibria was carried out in order to describe the retention behavior of the solute in the chromatographic system. The equilibrium constants corresponding to the transfer of the anionic and zwitterionic forms of the dansyl amino acids from the mobile to the stationary phase were determined. These values allowed one to explain the decrease in the retention factor and the associated increase in the separation factor as the eluent pH was increased. Thermodynamic parameter variations were calculated so that the driving forces of the solute association with the teicoplanin phase were derived. This approach indicated that the chiral discrimination was principally controlled by the interaction between the anionic form of the solute and the stationary phase.

Analysis of the band profiles of the enantiomers of phenylglycine in liquid chromatography on bonded teicoplanin columns using the stochastic theory of chromatography

The retention behaviour of the enantiomers of underivatized phenylglycine was studied on a Chirobiotic T column packed with amphoteric glycopeptide teicoplanin covalently bonded to the surface of silica gel. The retention and the selectivity of separation of the enantiomers increase with rising concentration of ethanol or of methanol in aqueous-organic mobile phases. The band profiles of the less retained L-phenylglycine are symmetrical, but the band profiles of the more strongly retained D-phenylglycine are tailing in all mobile phases tested. The band broadening does not diminish even at very low concentrations of phenylglycine, so that it cannot be attributed to possible column overload. The analysis of the band profile using the stochastic theory of chromatography suggests that the broadening can be attributed to at least two additional chiral centres of adsorption in the stationary phase contributing to the retention of the more strongly retained enantiomer in addition to the adsorption of the less retained one. This behaviour can be explained by the complex structure of the teicoplanin chiral stationary phase.

Effect of the mobile phase on the retention behaviour of optical isomers of carboxylic acids and amino acids in liquid chromatography on bonded Teicoplanin columns

Conditions for separation of enantiomers of underivatized amino acids phenyl glycine and tryptophan and of mandelic acid as test compounds were studied on a Chirobiotic T column packed with amphoteric glycopeptide Teicoplanin covalently bonded to the surface of silica gel. The effects of the mobile phase composition on the retention and selectivity under analytical conditions, on the profile of the adsorption isotherms of the enantiomers and on the overloaded separation were investigated. The concentration of ethanol or of methanol in aqueous-organic mobile phases and the pH of the mobile phase affect not only the retention and selectivity, the saturation capacity and the isotherm profile, but also the solubility of the acids, which should be taken into account in development of preparative separations. A compromise between the separation selectivity and the solubility should be made in selecting the mobile phase suitable to accomplish preparative separations at acceptable production rate and throughput of the operation.

Simultaneous analysis of underivatized chiral amino acids by liquid chromatography–ionspray tandem mass spectrometry using a teicoplanin chiral stationary phase

Simultaneous chiral separations of underivatized amino acids have been performed using a teicoplanin-based chiral stationary phase and ionspray tandem mass spectrometry for their ionisation and detection. Different amino acid enantiomer pairs were separated simultaneously, including those of positional isomeric amino acids (e.g., L,D-Leu/Ile, or L,D-Val/Iva). Due to the specificity of tandem mass spectrometry, co-eluting enantiomers of different amino acids could also be determined. Fifteen chiral underivatized proteinogenic and non-proteinogenic amino acids were analysed simultaneously under isocratic conditions (acetonitrile-water, 75:25) in less than 25 min. For maximum sensitivity, post-column addition of 500 mM aqueous HCOOH was necessary. Detection limits varied from 2.5 to 50 microg l(-1) depending on the amino acid. The signal vs. concentration relationship was linear for all D- and L-amino acids (0.9995 < or = r2 < or = 1) for three orders of magnitude.

Enantiomeric and diastereomeric high-performance liquid chromatographic separation of cyclic beta-substituted alpha-amino acids on a copper(II)-D-penicillamine chiral stationary phase

High-performance liquid chromatographic (HPLC) separation of stereoisomeric cyclic beta-substituted alpha-quaternary alpha-amino acids was performed by ligand-exchange on a copper(II)-D-penicillamine chiral stationary phase. The investigated amino acids are the 1-amino-2-methylcyclohexanecarboxylic acids, the 1-amino-2-hydroxycyclohexanecarboxylic acids, the 1-amino-2-methylcyclopentanecarboxylic acids and the trans-configured 1,2-diaminocyclohexanecarboxylic acids. The effects of the mobile phase composition (copper(II) concentration, type and content of organic modifier, pH) and the temperature on the enantio- and diastereoselectivity were studied and the conditions were optimised to resolve the four stereoisomers of each of the said amino acids in single chromatographic runs. A reversal of the elution order occurred for enantiomers of some of the amino acids in dependence on the acetonitrile content of the eluent. This phenomenon is explained by at least two different copper(II) complexes of the tridentate ligand penicillamine.