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

Recent advances in chiral selectors immobilization and chiral mobile phase additives in liquid chromatographic enantio-separations: A review

For decades now, the separation of chiral enantiomers of drugs has been gaining the interest and attention of researchers. In 1991, the first guidelines for development of chiral drugs were firstly released by the US-FDA. Since then, the development in chromatographic enantioseparation tools has been fast and variable, aiming at creating a suitable environment where the physically and chemically identical enantiomers can be separated. Among those tools, the immobilization of chiral selectors (CS) on different stationary phases and the chiral mobile phase additives (CMPA) which have been progressed and studied extensively. This review article highlights the major advances in immobilization of CS together with their different recognition mechanisms as well as CMPA as a cheaper and successful alternative for chiral stationary phases. Moreover, the role of molecular modeling tool as a pre-step in the choice of CS for evaluating possible interactions with different ligands has been pointed up. Illustrations of reported methods and updates for immobilized CS and CMPA have been included.

Capillary electrophoresis and molecular modeling of the chiral separation of aromatic amino acids using α/β-cyclodextrin and 18-crown-6

In this work, chiral separation of enantiomers of three amino acids was achieved using capillary electrophoresis technique with α-cyclodextrin (αCD) as a running buffer additive. Only tryptophan has exhibited baseline separation in the presence of αCD, while the enantiomers of the other two amino acids, phenylalanine and tyrosine, were only partially separated. The addition of 18-crown-6 (18C6) as a second additive imparted only slight improvement to the separation of all enantiomers. On the other hand, all three racemic amino acid mixtures demonstrated no indication of separation when the larger cavity cyclodextrin members, β- and γCD, are used as running buffer chiral additives. However, remarkable improvements in the separation of the enantiomers of phenylalanine and tyrosine were obtained when 18C6 is used together with βCD as a running buffer additive. Surprisingly, tryptophan enantiomers were not separated by the dual additive system of cyclodextrin and crown ether. Using electrospray ionization mass spectrometry (ESI-MS), all amino acids were found to form stable binary complexes with individual hosts as well as ternary compounds involving the crown ether and the cyclodextrin. Furthermore, we used molecular dynamics (MD) simulations to build a clear picture about the interaction between the guest and the hosts. Most of these complexes remained stable throughout the simulation times, and the molecular dynamics study allowed better understanding of these supramolecular assemblies.

Computational modeling of capillary electrophoretic behavior of primary amines using dual system of 18-crown-6 and β-cyclodextrin

Using capillary electrophoresis (CE) three chiral primary amine compounds 1-aminoindan (AI), 1-(1-naphthyl)ethylamine (NEA) and 1,2,3,4-tetrahydro-1-naphthylamine (THAN), exhibited only partial or no separation when β-cyclodextrin (βCD) was used as chiral selector. The use of 18-crown-6 (18C6) as a second additive with βCD resulted in an enhanced separation. A molecular modeling study, using molecular mechanics and the semiempirical PM6 calculations, was used to help explaining the mechanism of the enantiodifferentiation and to predict the separation process. Optimization of the structures of the complexes by the PM6 method indicate that the poor separation obtained in the presence of the βCD chiral selector alone is due to the small binding energy differences (ΔΔE) of 4.7, 1.1 and 1.2 kcal mol(-1) for AI, NEA and THAN, respectively. In the presence of 18C6 it was suggested that a sandwich compound between 18C6, amine and βCD is formed. Theoretical calculations show that a significant increase in the binding energy is obtained for the sandwich compounds indicating strong hydrophobic and van der Waals interactions that show enhanced enantiodifferentiation.

Enantioseparation of dansyl amino acids by HPLC on a monolithic column dynamically coated with a vancomycin derivative

In this work a chiral stationary phase was prepared by dynamically coating a monolithic reversed-phase HPLC column with a vancomycin-derivative as chiral selector. A hydrophobic alkyl-chain was attached to the vancomycin molecule, providing the immobilization of the chiral selector on the reversed-phase material. Dansyl amino acids were chosen as model analytes for testing the separation power of the dynamically coated phase. All investigated compounds were separated into their enantiomers. Compared with a conventionally packed vancomycin-CSP, a reversal of the enantiomer elution order was obtained.

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.

Chiral separation of three agrochemical toxins enantiomers by high-performance liquid chromatography on a vancomycin crystalline degradation products-chiral stationary phase

This work aims to evaluate for the enantiomeric separations of three agrochemical toxins: haloxyfop-methyl, fenoxaprop-p-ethyl and indoxacarb on crystalline degradation products-chiral stationary phase (CDP-CSP) of high-performance liquid chromatography (HPLC) under normal and polar organic phases. In the normal phase, the mobile phase was n-hexane with alcohols including methanol and isopropanol as polar modifiers. In the polar organic phase mode, the mobile phase was methanol with different percentages of triethylammunium acetate. The influence of flow rate (0.3-0.9 mL/min), analyte concentration and silica gel particle sizes (10, 15 and 30 microm) was investigated. This new chiral stationary phase showed excellent stereoselectivity for the two enantiomers of haloxyfop-methyl and fenoxaprop-p-ethyl and chiral recognition for indoxacarb under normal-phase mode. However, under polar organic phase, only indoxacarb was separated (alpha < 1.5). The chromatographic results were compared with commercial chiral columns.