Liquid chromatographic enantioseparation of carbocyclic β-amino acids possessing limonene skeleton on macrocyclic glycopeptide-based chiral stationary phases

Enantioseparation of ß2-amino acids by liquid chromatography using core-shell chiral stationary phases based on teicoplanin and teicoplanin aglycone

Enantioseparation of nineteen ß²-amino acids has been performed by liquid chromatography on chiral stationary phases based on native teicoplanin and teicoplanin aglycone covalently bonded to 2.7 µm superficially porous silica particles. Separations were carried out in unbuffered (water/methanol), buffered [aqueous triethylammonium acetate (TEAA)/methanol] reversed-phase (RP) mode, and in polar-ionic (TEAA containing acetonitrile/methanol) mobile phases. Effects of pH in the RP mode, acid and salt additives, as well as counter-ion concentrations on chromatographic parameters have been studied. The structure of selectands (ß²-amino acids possessing aliphatic or aromatic side chains) and selectors (native teicoplanin or teicoplanin aglycone) was found to have a considerable influence on separation performance. Analysis of van Deemter plots and determination of thermodynamic parameters were performed to further explore details of the separation performance.

Enantioselective Liquid Chromatographic Separations Using Macrocyclic Glycopeptide-Based Chiral Selectors

Numerous chemical compounds of high practical importance, such as drugs, fertilizers, and food additives are being commercialized as racemic mixtures, although in most cases only one of the isomers possesses the desirable properties. As our understanding of the biological actions of chiral compounds has improved, the investigation of the pharmacological and toxicological properties has become more and more important. Chirality has become a major issue in the pharmaceutical industry; therefore, there is a continuous demand to extend the available analytical methods for enantiomeric separations and enhance their efficiency. Direct liquid chromatography methods based on the application of chiral stationary phases have become a very sophisticated field of enantiomeric separations by now. Hundreds of chiral stationary phases have been commercialized so far. Among these, macrocyclic glycopeptide-based chiral selectors have proved to be an exceptionally useful class of chiral selectors for the separation of enantiomers of biological and pharmacological importance. This review focuses on direct liquid chromatography-based enantiomer separations, applying macrocyclic glycopeptide-based chiral selectors. Special attention is paid to the characterization of the physico-chemical properties of these macrocyclic glycopeptide antibiotics providing detailed information on their applications published recently.

Preparation and evaluation of biselector bonded-type multifunctional chiral stationary phase based on dialdehyde cellulose and 6-monodeoxy-6-monoamino-β-cyclodextrine derivatives

A novel biselector bonded-type multifunctional chiral stationary phase (MCSP) was prepared by covalently crosslinking dialdehyde cellulose (DAC) with 6-monodeoxy-6-monoamino-β-cyclodextrine (CD) via Schiff base reaction. The biselector bonded-type MCSP had good chiral and achiral chromatographic performance in normal phase (NP) and reversed phase (RP) modes. Seven and eight enantiomers were successfully separated on the prepared biselector bonded-type MCSP in NP and RP modes, respectively. The biselector bonded-type MCSP showed enhanced chiral resolution ability compared with single selector chiral stationary phases due to the simultaneous introduction of DAC and 6-monodeoxy-6-monoamino-β-CD on the surface of silica gel. Aromatic compounds including polycyclic aromatic hydrocarbons, anilines, phenols, phenylates, and aromatic acids were choosed as analytes to investigate the achiral chromatographic performance of the biselector bonded-type MCSP in NP and RP modes. Chromatographic evaluation results showed that the above aromatic compounds were essentially capable of achieving baseline separation by hydrophobic interaction, π-π interaction, and π-π electron-donor-acceptor interaction. Moreover, the host-guest inclusion effect of 6-monodeoxy-6-monoamino-β-CD and the multiple interactions made the biselector bonded-type MCSP have good steric selectivity. The preparation method of the biselector bonded-type MCSP was simple and provided a new idea and strategy for the preparation of the subsequent novel biselector MCSP.

Construction of β-Cyclodextrin Covalent Organic Framework-Modified Chiral Stationary Phase for Chiral Separation

Chiral covalent organic frameworks (CCOFs), built by the condensation reactions of organic building blocks with enantiomeric purity and linking subunits, have emerged as a marvelous category of porous crystalline material. In addition to stability and good porosity, CCOFs possess remarkable enantioselectivity, which would be exploited for asymmetric catalysis and chiral separation. β-cyclodextrin (β-CD) and its derivatives, a group of supramolecules with a hydrophobic cavity, have been widely applied to molecular specific recognitions. In this work, the β-CD covalent organic framework (COF) was exploited to construct chiral stationary phase (CSP) for chiral drugs analysis for the first time. We fabricated β-CD COF via the condensation reaction of heptakis(6-amino-6-deoxy)-β-CD and terephthalaldehyde at room temperature. β-CD COF-modified capillary columns were subsequently prepared by a photopolymerization method with shortened time and applied for separation of chiral drugs on capillary electrochromatography with good resolution and repeatability. Baseline separation for six enantiomers was achieved and the precisions (relative standard deviations) for intraday, interday, and column-to-column were <2.1%, 4.5%, and 7.3%, respectively. The results reveal that CCOFs-coated capillary columns show great prospect for chromatographic separation of chiral drugs.

Analysis of Enantiomers in Products of Food Interest

The separation of enantiomers has been started in the past and continues to be a topic of great interest in various fields of research, mainly because these compounds could be involved in biological processes such as, for example, those related to human health. Great attention has been devoted to studies for the analysis of enantiomers present in food products in order to assess authenticity and safety. The separation of these compounds can be carried out utilizing analytical techniques such as gas chromatography, high-performance liquid chromatography, supercritical fluid chromatography, and other methods. The separation is performed mainly employing chromatographic columns containing particles modified with chiral selectors (CS). Among the CS used, modified polysaccharides, glycopeptide antibiotics, and cyclodextrins are currently applied.

High-Performance Liquid Chromatography Enantioseparations Using Macrocyclic Glycopeptide-Based Chiral Stationary Phases: An Overview

Since their introduction by Daniel W. Armstrong in 1994, antibiotic-based chiral stationary phases have proven their applicability for the chiral resolution of various types of racemates. The unique structure of macrocyclic glycopeptides and their large variety of interactive sites (e.g., hydrophobic pockets, hydroxy, amino and carboxyl groups, halogen atoms, aromatic moieties) are the reasons for their wide-ranging selectivity. The commercially available Chirobiotic™ phases, which display complementary characteristics, are capable of separating a broad variety of enantiomeric compounds with good efficiency, good column loadability, high reproducibility, and long-term stability. These are the major reasons for the frequent use of macrocyclic antibiotic-based stationary phases in HPLC enantioseparations.This overview chapter provides a brief summary of general aspects of antibiotic-based chiral stationary phases including their preparation and their application to direct enantioseparations of various racemates focusing on the literature published since 2004.

Enantiomeric Resolution and Docking Studies of Chiral Xanthonic Derivatives on Chirobiotic Columns

A systematic study of enantioresolution of a library of xanthonic derivatives, prepared “in-house”, was successfully carried out with four commercially available macrocyclic glycopeptide-based columns, namely Chirobiotic^TM T, Chirobiotic^TM R, Chirobiotic^TM V and Chirobiotic^TM TAG. Evaluation was conducted in multimodal elution conditions: normal-phase, polar organic, polar ionic and reversed-phase. The effects of the mobile phase composition, the percentage of organic modifier, the pH of the mobile phase, the nature and concentration of different mobile phase additives on the chromatographic parameters are discussed. Chirobiotic^TM T and Chirobiotic^TM V, under normal-phase and reversed-phase modes, respectively, presented the best chromatographic parameters. Considering the importance of understanding the chiral recognition mechanisms associated with the chromatographic enantioresolution, and the scarce data available for macrocyclic glycopeptide-based columns, computational studies by molecular docking were also carried out.