Liquid and subcritical fluid chromatographic enantioseparation of N α -Fmoc proteinogenic amino acids on Quinidine -based zwitterionic and anion-exchanger type chiral stationary phases. A comparative study

Efficient enantioresolution of aromatic α-hydroxy acids with Cinchona alkaloid-based zwitterionic stationary phases and volatile polar-ionic eluents

Single enantiomers of mandelic acid (1), 3-phenyllactic acid (2), and 3-(4-hydroxyphenyl)lactic acid (3) are the subject of many fields of investigation, spanning from the pharmaceutical synthesis to that of biocompatible and biodegradable polymers, while passing from the interest towards their antimicrobial activity to their role as biomarkers of particular pathological conditions or occupational exposures to specific xenobiotics. All above mentioned issues justify the need for accurate analytical methods enabling the correct determination of the individual enantiomers. So far, all the developed liquid chromatography (LC) methods were not or hardly compatible with mass spectrometry (MS) detection. In this paper, a commercially available Cinchona-alkaloid derivative zwitterionic chiral stationary phase [that is, the CHIRALPAK® ZWIX(-)] was successfully used to optimize the enantioresolution of compounds 1-3 under polar-ionic (PI) conditions with a mobile phase consisting of an acetonitrile/methanol 95/5 (v/v) mixture with 80 mM formic acid. With the optimized conditions, enantioseparation and enantioresolution values up to 1.46 and 4.41, respectively, were obtained. In order to assess the applicability of the optimized enantioselective chromatography conditions in real-life scenarios and on MS-based systems, a proof-of-concept application was efficiently carried out by analysing dry urine spot samples spiked with 1 by means of a LC-MS system. The (S)<(R) enantiomer elution order (EEO) was established for compounds 1 and 2 by analysing a pure enantiomeric standard of known configuration. This was not possible for 3 because not commercially available. For this compound, the same EEO was identified applying a procedure based on ab initio time-dependent density-functional theory simulations coupled to electronic circular dichroism analyses. Moreover, a molecular dynamics simulation unveiled the role of the phenolic OH in compound 3 in the retention mechanism.

High-throughput enantioseparation of Nα-fluorenylmethoxycarbonyl proteinogenic amino acids through fast chiral chromatography on zwitterionic-teicoplanin stationary phases

In this study, 31 racemates of Nα-FMOC (fluorenylmethoxycarbonyl) amino acids (AAs) with different chemico-physical characteristics (neutral nonpolar, neutral polar, acidic and basic) have been successfully resolved in fast enantioselective chromatography on recently-developed zwitterionic-teicoplanin chiral stationary phases (CSPs). The CSPs were prepared by covalently bonding the teicoplanin selector on fully-porous particles of narrow dispersion particle-size distribution (particle diameter 1.9 µm) and superficially-porous particles (2.0 µm). Both the zwitterionic-teicoplanin CSPs have proved to be ideal media for the separation of this important class of compounds. In particular, the zwitterionic CSP prepared on superficially-porous particles exhibited superior enantioselectivity and resolution, compared to that made of fully porous particles, in virtue of more favorable thermodynamics. The zwitterionic nature of these CSPs allowed avoiding the annoying effect of Donnan's exclusion of enantiomers from the stationary phase. This effect, on the opposite, was frequently observed on a commercial teicoplanin CSP (Teicoshell) employed for comparative purposes. Noticeably, on the zwitterionic-teicoplanin CSPs, by using either acetonitrile- or methanol-rich mobile phases (MPs), it was possible to favor speed over enantioresolution and vice versa. This work gives further replies to the request for rapid determination of enantiomeric excess of Nα-FMOC proteinogenic (and non-proteinogenic) AAs, typically used as preferred chiral synthons in the solid-phase synthesis of therapeutic peptides.

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.

Cinchona Alkaloid-Based Zwitterionic Chiral Stationary Phases Applied for Liquid Chromatographic Enantiomer Separations: An Overview

For the early 2000s, chromatographic methods applying chiral stationary phases (CSPs) became the most effective techniques for the resolution of chiral compounds on both analytical and preparative scales. High-performance liquid chromatography (HPLC) employing various types of chiral selectors covalently bonded to silica-based supports offers a state-of-the-art methodology for "chiral analysis." Although a large number of CSPs are available nowadays, the design and development of new "chiral columns" are still needed since it is obvious that in practice one needs a good portfolio of different columns to face the challenging task of enantiomeric resolutions. The development of the unique chiral anion, cation, and zwitterion exchangers achieved by Lindner and his partners serves as an expansion of the range of the efficiently applicable CSPs.In this context this overview chapter discusses and summarizes direct enantiomer separations of chiral acids and ampholytes applying zwitterionic ion exchangers derived from Cinchona alkaloids. Our aim is to provide comprehensive information on practical solutions with focus on the molecular recognition and methodological variables.

Liquid chromatographic enantiomer separations applying chiral ion-exchangers based on Cinchona alkaloids

As the understanding of the various biological actions of compounds with different stereochemistry has grown, the necessity to develop methods for the analytical qualification and quantification of chiral products has become particularly important. The last quarter of the century has seen a vast growth of diverse chiral technologies, including stereocontrolled synthesis and enantioselective separation and analysis concepts. By the introduction of covalently bonded silica-based chiral stationary phases (CSPs), the so-called direct liquid chromatographic (LC) methods of enantiomer separation became the state-of-the-art methodology. Although a large number of CSPs is available nowadays, the design and development of new chiral selectors and CSPs are still needed since it is obvious that in practice one needs a good portfolio of different CSPs and focused "chiral columns" to tackle the challenging tasks. This review discusses and summarizes direct enantiomer separations of chiral acids and ampholytes applying anionic and zwitterionic ion-exchangers derived from Cinchona alkaloids with emphasis on literature data published in the last 10 years. Our aim is to provide an overview of practical solutions, while focusing on the integration of molecular recognition and methodological variables.