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.

Use of Gamithromycin as a Chiral Selector in Capillary Electrophoresis

In this short communication, we report the use of a second-generation macrolide antibiotic, gamithromycin (Gam), as a novel chiral selector for enantioseparation in capillary electrophoresis (CE). A preliminary analysis of the experiment results shows that Gam is especially suitable for the separation of chiral primary amines. Factors influencing enantioseparations were systematically investigated including the composition of the background electrolyte (BGE), concentration of Gam, the type and proportion of organic solvents, applied voltage, etc. In particular, N-Methylformamide (NMF) was successfully used as a non-aqueous solvent for Gam, and shown to be extremely effective for the separation of primaquine (PMQ) and 1-aminoindan (AMI) when used alone or mixed with other commonly used non-aqueous solvents (e.g. methanol). To our knowledge this was also the first application of NMF as a non-aqueous solvent for antibiotic chiral selectors in CE. The best separations were obtained with 100 mM Tris, 125 mM H₃BO₃ and 80 mM Gam in methanol/NMF (25:75) solvent for PMQ and AMI, or 80-100 mM Gam in methanol for the other model analytes. Among the analytes, the resolution (Rs) of amlodipine (AML) reached up to 15.65, which is to our knowledge the highest value ever reported in CE studies for this compound (except for using molecularly imprinted polymers technique).

Some thoughts about enantioseparations in capillary electrophoresis

In this overview the goal of the authors was to analyze from the historical perspective the reasons of success and failure of chiral capillary electrophoresis. In addition, the current trends are analyzed, unique advantages of capillary electrophoresis are highlighted and some future directions are discussed.

Recognition Mechanisms of Chiral Selectors: An Overview

Stereospecific recognition of chiral molecules plays an important role in nature as the basis of the interaction of chiral bioactive compounds with the chiral target structures. In separation sciences such as chromatographic and capillary electromigration techniques, interactions between chiral analytes and chiral selectors, i.e., the formation of transient diastereomeric complexes in thermodynamic equilibria, are the basis for chiral separations. Due to the large structural variety of chiral selectors, different structural features contribute to the overall chiral recognition process. This introductory chapter briefly summarizes the present understanding of the structural enantioselective recognition processes for various types of chiral selectors.

Chiral Separation by NACE Using Polyol Derivative-Boric Acid Complexes

Nonaqueous capillary electrophoresis (NACE) is an effective method for chiral separation. Many polyol derivatives (e.g., D-(+)-xylose, lactobionic acid, diacetone-D-mannitol, L-sorbose, and D-gluconic acid δ-lactone) can react with boric acid in methanol to produce polyol derivative-boric acid complexes which can be utilized as chiral selectors of enantioseparations. The enantiomers of more than a dozen basic analytes can be resolved under the optimized NACE using these chiral selectors.

Enantioseparation by Thin-Layer Chromatography

Despite the fact that high-performance liquid chromatography is the predominant technique for analytical and preparative enantioseparations, chiral thin-layer chromatography (TLC) may represent an alternative, especially if fast analysis with simple equipment is required. This chapter describes several approaches in chiral TLC for the separation of amino acids and basic drugs using DL-selenomethionine and β-adrenergic drugs as examples. Analytical approaches include the impregnation of the adsorbent with a chiral selector using pre-coated as well as custom-prepared TLC plates and addition of the selector to the mobile phase directly as well as in the form of copper metal complex. (-)-Quinine and L-amino acids were used as chiral selectors in different manners for enantioseparations.

Enantiomeric separation of adrenaline, noradrenaline, and isoprenaline by capillary electrophoresis using streptomycin-modified gold nanoparticles

Enantiomeric separations of the adrenergic compounds adrenaline, noradrenaline, and isoprenaline were studied. Electromigrative separations were performed in uncoated fused silica capillaries using streptomycin-modified gold nanoparticles (ST-AuNPs) as an additive to the background electrolyte. The ST-AuNPs are shown to serve as an effective chiral selector. The modified AuNPs were characterized in terms of size and zeta potential, and by IR and UV-vis spectra. The effects of ST-AuNP concentration, pH value, temperature, and separation voltage on the separations were systematically studied. Under optimized experimental conditions, racemic mixtures of the respective adrenergic drugs were baseline-separated within 7 min with a resolution of up to 7.5. The relative standard deviations of the resolution in inter-day and intra-day studies (n = 5) were generally <5%. Graphical abstract Schematic of the method for enantiomeric separations. (A): At low concentrations of streptavidinylated gold nanoparticles (ST-AuNPs), the better matching enantiomer is preferably "transported" by the ST-AuNPs; (B) ST-AuNP concentration increased to an optimal value; (C): The ST-AuNP concentration is too high; even poorly matching enantiomers will be transported simultaneously.

Enantioseparations in open-tubular capillary electrochromatography: Recent advances and applications

This review highlights recent advances and applications in open-tubular capillary electrochromatography (OT-CEC) for enantioseparations during the last decade. Although extensive research has been conducted in the area of separations by use of CEC, and a big number of reviews have been published, there is not a review on exclusively the use of chiral stationary phases (CSPs) in OT-CEC for enantioseparations. Therefore, in this review, the design and synthesis of different CSPs are presented, and their potential applications in OT-CEC for enantioseparations are discussed. The different approaches to CSP development include chiral nanomaterials, porous layers, molecular imprinting, sol-gel technique, polyelectrolyte multilayer coating, polymeric coating and others.

Chiral recognition in separation science - an update

Stereospecific recognition of chiral molecules is an important issue in various aspects of life sciences and chemistry including analytical separation sciences. The basis of analytical enantioseparations is the formation of transient diastereomeric complexes driven by hydrogen bonds or ionic, ion-dipole, dipole-dipole, van der Waals as well as π-π interactions. Recently, halogen bonding was also described to contribute to selector-selectand complexation. Besides structure-separation relationships, spectroscopic techniques, especially NMR spectroscopy, as well as X-ray crystallography have contributed to the understanding of the structure of the diastereomeric complexes. Molecular modeling has provided the tool for the visualization of the structures. The present review highlights recent contributions to the understanding of the binding mechanism between chiral selectors and selectands in analytical enantioseparations dating between 2012 and early 2016 including polysaccharide derivatives, cyclodextrins, cyclofructans, macrocyclic glycopeptides, proteins, brush-type selectors, ion-exchangers, polymers, crown ethers, ligand-exchangers, molecular micelles, ionic liquids, metal-organic frameworks and nucleotide-derived selectors. A systematic compilation of all published literature on the various chiral selectors has not been attempted.

Application of rifampicin as a chiral selector for enantioresolution of basic drugs using capillary electrophoresis

Rifampicin, a member of rifamycin sub-class of antibiotics which belongs to the naphthalenic ansamycin class of antibiotics, has a characteristic ansa structure, i.e., a ring structure or chromophore spanned by an aliphatic chain. The present work was designed to evaluate its potential as a chiral selector (CS) as its structure consisting of nine stereogenic centers, an aromatic moiety and several functional groups (i.e., one imine, one amide, one acetoxy residue, two aliphatic hydroxyl and three phenolic hydroxyl groups) was expected to instigate multiple enantioselective interactions, namely, hydrogen bonding and inclusion complexation with chiral analytes, and therefore resulting in efficient enantioseparations. Systematic experiments were performed to investigate the effects of concentration of CS, composition of background electrolyte (BGE) and applied voltage on chiral separation. Enantiomers of propranolol and metoprolol were baseline resolved using a BGE consisting of 20mM CS and 50/50 (v/v) iso-propanol/phosphate buffer (100mM, pH 7.0) whereas for enantiomers of sertraline, a BGE consisting of 23mM CS and 40/60 (v/v) iso-propanol/phosphate buffer (100mM, pH 7.0) resulted in baseline resolutions.

Enantiomer Separations by Capillary Electrophoresis

Capillary electrophoresis (CE) is a versatile and flexible technique for analytical enantioseparations. This is due to the large variety of chiral selectors as well as the different operation modes including electrokinetic chromatography, micellar electrokinetic chromatography, and microemulsion electrokinetic chromatography. The chiral selector, which is added to the background electrolyte, represents a pseudostationary phase with its own electrophoretic mobility allowing a variety of different separation protocols. The present chapter briefly addresses the basic fundamentals of CE enantioseparations as well as the most frequently applied chiral selectors and separation modes. The practical example illustrates the separation of the enantiomers of a positively charged analyte using native and charged cyclodextrin derivatives as chiral selectors.

Penicillin G as a novel chiral selector in capillary electrophoresis

The penicillin sub-class of β-lactam antibiotics has not been examined for its enantiodiscriminating abilities in capillary electrophoresis (CE) until date. The present work was therefore designed to evaluate penicillin G potassium salt (PenG) as an ion-pair chiral selector (CS) using CE for its several attributes, namely, high solubility in water and lower alcohols, structure allowing multiple interactions with analytes and cost-effectiveness. Systematic experiments were performed to investigate the effect of composition of background electrolyte, applied voltage and capillary temperature on chiral separation. Baseline resolutions of enantiomers of five basic chiral drugs (namely, darifenacin, citalopram, sertraline, propranolol and metoprolol) were attained using a background electrolyte composed of water:methanol (90:10, v/v) and consisting of 10.7 or 16.1mM CS at 20°C using an applied voltage of 5kV.