Chiral discrimination of sibutramine enantiomers by capillary electrophoresis and proton nuclear magnetic resonance spectroscopy

Stereoisomeric separation and chiral recognition mechanism study of star cyclodextrin polymer as the chiral stationary phase

BACKGROUND

As the derivatives of cyclodextrin (CD), cyclodextrin polymers (CDPs) effectively increase the concentration of CD units and construct supramolecular structures with unique stereoselectivity by the structure design. CDPs have shown significant potential in chiral separation, however, the process of stereoselective interactions on chiral stationary phases (CSPs) and the specific contribution of intermolecular forces are still a challenge issue. A comprehensive understanding of the chiral recognition mechanism of CDPs will help to optimize chiral separation conditions and design new CSPs.

RESULTS

The star CDP with a supermolecular structure was synthesized by grafting β-CD onto the external 6-position hydroxyl groups using β-CD as the parent nucleus. The enhanced host-guest recognition ability of CD supramolecular polymer structure provided better inclusion interaction and increased chiral recognition of the isomers. The Star-CD CSP with star CDP as a chiral ligand performed satisfactory stereoisomer separation ability with the separation factor (α) up to 2.0 for various quinoline alkaloid isomers and 1.89 for catechins. To elucidate its chiral separation mechanism, molecular docking was used to construct the three-dimensional visual models of the binding sites and the contribution of non-covalent interactions between Star-CD CSP and quinoline alkaloid isomers. In addition, the formation sites of non-covalent interactions on the CD monomers of the polymer side chains were confirmed from the actual geometric structure by analyzing the NMR chemical shift changes before and after the formation of complexes between Star-CD polymers and isomers. Combined with the mutual evidence of molecular simulation and chiral NMR, the specific recognition mechanism of selector-selectand complexes was comprehensively expounded.

SIGNIFICANCE

The multi-mode CSP based on cyclodextrin supramolecular structure provides new ideas for the stereoisomeric separation of complex chiral components with multiple chiral centers in natural products. Not limited to the macroscopic performance of the chromatographic separation, molecular docking explored the theoretical model of chiral recognition from the molecular level. The chiral NMR analysis confirmed the credibility of the model from the geometry structure, and then the recognition mechanism of multi-mode CSP was fully elaborated combining the above three aspects.

Single isomer cyclodextrins as chiral selectors in capillary electrophoresis

Since decades, cyclodextrins are one of the most powerful selectors in chiral capillary electrophoresis for the enantioseparation of diverse organic compounds. This review concerns papers published over the last decade (from 2009 until nowadays), dealing with the capillary electrophoretic application of single isomer cyclodextrin derivatives in chiral separations. Following a brief overview of their synthetic approaches, the inventory of the neutral, negatively and positively charged (including both permanently ionic and pH-tunable ionizable substituents) and zwitterionic CD derivatives is presented, with insights to underlying structural aspects by NMR spectroscopy and molecular modeling. CE represents an ideal tool to study the weak, non-covalent supramolecular interactions. The published methods are reviewed in the light of enantioselectivity, enantiomer migration order and the fine-tuning of enantiodiscrimination by the substitution pattern of the single entity selector molecules, which is hardly possible for their randomly substituted counterparts. All the reviewed publications herein support that cyclodextrin-based chiral capillary electrophoresis seems to remain a popular choice in pharmaceutical and biomedical analysis.

Enantiomeric Determination of Drugs in Pharmaceutical Formulations and Biological Samples by Electrokinetic Chromatography

Chirality is a relevant issue in the pharmaceutical field due to the different biological activity that enantiomers of a chiral drug can show. In fact, the desired biological or pharmaceutical activity might be present in only one of the enantiomers, while the other enantiomer(s) may have different biological activity, be inactive or even toxic. This has motivated in recent years the development of drugs marketed as pure enantiomers to avoid exposing the organism to the action of enantiomers that may not be active or even harmful to health. Thus, it is of high interest to develop enantioselective analytical methodologies to control the presence of enantiomeric impurities and to understand the enantioselective metabolism of chiral drugs. This review gives an overview about the analytical strategies developed by electrokinetic chromatography (EKC) from 2010 to June 2019 for the enantiomeric determination of drugs in both pharmaceutical formulations and biological samples. The types of chiral selectors used, the migration order of enantiomers, their resolution, the detection technique employed and the sensitivity achieved are revised and compared. Also, applications to assess the enantiomeric purity control of pharmaceutical formulations and to determine chiral drugs in biological samples to study their metabolism are included. Advantages and limitations of the chiral methods developed by EKC are also discussed.

Effect of sulfobutyl ether-β-cyclodextrin and propylene glycol alginate on the solubility of clozapine

Clozapine (CLZ) is an atypical antipsychotic medication used in the treatment of schizophrenia and is poorly soluble in water (0.05 mM). In this study, we have investigated the effect of β-cyclodextrin (CD) and its derivatives on the solubility of CLZ. The solubility of the CLZ was measured to generate a phase solubility diagram, and the interaction between CLZ and sulfobutyl ether-β-cyclodextrin (SBE-β-CD) in aqueous solution was observed by ¹H- and 2D rotating-frame Overhauser enhancement spectroscopy (ROESY)-NMR methods. Moreover, the synergistic effect of SBE-β-CD and water-soluble polymers, including polyvinylpyrrolidone, hydroxypropyl methylcellulose, carboxymethylcellulose sodium salt, polyvinyl alcohol, sodium alginate, and propylene glycol alginate (PGA), on the solubility of CLZ was investigated. The results show that the solubility of CLZ with 1 w/v% PGA was 7.6 mM, which was almost four times greater than that of CLZ without PGA in a 15 mM SBE-β-CD solution. In contrast, the solubility of CLZ with 1 w/v % PGA in an aqueous solution decreased by one-third relative to that of CLZ in a 15 mM SBE-β-CD solution. 2D ROESY-NMR indicated that a CLZ/SBE-β-CD/PGA ternary complex formed. It was found that the combination of PGA and SBE-β-CD enhanced the solubility of CLZ.

Contemporary theory of enantioseparations in capillary electrophoresis

The first separation of enantiomers in capillary electrophoresis (CE) counts slightly longer than three decades. Fast development of the practice and theory of chiral CE occurred in the past 30 years and today one can consider this technology to have a solid and mature theoretical background. The goal of the present review is not only to summarize the history and contemporary theory of enantioseparations by using CE but also to present the authors personal view where shall we head to with this attractive technology not only from the viewpoint of separation of enantiomers but also for better understanding the mechanisms of non-covalent (enantioselective) interactions in chemistry, biology, medicine and related disciplines.

Chiral discrimination in nuclear magnetic resonance spectroscopy

Chirality is a fundamental property of molecules whose spatial symmetry is characterized by the absence of improper rotations, making them not superimposable to their mirror image. Chiral molecules constitute the elementary building blocks of living species and one enantiomer is favoured in general (e.g. L-aminoacids and D-sugars pervade terrestrial homochiral biochemistry) because most chemical reactions producing natural substances are enantioselective. Since the effect of chiral chemicals and drugs on living beings can be markedly different between enantiomers, the quest for practical spectroscopical methods to scrutinize chirality is an issue of great importance and interest. Nuclear magnetic resonance (NMR) is a topmost analytical technique, but spectrometers currently used are 'blind' to chirality, i.e. unable to discriminate the two mirror-image forms of a chiral molecule, because, in the absence of a chiral solvent, the spectral parameters, chemical shifts and spin-spin coupling constants are identical for enantiomers. Therefore, the development of new procedures for routine chiral recognition would offer basic support to scientists. However, in the presence of magnetic fields, a distinction between true and false chirality is mandatory. The former epitomizes natural optical activity, which is rationalized by a time-even pseudoscalar, i.e. the trace of a second-rank tensor, the mixed electric dipole/magnetic dipole polarizability. The Faraday effect, magnetic circular dichroism and magnetic optical activity are instead related to a time-odd axial vector. The present review summarizes recent theoretical and experimental efforts to discriminate enantiomers via NMR spectroscopy, with the focus on the deep connection between chirality and symmetry properties under the combined set of fundamental discrete operations, namely charge conjugation, parity (space inversion) and time (motion) reversal.

Rapid Surface Enhanced Raman Scattering (SERS) Detection of Sibutramine Hydrochloride in Pharmaceutical Capsules with a β-Cyclodextrin- Ag/Polyvivnyl Alcohol Hydrogel Substrate

Sibutramine hydrochloride (SH) is a banned weight-loss drug, but its illegal addition to health products is still rampant. This suggests a very urgent need for a fast and precise detection method for SH. Surface Enhanced Raman Scattering (SERS) is a promising candidate for this purpose, but the weak affinity between SH and bare metal limits its direct SERS detection. In the present work, β-cyclodextrin was capped in situ onto the surface of Ag nanoparticles to function as a scaffold to capture SH. The obtained Ag nanoparticles were encapsulated into polyvinyl alcohol (PVA) to fabricate a SERS active hydrogel with excellent reproducibility. A facile SERS strategy based on such substrate was proposed for trace SH quantification with a linear range of 7.0–150.0 µg·mL^–1, and a detection limit low to 3.0 µg·mL⁻¹. It was applied to analyze seven types of commercial slimming capsules with satisfactory results, showing good prospect for real applications.

Analytical Methodologies for the Stereoselective Determination of Sibutramine: An Overview

Sibutramine is a chiral anti-obesity drug which decreases food intake and increases energy expenditure. In therapy it is used as a racemic mixture; however both pharmacokinetic and pharmacodynamic data have revealed enantioselective behavior of sibutramine and its major active metabolites. Several chromatographic and electrophoretic analytical methods have been published so far for the chiral determination of sibutramine from pharmaceutical preparations and biological samples. The current paper aims to provide a systematic review of the stereochemical aspects and analytical methods used for the enantiodetermination of sibutramine and its active enantiomers covering the last 15 years.

Applications of nuclear magnetic resonance spectroscopy for the understanding of enantiomer separation mechanisms in capillary electrophoresis

This review deals with the applications of nuclear magnetic resonance (NMR) spectroscopy to understand the mechanisms of chiral separation in capillary electrophoresis (CE). It is accepted that changes observed in the separation process, including the reversal of enantiomer migration order (EMO), can be caused by subtle modifications in the molecular recognition mechanisms between enantiomer and chiral selector. These modifications may imply minor structural differences in those selector-selectand complexes that arise from the above mentioned interactions. Therefore, it is mandatory to understand the fine intermolecular interactions between analytes and chiral selectors. In other words, it is necessary to know in detail the structures of the complexes formed by the enantiomer (selectand) and the selector. Any differences in the structures of these complexes arising from either enantiomer should be detected, so that enantiomeric bias in the separation process could be explained. As to the nature of these interactions, those have been extensively reviewed, and it is not intended to be discussed here. These interactions contemplate ionic, ion-dipole and dipole-dipole interactions, hydrogen bonding, van der Waals forces, π-π stacking, steric and hydrophobic interactions. The main subject of this review is to describe how NMR spectroscopy helps to gain insight into the non-covalent intermolecular interactions between selector and selectand that lead to enantiomer separation by CE. Examples in which diastereomeric species are created by covalent (irreversible) derivatization will not be considered here. This review is structured upon the different structural classes of chiral selectors employed in CE, in which NMR spectroscopy has made substantial contributions to rationalize the observed enantioseparations. Cases in which other techniques complement NMR spectroscopic data are also mentioned.

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.

Recent advances in capillary electrophoretic migration techniques for pharmaceutical analysis

Since the introduction about 30 years ago, CE techniques have gained a significant impact in pharmaceutical analysis. The present review covers recent advances and applications of CE for the analysis of pharmaceuticals. Both small molecules and biomolecules such as proteins are considered. The applications range from the determination of drug-related substances to the analysis of counterions and the determination of physicochemical parameters. Furthermore, general considerations of CE methods in pharmaceutical analysis are described.