Enantioselective stabilization of inclusion complexes of metoprolol in carboxymethylated beta-cyclodextrin

Combination of capillary electrophoresis and molecular modeling to study the enantiomer affinity pattern between β-blockers and anionic cyclodextrin derivatives in a methanolic and water background electrolyte

In order to have deep insights into the mechanisms of enantiomer affinity pattern in both aqueous and non-aqueous systems, an approach combining capillary electrophoresis and molecular modeling was undertaken. A chiral β-blocker; acebutolol, was enantioseparated in aqueous capillary electrophoresis and non-aqueous capillary electrophoresis using two anionic β-cyclodextrin derivatives. The enantiomer affinity pattern of acebutolol was found to be opposite when an aqueous background electrolyte was replaced with non-aqueous background electrolyte in the presence of heptakis(2,3-di-O-acetyl-6-sulfo)-β-cyclodextrin but remained the same in the presence of heptakis(2,3-di-O-methyl-6-sulfo)-β-cyclodextrin. Molecular docking of acebutolol into two β-cyclodextrin derivatives indicated two distinct binding modes called 'up' and 'down' conformations. After structure optimization by molecular dynamics and energy minimization, both enantiomers of acebutolol were preferred to the 'up' conformation with heptakis(2,3-di-O-methyl-6-sulfo)-β-cyclodextrin while 'down' conformation with heptakis(2,3-di-O-acetyl-6-sulfo)-β-cyclodextrin. The further calculation of the complex energy with solvent effect indicated that heptakis(2,3-di-O-acetyl-6-sulfo)-β-cyclodextrin had higher affinity to S-acebutolol than R-acebutolol in non-aqueous capillary electrophoresis while it showed better binding to R-acebutolol in aqueous capillary electrophoresis. However, the heptakis(2,3-di-O-methyl-6-sulfo)-β-cyclodextrin bound better to R-acebutolol in both aqueous and non-aqueous capillary electrophoresis, implying that the binding mode played more important role in chiral separation of heptakis(2,3-di-O-methyl-6-sulfo)-β-cyclodextrin while the solvent effect had prevailing impact on heptakis(2,3-di-O-acetyl-6-sulfo)-β-cyclodextrin.

Study of the enantioselectivity and recognition mechanism of chiral dual system based on chondroitin sulfate D in capillary electrophoresis

Several chiral reagents including cyclodextrins (CDs) and derivatives, crown ethers, proteins, chiral surfactants, and polymers have been involved in dual-selector systems for enantioseparation of a series of compounds by capillary electrophoresis (CE). In this paper, chondroitin sulfate D-based dual-selector system (CSD/CM-β-CD) was firstly established and investigated for the enantioseparation of six basic racemic drugs in capillary electrophoresis. Compared to the single-selector systems, synergistic effect and significantly improved separations for all tested analytes were observed in CSD/CM-β-CD system. The effect of several parameters, such as buffer pH, chiral selector concentration, and applied voltage, was systematically optimized. Meanwhile, to investigate the possible chiral recognition mechanisms in CSD/CM-β-CD synergistic system, we tried to apply the molecular docking method to simulate the host-guest binding procedures of the polysaccharide-based dual system for the first time. The difference in binding free energy was found to correspond to the chiral selectivity factor. The existence of CSD-CM-β-CD complex may give rise to a higher discriminatory ability against the enantiomers, indicating the synergistic effect in CSD/CM-β-CD system. Graphical abstract ᅟ.

Effectively enhancing the enantioseparation ability of β-cyclodextrin derivatives by de novo design and molecular modeling

Rational engineering of native β-CD as an ideal chiral selector for a definite analyte in capillary electrophoresis represents a challenge in separation science. Herein, a rational and systematic strategy that combines the de novo design and molecular modeling is firstly described to expedite the manipulation and selection of effective selector for enantioseparation in capillary electrophoresis. Using β-adrenoreceptor agonists as model analytes, we demonstrate how this strategy efficiently improves the enantiorecognition in chiral discrimination sites of inclusion complexes. The evolved β-CD derivative could be utilized as a chiral receptor to achieve the effective enantioseparation (R_s > 1.5) of racemic β-adrenoreceptor agonists. We highlight a novel strategy for efficiently and rapidly manipulating native CD based on the characteristics of analyte so as to gain an excellent chiral selector.

Phosphated cyclodextrins as water-soluble chiral NMR solvating agents for cationic compounds

The utility of phosphated α-, β- and γ-cyclodextrins as water-soluble chiral NMR solvating agents for cationic substrates is described. Two sets of phosphated cyclodextrins, one with degrees of substitution in the 2–6 range, the other with degrees of substitution in the 6–10 range, are examined. Results with 33 water-soluble cationic substrates are reported. We also explored the possibility that the addition of paramagnetic lanthanide ions such as praseodymium(III) and ytterbium(III) further enhances the enantiomeric differentiation in the NMR spectra. The chiral differentiation with the phosphated cyclodextrins is compared to prior results obtained with anionic carboxymethylated cyclodextrins. There are a number of examples where a larger differentiation is observed with the phosphated cyclodextrins.

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.

Molecular modeling study of chiral separation and recognition mechanism of β-adrenergic antagonists by capillary electrophoresis

Chiral separations of five β-adrenergic antagonists (propranolol, esmolol, atenolol, metoprolol, and bisoprolol) were studied by capillary electrophoresis using six cyclodextrins (CDs) as the chiral selectors. Carboxymethylated-β-cyclodextrin (CM-β-CD) exhibited a higher enantioselectivity power compared to the other tested CDs. The influences of the concentration of CM-β-CD, buffer pH, buffer concentration, temperature, and applied voltage were investigated. The good chiral separation of five β-adrenergic antagonists was achieved using 50 mM Tris buffer at pH 4.0 containing 8 mM CM-β-CD with an applied voltage of 24 kV at 20 °C. In order to understand possible chiral recognition mechanisms of these racemates with CM-β-CD, host-guest binding procedures of CM-β-CD and these racemates were studied using the molecular docking software Autodock. The binding free energy was calculated using the Autodock semi-empirical binding free energy function. The results showed that the phenyl or naphthyl ring inserted in the hydrophobic cavity of CM-β-CD and the side chain was found to point out of the cyclodextrin rim. Hydrogen bonding between CM-β-CD and these racemates played an important role in the process of enantionseparation and a model of the hydrogen bonding interaction positions was constructed. The difference in hydrogen bonding formed with the -OH next to the chiral center of the analytes may help to increase chiral discrimination and gave rise to a bigger separation factor. In addition, the longer side chain in the hydrophobic phenyl ring of the enantiomer was not beneficial for enantioseparation and the chiral selectivity factor was found to correspond to the difference in binding free energy.

Advances in the enantioseparation of β-blocker drugs by capillary electromigration techniques

β-Blocker drugs or β-adrenergic blocking agents are an important class of drugs, prescribed with great frequency. They are used for various diseases, particularly for the treatment of cardiac arrhythmias, cardioprotection after myocardial infarction (heart attack), and hypertension. Almost all β-blocker drugs possess one or more stereogenic centers; however; only some of them are administered as single enantiomers. Since both enantiomers can differ in their pharmacological and toxicological properties, enantioselective analytical methods are required not only for pharmacodynamic and pharmacokinetic studies but also for quality control of pharmaceutical preparations with the determination of enantiomeric purity. In addition to the chromatographic tools, in recent years, capillary electromigration techniques (CE, CEC, and MEKC) have been widely used for enantioselective purposes employing a variety of chiral selectors, e.g. CDs, polysaccharides, macrocyclic antibiotics, proteins, chiral ion-paring agents, etc. The high separation efficiency, rapid analysi,s and low consumption of reagents of electromigration methods make them a very attractive alternative to the conventional chromatographic methods. In this review, the development and applications of electrodriven methods for the enantioseparation of β-blocker drugs are reported. The papers concerning this topic, published from January 2000 until December 2010, are summarised here. Particular attention is given to the coupling of chiral CE and CEC methods to MS, as this detector provides high sensitivity and selectivity.

Spectroscopic investigations and crystal structure from synchrotron powder data of the inclusion complex of beta-cyclodextrin with atenolol

Inclusion complexes of atenolol with beta-cyclodextrin (beta-CD) in aqueous solution have been investigated with 1H NMR and UV-vis spectroscopy. The stoichiometry of this inclusion complex was established to be equimolar (1:1) and its stability constant was determined by UV-vis spectroscopy. The crystal structure of the beta-CD-atenolol (1:1) inclusion compound has been solved from synchrotron powder diffraction data using direct-space search techniques. The crystal structure model and 1H NMR data are in good agreement and, with support of Hyperchem MM+ molecular dynamics results, suggest which protons are likely to be involved in the inclusion process that leads to the supramolecular architecture of this guest-host complex.

UV–vis and FTIR–ATR spectroscopic techniques to study the inclusion complexes of genistein with β-cyclodextrins

The effect of beta-cyclodextrin (beta-CyD), (2-hydroxypropyl)-beta-cyclodextrin (HP-beta-CyD) and methyl-beta-cyclodextrin (Me-beta-CyD) complexation on the UV absorption of genistein (Gen) was studied in pure water. A phase solubility study was performed, according to the method reported by Higuchi and Connors, to evaluate the changes of isoflavone in the complexation state and the obtained diagrams suggested that it forms complexes with a stoichiometry of 1:1. Then, the solid complexes of genistein with these macrocycles in 1:1 molar ratio were prepared by the co-precipitation method and characterized by FTIR absorption spectroscopy in ATR geometry. The host-guest interactions have been evidenced by monitoring, in the FTIR-ATR spectra, the changes in some guest molecule bands relative to those observed in the spectra of the 1:1 physical mixtures and complexes. In particular, for the high-frequency O-H stretching band, a quantitative vibrational assignment of the observed sub-bands has been made. From the results, the inclusion phenomena have been discussed.

The rutin/β-cyclodextrin interactions in fully aqueous solution: spectroscopic studies and biological assays

In the present work the feasibility of beta-cyclodextrin complexation was explored, as a tool for improving the aqueous solubility and antioxidant efficacy of rutin. By means of 1H NMR, UV-vis and circular dichroism spectroscopy the single aromatic ring of rutin was found to be inserted into the beta-cyclodextrin cavity to form a 1:1 inclusion complex. The effect of beta-cyclodextrin on the spectral features of rutin was quantitatively investigated, in fully aqueous medium, by holding the concentration of the guest constant and varying the host concentration. The associated binding constants were estimated to be 142+/-20 and 153+/-20 M(-1), respectively, on the basis of the observed UV-vis absorption and circular dichroism intensities. The antioxidant activity of rutin was also investigated, as affected by molecular encapsulation within beta-cyclodextrin (batophenanthroline test; comet assay; lipid peroxidation); the inclusion complex revealed improved antioxidant efficacy that may be in part explained by an increased solubility in the biological moiety.

Determination of stability constants of the inclusion complexes of β-blockers in heptakis (2,3-dimethyl-6-sulfato)-β-cyclodextrin

The beta-blockers possess at least one chiral center and the S(-)-enantiomer shows higher affinity for binding to the beta-adrenergic receptors than antipode. The stability constants of acebutolol, celiprolol, propranolol and terbutaline in the inclusion complexes with single-isomer heptakis (2,3-dimethyl-6-sulfato)-beta-cyclodextrin (HDMS-beta-CD) were determined by capillary electrophoresis. The approximation and linear double reciprocal methods were adapted with comparable results. Among the beta-blockers studied, propranolol had the lowest stability constant but the highest enantioselectivity, indicating that the magnitudes of the stability constants carried little information about enantioseparation. The magnitudes of enantioselectivities between the enantiomer pair were in the order of propranolol > celiprolol > terbutaline > acebutolol.

NMR spectroscopic characterization of metoprolol/cyclodextrin complexes in aqueous solution: Cavity size dependency

The inclusion complex formation of a water-soluble beta(1)-selective adrenoreceptor antagonist Metoprolol (Met) with alpha-cyclodextrin (alpha-CyD), beta-cyclodextrin (beta-CyD), gamma-cyclodextrin (gamma-CyD), and 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CyD) in aqueous solution was studied by ultraviolet (UV), circular dichroism (CD), and nuclear magnetic resonance (NMR) spectroscopies and the modes of interaction were assessed. Continuous variation plots revealed that Met forms the inclusion complexes with alpha-CyD, beta-CyD, and HP-beta-CyD in a stoichiometry of 1:1, whereas gamma-CyD forms the 2:1 complex where two Met molecules are included in one gamma-CyD cavity. NMR spectroscopic studies, including ROESY and GROESY techniques, clearly indicated that alpha-CyD with the small cavity includes the methoxyethylbenzene moiety of Met molecule shallowly in the cavity, depositing the benzene and the methoxy moieties around the secondary and primary sides, respectively, of the cavity. In the case of the beta-CyD complex, the methoxyethylbenzene moiety is more deeply included in the cavity, and it is feasible that Met may be able to enter from both primary and secondary hydroxyl sides of the cavity, forming the 1:1 complex. On the other hand, two Met molecules are included probably in an antiparallel orientation in the large gamma-CyD cavity, and the benzene moieties of Met are in contact with each other.

NMR studies of chiral recognition by cyclodextrins

Chiral recognition by cyclodextrins is of considerable importance, especially for pharmaceutical industry, in view of the possible side effects of the second enantiometer of chiral drugs. In general, it manifests itself in all NMR parameters (chemical shifts, coupling constants, NOE and ROE effects, and relaxation rates) on one hand. On the other hand, it allows one to determine the thermodynamic parameters characterizing diastereomeric complexes formed by cyclodextrins with enantiomeric guests. After an introduction and a general discussion of NMR manifestations of chiral recognition by cyclodextrin, the existing literature data on this problem will be discussed herein. Chirality 16:90-105, 2004.