Fast separations of chiral β-blockers on a cellulose tris(3,5-dimethylphenylcarbamate)-coated zirconia monolithic column by capillary electrochromatography

Trends in monoliths: Packings, stationary phases and nanoparticles

Monoliths media are gaining interest as excellent substitutes to conventional particle-packed columns. Monolithic columns show higher permeability and lower flow resistance than conventional liquid chromatography columns, providing high-throughput performance, resolution and separation in short run times. Monolithic columns with longer length, smaller inner diameter and specific selectivity to peptides or enantiomers have been played important role in hyphenated system. Monolithic stationary phases possess great efficiency, resolution, selectivity and sensitivity in the separation of complex biological samples, such as the complex mixtures of peptides for proteome analysis. The development of monolithic stationary phases has opened the new avenue in chromatographic separation science and is in turn playing much more important roles in the wide application area. Monolithic stationary phases have been widely used in fast and high efficiency one- and multi-dimensional separation systems, miniaturized devices, and hyphenated system coupled with mass spectrometers. The developing technology for preparation of monolithic stationary phases is revolutionizing the column technology for the separation of complex biological samples. These techniques using porous monoliths offer several advantages, including miniaturization and on-line coupling with analytical instruments. Additionally, monoliths are ideal support media for imprinting template-specific sites, resulting in the so-called molecularly-imprinted monoliths, with ultra-high selectivity. In this review, the origin of the concept, the differences between their characteristics and those of traditional packings, their advantages and drawbacks, theory of separations, the methods for the monoliths preparation of different forms, nanoparticle monoliths and metal-organic framework are discussed. Two application areas of monolithic metal-organic framework and nanoparticle monoliths are provided. The review article discusses the results reported in a total of 218 references. Other older references were included to illustrate the historical development of monoliths, both in preparation and types, as well as separation mechanism.

Chiral separation of β-blockers by supercritical fluid chromatography using Chiralpak-IG and Chiralpak IBN-5 columns

Chiral separation of β-blockers is performed by utilizing the supercritical fluid chromatographic method. The chiral columns utilized were Chiralpak IG and Chiralpak IBN-5. The finest mobile phase was CO₂ -0.2% TEA in methanol (60:40). The values atenolol enantiomers retention factors were 6.39 and 8.98. These values for propranolol enantiomers were 3.39 and 4.06. These values for betaxolol enantiomers were 4.08 and 4.68. The separation and resolution factor values for atenolol, propranolol, and betaxolol were 1.41 and 3.33, 1.19 and 2.23, and 1.15 and 1.87, separately and respectively. By comparison, it was observed that Chiralpak IG column is better than Chiralpak IBN-5 column. Supercritical fluid chromatography has been found as the best analytical technique due to its high speed, being eco-friendly, and being economic. The various most probable interactions responsible for the chiral resolution are hydrogen bonding, dipole-dipole interactions, steric effect, and π-π interactions. The reported methods are effective, efficient, and reproducible and may be used to separate and identify atenolol, propranolol, and betaxolol in any unknown samples.

Enantiomeric separation of six beta-adrenergic blockers on Chiralpak IB column and identification of chiral recognition mechanisms by molecular docking technique

Enantiomeric separation of six β-adrenergic blockers was systematically studied for the first time on a polysaccharide-based chiral stationary phase, i.e. Chiralpak IB, under the normal-phase mode. The effect of alcohol modifiers, alcohol content and basic additive on enantiomeric separation was evaluated and optimized. Under the optimal conditions, the enantiomers of atenolol, bevantolol, cartelol, esmolol, metoprolol and propranolol were all baseline resolved with resolutions of 1.50, 8.56, 2.05, 2.11, 3.56 and 4.02, respectively. Additionally, molecular docking was tested to explain chiral recognition mechanisms of this set of the drug enantiomers on Chiralpak IB. The details of the various interactions affecting enantiomeric separation were confirmed from the molecular level and the modeling data were in agreement with the chromatographic results concerning the enantioselectivity.

Recent Trends in Fast Liquid Chromatography for Pharmaceutical Analysis

Background:

Liquid chromatography is the workhorse of analytical laboratories of pharmaceutical companies for analysis of bulk drug materials, intermediates, drug products, impurities and degradation products. This efficient technique is impeded by its long and tedious analysis procedures. Continuous efforts of scientists to reduce the analysis time resulted in the development of three different approaches namely, HTLC, chromatography using monolithic columns and UHPLC.

Methods:

Modern column technology and advances in chromatographic stationary phase including silica-based monolithic columns and reduction in particle and column size (UHPLC) have not only revolutionized the separation power of chromatographic analysis but also have remarkably reduced the analysis time. Automated ultra high-performance chromatographic systems equipped with state-ofthe- art software and detection systems have now spawned a new field of analysis, termed as Fast Liquid Chromatography (FLC). The chromatographic approaches that can be included in FLC are hightemperature liquid chromatography, chromatography using monolithic column, and ultrahigh performance liquid chromatography.

Results:

This review summarizes the progress of FLC in pharmaceutical analysis during the period from year 2008 to 2017 focusing on detecting pharmaceutical drugs in various matrices, characterizing active compounds of natural products, and drug metabolites. High temperature, change in the mobile phase, use of monolithic columns, new non-porous, semi-porous and fully porous reduced particle size of/less than 3μm packed columns technology with high-pressure pumps have been extensively studied and successively applied to real samples. These factors revolutionized the fast high-performance separations.

Conclusion:

Taking into account the recent development in fast liquid chromatography approaches, future trends can be clearly predicated. UHPLC must be the most popular approach followed by the use of monolithic columns. Use of high temperatures during analysis is not a feasible approach especially for pharmaceutical analysis due to thermosensitive nature of analytes.

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.

Cellulose type chiral stationary phase based on reduced graphene oxide@silica gel for the enantiomer separation of chiral compounds

The graphene oxide (GO) was covalently coupled to the surfaces of silica gel (SiO2 ) microspheres by amide bond to get the graphene oxide@silica gel (GO@SiO2 ). Then, the GO@SiO2 was reduced with hydrazine to the reduced graphene oxide@silica gel (rGO@SiO2 ), and the cellulose derivatives were physically coated on the surfaces of rGO@SiO2 to prepare a chiral stationary phase (CSP) for high performance liquid chromatography. Under the optimum experimental conditions, eight benzene-enriched enantiomers were separated completely, and the resolution of trans-stilbene oxide perfectly reached 4.83. Compared with the blank column of non-bonded rGO, the separation performance is better on the new CSP, which is due to the existence of rGO to produce special retention interaction with analytes, such as π-π stacking, hydrophobic effect, π-π electron-donor-acceptor interaction, and hydrogen bonding. Therefore, the obtained CSP shows special selectivity for benzene-enriched enantiomers, improves separation selectivity and efficiency, and rGO plays a synergistic effect with cellulose derivatives on enantioseparation.

Approaches for enantioselective resolution of pharmaceuticals by miniaturised separation techniques with new chiral phases based on nanoparticles and monolithis

This article discusses new developments in the preparation of nanoparticles and monoliths with emphasis upon their application as the stationary and pseudo-stationary phases for miniaturised liquid phase separation techniques, which have occurred in the last 10 years (from 2006 to the actuality). References included in this review represent current trends and state of the art in the application of these materials to the analysis, by EKC, CEC and miniaturised chromatography, of chiral compounds with environmental interest such as pharmaceuticals. Due to their extraordinary properties, columns prepared with these new chiral stationary or pseudo-stationary phases, based on materials such as gold nanoparticles, metal-organic frameworks, ordered mesoporous silicas, carbonaceous materials, polymeric-based and silica-based monoliths or molecularly imprinted materials, can usually show some improvements in the separation selectivity, column efficiency and chemical stability in comparison with conventional chiral columns available commercially.

Capillary electrochromatographic fast enantioseparation based on a chiral metal-organic framework

Metal-organic frameworks (MOFs) have received great attention because of their unusual properties and fascinating structures in separation sciences. However, to the best of our knowledge, there has been no attempt to utilize chiral MOFs as stationary phases in packed-CEC. Here, a chiral MOF [In3 O(obb)3 (HCO2 )(H2 O)]·solvent (4,4'-oxybisbenzoic acid) was explored as the chiral stationary phase in packed-CEC for separation of chiral compounds and isomers. The fabricated [In3 O(obb)3 (HCO2 )(H2 O)]·solvent packed capillary columns gave fast enantioseparation of (±)-hydrobenzoin, (±)-1-phenyl-1,2-ethanediol, and clenbuterol within 3 min in CEC. Besides, the baseline separations of nitrophenol isomers within 6 min were also achieved. The RSDs for the retention time of run-to-run, day-to-day, and column-to-column reproducibility were 1.51-3.63, 1.83-3.98, and 3.42-5.66%, respectively. These results demonstrate that chiral MOFs are promising for enantioseparation in CEC.

Recent advances on capillary columns, detectors, and two-dimensional separations in capillary electrochromatography

As a typical miniaturized analytical technique, CEC has attracted much attention because of its low sample and solvent consumption, high efficiency, high selectivity, high resolution, and fast speed. In this review, we mainly cover the development of capillary columns and detection techniques in the CEC since 2009. Herein, three types of capillary columns, namely, open-tubular capillary columns, monolithic columns and packed columns, and several types of detectors are reviewed in detail. Moreover, a 2D separation system based on CEC is also reported.

Synthesis and characterization of cellulose 3,5-dimethylphenylcarbamate silica hybrid spheres for enantioseparation of chiral β-blockers

A cellulose derivative-based chiral stationary phase (CSP) is considered one of the most widely applied CSPs due to its powerful enantioseparation ability. The high loading capacity and mechanical strength of CSPs are crucial for their application in preparative chromatography, such as a simulated moving bed. Compared to traditional cellulose-based CSPs that have been adsorbed onto chromatographic supports, organic-inorganic hybrid CSPs exhibit a potentially higher loading capacity and mechanical strength by increasing the density of chiral recognition groups. A hybrid cellulose 3,5-dimethylphenylcarbamate chiral stationary phase (organic/inorganic: 70/30, w/w) was prepared via a sol-gel method and characterized with several analytical techniques, including Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and (29)Si cross polarization/magic angle spinning nuclear magnetic resonance ((29)Si CP/MAS NMR). In addition, the as-synthesized hybrid chiral silica spheres were treated with an end-capping process to mask the residual silica hydroxyl groups. Compared to a commercial Chiralpak IB column, better separation of β-blocker drugs, including pindolol (selectivity of 5.55), metoprolol (2.30), propranolol (1.96), bisoprolol (1.74) and atenolol (1.46), on the end-capped CSP was achieved using liquid chromatography, which suggests that the packing material synthesized in this work has sufficient chiral discriminating ability for the effective separation of β-blocker drugs.

Chiral selectors in CE: recent developments and applications

This review article provides an overview of the recent advances in enantioanalysis by use of electrophoretic techniques. Due to the big number of publications in the subject mentioned above, this article is focused on chiral method developments and applications published from 2008 until 2011, and it demonstrates chiral selectors used in CE. Numerous chiral selectors have been used over the years, and these include the cyclic and the linear oligo- and polysaccharides, the branched polysaccharides, the polymeric and monomeric surfactants, the macrocyclic and other antibiotics, and the crown ethers. Different dual-selector systems are also presented in this article, and the results are compared with those obtained by use of a single chiral selector. Finally, several pharmaceutical and biomedical applications based on chiral recognition are summarized.

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.