Recent advances in chiral separation principles in capillary electrophoresis and capillary electrochromatography

Fabrication strategies for chiral self-assembly surface

Chiral self-assembly is the spontaneous organization of individual building blocks from chiral (bio)molecules to macroscopic objects into ordered superstructures. Chiral self-assembly is ubiquitous in nature, such as DNA and proteins, which formed the foundation of biological structures. In addition to chiral (bio) molecules, chiral ordered superstructures constructed by self-assembly have also attracted much attention. Chiral self-assembly usually refers to the process of forming chiral aggregates in an ordered arrangement under various non-covalent bonding such as H-bond, π-π interactions, van der Waals forces (dipole-dipole, electrostatic effects, etc.), and hydrophobic interactions. Chiral assembly involves the spontaneous process, which followed the minimum energy rule. It is essentially an intermolecular interaction force. Self-assembled chiral materials based on chiral recognition in electrochemistry, chiral catalysis, optical sensing, chiral separation, etc. have a broad application potential with the research development of chiral materials in recent years.

Ultra-Fast Portable and Wearable Sensing Design for Continuous and Wide-Spectrum Molecular Analysis and Diagnostics

The design and characterization of spatiotemporal nano-/micro-structural arrangement that enable real-time and wide-spectrum molecular analysis is reported and demonestrated in new horizons of biomedical applications, such as wearable-spectrometry, ultra-fast and onsite biopsy-decision-making for intraoperative surgical oncology, chiral-drug identification, etc. The spatiotemporal sesning arrangement is achieved by scalable, binder-free, functionalized hybrid spin-sensitive (<↑| or <↓|) graphene-ink printed sensing layers on free-standing films made of porous, fibrous, and naturally helical cellulose networks in hierarchically stacked geometrical configuration (HSGC). The HSGC operates according to a time-space-resolved architecture that modulate the mass-transfer rate for separation, eluation and detection of each individual compound within a mixture of the like, hereby providing a mass spectrogram. The HSGC could be used for a wide range of applictions, including fast and real-time spectrogram generator of volatile organic compounds during liquid-biopsy, without the need of any immunochemistry-staining and complex power-hungry cryogenic machines; and wearable spectrometry that provide spectral signature of molecular profiles emiited from skin in the course of various dietry conditions.

Past, present, and future developments in enantioselective analysis using capillary electromigration techniques

Enantioseparation of chiral products has become increasingly important in a large diversity of academic and industrial applications. The separation of chiral compounds is inherently challenging and thus requires a suitable analytical technique that can achieve high resolution and sensitivity. In this context, CE has shown remarkable results so far. Chiral CE offers an orthogonal enantioselectivity and is typically considered less costly than chromatographic techniques, since only minute amounts of chiral selectors are needed. Several CE approaches have been developed for chiral analysis, including chiral EKC and chiral CEC. Enantioseparations by EKC benefit from the wide variety of possible pseudostationary phases that can be employed. Chiral CEC, on the other hand, combines chromatographic separation principles with the bulk fluid movement of CE, benefitting from reduced band broadening as compared to pressure-driven systems. Although UV detection is conventionally used for these approaches, MS can also be considered. CE-MS represents a promising alternative due to the increased sensitivity and selectivity, enabling the chiral analysis of complex samples. The potential contamination of the MS ion source in EKC-MS can be overcome using partial-filling and counter-migration techniques. However, chiral analysis using monolithic and open-tubular CEC-MS awaits additional method validation and a dedicated commercial interface. Further efforts in chiral CE are expected toward the improvement of existing techniques, the development of novel pseudostationary phases, and establishing the use of chiral ionic liquids, molecular imprinted polymers, and metal-organic frameworks. These developments will certainly foster the adoption of CE(-MS) as a well-established technique in routine chiral analysis.

Enantioseparation of ketoconazole and miconazole by capillary electrophoresis and a study on their inclusion interactions with β-cyclodextrin and derivatives

A chiral separation method coupled with capillary electrophoresis (CE) analysis for ketoconazole and miconazole enantiomers using chiral selectors such as β-cyclodextrin (β-CD) and hydroxypropyl-β-CD (HP-β-CD) was developed in this study, which included the optimisation, validation and application of the method on the antifungal cream samples. The formation of inclusion complex between the hosts (β-CD and HP-β-CD) and guests (ketoconazole and miconazole) were compared and analysed using ultraviolet-visible spectrophotometry, nuclear magnetic resonance (NMR) spectroscopy and molecular docking methods. Results from the study showed that in a concentration that ranged between 0.25 and 50 mg L^-1 , the linear calibration curves of each enantiomer had a high coefficient of regression (R² > 0.999), low limit of detection (0.075 mg L^-1 ) and low limit of quantification (0.25 mg L^-1 ). The relative standard deviation (RSD) of the intraday and interday analyses ranged from 0.79% to 8.01% and 3.30% to 11.43%, respectively, while the recoveries ranged from 82.0% to 105.7% (RSD < 7%, n = 3). The most probable structure of the inclusion complexes was proposed based on the findings from the molecular docking studies conducted using the PatchDock server.

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.

Enantiomeric Recognition and Separation by Chiral Nanoparticles

Chiral molecules are stereoselective with regard to specific biological functions. Enantiomers differ considerably in their physiological reactions with the human body. Safeguarding the quality and safety of drugs requires an efficient analytical platform by which to selectively probe chiral compounds to ensure the extraction of single enantiomers. Asymmetric synthesis is a mature approach to the production of single enantiomers; however, it is poorly suited to mass production and allows for only specific enantioselective reactions. Furthermore, it is too expensive and time-consuming for the evaluation of therapeutic drugs in the early stages of development. These limitations have prompted the development of surface-modified nanoparticles using amino acids, chiral organic ligands, or functional groups as chiral selectors applicable to a racemic mixture of chiral molecules. The fact that these combinations can be optimized in terms of sensitivity, specificity, and enantioselectivity makes them ideal for enantiomeric recognition and separation. In chiral resolution, molecules bond selectively to particle surfaces according to homochiral interactions, whereupon an enantiopure compound is extracted from the solution through a simple filtration process. In this review article, we discuss the fabrication of chiral nanoparticles and look at the ways their distinctive surface properties have been adopted in enantiomeric recognition and separation.

Separation of enantiomers by their enantiospecific interaction with achiral magnetic substrates

It is commonly assumed that recognition and discrimination of chirality, both in nature and in artificial systems, depend solely on spatial effects. However, recent studies have suggested that charge redistribution in chiral molecules manifests an enantiospecific preference in electron spin orientation. We therefore reasoned that the induced spin polarization may affect enantiorecognition through exchange interactions. Here we show experimentally that the interaction of chiral molecules with a perpendicularly magnetized substrate is enantiospecific. Thus, one enantiomer adsorbs preferentially when the magnetic dipole is pointing up, whereas the other adsorbs faster for the opposite alignment of the magnetization. The interaction is not controlled by the magnetic field per se, but rather by the electron spin orientations, and opens prospects for a distinct approach to enantiomeric separations.

Enantioselective analysis of proteinogenic amino acids in cerebrospinal fluid by capillary electrophoresis-mass spectrometry

d-Amino acids (AAs) are increasingly being recognized as essential molecules in biological systems. Enantioselective analysis of proteinogenic AAs in biological samples was accomplished by CE-MS employing β-CD as chiral selector and ESI via sheath-liquid (SL) interfacing. Prior to analysis, AAs were fully derivatized with FMOC, improving AA-enantiomer separation and ESI efficiency. In order to optimize the separation and MS detection of FMOC-AAs, the effects of type and concentration of CD in the BGE, the composition of the SL, and MS-interfacing parameters were evaluated. Using a BGE of 10 mM β-CD in 50 mM ammonium bicarbonate (pH 8) containing 15% v/v isopropanol, a SL of isopropanol-water-1 M ammonium bicarbonate (50:50:1, v/v/v) at a flow rate of 3 μL/min, and a nebulizer gas pressure of 2 psi, 15 proteinogenic AAs could be detected with enantioresolutions up to 3.5 and detection limits down to 0.9 μM (equivalent to less than 3 pg AA injected). The selectivity of the method was demonstrated by the analysis of spiked cerebrospinal fluid, allowing specific detection of d-AAs. Repeatability and linearity obtained for cerebrospinal fluid were similar to standard solutions, with peak area and migration-time RSDs (n = 5) below 16.2 and 1.6%, respectively, and a linear response (R(2) ≥ 0.977) in the 3-90 μM range.

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.

One-step preparation and application of mussel-inspired poly(norepinephrine)-coated polydimethylsiloxane microchip for separation of chiral compounds

In this paper, using the self-polymerization of norepinephrine (NE) and its favorable film-forming property, a simple and green preparation approach was developed to modify a PDMS channel for enantioseparation of chiral compounds. After the PDMS microchip was filled with NE solution, poly(norepinephrine) (PNE) film was gradually formed and deposited on the inner wall of microchannel as permanent coating via the oxidation of NE by the oxygen dissolved in the solution. Due to possessing plentiful catechol and amine functional groups, the PNE-coated PDMS microchip exhibited much better wettability, more stable and suppressed EOF, and less nonspecific adsorption. The water contact angle and EOF of PNE-coated PDMS substrate were measured to be 13° and 1.68 × 10(-4) cm(2) V(-1) s(-1) , compared to those of 108° and 2.24 × 10(-4) cm(2) V(-1) s(-1) from the untreated one, respectively. Different kinds of chiral compounds, such as amino acid enantiomer, drug enantiomer, and peptide enantiomer were efficiently separated utilizing a separation length of 37 mm coupled with in-column amperometric detection on the PNE-coated PDMS microchips. This facile mussel-inspired PNE-based microchip system exhibited strong recognition ability, high-performance, admirable reproducibility, and stability, which may have potential use in the complex biological analysis.

Molecular imprinting science and technology: a survey of the literature for the years 2004-2011

Herein, we present a survey of the literature covering the development of molecular imprinting science and technology over the years 2004-2011. In total, 3779 references to the original papers, reviews, edited volumes and monographs from this period are included, along with recently identified uncited materials from prior to 2004, which were omitted in the first instalment of this series covering the years 1930-2003. In the presentation of the assembled references, a section presenting reviews and monographs covering the area is followed by sections describing fundamental aspects of molecular imprinting including the development of novel polymer formats. Thereafter, literature describing efforts to apply these polymeric materials to a range of application areas is presented. Current trends and areas of rapid development are discussed.

Simulation of the effects of complex- formation equilibria in electrophoresis: II. experimental verification

The complete mathematical model of electromigration in systems with complexation agents introduced in the Part I of this article (V. Hruška et al., Eletrophoresis, 2012, 33, this issue), which was implemented into our simulation program Simul 5, was verified experimentally. Three different chiral selector (CS) systems differing in the type of the CS, the magnitude of the complexation constants as well as in the experimental conditions were selected for verification. The experiments and simulations were performed at various concentrations of the CSs in order to discuss the influence of the concentration of the CS on the separation. The simulated and experimental electropherograms show very good agreement in the position, shape and amplitude of the analyte peaks. The new Simul 5 Complex offers a deep insight into electrophoretical separations that take place in systems containing complexing agents, for example into enantiomer separations. Using Simul 5 Complex we were able to predict and explain the significant electromigration dispersion of analyte peaks. It was clarified that the electromigration dispersion in these systems results directly from complexation. The new Simul 5 Complex was also shown to be a useful and powerful tool for the prediction of the results of enantioseparations.

Dynamical solutions for migration of chiral DNA-type objects in shear flows

We present a dynamical analysis of chiral object motions to explain physical mechanisms and give quantitative predictions on the shear-induced drift motions of chiral objects and the chiral separation of enantiomers using shear flows. For objects well represented by the uniaxial approximation, such as DNA and chiral disk hexamers, dynamical motions in low-Reynolds-number shear flows are solved analytically, in terms of steady-state object-flow interacting parameters, which can be calculated numerically by well-established methods. The shear-induced drifting speed of long helices are evaluated. Good agreements are found between our results and those obtained from dynamical simulations [Makino and Doi, Phys. Fluids 17, 103605 (2005)]. We also compare our results with those obtained experimentally [Marcos, Fu, Powers, and Stocker, Phys. Rev. Lett. 102, 158103 (2009)]. The analysis may also be extended to study other important chiral-flow interactions in nature environments and microfluidic devices, such as the particle-wall and interparticle interactions.

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.

Surfactant-bound monolithic columns for CEC

A novel anionic surfactant bound monolithic stationary phase based on 11-acrylaminoundecanoic acid is designed for CEC. The monolith possessing bonded undecanoyl groups (hydrophobic sites) and carboxyl groups (weak cationic ion-exchange sites) were evaluated as a mixed-mode stationary phase in CEC for the separation of neutral and polar solutes. Using a multivariate D-optimal design the composition of the polymerization mixture was modeled and optimized with five alkylbenzenes and seven alkyl phenyl ketones as test solutes. The D-optimal design indicates a strong dependence of electrochromatographic parameters on the concentration of 11-acrylaminoundecanoic acid monomer and porogen (water) in the polymerization mixture. A difference of 6, 8 and 13% RSD between the predicted and the experimental values in terms of efficiency, resolution and retention time, respectively, indeed confirmed that the proposed approach is practical. The physical (i.e. morphology, porosity and permeability) and chromatographic properties of the monolithic columns were thoroughly investigated. With the optimized monolithic column, high efficiency separation of N-methylcarbamates pesticides and positional isomers was successfully achieved. It appears that this type of mixed-mode monolith (containing both chargeable and hydrophobic sites) may have a great potential as a new generation of CEC stationary phase.

Inherent peak compression of charged analytes in electrochromatography

This work resolves peak compression of charged analytes in CEC with strong cation-exchange stationary phase particles. By combining electrochromatographic peak shape analysis with the results of numerical simulations and confocal laser scanning microscopy in the packed capillaries, we identify electrical field-induced concentration polarization as the key physical phenomenon responsible for the inherent existence of local electrical field gradients on the scale of an individual support particle. Consequently, positive and negative field gradients exist between and inside the particles along the whole packing. Their intensity depends on the particles cation-selectivity (governed by the particles volume charge density and the mobile phase ionic strength) and the applied field strength. The interplay of these local field gradients with the analytes retention (intraparticle adsorption) determines whether fronting, tailing, or spiked analyte peaks are observed, and it provides a mechanism by which strongly retained analytes can be eluted over long distances with little zone dispersion. Our analysis explains the "anomalous" peak compression effects with strong cation-exchange particles, which have been reported more than a decade ago (Smith, N. W., Evans, M. B., Chromatographia 1995, 41, 197-203) and since then remained largely unresolved.

Advanced CE for chiral analysis of drugs, metabolites, and biomarkers in biological samples

An analysis of recent trends indicates that CE can show real advantages over chromatographic methods in ultratrace enantioselective determination of biologically active compounds in complex biological matrices. It is due to high separation efficiency and many applicable in-capillary electromigration effects in CE (countercurrent migration, stacking effects) enhancing significantly (enantio)separability and enabling effective sample preparation (preconcentration, purification, analyte derivatization). Other possible on-line combinations of CE, such as column coupled CE-CE techniques and implementation of nonelectrophoretic techniques (extraction, membrane filtration, flow injection) into CE, offer additional approaches for highly effective sample preparation and separation. CE matured to a highly flexible and compatible technique enabling its hyphenation with powerful detection systems allowing extremely sensitive detection (e.g. LIF) and/or structural characterization of analytes (e.g. MS). Within the last decade, more as well as less conventional analytical on-line approaches have been effectively utilized in this field and their practical potentialities are demonstrated on many new application examples in this article. Here, three basic areas of (enantioselective) drug bioanalysis are highlighted and supported by a brief theoretical description of each individual approach in a compact review structure (to create integrated view on the topic), including (i) progressive enantioseparation approaches and new enantioselective agents, (ii) in-capillary sample preparation (preconcentration, purification, derivatization), and (iii) detection possibilities related to enhanced sensitivity and structural characterization.

Strategies in method development to quantify enantiomeric impurities using CE

The growing number of chiral new drug substances requires increasing efforts in developing enantioselective methods. According to International conference on Harmonization guidelines, one should quantify the enantiomeric impurity of 0.1% relative to the major constituent. Capillary electrophoresis has evolved into an important tool for the separation of chiral drugs. The common strategies consist of two steps: firstly, initial separation conditions are evaluated. This screening usually focuses on the selection of the appropriate chiral selector. In our study 22 neutral, anionic or cationic cyclodextrins were dissolved in phosphate buffer (pH 2.5, 50 mM, CD conc.: 2.0%). Then they were investigated for the separation of 14 chiral compounds. Secondly, the obtained initial conditions for the enantiomeric separation were optimized in terms of resolution and analysis time. In our approach, important optimized factors including the concentration of the chiral selector (1-10%), the pH of the buffer (2.0-9.0), and the percentage of organic modifier (0-15%) were studied. This common strategy was completed by elaborating final requirements for the quantification of the enantiomeric impurity. A resolution between 3 and 4 was found to be necessary for the racemic mixture during the screening and optimization steps, in order to later allow for peak overloading and thus to sufficiently increase the signal-to-noise ratio. The complete strategy was conducted for atenolol, isoprenaline, verapamil and mandelic acid.