A feasibility study on the use of hydrophobic eutectic solvents as pseudo-stationary phases in capillary electrophoresis for chiral separations

Theoretical study of modified multiple dual-mode elution counter-current chromatography based on chiral separation

The fact that both phases are liquids makes it simple to implement counter-current chromatography (CCC), which involves alternating elution modes and phase roles during the separation process. Using two distinct multiple dual-mode (MDM) elution techniques for chiral separation, the team demonstrated a superior separation effect in a prior study. When selecting a chiral selector with high enantiomer recognition capability is difficult, the separation efficiency of the CCC column can be enhanced through multiple cycles. This approach facilitates the preparation of enantiomers that are challenging to isolate under classical CCC conditions. A straightforward mathematical model was created in this study to forecast the number of cycles and the mode conversion time needed for the separation process after the modified MDM method was further examined theoretically. After modeling molecules, the theoretical model was confirmed, and the outcomes generally agreed with the calculations from the theoretical model. This offers substantial support for the modified MDM method's real-world implementation in chiral separation.

Capillary electrophoresis separations with Betaine:Urea, a deep eutectic solvent as the separation medium

BACKGROUND

Capillary electrophoresis (CE) is a highly versatile separation technique widely used in analytical chemistry. Traditionally, CE can be categorized as either aqueous or non-aqueous systems based on the buffer solvents employed. For decades, non-aqueous CE has been predominantly associated with the use of organic solvents, a perception deeply ingrained in the scientific community. However, growing concerns about the health and environmental impacts of these solvents, driven by the principles of green chemistry, have prompted a reevaluation of their use. In response to these concerns, our group recently introduced a deep eutectic solvent (DES), specifically Proline:Urea, as an innovative and eco-friendly separation medium for CE. This approach not only enhances the sustainability of CE separations but also offers a new perspective for the development of innovative CE separation media.

RESULTS

Building on our previous work, here we report the use of the second DES, Betaine:Urea (BU), as a new separation medium that offers further improved performance for CE applications. The DES was systematically characterized, with key physical properties relevant to CE separations, such as thermal properties, viscosity, dielectric constant, Joule heating effect, and UV transmittance, being thoroughly examined. Using a complex sample of 10 structurally similar naphthalene derivatives, we demonstrated the efficiency of BU in capillary zone electrophoresis (CZE) for separating analytes with varying charges (including cations, neutrals, and anions) and sizes. Additionally, we established the first micellar electrokinetic chromatography (MEKC) system in this DES using sodium dodecyl sulfate (SDS) as the surfactant. This system successfully resolved 6 structurally similar neutrals that could not be separated by conventional aqueous SDS-MEKC, highlighting the versatility of this DES-type separation medium. Furthermore, BU showed several advantages over the previously reported DES, Proline:Urea, particularly in terms of stability, viscosity, and Joule heating effects.

SIGNIFICANCE

This study holds the potential to challenge the traditional notion that "CE separation media are merely categorized into aqueous and organic solvents". Given that DESs are "designer" solvents with highly tunable properties and environmentally friendly characteristics, the introduction of BU as a viable alternative to traditional solvents not only expands the media available for CE separations, but also offers a more efficient and potentially more sustainable option for specific analyses.

Cyclodextrins-based deep eutectic supramolecules as chiral selectors for enhanced enantioseparation in capillary electrophoresis

The joint use of deep eutectic solvents (DESs) and cyclodextrins (CDs) has been well demonstrated to have a promoting effect on chiral separation in capillary electrophoresis (CE). These studies focused on constructing synergistic separation systems by adding DESs and CDs to the buffer solution respectively. In this work, for the first time, β-cyclodextrin (β-CD), methyl-β-cyclodextrin (M-β-CD), and hydroxypropyl-β-cyclodextrin (HP-β-CD) were directly used as precursors to prepare several CDs-based deep eutectic supramolecules (DESUPs) by assembling with two organic acids (L-lactic acid and L-malic acid) in different ratios through a simple two-phase mixing. These CDs-based DESUPs were further employed as chiral selectors in CE to separate six racemic chiral drugs. Compared with the unmodified CDs systems, the separations of model drugs in the DESUPs separation systems were significantly improved. We calculated the binding constants of HP-β-CD with enantiomers before and after preparation as DESUPs, and investigated the chiral recognition mechanism of DESUPs chiral selectors using UV spectroscopy and nuclear magnetic resonance method. The enhanced enantioselectivity of CDs-based DESUPs was attributed to several factors. This study has opened up a new path for the exploration of high-performance chiral materials.

Clindamycin phosphate-based deep eutectic solvent as a chiral selector for enantioseparation of amino alcohol drugs in nonaqueous capillary electrophoresis

Clindamycin phosphate (CP) exhibits good enantioselectivity for many basic drugs, but its separation effect for most amino alcohol drugs is not satisfactory. In this work, a deep eutectic solvent (DES) chiral selector based on CP was prepared for the first time and utilized as a single chiral selector in nonaqueous capillary electrophoresis (NACE) to separate twelve amino alcohol drugs. Compared with unmodified CP, the separations of model drugs in the DES chiral selector system were significantly improved. Most amino alcohol drugs could be completely separated, and the peak shapes were good. In addition, we used infrared spectroscopy and nuclear magnetic resonance method to study the specific separation mechanism and explored the reasons why DES chiral selector has better enantioselectivity. This work is the first to directly modify antibiotic chiral selector into DES, indicating a direction for us to develop novel chiral recognition materials.

Deep eutectic solvents as a green alternative to organic solvents for β-cyclodextrin pseudo-stationary phase in capillary electrophoresis

β-cyclodextrin (β-CD), as an important pseudo-stationary phase (PSP) in capillary electrophoresis (CE), frequently confronts challenges stemming from its limited water solubility, particularly when high concentrations are required for resolving complex analytes. Traditionally, researchers often resort to the use of (toxic) organic solvents to enhance the solubility of β-CD, establishing non-aqueous capillary electrophoresis (NACE) for specific separations. However, such practices are hazardous to health and run counter to the principles of green analytical chemistry. In this study, we demonstrate a deep eutectic solvent (DES), Proline:Urea (PU), as a promising alternative to conventional organic solvents for β-CD-based CE separations. The DES exhibits a solubility of up to 30% for β-CD, a significant improvement compared to the 1.8% solubility in the aqueous phase. Utilizing this DES-type separation medium, we achieved simultaneous baseline separation of a complex analyte composed of eight structurally similar naphthoic acid derivatives. Furthermore, we conducted a systematic comparison of β-CD's performance in aqueous CE buffers, organic solvents, and DESs, highlighting the superiority of this novel and environmentally friendly CE separation medium.

Update on chiral recognition mechanisms in separation science

The stereospecific analysis of chiral molecules is an important issue in many scientific fields. In separation sciences, this is achieved via the formation of transient diastereomeric complexes between a chiral selector and the selectand enantiomers driven by molecular interactions including electrostatic, ion-dipole, dipole-dipole, van der Waals or π-π interactions as well as hydrogen or halogen bonds depending on the nature of selector and selectand. Nuclear magnetic resonance spectroscopy and molecular modeling methods are currently the most frequently applied techniques to understand the selector-selectand interactions at a molecular level and to draw conclusions on the chiral separation mechanism. The present short review summarizes some of the recent achievements for the understanding of the chiral recognition of the most important chiral selectors combining separation techniques with molecular modeling and/or spectroscopic techniques dating between 2020 and early 2024. The selectors include polysaccharide derivatives, cyclodextrins, macrocyclic glycopeptides, proteins, donor-acceptor type selectors, ion-exchangers, crown ethers, and molecular micelles. The application of chiral ionic liquids and chiral deep eutectic solvents, as well as further selectors, are also briefly addressed. A compilation of all published literature on chiral selectors has not been attempted.

Evaluation of deep eutectic solvents chiral selectors based on lactobionic acid in capillary electrophoresis

Recently, deep eutectic solvents (DESs) have attracted considerable interest in analytical chemistry. This work described the enantioseparations of twenty amino alcohol drugs with several DESs based on lactobionic acid (LA) as the sole chiral selector in capillary electrophoresis (CE) firstly. Compared to the single LA system and the ionic liquid/LA synergistic system, the DES system exhibited considerably improved separations. The influences of some key parameters on separations were investigated in detail. This work also experimentally demonstrated that the carboxyl group was indispensable in the process of chiral recognition. The mechanisms of the improvements of DESs on enantioseparations were studied via ultraviolet spectroscopy. Furthermore, the proposed method was used to determine the enantiomeric purity of propranolol hydrochloride successfully. This is the first time that chiral DESs were utilized as the sole chiral selectors in CE, and this strategy has opened up a new prospect for the use of DESs in enantioseparation.

Capillary electrophoresis separations with deep eutectic solvents as greener separation media: A proof-of-concept study

Capillary electrophoresis (CE) has conventionally been classified into aqueous and non-aqueous categories based on the types of buffer solvents employed. Traditionally, non-aqueous CE has always been associated with the use of organic solvents, which are considered hazardous to health and environmentally detrimental. In this work, we introduce deep eutectic solvents (DESs) as CE separation media for the first time, presenting a novel and environmentally friendly approach to CE separations. The DES employed consists of proline and urea (Proline:Urea, PU), both of which are naturally occurring compounds that are readily available, cost-effective, and environmentally benign. Various fundamental aspects of the DES-type CE media were investigated, including thermal property, viscosity, electroconductivity, Joule heating effect, and compatibility with detectors. A simulated complex mixture of ten naphthalene-based compounds with varied charges and sizes was separated using the DES-based medium in capillary zone electrophoresis (CZE) mode. Moreover, we also established a DES-based micellar electrokinetic chromatography (MEKC) system utilizing Tween-20 as the surfactant. Six structurally similar naphthalene derivatives (isomers) that couldn't be resolved by CZE were effectively separated due to their strong hydrophobic interaction with Tween-20 micelles within the DES medium. Given that DESs are "designer" solvents with highly tunable properties and environmentally friendly characteristics, this study demonstrates the potential of employing DESs as an alternative to organic solvents for greener CE separations.

Investigation on improvement of enantioseparation based on clindamycin phosphate by chiral deep eutectic solvents in capillary electrophoresis

In this work, the potential synergetic effect between deep eutectic solvents and an antibiotic chiral selector (clindamycin phosphate) for enantioseparation was investigated in capillary electrophoresis. We synthesized a series of deep eutectic solvents with choline chloride as hydrogen bond acceptor and three α-hydroxyl acids (l-lactic acid, l-malic acid, and l-tartaric acid) as hydrogen bond donors. Compared to the single clindamycin phosphate separation system, significantly improved separations of model drugs were observed in several synergetic systems. Compared to deep eutectic solvents with a single hydrogen bond donor, deep eutectic solvents with mixed-type hydrogen bond donors were superior. The influences of several key parameters including the type and proportion of organic modifier, clindamycin phosphate concentrations, deep eutectic solvents concentrations, and buffer pH were investigated in detail. The mechanism of the enhanced separations in deep eutectic solvents systems was investigated by means of electroosmotic flow analysis, nuclear magnetic resonance analysis, and molecular modeling. It was the first time that the synergetic systems between deep eutectic solvents and antibiotic chiral selector were established in capillary electrophoresis, and these deep eutectic solvents were demonstrated to have a good synergetic effect with clindamycin phosphate for enantioseparation.

Deep eutectic solvents combined with beta-cyclodextrin derivatives for chiral separation of typical adrenergic receptor agonists by capillary electrophoresis with amperometric detection

Enantioseparation has always been one of the research hotspots and difficulties in the field of modern separation science. In this work, a binary chiral electrophoretic separation system was constructed using deep eutectic solvents (DESs) coupled with beta-cyclodextrin derivatives based on capillary electrophoresis with amperometric detection system, and five groups of typical adrenergic receptor agonists (adrenaline, salbutamol, isoproterenol, norepinephrine and terbutaline) were selected as the model enantiomers. The effects of additive types and contents of DESs and cyclodextrins, and the pH value and concentration of the running buffer on the resolution of the selected chiral compounds were investigated in detail. The mechanism of DESs improving separation was explored preliminarily by means of UV spectrophotometry, which was further verified based on the comparison of single and mixed components of choline chlorine-urea DES. Under the optimum conditions, the relative standard deviations for inter-day and intra-day repeatability of the migration time, peak area and resolution for adrenaline and salbutamol were within 8.7%, and the limits of detection reached 0.030 μg mL-1 (S/N = 3). The recovery data were in the range of 96.3-118.7%. The developed methods have been applied for the analyses of (+)-adrenaline hydrochloride injection and (±)-salbutamol aerosol. This binary chiral electrophoretic separation system by CE-AD has high detection sensitivity and low analytical cost, providing an alternative for the separation and analysis of chiral drugs.

Strategies for capillary electrophoresis: Method development and validation for pharmaceutical and biological applications-Updated and completely revised edition

This review is in support of the development of selective, precise, fast, and validated capillary electrophoresis (CE) methods. It follows up a similar article from 1998, Wätzig H, Degenhardt M, Kunkel A. "Strategies for capillary electrophoresis: method development and validation for pharmaceutical and biological applications," pointing out which fundamentals are still valid and at the same time showing the enormous achievements in the last 25 years. The structures of both reviews are widely similar, in order to facilitate their simultaneous use. Focusing on pharmaceutical and biological applications, the successful use of CE is now demonstrated by more than 600 carefully selected references. Many of those are recent reviews; therefore, a significant overview about the field is provided. There are extra sections about sample pretreatment related to CE and microchip CE, and a completely revised section about method development for protein analytes and biomolecules in general. The general strategies for method development are summed up with regard to selectivity, efficiency, precision, analysis time, limit of detection, sample pretreatment requirements, and validation.