Chemically L-phenylalaninamide-modified monolithic silica column prepared by a sol-gel process for enantioseparation of dansyl amino acids by ligand exchange-capillary electrochromatography

Polydopamine-Assisted Rapid One-Step Immobilization of L-Arginine in Capillary as Immobilized Chiral Ligands for Enantioseparation of Dansyl Amino Acids by Chiral Ligand Exchange Capillary Electrochromatography

Herein, a novel L-arginine (L-Arg)-modified polydopamine (PDA)-coated capillary (PDA/L-Arg@capillary) was firstly fabricated via the basic amino-acid-induced PDA co-deposition strategy and employed to constitute a new chiral ligand exchange capillary electrochromatography (CLE-CEC) method for the high-performance enantioseparation of D,L-amino acids (D,L-AAs) with L-Arg as the immobilized chiral ligand coordinating with the central metal ion Zn(II) as running buffer. Assisted by hydrothermal treatment, the robust immobilization of L-Arg on the capillary inner wall could be facilely achieved within 1 h, prominently improving the synthesis efficiency and simplifying the preparation procedure. The successful preparation of PDA/L-Arg coatings in the capillary was systematically characterized and confirmed using several methods. In comparison with bare and PDA-functionalized capillaries, the enantioseparation capability of the presented CLE-CEC system was significantly enhanced. Eight D,L-AAs were completely separated and three pairs were partially separated under the optimal conditions. The prepared PDA/L-Arg@capillary showed good repeatability and stability. The potential mechanism of the greatly enhanced enantioseparation performance obtained by PDA/L-Arg@capillary was also explored. Moreover, the proposed method was further utilized for studying the enzyme kinetics of L-glutamic dehydrogenase, exhibiting its promising prospects in enzyme assays and other related applications.

Advancements in the preparation and application of monolithic silica columns for efficient separation in liquid chromatography

Fast and efficient separation remains a big challenge in high performance liquid chromatography (HPLC). The need for higher efficiency and resolution in separation is constantly in demand. To achieve that, columns developed are rapidly moving towards having smaller particle sizes and internal diameters (i.d.). However, these parameters will lead to high back-pressure in the system and will burden the pumps of the HPLC instrument. To address this limitation, monolithic columns, especially silica-based monolithic columns have been introduced. These columns are being widely investigated for fast and efficient separation of a wide range of molecules. The present article describes the current methods developed to enhance the column efficiency of particle packed columns and how silica monolithic columns can act as an alternative in overcoming the low permeability of particle packed columns. The fundamental processes behind the fabrication of the monolith including the starting materials and the silica sol-gel process will be discussed. Different monolith derivatization and end-capping processes will be further elaborated and followed by highlights of the performance such monolithic columns in key applications in different fields with various types of matrices.

Chiral ligand exchange capillary electrochromatography with dual ligands for enantioseparation of D,L-amino acids

Utilizing block copolymers as coatings, a protocol of chiral ligand exchange capillary electrochromatography (CLE-CEC) protocol was designed and developed with dual ligands for D,L-amino acids enantioseparation. Four block copolymers including poly maleic anhydride-co-styrene-co-N-methacryloyl-L-histidine methyl ester [P(MAn-St-MAH)], poly maleic anhydride-co-styrene-co-N-methacryloyl-L-lysine methyl ester [P(MAn-St-MAL)], poly maleic anhydride-co-styrene-co-N-methacryloyl-L-phenylalanine methyl ester [P(MAn-St-MAP)] and poly maleic anhydride-co-styrene-co-N-methacryloyl-L-threonine methyl ester [P(MAn-St-MAT)] were synthesized by reversible addition fragmentation chain transfer polymerization reaction. Key factors affecting the enantioresolution were optimized, including the concentration of Zn (II) central ion, pH value of buffer solution and monomers of the block copolymers. The enantioresolution of the proposed CLE-CEC system could be enhanced dramatically by employing P(MAn-St-MAH) as the immobilized chiral ligand and by coordinating the synergistic effect of free ligand in buffer solution. The principle of improved enantioresolution of the CLE-CEC system with dual ligands was discussed. Well enantioseparation was successfully realized with 7 pairs of D,L-amino acids enantiomers baseline separation and 5 pairs part separation. For quantitative analysis of D,L-alanine, a good linearity was established in the range of 9.4 μM to 1.5 mM (r² = 0.997) with the limits of detection (LODs) 3.7 μM of D-alanine, 2.0 μM for L-alanine, and limits of quantification (LOQs) 9.0 μM for D-alanine and 6.0 μM for L-alanine. The peak area and migration time reproducibility (n = 6) were 4.1% and 3.5% for D-alanine, 3.7% and 3.1% for L-alanine. Further, the enzyme kinetics study of alanine aminotransferase was investigated with the constructed CLE-CEC system.

Construction of chiral ligand exchange capillary electrochromatography for d,l-amino acids enantioseparation and its application in glutaminase kinetics study

A chiral ligand exchange capillary electrochromatography (CLE-CEC) protocol was designed and implemented for d,l-amino acids enantioseparation with poly(maleic anhydride-styrene-methacryloyl-l-arginine methyl ester) as the coating. The block copolymer was synthesized through the reversible addition fragmentation chain transfer reaction. In the constructed CLE-CEC system, poly (methacryloyl-l-arginine methyl ester) moiety of the block copolymer played the role as the immobilized chiral ligand and Zn (II) was used as the central ion. Key factors, including pH of buffer solution, ratio of Zn (II) to ligands, the mass ratio of monomers in the block copolymer, which affect the enantioresolution were investigated. Comparing with the bare capillary, the CLE-CEC enantioresolution was enhanced greatly with the coating one. 5 Pairs of d,l-amino acids enantiomers obtained baseline separation with 5 pairs partly separated. The mechanism of enhancement enantioresolution of the developed CLE-CEC system was explored briefly. Further, good linearities were achieved in the range of 25.0 μM-5.0 mM for quantitative analysis of d-glutamine (r² = 0.997) and l-glutamine (r² = 0.991). Moreover, the proposed CLE-CEC assay was successfully applied in the kinetics study of glutaminase by using l-glutamine as the substrate.

Negative and Positive Confinement Effects in Chiral Separation Chromatography Monitored with Molecular-Scale Precision by In-Situ Electron Paramagnetic Resonance Techniques

Separation of compounds using liquid chromatography is a process of enormous technological importance. This is true in particular for chiral substances, when one enantiomer has the desired set of properties and the other one may be harmful. The degree of development in liquid chromatography is extremely high, but still there is a lack in understanding based on experimental data how selectivity works on a molecular level directly at the surfaces of a porous host material. We have prepared amino-acid containing organosilica as such host materials. Watching the rotational dynamics of chiral spin probes using electron paramagnetic resonance spectroscopy allows us to differentiate between surface adsorbed and free guest species. Diastereotopic selectivity factors were determined, and the influence of chiral surface group density, chemical character of the surface groups, pore-size, and temperature was investigated. We found higher selectivity values in macroporous solids with a rather rigid organosilica network and at lower temperature, indicating the significant effect of confinement effects. In mesoporous materials features are opposed with regards to the T-dependent behavior. From EPR imaging techniques and the resulting (macroscopic) diffusion coefficients, we could confirm that the correlations found on the microscopic level transform also to the macroscopic behavior. Thus, our study is of value for the development of future chromatography materials by design.

Recent Progress in Monolithic Silica Columns for High-Speed and High-Selectivity Separations

Monolithic silica columns have greater (through-pore size)/(skeleton size) ratios than particulate columns and fixed support structures in a column for chemical modification, resulting in high-efficiency columns and stationary phases. This review looks at how the size range of monolithic silica columns has been expanded, how high-efficiency monolithic silica columns have been realized, and how various methods of silica surface functionalization, leading to selective stationary phases, have been developed on monolithic silica supports, and provides information on the current status of these columns. Also discussed are the practical aspects of monolithic silica columns, including how their versatility can be improved by the preparation of small-sized structural features (sub-micron) and columns (1 mm ID or smaller) and by optimizing reaction conditions for in situ chemical modification with various restrictions, with an emphasis on recent research results for both topics.

Facile one-pot preparation of chiral monoliths with a well-defined framework based on the thiol–ene click reaction for capillary liquid chromatography

A novel chiral cyclodextrin (CD) monolith was easily prepared via a one-pot process based on the thiol–ene click reaction of allyl-β-CD with pentaerythritol tetra-(3-mercaptopropionate) in a fused-silica capillary.

Single-step approach for fabrication of vancomycin-bonded silica monolith as chiral stationary phase

A vancomycin-bonded silica monolithic column for capillary electrochromatography (CEC) was prepared by a single-step in situ sol-gel approach. This sol-gel process incorporates a synthetic sol-gel precursor which contains a macrocyclic antibiotic, vancomycin, to form a porous silica network inside a fused-silica capillary. To avoid degradation of vancomycin during the column fabrication, a mild step was adopted into the sol-gel process. The performance of the vancomycin chiral stationary phase was investigated by CEC in both the reversed-phase mode and the normal-phase mode. The vancomycin chiral stationary phase was optimized with respect to vancomycin loading in the reversed-phase mode for chiral separation of thalidomide enantiomers. The best efficiency and resolution values of 94600plates/m and 5.79, respectively, were achieved. The optimized column was further applied to chiral separation of alprenolol enantiomers. A plate height of less than 7μm for the first eluted enantiomer of alprenolol was obtained in an aqueous mobile phase at a flow rate of 0.74mm/s. Using enantiomers of seven β-blockers and some other basic enantiomers as test analytes, separation efficiencies of up to 148100plates/m in the reversed-phase mode and up to 138100plates/m in the normal-phase mode were achieved.

Chiral separation-based ligand exchange by open-tubular capillary electrochromatography

Chiral ligand-exchange enantioseparation of aliphatic and aromatic amino acids was successfully performed using a new open-tubular zwitterionic column with tentacle-type polymer stationary phase. The polymeric stationary phase was prepared using 3-chloro-2-hydroxypropyl methacrylate (HPMA-Cl), a new reactive monomer. The preparation procedure of the open-tubular column included silanization, in situ graft polymerization with HPMA-Cl, and L-histidine (L-His) modification. L-His was used as a chiral ligand-exchange selector and copper(II) as a central ion. Successful enantioseparation of six pairs of amino acid enantiomers was achieved with a buffer of 5 mM CuSO₄, 20 mM (NH4)₂SO₄ at pH 3.0.

Application of chiral ligand-exchange stationary phases in capillary electrochromatography

The chiral separation principle ligand-exchange is applicable for HPLC, capillary electrophoresis as well as for capillary electrochromatography. Among other chiral separation principles, ligand-exchange permits simple method optimization and can be used particularly for chiral amino acids, amino alcohols or α-hydroxy acids. Up to now, three different approaches how to prepare capillaries for ligand-exchange electrochromatography have been published: The use of packed columns, monolithic columns, and particle-loaded columns. This chapter describes examples for the preparation of such columns for all three approaches and illustrates their use for chiral separations.

Chiral separation using capillary electromigration techniques based on ligand exchange principle

Over the last couple of decades, researchers have developed diverse chiral separation methods emerged from a few chiral separation principles. This review article is primarily focused on the application of chiral ligand-exchange (CLE) principle in capillary electromigration techniques, such as capillary electrophoresis (CE) and capillary electrochromatography (CEC). First, the most commonly used CLE-CZE separation mode by using different kinds of central ions, such as Cu(II), Zn(II), borate ion, and other metal ions, has been introduced. Meanwhile, several kinds of surfactants have been applied as the micelle-forming agents in the CLE micellar electrokinetic chromatography mode. The highlight of recent research of CLE-CEC is the exploitation of novel columns for chiral separation. Then, two kinds of capillary columns, packed capillary and monolithic capillary column, have been briefly described. Finally, the effective application of these chiral separation methods has been presented, including the application in life science and food analysis area.

A novel chiral ligand exchange capillary electrophoresis system with amino acid ionic liquid as ligand and its application in screening D-amino-acid oxidase inhibitors

A novel amino acid ionic liquid (AAIL) with L-ornithine (L-Orn) as anion was successfully synthesized, and subsequently applied as an available chiral ligand coordinated with Zn(II) in a chiral ligand exchange capillary electrophoresis (CLE-CE) system for the enantioseparation of dansyl amino acids (Dns-D,L-AAs). The influence of key parameters, such as buffer pH, concentration ratio of Zn(II) to ligand and complex concentration, was investigated in detail. Eleven pairs of Dns-D,L-AAs enantiomers were baseline separated and three pairs were partly separated under the optimum conditions. For exploring its potential application, the quantitative features of this proposed method were studied. Good linearity (r(2) = 0.999) and favorable repeatability (RSD ≤ 3.4%) were obtained by using Dns-D,L-Met as the test analyte. Finally, this method was employed to investigate the inhibition efficiency of d-amino acid oxidase (DAAO) inhibitors, which may pave a new way for the high-throughput screening of enzyme inhibitors and relevant drug discovery.

Chiral metal-ion complexes for enantioseparation by capillary electrophoresis and capillary electrochromatography: a selective review

This review gives an overview about chiral separation by capillary electrophoresis and capillary electrochromatography using different chiral metal-ion complexes. The topic enantioseparation is still of big interest for chiral drugs and natural compounds. Regarding chiral drugs it is often the case that the enantiomers differ in activity. The chiral separation principle of ligand-exchange (LE) can be enabled for liquid chromatography, capillary electrophoresis as well as for capillary electrochromatography. Ligand-exchange can be applied particularly for chiral amino acids, amino alcohols or α-hydroxy acids. Examples and applications are given along with the latest developments.

Ordered, mesoporous metal phosphonate materials with microporous crystalline walls for selective separation techniques

Ordered, hexagonal, mesoporous metal (Ti, Zr, V, Al)-phosphonate materials with microporous crystalline walls are synthesized through a microwave-assisted procedure by using triblock copolymer F127 as the template. Corresponding metal chlorides and ethylene diamine tetra(methylene phosphonic acid) are chosen as the inorganic precursors and the coupling molecule, respectively. X-ray diffractometry, transmission electron microscopy, N(2) sorption, and thermogravimetry measurements confirm that the obtained metal phosphonates possess a hierarchically porous structure with pore sizes of 7.1-7.5 nm and 1.3-1.7 nm for mesopores and micropores, respectively, and the metal phosphonate materials are thermally stable up to around 450 °C with the pore structure and hybrid framework well preserved. Magic angle spinning NMR, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy analyses indicate that the phosphonate groups are homogenously incorporated into the hybrid framework of the obtained materials. For the first time, the mesoporous hybrid materials are employed as the stationary phase in open tubular capillary electrochromatography technique for the separation of various substances including acidic, basic, and neutral compounds. These materials show good selectivity and reproducibility for this application.

Enantioseparation of amino acids, alpha-hydroxy acids, and dipeptides by ligand-exchange CEC using silica-based chiral stationary phases

This work deals with the application of silica-based ligand-exchange chiral stationary phases (CSPs) for the enantioseparation of underivatized amino acids, alpha-hydroxy acids, and dipeptides with packed CEC. Two different possibilities of preparing silica-based CSPs are presented. One phase contains L-4-hydroxyproline chemically bonded via a spacer to 3 mum silica material. The other approach makes use of N-decyl-L-4-hydroxyproline dynamically coated on a reversed-phase packed capillary. Dynamical coating of reversed-phase material represents a simple alternative to prepare CSP. A comparison of the chemically bonded phase with the dynamically coated CSP by means of resolution of complex-forming analytes is presented. The chemically bonded phase was found to be superior to the dynamically coated phase in terms of resolution of amino acids and dipeptides. However, the dynamically coated CSP was found to be especially suitable for the separation of alpha-hydroxy acids. Both techniques are applicable for enantiomer purity tests.

Electroosmotic pumps for microflow analysis

With rapid development in microflow analysis, electroosmotic pumps are receiving increasing attention. Compared to other micropumps, electroosmotic pumps have several unique features. For example, they are bi-directional, can generate constant and pulse-free flows with flow rates well suited to microanalytical systems, and can be readily integrated with lab-on-chip devices. The magnitude and the direction of flow of an electroosmotic pump can be changed instantly. In addition, electroosmotic pumps have no moving parts. In this article, we discuss common features, introduce fabrication technologies and highlight applications of electroosmotic pumps.

Preparation and evaluation of a lysine-bonded silica monolith as polar stationary phase for hydrophilic interaction pressurized capillary electrochromatography

A silica-based monolith as polar stationary phase was described for hydrophilic interaction pressurized capillary electrochromatography (HI-pCEC). The polar monolithic column was prepared by on-column reaction of lysine with epoxy groups on a gamma-glycidoxypropyltrimethosysilane-modified silica monolith. The stationary phase yielded strong hydrophilic interaction due to the slightly polar hydroxyl groups, and the strong polar lysine ligand with amino groups and carboxylic groups contained on the surface of the monolith. In order to evaluate the hydrophilic character of lysine ligand, the chromatographic behaviors of epoxy monolith (before lysine bonded) and diol monolith (hydroxyl groups contained) were also investigated. Two groups of comparative experiment were developed in terms of the separation of typical neutral non-polar and polar compounds performed in a mobile phase of aqueous-acetonitrile solution. Results showed that the lysine monolith was much more hydrophilic than the diol monolith, which presented less hydrophobic than the epoxy monolith. For further study on its hydrophilic character, the lysine monolith was demonstrated in the HI-pCEC mode for the separations of various polar compounds such as phenols, nucleic acid bases and nucleosides.

Dynamically coated silica monolith with ionic liquids for capillary electrochromatography

An ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) was introduced as dynamic coating of a silica monolithic column for capillary electrochromatography of phenols and nucleoside monophosphates. The run-to-run and column-to-column repeatability of migration time for six phenols were satisfactory on this column with relative standard deviation values less than 0.90 and 4.31%, respectively. Anodic electroosmotic flow (EOF) was observed, which increased with the increase of [BMIM][BF4] concentration within 120 mM and when [BMIM][BF4] concentration was above 120 mM, EOF leveled off due to the saturation of [BMIM][BF4] on the monolith. Efficient separation of phenols and nucleoside monophosphates on this dynamically coated monolithic column was obtained, compared with a dynamically coated fused-silica column and unmodified silica monolithic column. The retention behavior of uncharged phenols is mainly manipulated by hydrophobic interactions due to the presence of butyl groups, and that of nucleoside monophosphates is governed by the electrostatic attraction mechanism based on the interaction between positively charged [BMIM][BF4] moieties and negatively charged phosphate groups. In addition, silica matrix also contributes to the separation resolution.