Chiral separation of amino acids derivatized with fluoresceine-5isothiocyanate by capillary electrophoresis and laser-induced fluorescence detection using mixed selectors of beta-cyclodextrin and sodium taurocholate

Cyclodextrins and their applications in pharmaceutical and related fields

This chapter presents an overall account of cyclodextrins (CDs) with a brief description of the history, classification, and properties of these macromolecules. CDs act as complexing agents for drugs to form CD-drug inclusion complexes by various techniques. These complexes lead to the modification of the physicochemical properties of drugs to make them more soluble, chemically, and photochemically stable, and less toxic. It focuses in detail on various pharmaceutical uses of CDs and their derived forms in drug solubility, bioavailability, drug stability, drug delivery, and drug safety which have been specifically highlighted. The role of CDs and derivatives as excipients in the drug formulation of solid dosage forms, parenteral dosage forms, and anticancer drugs has been emphasized. Some other applications of CDs in cosmetics, environmental protection, food technology, and analytical methods have been described.

Conductometric Studies of Formation the Inclusion Complexes of Phenolic Acids with β-Cyclodextrin and 2-HP-β-Cyclodextrin in Aqueous Solutions

An attempt was made to evaluate the possibility of creating and assessing the stability of inclusion complexes of selected phenolic acids [trans-4-hydroxycinnamic acid (trans-p-coumaric acid), trans-3,4-dihydroxycinnamic acid (trans-caffeic acid), trans-4-hydroxy-3-methoxycinnamic acid, (trans-ferulic acid) and trans-3-phenylacrylic acid (trans-cinnamic acid)] with β-cyclodextrin and 2-HP-β-cyclodextrin in aqueous solutions in a wide temperature range 283.15 K-313.15 K. On the basis of the values of the limiting molar conductivity (Λ_CDNaDod), calculated from the experimental data, the values of the formation constants and the thermodynamic functions of formation (standard enthalpy, entropy, and Gibs standard enthalpy) of the studied complexes were determined. It has been found that the stability of the studied complexes increases with lowering of the molar mass of cyclodextrin and lowering of the temperature.

Capillary array electrophoresis imaging of biochemicals in tissue sections

It is of great significance to reveal the molecular distribution images in biological tissues, which has led to the bloom of mass spectrometry imaging. Unfortunately, its application is encountering the resistance of high technical barriers and equipment cost, as well as the inability to image substances that cannot be desorbed or ionized, or cannot be separated by their mass-to-charge ratios. Herein presented is a complementary and cost-effective method called capillary array electrophoresis (CAE) imaging. To have the information of molecules and their spatial location, a gridding cutter was fabricated to orderly dissect a tissue section into a leakproof array of micro wells enclosed by the grid-blade arrays. After in situ extraction and fluorophore-labeling of analytes, the samples in the wells were directly subjected to CAE-LIF (laser-induced fluorescence), and the molecular distribution images were depicted with the separated peaks. The practicability was demonstrated by CAE imaging of rat brain tissue sections with amino acid neurotransmitters (e.g., glutamine, 4-aminobutyric acid, alanine, glutamic acid and aspartic acid) as targets. The resultant images showed the global differences of molecular distributions, with a spatial resolution of 1000 μm that was presently determined by the well width but ultimately by the bore size of capillary (down to 10-50 μm). CAE imaging can hence be promising for its low cost, low technical barriers and abundant mechanisms to separate the charged and non-charged, chiral and non-chiral substances.

Bile Salts in Chiral Micellar Electrokinetic Chromatography: 2000-2020

Bile salts are naturally occurring chiral surfactants that are able to solubilize hydrophobic compounds. Because of this ability, bile salts were exploited as chiral selectors added to the background solution (BGS) in the chiral micellar electrokinetic chromatography (MEKC) of various small molecules. In this review, we aimed to examine the developments in research on chiral MEKC using bile salts as chiral selectors over the past 20 years. The review begins with a discussion of the aggregation of bile salts in chiral recognition and separation, followed by the use of single bile salts and bile salts with other chiral selectors (i.e., cyclodextrins, proteins and single-stranded DNA aptamers). Advanced techniques such as partial-filling MEKC, stacking and single-drop microextraction were considered. Potential applications to real samples, including enantiomeric impurity analysis, were also discussed.

Theoretical electronic circular dichroism investigations of chiral amino acids and development of separation and identification methods independent of standards

Through an appropriate computational protocol and environmental simulation, a satisfactory fit was observed for the theoretical electronic circular dichroism (ECD) spectra of 19 chiral amino acids (AAs), which correspondeds to the forms of the AAs in aqueous solution. Methods for enantioseparation of these chiral AAs by capillary electrophoresis (CE) and high-performance liquid chromatography (HPLC) were developed. Combining ECD with chromatographic separation methods, enantiomers were identified and quantified independent of a single enantiomer standard.

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.

Chiral Micellar Electrokinetic Chromatography

The potential of Micellar Electrokinetic Chromatography to achieve enantiomeric separations is reviewed in this article. The separation principles and the most frequently employed separation strategies to achieve chiral separations by Micellar Electrokinetic Chromatography are described. The use of chiral micellar systems alone or combined with other micellar systems or chiral selectors, as well as of mixtures of achiral micellar systems with chiral selectors is discussed together with the effect of different additives present in the separation medium. Indirect methods based on the derivatization of analytes with chiral derivatizing reagents and the use of achiral micelles are also considered. Preconcentration techniques employed to improve sensitivity and the main approaches developed to facilitate the coupling with Mass Spectrometry are included. The most recent and relevant methodologies developed by chiral Micellar Electrokinetic Chromatography and their applications in different fields are presented.

Amino acid chiral ionic liquids combined with hydroxypropyl-β-cyclodextrin for drug enantioseparation by capillary electrophoresis

Four amino acid chiral ionic liquids were evaluated in dual systems with hydroxypropyl-β-cyclodextrin to investigate the enantioseparation by CE of a group of seven drugs as model compounds (duloxetine, verapamil, terbutaline, econazole, sulconazole, metoprolol, and nadolol). The use of two of these chiral ionic liquids (tetramethylammonium L-Lysine ([TMA][L-Lys]) and tetramethylammonium L-glutamic acid ([TMA][L-Glu])) as modifiers in CE is reported for the first time in this work whereas tetrabutylammonium L-lysine ([TBA][L-Lys]) and tetrabutylammonium L-glutamic acid ([TBA][L-Glu]) were employed previously in CE although very scarcely. The effect of the nature and the concentration of each ionic liquid added to the separation buffer containing the neutral cyclodextrin on the enantiomeric resolution and the migration time obtained for each drug, was investigated. A synergistic effect was observed when combining each chiral ionic liquid with hydroxypropyl-β-cyclodextrin in the case of the five compounds for which the cyclodextrin showed enantiomeric discrimination power when used as sole chiral selector (duloxetine, verapamil, terbutaline, econazole, sulconazole). Buffer concentration and pH, temperature and separation voltage were varied in order to optimize the enantiomeric separation of these five compounds using dual systems giving rise to resolutions ranging from 1.1 to 6.6.

A compact short-capillary based high-speed capillary electrophoresis bioanalyzer

Here, a compact high-speed CE bioanalyzer based on a short capillary has been developed. Multiple modules of picoliter scale sample injection, high-speed CE separation, sample changing, LIF detection, as well as a custom designed tablet computer for data processing, instrument controlling, and result displaying were integrated in the bioanalyzer with a total size of 23 × 17 × 19 cm (length × width × height). The high-speed CE bioanalyzer is capable of performing automated sample injection and separation for multiple samples and has been successfully applied in fast separations of amino acids, chiral amino acids, proteins and DNA fragments. For instance, baseline separation of six FITC-labeled amino acids and ultrahigh-speed separation of three amino acids could be achieved within 7 and 1 s, respectively. The separation speed and efficiency of the optimized high-speed CE system are comparable to or even better than those reported in microchip-based CE systems. We believe this bioanalyzer could provide an advanced platform for fundamental research in bioscience and clinical diagnosis, as well as in quality control for drugs, foods, and feeds.

Study on clarithromycin lactobionate based dual selector systems for the enantioseparation of basic drugs in capillary electrophoresis

In this paper, the use of clarithromycin lactobionate, a kind of antibiotic chiral selector, in combination with four neutral cyclodextrin derivatives (glucose-β-cyclodextrin, hydroxyethyl-β-cyclodextrin, methyl-β-cyclodextrin and hydroxypropyl-β-cyclodextrin) was reported for the first time. As a result, these dual systems gave much better resolution of nefopam (the Rs increased to 3.58, 2.72, 1.49 and 1.42, respectively) compared to the single systems. The effects of buffer pH and selector concentration on the separation of nefopam were also investigated. Additionally, some other basic drugs including metoprolol, atenolol, propranolol, bisoprolol, esmolol and ritodrine were tested for the investigation and evaluation of the enantiorecognition capability of the four dual systems. As expected, the synergistic effect was observed in four systems. Different results of these dual systems were also summarized.

Enantioseparation of dansyl amino acids and dipeptides by chiral ligand exchange capillary electrophoresis based on Zn(II)-L-hydroxyproline complexes coordinating with γ-cyclodextrins

A chiral ligand exchange capillary electrophoresis (CLE-CE) method using Zn(II) as the central ion and L-4-hydroxyproline as the chiral ligand coordinating with γ-cyclodextrin (γ-CD) was developed for the enantioseparation of amino acids (AAs) and dipeptides. The effects of various separation parameters, including the pH of the running buffer, the ratio of Zn(II) to L-4-hydroxyproline, the concentration of complexes and cyclodextrins (CDs) were systematically investigated. After optimization, it has been found that eight pairs of labeled AAs and six pairs of labeled dipeptides could be baseline-separated with a running electrolyte of 100.0mM boric acid, 5.0mM ammonium acetate, 3.0mM Zn(II), 6.0mM L-hydroxyproline and 4.0mM γ-CD at pH 8.2. The quantitation of AAs and dipeptides was conducted and good linearity (r(2)≥0.997) and favorable repeatability (RSD≤3.6%) were obtained. Furthermore, the proposed method was applied in determining the enantiomeric purity of AAs and dipeptides. Meanwhile, the possible enantiorecognition mechanism based on the synergistic effect of chiral metal complexes and γ-CD was explored and discussed briefly.

One-step packing of anti-voltage photonic crystals into microfluidic channels for ultra-fast separation of amino acids and peptides

Packing of stable and crack-free photonic crystals (PCs) into micro channels is a prerequisite for ideal separation, but often takes several days and many steps, including assembly and immobilization. This work was dedicated to finding a fast, one-step solution. Simply by heating and blowing away the vapor, the packing of silica PCs into micro channels by classic evaporation-induced assembly was greatly accelerated and could unite the immobilization into one step. An apt method was thus established, which was able to pack 2 cm PCs into microfluidic channels in 15 min, saving a lot of time. The packed PCs showed no evident cracks along the borders of their continuous domain, therefore they are capable of withstanding an anti-electrical field at 2000 V cm(-1) for 5 h and storage in water for 2 months. This enables ultra-fast separation of amino acids along a 2.5 mm PC in 4 s, and peptides along a 10 mm PC in 12 s. The separation was highly efficient and reproducible, with a 300 nm plate height and 0.24%-0.35% relative standard deviation of migration time. This one-step approach is extendable to other gelling particles, and the resulted stable, crack-free PCs would have large potential in ultra-fast separation of other analytes.

Separation and determination of chiral composition in penicillamine tablets by capillary electrophoresis in a broad pH range

A chiral capillary electrophoretic method with nearly full pH window was explored for the separation and determination of dl-penicillamine. A facile one-pot labeling technique was coupled in the method for introduction of chromophore and charge groups onto the analytes to facilitate the electromigration and sensitive detection. By using simply a cost-effective neutral β-cyclodextrin as chiral selector, baseline separation of the dl-penicillamine was achieved from pH 2.0 to over pH 10. Quantification of standard d- and l-penicillamines was demonstrated by taking pH 4.5, 7.4, and 9.7 as the representatives of acidic, neutral, and basic conditions. The working curves were constructed between peak area and concentration, having linear ranges of 8.56-8.56 × 10(2) μg/mL for pH 4.5 and 8.56-1.71 × 10(3) μg/mL for pH 7.4 and 9.7, with correlation coefficients all better than 0.999. The limit of detection (S/N = 3) was 2.58 μg/mL in acidic and neutral conditions or 1.41 μg/mL in basic condition. The method was further validated by assaying the commercial penicillamine tablets, applicable to quantification of the effective enantiomer and the trace impurity of l-penicillamine at a content of down to 0.2, 0.6, and 2.0% for pH 9.7, 4.5, and 7.4, respectively. The recovery determined by spiking technique was in a range from 93.1 to 105 %. The method is easily extendable to the analysis of other chiral amines or amino acids.

Capillary electrophoresis-laser-induced fluorescence (CE-LIF) assay for measurement of intracellular D-serine and serine racemase activity

An enantioselective capillary electrophoresis-laser-induced fluorescence (CE-LIF) method for the analysis of D-serine (D-Ser) in cellular matrices has been developed. The assay involves derivatization with FITC followed by CE-LIF using 0.5 mM hydroxyl propyl-β-cyclodextrin in borate buffer [80 mM, pH 9.3]. The method was able to resolve D-Ser and L-Ser with an enantioselectivity (α) of 1.03 and a resolution (R(s)) of 1.37. Linearity was established from 0.25 to 100.00 μM. The assay was also able to enantioselectively resolve 6 additional amino acid racemates. The method was applied to the determination of intracellular D-Ser concentrations in PC-12, C6, 1312N1, and HepG2 cell lines. This method was used to determine the concentration-dependent increases in D-Ser and associated EC₅₀ values produced by L-Ser and the concentration-dependent decreases in d-Ser and associated IC₅₀ values produced by glycine, a competitive inhibitor of serine racemase (SR). Western blot analysis determined that the PC-12 and C6 cell lines contained monomeric and dimeric forms of SR while the 1321N1 and HepG2 cells contained only the monomeric form. Although the SR dimer has been identified as the active form of the enzyme, all four of the tested cell lines expressed enzymatically active SR.

Multidimensional separation of chiral amino acid mixtures in a multilayered three-dimensional hybrid microfluidic/nanofluidic device

Microscale total analysis systems (microTAS) allow high-throughput analyses by integrating multiple processes, parallelization, and automation. Here we combine unit operations of microTAS to create a device that can perform multidimensional separations using a three-dimensional hybrid microfluidic/nanofluidic device composed of alternating layers of patterned poly(methyl methacrylate) and nanocapillary array membranes constructed from nuclear track-etched polycarbonate. Two consecutive electrophoretic separations are performed, the first being an achiral separation followed by a chiral separation of a selected analyte band. Separation conditions are optimized for a racemic mixture of fluorescein-isothiocyanate-labeled amino acids, serine and aspartic acid, chosen because there are endogenous D-forms of these amino acids in animals. The chiral separation is implemented using micellar electrokinetic chromatography using beta-cyclodextrin as the chiral selector and sodium taurocholate as the micelle-forming agent. Analyte separation is monitored by dual-beam laser-induced fluorescence detection. After separation in the first electrophoretic channel, the preselected analyte is sampled by the second-stage separation using an automated collection sequence with a zero-crossing algorithm. The controlled fluidic environment inherent to the three-dimensional architecture enables a series of separations in varying fluidic environments and allows sample stacking via different background electrolyte pH conditions. The ability to interface sequential separations, selected analyte capture, and other fluidic manipulations in the third dimension significantly improves the functionality of multilayer microfluidic devices.

Mucoadhesive microspheres for nasal administration of cyclodextrins

The aim of this study was to examine the in vitro capacity of cyclodextrins to interfere on the beta-amyloid fibril formation; then, mucoadhesive microspheres containing cyclodextrins were prepared and characterised as nasal delivery system for brain targeting. Eight batches of microspheres containing chitosan or alginate loaded with beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin in two different cyclodextrin to polymer ratios were produced by spray drying. The results show that none of the tested CDs has direct cellular toxicity and they protect the cell viability from beta-peptide. The microspheres prepared are characterised by small particle sizes, ability to absorb water and to delay the in vitro dissolution rate of the CDs; good ex vivo mucoadhesive properties of the formulations are assessed. The microsphere properties are influenced by the kind of polymer, of cyclodextrin and by cyclodextrin to polymer ratio used. In particular, the alginate formulation containing the higher cyclodextrin content shows the best performance.

Biomedical applications of amino acid detection by capillary electrophoresis

Capillary electrophoresis (CE) is an efficient tool for amino acid (AA) analysis. However, its role can be fully accounted for only by examining the applications on real matrices. Methods must be successfully transferred into working environments for use by non-CE experts before their power can be realized. This transfer of technology is rapidly increasing. In this chapter, some applications to real samples are presented with the precise intent to illustrate the great capabilities of CE to AA analysis in clinical applications.

On-column labeling technique and chiral CE of amino acids with mixed chiral selectors and UV detection

A novel method has been developed for the on-column labeling of amino acid enantiomers with 9-fluoroenylmethyl chloroformate (FMOC), followed by chiral CE with a binary chiral selector system and UV detection. Efficient labeling was achieved by sequential injection of amino acids, borate buffer, and FMOC labeling solution at 0.2 psi for 6 s. After injection, the sandwich sections were electrically mixed at 250 V/cm for 6 s and allowed to react (electric field-free) at room temperature for 2 min. With this procedure, successful online-labeling and chiral CE separation of 19 pairs of amino acids (AA) have been conducted, giving 17 pairs fully enantioresolved (R(s) = 1.73-5.79) and two pairs partially resolved (Ala, R(s) = 0.39 and Arg, R(s) = 1.15) using a running buffer of 150 mM borate containing 30 mM beta-CD, 30 mM sodium taurodeoxycholate (STDC), and 15% isopropanol (IPA) at pH 9.0. Chiral CE of some mixed pairs was also demonstrated, much the same as using precolumn labeling. Surprisingly, Met, Asp, Asn, Gln, and His gained even higher enantioresolution (up to 2.5%) compared with the case of precolumn labeling. As validated by both artificially prepared solutions and serum samples, the method was applicable to the quantitative determination of AA, with LODs down to 4.0 microM. The method allowed the determination of D-AA at the ratio of 1:100 (D:L).

Degree of supersaturation-regulated chiral symmetry breaking in one crystal

This paper aimed at studying chiral symmetry-breaking phenomena in one crystal. Preferential crystallization of racemic asparagines was carried out in nonseeded stagnant solutions through slow cooling. By varying the supersaturation, only one transparent crystal could be obtained at enough low supersaturation of dl-asparagine, and the crystal was not pure enantiomer with crystal enantiomeric excess increasing inversely with the degree of supersaturation. Crystal enantiomeric excess can amount up to 85% in one transparent crystal. Because no secondary nucleation occurred except for stochastic primary nucleation, we suggest that primary nucleation and competition between l- and d-nuclei were considered to be a mechanism for asymmetry amplification. High-performance capillary electrophoresis coupled with laser-induced fluorescence was used to separate and quantify l- and d-asparagine and the enantiomeric excess value can be calculated according to their concentration.

Enantioselective screen-printed amperometric biosensor for the determination of d-amino acids

D-amino acids are generally considered to be important markers of bacterial contamination of food products. A screen-printed amperometric biosensor for the detection of D-amino acids has been constructed by the immobilization of D-amino acid oxidase on a graphite working electrode of a screen-printed strip modified with Prussian Blue and Nafion layers. Enzyme immobilization was then carried out by cross-linking of a mixture of the enzyme and bovine serum albumin with glutaraldehyde. As a result of the mediator addition and because of the multi-layer construction of the biosensor, including a polymer layer to avoid the interferences, the limit of the detection of the developed biosensor was two orders of magnitude improved in comparison to other screen-printed biosensors, as far as the determination of amino acids is concerned. Additional modification of the graphite electrode with carbon nanotubes led to a significant enhancement of the signal magnitude. A fast linear response of the developed biosensor was subsequently observed in static measurements for D-alanine in the concentration range from 5 to 200 microM. Excellent enantioselectivity towards D-amino acids was discovered. During the experiment, D-amino acids were detected in fruit juices and some milk samples. The complex matrix of natural milk samples had no influence on the response of the biosensor. The results were in good agreement with those obtained by capillary electrophoresis measurements.

Development of micellar electro kinetic chromatography for the separation and quantitation of L-valine, L-leucine, L-isoleucin and L-phenylalanine in human plasma and comparison with HPLC

Phenylketonuria (PKU) and Maple Syrup Urine Disease (MSUD) are two inborn metabolic diseases which are carried by autosomal recessive genes in man. These genetic errors result in accumulation of phenylalanine (in PKU) or valine, leucine and isoluecin (in MSUD). At high concentrations, amongst other problems, these amino acids cause mental retardation. However if detected early after birth, using special diets and other forms of therapy, mental abnormalities can be prevented. As a result in many countries screening of infants for MSUD and PKU, by measuring plasma amino acids has become a routine neonatal test. Capillary Electrophoresis (CE) assays have a number of advantages over the traditional chromatography techniques (such as GC or HPLC). These include low cost, high speed of analysis and high resolution. These characteristics, make CE an ideal method for the screening of inborn errors of metabolism. We developed a CE assay based on pre-column derivatisation of amino acids with phenylisothiocyanate. This conjugate has strong absorbance at 254 nm. CE was carried out using a Spectraphoresis 1000 instrument, fitted with 40 cm of a 25 microm capillary, at 17 degrees C. A running voltage of 18KV was used to separate the amino acid mixture in an electrophoretic buffer containing 45 mM imidazole, 6 mM borate and 208 mM SDS, fixed at pH 9 with 2-N-morpholino ethane sulfonic acid. The assay was calibrated using various concentrations of amino acid standards. LOD, LOQ, recovery, inter-day and intra-day variations of the assay were determined. Also, levels of the 4 amino acids in normal and abnormal plasma were determined and compared with HPLC.

Enantiomeric separation using an l-RNA aptamer as chiral additive in partial-filling capillary electrophoresis

In this paper, we report the chiral resolution of arginine using an anti-arginine l-RNA aptamer chiral selector in partial-filling CE. The effects of the capillary temperature, sample load, and aptamer plug length on the enantiomeric separation were assessed. Very high chiral resolving capability was observed at low or moderate capillary temperatures (the target peak being not detected in the separation window), whereas the practical chiral resolution was achieved only at high enough temperatures (50-60 degrees C). Over this high-temperature range, the electrophoretic behavior of the target enantiomer appeared to result from a combination of binding site heterogeneity, slow desorption kinetics, and concentration overload of aptamer binding sites. From additional thermal UV melting experiments, three RNA conformations were identified for the 50-60 degrees C temperatures. It was suggested that the presence of these different RNA conformations was a plausible source of the binding site heterogeneity.

CE coupling with end-column electrochemiluminescence detection for chiral separation of disopyramide

CE with electrochemiluminescence (ECL) detection technique was successfully applied for the chiral separation of a kind of class IA antiarrhythmic racemic drug. To the best of our knowledge, this is the first report of ECL detection used in chiral CE. To get better detection sensitivity and good enantioresolution at the same time, the conditions of capillary inlet and outlet buffer were systematically optimized. Unlike the traditional chiral separation method, the buffers we used in the capillary inlet and outlet differed from each other in terms of buffer pH, ionic strength, type of BGE as well as buffer composition. Under the optimum conditions, baseline enantioseparation and highly sensitive detection of the enantiomers were achieved. Wide linear relationship of each enantiomer was achieved in the range of 5 x 10(-7) to 2 x 10(-5) mol/L with relative coefficients of 0.996 and 0.997, respectively. The detection limits were estimated to be 8 x 10(-8) and 1.0 x 10(-7) mol/L (S/N = 3) for the enantiomers, respectively. In addition, a successful application of this new method to the chiral separation of the racemic drug in spiked plasma samples confirmed the validity and applicability of the chiral CE-ECL method.

Chiral capillary electrophoretic separation of amino acids derivatized with 9-fluorenylmethylchloroformate using mixed chiral selectors of beta-cyclodextrin and sodium taurodeoxycholate

Chiral separation of 19 pairs of amino acid (AA) enantiomers derivatized with 9-fluorenylmethylchloroformate (FMOC) was successfully conducted by capillary electrophoresis using the mixture of beta-CD and sodium taurodeoxycholate (STDC) as selectors. Resolution was considerably superior to that obtained by using either beta-CD or STDC alone. After a systematic inspection, a buffer composed of 150 mM borate and 18% v/v isopropanol at pH 8.0, dissolved with 30 mM beta-CD and 30 mM STDC, was adopted and able to generate baseline resolution (>1.50) for 17 pairs of FMOC-AA enantiomers and somewhat lower resolution for arginine (1.36) and alanine (1.18), respectively. Experimental data revealed that the addition of the second selector did not increase the mobility difference between a pair of enantiomers (Delta mu = mu(D) - mu(L) and the number of theoretical plates (N), but decreased the summed apparent mobility of a pair of enantiomers (Sigma mu = mu(D) - mu(L)), which was mainly due to the decrease of the electroosmotic flow. The variation of Sigma mu was thus the major reason responsible for the improvement of chiral resolution in this study. The result demonstrated that not only the intrinsic selectivity of the selectors was the basis of the chiral separation, but also the non-chiral effect of the selectors, the change of the electroosmotic flow, was an important factor in enhancing the enantioseparation resolution. This study could probably help to explain the reasons for resolution improvement in some dual selectors systems, which are not very clear at present.

Capillary electrophoresis method for determination ofD-serine and its application for monitoring of serine racemase activity

Serine racemase (SR) is an enzyme responsible for the biosynthesis of D-serine, the coagonist of the N-methyl-D-aspartate receptor, in the brain. Therefore, it has been suggested as a possible therapeutic target for the treatment of various neurodegenerative diseases. To develop a potent inhibitor of SR, a simple, sensitive, fast, and robust assay is needed. In this paper, a new CE method for the determination of D-serine is described. Serine enantiomers are resolved in the form of o-phthaldialdehyde (OPA)/2-mercaptoethanol (2-ME) derivatives in an alkaline BGE composed of 50 mM sodium tetraborate, pH 9.7, and containing 40 mM 2-hydroxypropyl-gamma-CD as a chiral selector. The problem of time-limited stability of OPA/2-ME derivatives has been overcome by employing in-capillary derivatization of the sample, i.e., the derivatization reaction was carried out directly in the separation capillary in the first phase of the CE run. UV-absorption detection at 230 nm allowed concentration detection limit of 3 microM. Baseline resolution of D- and L-serine derivatives was achieved in less than 10 min. This fact, together with the simple sample pretreatment, allowed application of the method to medium-throughput monitoring of SR activity, such as the screening of potential SR inhibitors. A good agreement was achieved between the developed CE method and the previously established HPLC method for determination of the inhibition constant, K(i), of a new SR inhibitor, L-erythro-3-hydroxyaspartate.