Enantiomeric separation of Novel Psychoactive Substances by capillary electrophoresis using (+)-18-crown-6-tetracarboxylic acid as chiral selector

In the recent years, hundreds of Novel Psychoactive Substances (NPS) have entered both the European and the global drug market. These drugs, which are mainly used for recreational matters, have caused serious social problems. Every year, the spectrum of these misused drugs is enlarged by new derivatives, which are produced by modifications of basic structures of already well-known substances. Additionally, a lot of them possess a stereogenic center which leads to 2 enantiomeric forms. The fact that the pharmacological effects and potencies of the enantiomers of these chiral NPS may differ can be assumed from a broad spectrum of active pharmaceutical ingredients. For this reason, analytical method development regarding enantiomeric separation for these classes of substances is of great pharmaceutical and medical interest. The aim of this work was to create an easy-to-prepare chiral capillary electrophoresis method for the enantioseparation of NPS which contains a primary amino group by means of (+)-18-crown-6-tetracarboxylic acid as chiral selector. Novel Psychoactive Substances were purchased at various Internet stores or represent samples seized by Austrian police. The effects of selector concentration, the electrolyte composition, and the addition of organic modifiers to the background electrolyte on enantioseparation were investigated. Under optimized conditions, the use of 20-mM (+)-18-crown-6-tetracarboxylic acid, 10-mM Tris, and 30-mM citric acid buffer at pH 2.10 turned out to be effective. Fifteen of 24 tested NPS were resolved in their enantiomers within 15 minutes. It was found that all NPS were traded as racemic mixtures.

Chiral capillary electrophoresis with UV-excited fluorescence detection for the enantioselective analysis of 9-fluorenylmethoxycarbonyl-derivatized amino acids

The potential of capillary electrophoresis (CE) with ultraviolet (UV)-excited fluorescence detection for sensitive chiral analysis of amino acids (AAs) was investigated. DL-AAs were derivatized with 9-fluorenylmethoxycarbonyl chloride (FMOC)-Cl to allow their fluorescence detection and enhance enantioseparation. Fluorescence detection was achieved employing optical fibers, leading UV excitation light (< 300 nm) from a Xe-Hg lamp to the capillary window, and fluorescence emission to a spectrograph equipped with a charge-coupled device (CCD). Signal averaging over time and emission wavelength intervals was carried out to improve the signal-to-noise ratio of the FMOC-AAs. A background electrolyte (BGE) of 40 mM sodium tetraborate (pH 9.5), containing 15% isopropanol (v/v), 30 mM sodium dodecyl sulfate (SDS), and 30 mM β-cyclodextrin (β-CD), was found optimal for AA chemo- and enantioseparation. Enantioresolutions of 1.0 or higher were achieved for 16 proteinogenic DL-AAs. Limits of detection (LODs) were in the 10-100-nM range (injected concentration) for the D-AA enantiomers, except for FMOC-D-tryptophan (536 nM) which showed intramolecular fluorescence quenching. Linearity (R² > 0.997) and repeatability for peak height (relative standard deviations (RSDs) < 7.0%; n = 5) and electrophoretic mobility (RSDs < 0.6%; n = 5) of individual AA enantiomers were established for chiral analysis of DL-AA mixtures. The applicability of the method was investigated by the analysis of cerebrospinal fluid (CSF). Next to L-AAs, endogenous levels of D-glutamine and D-aspartic acid could be measured in CSF revealing enantiomeric ratios of 0.35 and 19.6%, respectively. This indicates the method's potential for the analysis of low concentrations of D-AAs in presence of abundant L-AAs.

Chiral selectivity of guanosine media in capillary electrophoresis

Gels formed by self-association of monomeric guanosine compounds join numerous other agents such as cyclodextrins, crown ethers, chiral surfactants, antibiotics, proteins, and polysaccharides for chiral separations. Guanosine gels (G-gels) are self-assembled networks of hydrogen-bonded tetrads formed by guanosine nucleotides and their derivatives. The tetrads stack upon themselves to form columnar, helical aggregates that are stabilized by π-π interactions and centrally located cations. Previous work showed the effectiveness of G-gels formed by guanosine-5'-monophophate for separation of the enantiomers of the cationic drug propranolol using capillary electrophoresis. Subsequently, it was found that not all chiral compounds could be resolved into their enantiomers, leading us to investigate in this work the structural features that appear to be correlated to enantiomerically selective interactions of chiral compounds with G-gels. For those compounds (anionic 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate and zwitterionic tryptophan) for which enantiomeric resolution was achieved, the effects of experimental conditions and G-gel composition were examined. For other compounds with no net charge (hydrobenzoin and zwitterionic amino acids and derivatives), the migration times were used as an indicator of the extent of interaction with the G-gel run buffer. It was found that the extent of interaction alone does not determine the chiral selectivity of the G-gel, indicating that the mechanism of chiral separation involves particular structural characteristics of the chiral compounds.

Computational modeling of capillary electrophoretic behavior of primary amines using dual system of 18-crown-6 and β-cyclodextrin

Using capillary electrophoresis (CE) three chiral primary amine compounds 1-aminoindan (AI), 1-(1-naphthyl)ethylamine (NEA) and 1,2,3,4-tetrahydro-1-naphthylamine (THAN), exhibited only partial or no separation when β-cyclodextrin (βCD) was used as chiral selector. The use of 18-crown-6 (18C6) as a second additive with βCD resulted in an enhanced separation. A molecular modeling study, using molecular mechanics and the semiempirical PM6 calculations, was used to help explaining the mechanism of the enantiodifferentiation and to predict the separation process. Optimization of the structures of the complexes by the PM6 method indicate that the poor separation obtained in the presence of the βCD chiral selector alone is due to the small binding energy differences (ΔΔE) of 4.7, 1.1 and 1.2 kcal mol(-1) for AI, NEA and THAN, respectively. In the presence of 18C6 it was suggested that a sandwich compound between 18C6, amine and βCD is formed. Theoretical calculations show that a significant increase in the binding energy is obtained for the sandwich compounds indicating strong hydrophobic and van der Waals interactions that show enhanced enantiodifferentiation.

Separation of enantiomers in capillary electrophoresis with contactless conductivity detection

Contactless conductivity detection is successfully demonstrated for the enantiomeric separation of basic drugs and amino acids in capillary electrophoresis (CE). Derivatization of the compounds or the addition of a visualization agent as for indirect optical detection schemes were not needed. Non-charged chiral selectors were employed, hydroxypropylated cyclodextrin (CD) for the more lipophilic basic drugs and 18-crown-6-tetracarboxylic acid (18C6H4) for the amino acids. Acidic buffer solutions based on lactic or citric acid were used. The detection limits were determined as 0.3 microM for pseudoephedrine as an example of a basic drug and were in the range from 2.5 to 20 microM for the amino acids.

On-line sample cleanup and chiral separation of gemifloxacin in a urinary solution using chiral crown ether as a chiral selector in microchip electrophoresis

In chiral capillary electrophoresis of primary amine enantiomers using (+)-18-crown-6-tetracarboxylic acid (18C6H4) as a chiral selector, the presence of alkaline metal ions in the sample solution as well as in the run buffer is undesirable due to their strong competitive binding with 18C6H4. A channel-coupled microchip electrophoresis device was designed to clean up alkaline metal ions from a sample matrix for the chiral analysis of amine. In the first channel, the metal ions in the sample were monitored by indirect detection using quinine as a chromophore and drained to the waste. In the second separation channel, gemifloxacin enantiomers, free of the alkaline metal ions, were successfully separated using only a small amount of the chiral selector (50 microM 18C6H4).

Chiral separation of gemifloxacin in sodium-containing media using chiral crown ether as a chiral selector by capillary and microchip electrophoresis

Chiral crown ether, (+)-(18-crown-6)-tetracarboxylic acid (18C6H(4)), is an effective chiral selector for resolving enantiomeric primary amines owing to the difference in affinities between 18C6H(4) and each of the amine enantiomers. In addition to the destacking effect of sodium ion in the sample solution, the strong affinity of sodium ion to the polyether ring of crown ether is unfavorable to chiral capillary electrophoresis using 18C6H(4) as a chiral selector. In this report, the chiral separation of gemifloxacin dissolved in a saline sample matrix using 18C6H(4) was investigated. Adding a chelating agent, ethylenediaminetetraacetic acid (EDTA), to the run buffer greatly improved the separation efficiencies and peak shapes. The successful chiral separation of gemifloxacin in a urinary solution was demonstrated for both capillary and microchip electrophoresis.

Comparative studies of various run buffers for chiral capillary electrophoresis using chiral crown ether as a chiral selector

In the capillary electrophoretic separation of primary amine enantiomers using (+)-(18-crown-6)-tetracarboxylic acid (18C6H4) as a chiral selector, the presence of run buffer constituents such as tris(hydroxymethyl)aminomethane (Tris) or Na+ competing with analytes for 18C6H4, diminishes the effectiveness of 18C6H4. In order to determine appropriate buffer systems for 18C6H4, various run buffer cationic components including Tris, 1,3-bis[tris(hydroxymethyl)methylamino]propane, bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane, triethanolamine, tetramethylammonium, and Na+ were compared. Quantitative studies of the effects of the competitive constituents were carried out by measuring the electrophoretic mobilities of histidine as a function of the 18C6H4 concentration. We also derived a simple equation to estimate the optimal chiral selector concentration for a maximum mobility difference in the presence of a competitive inhibitor.

Mechanistic study of enantiomeric recognition of primary amino compounds using an achiral crown ether with cyclodextrin by capillary electrophoresis and nuclear magnetic resonance

A model and theoretical equations are presented to investigate the enantiomeric recognition mechanism of primary amino compounds using an achiral crown ether with cyclodextrin by capillary electrophoresis (CE) and nuclear magnetic resonance (NMR). Association constants were calculated from CE and 1H NMR experiment results on the basis of the model and the equations. The key step of chiral recognition was identified from those values. Using CE analyses of three primary amino compounds [1-(1-naphthyl)ethylamine; 1-aminoindan; 1,2,3,4-tetrahydro-1-naphthylamine], the key step was identified with the equilibrium where the complex of a primary amino compound and 18-crown-6 becomes associated with 2,6-di-O-methyl-beta-cyclodextrin for all the three compounds. From the 1H NMR analyses of 1-(1-naphthyl)ethylamine, the key step was identified with the equilibrium where the complex of 1-(1-naphthyl)ethylamine and 18-crown-6 becomes associated with beta-cyclodextrin.

Interaction between 18-crown-6-tetracarboxylic acid and positional substituents of enantiomers and simultaneous separation of positional enantiomers of methyl-DL-tryptophans by capillary electrophoresis

18-Crown-6-tetracarboxylic acid (18C6H4) is a chiral selector with high selectivity for the enantioseparation of solutes bearing the primary amine function. This work presents the simultaneous separation of positional enantiomers of methyl-DL-tryptophans by using 18C6H4 as an additive to the background electrolyte. Separation conditions such as pH, the concentration of 18C6H4, and the applied voltages have critical inference on the simultaneous separation. The addition of cyclodextrins as anionic surfactants to the background electrolyte did not improve the separation. The selector-selectand interactions between 18C6H4 and the positional enantiomers have been investigated. It was observed that both the position and type of substituents contribute to the enantioselectivity. The migration order and resolution depended on the distance from the substituents to the asymmetric carbon of the enantiomers.

Study of a novel cationic calix[4]arene used a selectivity modifier in capillary electrophoresis with electrochemical detection

The use of a novel cationic calixarene, p-(quaternary ammonium) calix[4]arene, as selectivity modifier in capillary electrophoresis coupled with electrochemical detection was reported. The calixarene displayed good selectivity for the positional isomers of benzenediol and aminophenol and their successful separation was obtained under optimum conditions. The interaction mechanism between p-(quaternary ammonium) calix[4]arene and the solutes is discussed using the molecular modeling method. The detection limits by electrochemical detection for the most solutes studied here were below picogram level, which was approximately 2 orders of magnitude lower than those reported in the literature using UV detection. The results showed that electrochemical detection is especially suitable for an electrophoresis system where calixarenes are used as modifier.

Enantiomeric separations of primary amino compounds by capillary electrochromatography with monolithic chiral stationary phases of chiral crown ether-bonded negatively charged polyacrylamide gels

A novel enantiomeric separation method by capillary electrochromatography with chiral crown ether-bonded negatively charged polyacrylamide gels is presented. Two kinds of chiral crown ether derivatives, (+)-tetraallyl 18-crown-6 carboxylate and (+)-18-crown-6 tetracarboxylic acid 2-allyl ester were synthesized and allowed to covalently bind to a negatively charged polyacrylamide gel, a so-called monolithic stationary phase, respectively. The gel was placed in fused-silica tubing, the walls of which had been activated with a bifunctional reagent to make the resulting gel bind covalently to the inner surface. Enantiomeric separations of 12 primary amino compounds were achieved using these columns and mobile phases of 200 mM triethanolamine-300 mM boric acid buffers with high efficiencies of up to 135000 plates m(-1). Both the within- and between-run reproducibilities of retention time and separation factor were good. The reproducibilities of retention time and separation factor for three different columns prepared from a different batch of monomers were acceptable. The gel-filled capillaries were stable for at least 13 months with intermittent use for 3 months followed by storage at room temperature for 10 months. The result of the optical purity test of alanine-2-naphthylamide is also described.

Recent progress in chiral separation principles in capillary electrophoresis

This review summarizes recent developments in the field of chiral separations by electromigration techniques including capillary zone electrophoresis (CZE), capillary gel electrophoresis (CGE), isotachophoresis (ITP), electrokinetic chromatography (EKC), and capillary electrochromatography (CEC). This overview focuses on the development of new chiral selectors and the introduction of new techniques rather than applications of already established selectors and methods. The mechanisms of the different chiral separation principles are discussed.

Competitive binding of a Tris run buffer with chiral crown ether in chiral capillary electrophoresis

In capillary electrophoresis of primary amine racemates using (+)-(18-crown-6)-tetracarboxylic acid (18C6H4) as a chiral selector, chiral recognition emanates from the differences in the complex formation between 18C6H4 and the two protonated amine enantiomers. The presence of buffer constituents such as tris(hydroxymethyl)aminomethane (Tris) or Na+, capable of forming complexes with 18C6H4, is thus detrimental to the chiral separation of primary amines. Such a competitive binding of buffer constituents was studied by comparing the electrophoretic mobilities of racemic analytes obtained in Tris/citric acid and triethylamine/citric acid buffers. We developed a simple fitting method to determine the competitive binding constant and applied it to the Tris buffer system. The competitive binding constant of Tris with 18C6H4 obtained at pH 3.0 was 27 +/- 4.

Chiral separation of amino acids and peptides by capillary electrophoresis

Chiral separation of amino acids and peptides by capillary electrophoresis (CE) is reviewed regarding the separation principles of different approaches, advantages and limitations, chiral recognition mechanisms and applications. The direct approach details various chiral selectors with an emphasis on cyclodextrins and their derivatives, antibiotics and chiral surfactants as the chiral selectors. The indirect approach deals with various chiral reagents applied for diastereomer formation and types of separation media such as micelles and polymeric pseudo-stationary phases. Many derivatization reagents used for high sensitivity detection of amino acids and peptides are also discussed and their characteristics are summarized in tables. A large number of relevant examples is presented illustrating the current status of enantiomeric and diastereomeric separation of amino acids and peptides. Strategies to enhance the selectivity and optimize separation parameters by the application of experimental designs are described. The reversal of enantiomeric elution order and the effects of organic modifiers on the selectivity are illustrated in both direct and indirect methods. Some applications of chiral amino acid and peptide analysis, in particular, regarding the determination of trace enantiomeric impurities, are given. This review selects more than 200 articles published between 1988 and 1999.

Separation of enantiomers by capillary electrophoresis-mass spectrometry employing a partial filling technique with a chiral crown ether

Enantiomer separations were performed by capillary electrophoresis-mass spectrometry (CE-MS) with (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18C6H4) as a chiral selector. In order to prevent the introduction of the nonvolatile chiral, selector, 18C6H4, into the nozzle of the CE-MS interface and/or the orifice plate, a partial filling technique was employed in this study. By the partial filling technique, the contamination caused by the nonvolatile chiral selector was avoided not only during the analysis but also during the washing of capillary with the separation solution prior to the run. Several racemic compounds having a primary amino group were successfully separated. Racemic 3-aminopyrrolidine and racemic alpha-amino-epsilon-caprolactam have no strong UV absorption, but such compounds were detected with a high sensitivity by MS detection. In this paper, the effects of the length of separation zone and those of the 18C6H4 concentration were described. As the length of the separation zone was longer or as the concentration of 18C6H4 was higher, the enantiomer resolution was enhanced more and more. However, the optimization of 18C6H4 concentration was practically enough to obtain the baseline separation.

Recent advancements in amino acid analysis using capillary electrophoresis

Recent advances in the analysis of amino acids using capillary electrophoresis are addressed. This area of research continues to receive increased attention as is evident from the 62 references reviewed. This review discusses current detection strategies including UV absorbance, laser-induced fluorescence, electrochemical, and others. Separation methodologies for both derivatized and underivatized amino acids are reviewed. Both direct and indirect enantiomeric resolution of amino acids are addressed. Applications utilizing capillary electrophoresis for the analysis of amino acids are discussed. This review covers literature published in 1997 and 1998.

Overview of capillary electrophoresis and capillary electrochromatography

This paper provides an overview on the current status of capillary electrophoresis (CE) and capillary electrochromatography (CEC). The focus is largely on the current application areas of CE where routine methods are now in place. These application areas include the analysis of DNA, clinical and forensic samples, carbohydrates, inorganic anions and metal ions, pharmaceuticals, enantiomeric species and proteins and peptides. More specific areas such the determination of physical properties, microchip CE and instrumentation developments are also covered. The application, advantages and limitations of CEC are covered. Recent review articles and textbooks are frequently cited to provide readers with a source of information regarding pioneering work and theoretical treatments.

Enantiomeric separation by capillary electrophoresis using a crown ether as chiral selector

This paper reviews chiral separations of primary amines by capillary electrophoresis and crown ether as chiral selector. Two possible mechanisms of chiral recognition by host-guest complexation are discussed: (i) The substituents of the crown ether act as barriers for the guest compounds, and (ii) lateral electrostatic interactions between host and guest occur. Experimental conditions affecting the separation are discussed in detail. A literature overview of practical applications is presented as well. More than 80 different primary amines were analyzed, whereupon the majority could be resolved using a screening method. It is shown that a synergistic effect on the resolution of chiral amines is observed when the chiral crown ether and cyclodextrins are simultaneously used in the same buffer system. This approach opens interesting perspectives for further method optimization.

Comparison of highly sulfated alpha-, beta-, and gamma-cyclodextrins and 18-crown-6-tetracarboxylic acid for the enantiomeric separation of some amino acids and derivatives by capillary electrophoresis

18-Crown-6-tetracarboxylic acid (18C6H4) and highly sulfated cyclodextrins (HS-alpha-, beta-, gamma-CDs) are highly selective chiral selectors for the enantioseparation of solutes bearing the primary amino function. Excellent resolutions were obtained for all solutes on HS-gamma-CD and on 18C6H4. The former, however, is by far the best chiral selector for the solutes studied in this work because the highest resolution is obtained with the shortest migration times. The reversal of the D- and L-migration order on HS-CDs compared to 18C6H4 is an interesting feature for the determination of enantiomeric excess.