Enantiomeric Separation of 13 New Amphetamine‐Like Designer Drugs by Capillary Electrophoresis,Using Modified‐‐Cyclodextrins

An easy‐to‐prepare chiral CE method for the enantiomeric separation of 13 new amphetamine‐like designer drugs, using CDs as chiral selectors, was developed. Sulfated‐β‐CD was found to be the best chiral selector among the three used (sulfated‐β‐CD, caroboxymethyl‐β‐CD, dimethyl‐β‐CD). The separation of the analytes was achieved in a fused‐silica gel capillary at 20 °C using an applied voltage of +25 kV. The optimized background electrolyte consisted of 63.5 mM H₃PO₄ and 46.9 mM NaOH in water. Several electrophoretic parameters such as CD type, CD concentration (1 ? 40 mg/mL), buffer pH (2.6, 3.6, 5.0, 6.0), length of the capillary (70 ? 40 cm total length), amount of the organic solvent (methanol and acetonitrile) were investigated and optimized. Chirality 25:617–621, 2013. © 2013 Wiley Periodicals, Inc.     

Nonaqueous Capillary Electrophoretic Enantioseparation of Water Insoluble Tröger's Base Derivatives Using β‐Cyclodextrin as Chiral Selector

Racemic mixtures of six Tröger's base derivatives were separated by chiral nonaqueous capillary electrophoresis. The separation protocol was optimized first for suitable solvents. Then the applicability of various salts dissolved in organic solvents and their mixtures was evaluated. As chiral selectors β‐cyclodextrin and heptakis(2,6‐di‐O‐methyl)‐β‐cyclodextrin at various concentrations were used. The best enantioselectivity for the studied analytes was obtained utilizing formamide as organic nonaqueous solvent containing a mixture of sodium citrate and tris(hydroxymethyl)aminomethane acetate as electrolytes, and β‐cyclodextrin as chiral additive. The experimental results demonstrated the feasibility of nonaqueous capillary electrophoresis for enantioseparation of Tröger's base derivatives. This technique represents a suitable alternative to more commonly used capillary electrophoresis in aqueous environment. Chirality 25:810–813, 2013. © 2013 Wiley Periodicals, Inc.     

Enantiomeric separation of N‐protected amino acids by non‐aqueous capillary electrophoresis using quinine or Tert‐butyl carbamoylated quinine as chiral additive

A capillary electrophoretic (CE) method for the enantioseparation of N‐protected chiral amino acids was developed using quinine and tert‐butyl carbamoylated quinine as chiral selectors added to nonaqueous electrolyte solutions (NACE). A series of various N‐derivatized amino acids were tested as chiral selectands, and in order to optimize the CE enantioseparation of these compounds, different parameters were investigated: the nature of the organic solvent, the combination of different solvents, the nature and the concentration of the background electrolyte, the selector concentration, the capillary temperature, and the applied voltage. The influence of these factors on the separation of the analyte enantiomers and the electroosmotic flow was studied. Generally, with tert‐butyl carbamoylated quinine as chiral selector, better enantioseparations were achieved than with unmodified quinine. Optimum experimental conditions were found with a buffer made of 12.5 mM ammonia, 100 mM octanoic acid, and 10 mM tert‐butyl carbamoylated quinine in an ethanol–methanol mixture (60:40 v/v). Under these conditions, DNB‐Leu enantiomers could be separated with a selectivity factor (α) of 1.572 and a resolution (Rs) of 64.3; a plate number (N) of 127,000 and an asymmetry factor (As) of 0.93 were obtained for the first migrating enantiomer. Chirality 11:622–630, 1999. © 1999 Wiley‐Liss, Inc.     

Enantiomeric separation of dansyl amino acids using macrocyclic antibiotics as chiral mobile phase additives by narrow-bore high-performance liquid chromatography

Seven macrocyclic antibiotics were evaluated as chiral selectors for the enantiomeric separation of 11 dansyl amino acids using narrow-bore high-performance liquid chromatography (HPLC). The macrocyclic antibiotics were incorporated as mobile phase additives to determine the enantioselective effects on the chiral analytes. The resolution and capacity factor (k') of each analyte were assessed while varying the structure of macrocyclic antibiotic and the mobile phase buffer pH. The selectivity of the chiral selectors was measured as a function of changes in these parameters. All 11 dansyl amino acids were separated by at least one of the chiral selectors. Three-dimensional computer modeling of the more effective chiral selectors illustrated the importance of macrocyclic antibiotic structure concerning stereospecific analyte interaction.     

Chiral Separation of the Clinically Important Compounds Fucose and Pipecolic Acid Using CE: Determination of the Most Effective Chiral Selector

In this study, simple electrophoretic methods were developed for the chiral separation of the clinically important compounds fucose and pipecolic acid. In recent years, these analytes, and particularly their individual enantiomers, have attracted considerable attention due to their role in biological functions and disorders. The detectability and sensitivity of pipecolic acid and fucose were improved by reacting them with fluorenylmethyloxycarbonyl chloride (FMOC‐Cl) and 5‐amino‐2‐naphthalene‐sulfonic acid (ANSA), respectively. The enantioseparation conditions were optimized by initially investigating the type of the chiral selector. Different chiral selectors, such as polymeric surfactants and cyclodextrins, were used and the most effective ones were determined with regard to resolution and analysis time. A 10‐mM β‐cyclodextrin was able to separate the enantiomers of ANSA‐DL‐fucose and the polymeric surfactant poly(sodium N‐undecanoyl‐LL‐leucine‐valinate) was able to separate the enantiomers of FMOC‐DL‐pipecolic acid, with resolution values of 3.45 and 2.78, respectively. Additional parameters, such as the concentration and the pH of the background electrolyte (BGE), the concentration of the chiral selector, and the addition of modifiers were examined in order to optimize the separations. The addition of the chiral ionic liquid D‐alanine tert‐butyl ester lactate into the BGE was also investigated, for the first time, in order to improve resolution of the enantiomers. Chirality 25:556–560, 2013. © 2013 Wiley Periodicals, Inc.     

Enantioselectivity of potentiometric sensors with application of different mechanisms of chiral discrimination

In the recent years, numerous successful applications of various chiral selectors in high performance separation methods have generated an increasing interest in the application of some of these compounds as electroactive species in potentiometric sensors. The objective of this work was to examine the enantioselectivy of several different sensors employing substituted cyclodextrins, example antibiotic teicoplanin and electrodeposited conductive polymers for various chiral analytes. Varying degrees of enantioselectivity were found for the ion-selective electrodes examined, depending on the chiral selector used and the target analyte.     

Establishment and Evaluation of the Novel Tetramethylammonium‐L‐Hydroxyproline Chiral Ionic Liquid Synergistic System Based on Clindamycin Phosphate for Enantioseparation by Capillary Electrophoresis

Much attention has been paid to chiral ionic liquids (ILs) in analytical chemistry, especially its application in capillary electrophoresis (CE) enantioseparation. However, the investigation of chiral ionic liquids synergistic systems based on antibiotic chiral selectors has been reported in only one article. In this work, a novel chiral ionic liquid, tetramethylammonium‐L‐hydroxyproline (TMA‐L‐Hyp), was applied for the first time in CE chiral separation to evaluate its potential synergistic effect with clindamycin phosphate (CP) as the chiral selector. As observed, significantly improved separation was obtained in this TMA‐L‐Hyp/CP synergistic system compared to TMA‐L‐Hyp or a CP single system. Several primary factors that might influence the separation were investigated, including CP concentration, TMA‐L‐Hyp concentration, buffer pH, types and concentrations of organic modifier, applied voltage, and capillary temperature. The best results were obtained with a 40 mM borax buffer (pH 7.6) containing 30 mM TMA‐L‐Hyp, 80 mM CP, and 20% (v/v) methanol, while the applied voltage and temperature were set at 20 kV and 20°C, respectively. Chirality 27:598–604, 2015. © 2015 Wiley Periodicals, Inc.     

Chiral separation of 2,3-allenoic acid by capillary zone electrophoresis using cyclodextrin derivatives

In this paper, five of six samples of 2,3-allenoic acid enantiomers were separated by capillary zone electrophoresis (CZE) using hydroxypropyl-beta-cyclodextrin (HP-beta-CD) and hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD) as chiral selectors. Using HP-beta-CD for chiral separation, three of the six enantiomers were separated. Five experimental conditions including HP-beta-CD concentration, pH, buffer concentration, temperature, and running voltage were investigated for their influence on separation and migration using enantiomers of 2-methyl-4-phenyl-2,3-butadienoic acid (A) and 2-(n-propyl)-4-phenyl-2,3-butadienoic acid (B) as samples. Good separation results were observed when [HP-beta-CD] = 3-12 mmol/L and pH = 7-9 for samples A and B. The temperature range of 15-25 degrees C can be selected for convenience. According to the chiral separation results, HP-beta-CD and HP-gamma-CD should be valuable selectors to separate 2,3-allenoic acids and HP-gamma-CD was suggested to separate the 2,3-allenoic acid samples with a group at 4-position bulkier than phenyl.     

Determination of the chiral status of different novel psychoactive substance classes by capillary electrophoresis and β-cyclodextrin derivatives

Besides the abuse of well-known illicit drugs, consumers discovered new synthetic compounds with similar effects but minor alterations in their chemical structure. Originally, these so-called novel psychoactive substances (NPS) have been created to circumvent law of prosecution because of illicit drug abuse. During the past decade, such compounds came up in generations, the most popular compound was a synthetic cathinone derivative named mephedrone. Cathinones are structurally related to amphetamines; to date, more than 120 completely new derivatives have been synthesized and are traded via the Internet. Cathinones possess a chiral center; however, only little is known about the pharmacology of their enantiomers. However, NPS comprise further chiral compound classes such as amphetamine derivatives, ketamines, 2-(aminopropyl)benzofurans, and phenidines. In continuation of our project, a cheap and easy-to-perform chiral capillary zone electrophoresis method for enantioseparation of cathinones presented previously was extended to the aforementioned compound classes. Enantioresolution was achieved by simply adding native β-cyclodextrin, acetyl-β-cyclodextrin, 2-hydroxypropyl-β-cyclodextrin, or carboxymethyl-β-cyclodextrin as chiral selector additives to the background electrolyte. Fifty-one chiral NPS served as analytes mainly purchased from online vendors via the Internet. Using 10 mM of the aforementioned β-cyclodextrins in a 10 mM sodium phosphate buffer (pH 2.5), overall, 50 of 51 NPS were resolved. However, chiral separation ability of the selectors differed depending on the analyte. Additionally, simultaneous enantioseparations, the determination of enantiomeric migration orders of selected analytes, and a repeatability study were performed successfully. It was proven that all separated NPS were traded as racemic mixtures.     

Separation of Selected Imidazole Enantiomers Using Dual Cyclodextrin System in Micellar Electrokinetic Chromatography

Cyclodextrin‐modified micellar electrokinetic chromatography (CD‐MEKC) method was developed for simultaneous enantioseparation of three imidazole drugs namely tioconazole, isoconazole and fenticonazole. Three easily available and inexpensive cyclodextrins namely 2‐hydroxypropyl‐β‐cyclodextrin (HP‐β‐CD), 2‐hydroxypropyl‐γ‐cyclodextrin (HP‐γ‐CD) and heptakis(2,6‐di‐O‐methyl)‐β‐cyclodextrin (DM‐β‐CD) were evaluated to discriminate the six stereoisomers of the drugs. However, none of the three CDs gave a complete enantioseparation of the drugs. Effective enantioseparation of tioconazole, isoconazole and fenticonazole was achieved using a combination of 35 mM HP‐γ‐CD and 10 mM DM‐β‐CD as chiral selectors. The best separation using both HP‐γ‐CD and DM‐β‐CD (35 mM:10 mM) as chiral selectors were accomplished in background electrolyte (BGE) containing 35 mM phosphate buffer (pH 7.0), 50 mM sodium dodecyl sulfate (SDS) and 15% (v/v) acetonitrile at 27 kV and 30 °C with all peaks resolved in less than 15 min with resolutions, R_s 1.90‐27.22 and peak efficiencies, N > 180 000. The developed method was linear over the concentration range of 25–200 mg l^‐1 (r² > 0.998) and the detection limits (S/N = 3) of the three imidazole drugs were found to be 2.7‐7.7 mg l^‐1. The CD‐MEKC method was successfully applied to the determination of the three imidazole drugs in spiked human urine sample and commercial cream formulation of tioconazole and isoconazole with good recovery (93.6‐106.2%) and good RSDs ranging from 2.30‐6.8%. Chirality 25:328–335, 2013. © 2013 Wiley Periodicals, Inc.     

Combined use of [TBA][L-ASP] and hydroxypropyl-β-cyclodextrin as selectors for separation of Cinchona alkaloids by capillary electrophoresis

In this paper, a new capillary electrophoresis (CE) separation and detection method was developed for the chiral separation of the four major Cinchona alkaloids (quinine/quinidine and cinchonine/cinchonidine) using hydroxypropyl-β-cyclodextrin (HP-β-CD) and chiral ionic liquid ([TBA][L-ASP]) as selectors. Separation parameters such as buffer concentrations, pH, HP-β-CD and chiral ionic liquid concentrations, capillary temperature, and separation voltage were investigated. After optimization of separation conditions, baseline separation of the three analytes (cinchonidine, quinine, cinchonine) was achieved in fewer than 7 min in ammonium acetate background electrolyte (pH 5.0) with the addition of HP-β-CD in a concentration of 40 mM and [TBA][L-ASP] of 14 mM, while the baseline separation of cinchonine and quinidine was not obtained. Therefore, the first-order derivative electropherogram was applied for resolving overlapping peaks. Regression equations revealed a good linear relationship between peak areas in first-order derivative electropherograms and concentrations of the two diastereomer pairs. The results not only indicated that the first-order derivative electropherogram was effective in determination of a low content component and of those not fully separated from adjacent ones, but also showed that the ionic liquid appeared to be a very promising chiral selector in CE.     

Cellulose tris(3,5-dimethylphenylcarbamate)-based chiral stationary phases as effective tools for enantioselective HPLC separation of structurally different disubstituted binaphthyls

Cellulose tris(3,5-dimethylphenylcarbamate)-based chiral stationary phases (CSPs) were used for a study of the HPLC retention and enantioseparation behavior of 2,2'-disubstituted or 3,2,2'-trisubstituted 1,1'-binaphthyls and 8,3'-disubstituted 1,2'-binaphthyls. The effects of the mobile phase composition in normal- (NP) and reversed-phase (RP) separation modes were investigated. The NP mobile phases contained n-hexane and propane-2-ol at various volume ratios, the RP ones were obtained by mixing acetonitrile with water or a 20 mM phosphate buffer of pH 6.0 or 3.0. The RP separation mode has been found more suitable for enantioresolution of most of the analytes. The best enantioseparation of 2,2'-diacetyl-1,1'-binaphthyl, 2-hydroxy-2'-(phenylamino)-1,1'-binaphthyl-3-carboxylic acid and 2-amino-2'-hydroxy-1,1'-binaphthyl-3-carboxylic acid was obtained in the mobile phase of ACN/20 mM phosphate buffer, pH 3.0, 40/60 (v/v), whereas N-(2'-hydroxy-1,1'-binaphthyl-2-yl)acetamide, N-(3'-methoxy-1,2'-binaphthyl-8-yl)acetamide, and N-(3'-hydroxy-1,2'-binaphthyl-8-yl)acetamide yielded better results in ACN/water at the same v/v ratio. The analyte-CSP interaction mechanism was found to be temperature independent but the enantioresolution improved at an elevated temperature. The mechanism of the enantioselective discrimination is discussed on the basis of the thermodynamic parameters obtained. Semi-preparative separation conditions have been proposed for 2-amino-2'-hydroxy-1,1'-binaphthyl-3-carboxylic acid, N-(3'-methoxy-1,2'-binaphthyl-8-yl)acetamide, and N-(3'-hydroxy-1,2'-binaphthyl-8-yl)acetamide.     

Development of a validated capillary electrophoresis method for enantiomeric purity control and quality control of levocetirizine in a pharmaceutical formulation

A chiral capillary electrophoresis method has been developed for the quantification of 0.1% of the enantiomeric impurity (dextrocetirizine) in levocetirizine and determination of both in pharmaceuticals using sulfated-β-cyclodextrins (CDs) as chiral selector. Several parameters affecting the separation were studied such as the type and concentration of chiral selectors, buffer composition and pH, organic modifier, mixtures of two CDs in a dual system, voltage, and temperature. The optimal separation conditions were obtained using a 50 mM tetraborate buffer (pH 8.2) containing 1% (w/v) sulfated-β-CDs on a fused-silica capillary. Under these conditions, the resolution of two enantiomers was higher than 3. To validate the method, the stability of the solutions, robustness (two level half fraction factorial design for 5 factors using 19 experiments [2(n-1)+3]), precision, linearity (dextrocetirizine 0.25-2.5 μg/ml, R(2) = 0.9994, y = 0.0375x + 0.0008; levocetirizine 15-100 μg/ml, R(2) = 0.9996, y = 0.0213x + 0.0339), limit of detection (0.075 μg/ml, 0.03% m/m), limit of quantification (0.25 μg/ml, 0.1% m/m), accuracy (dextrocetirizine 84-109%, levocetirizine 97.3-103.1%), filter effect, and different CD batches were examined. The validated method was further applied to bulk drug and tablets of levocetirizine.     

Optimization of the separation lactic acid enantiomers in body fluids by capillary electrophoresis

The optimization of the separation conditions of the two optical isomers of lactic acid by a factorial design is reported. Initially, different chiral selectors were systematically investigated and then a experimental design with three quantitative factors (cyclodextrin concentration and background buffer pH and concentration) were evaluated. Optimal conditions for obtaining a resolution higher than 1.5 were: phosphate buffer 200 mM at pH=6.0 with 413 mM 2-hydroxypropyl-beta-cyclodextrin added (HP-beta-CD), 20 degrees C, -20 kV of applied potential and polyacrylamide-coated capillary. The method was validated for the measurement in plasma and it was applied to the identification of both isomers in body fluids such as urine, amniotic fluid and cerebrospinal fluid. Samples were centrifuged and diluted (1:4) prior to the analysis.     

Liquid chromatographic direct resolution of flecainide and its analogs on a chiral stationary phase based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid

Flecainide, an antiarrythmic agent, and its analogs were resolved on a high performance liquid chromatographic chiral stationary phase (CSP) based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid with the use of a mobile phase consisting of methanol‐acetonitrile‐trifluoroacetic acid‐triethylamine (80/20/0.1/0.3, v/v/v/v). The chiral resolution was quite successful, the separation factors (α) and the resolutions (R_S) for 20 analytes including flecainide being in the range of 1.19–1.82 and 1.73–6.80, respectively. The ortho‐substituent of the benzoyl group of analytes was found to cause decrease in the retention times of analytes probably because of the conformational deformation of analytes originated from the steric hindrance exerted by the ortho‐substituent. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Quinine‐Based Zwitterionic Chiral Stationary Phase as a Complementary Tool for Peptide Analysis: Mobile Phase Effects on Enantio‐ and Stereoselectivity of Underivatized Oligopeptides

Peptide stereoisomer analysis is of importance for quality control of therapeutic peptides, the analysis of stereochemical integrity of bioactive peptides in food, and the elucidation of the stereochemistry of peptides from a natural chiral pool which often contains one or more D‐amino acid residues. In this work, a series of model peptide stereoisomers (enantiomers and diastereomers) were analyzed on a zwitterionic ion‐exchanger chiral stationary phase (Chiralpak ZWIX(+) 5 µm), in order to investigate the retention and separation performance for such compounds on this chiral stationary phase and elucidate its utility for this purpose. The goal of the study focused on 1) investigations of the effects of the sample matrix used to dissolve the peptide samples; 2) optimization of the mobile phase (enabling deriving information on factors of relevance for retention and separation); and 3) derivation of structure–selectivity relationships. It turned out that small di‐ and tripeptides can be well resolved under optimized conditions, typically with resolutions larger than 1.5. The optimized mobile phase often consisted of methanol–tetrahydrofuran–water (49:49:2; v/v/v) with 25 mM formic acid and 12.5 mM diethylamine. This work proposes some guidance on which mobile phases can be most efficiently used for peptide stereoisomer separations on Chiralpak ZWIX. Chirality 28:5–16, 2016. © 2015 Wiley Periodicals, Inc.     

Liquid chromatographic enantiomer resolution of N-hydrazide derivatives of 2-aryloxypropionic acids on a crown ether derived chiral stationary phase

The liquid chromatographic separation of the enantiomers of several N-hydrazide derivatives of 2-aryloxypropionic acids was performed on a crown ether type chiral stationary phase derived from (18-crown-6)-2,3,11,12-tetracarboxylic acid. The behavior of chromatographic parameters by the change of mobile phases and additives for the resolution of these analytes was investigated. The enantiomers of all analytes were base-line resolved with a mobile phase of 100% methanol containing 20 mM H2SO4. These results are the first reported for enantiomer resolution of chiral acids of 2-aryloxypropionic acids as their N-hydrazide derivatives.     

Comparison of polysaccharide-based and protein-based chiral liquid chromatography columns for enantioseparation of drugs

Two different columns—Lux Cellulose-1 and Chiralpak CBH—were evaluated for their chiral recognition abilities for eight drugs comprising three β-blockers, one antacid, and four cathinones in polar-organic elution mode and reversed-phase elution mode, respectively. The factors that affected the enantioseparation were tested and optimized to develop a suitable chiral separation method whose LC conditions are compatible with MS detection. In polar-organic elution mode with the Lux Cellulose-1 column, methanol and acetonitrile were tested as the main components of the mobile phase. In addition, the effects of adding isopropanol as organic modifier, acidic additives (formic acid), and basic additives (diethylamine) were evaluated. In reversed-phase elution mode with the Chiralpak CBH column, the effect of type and concentration of organic modifier (isopropanol, acetonitrile, and methanol), the mobile phase pH (6.4 and 5.0), and buffer concentration (1mM-20mM ammonium acetate) were evaluated. The best enantioseparation was achieved with the Chiralpak CBH column with a mobile phase composed of 5mM ammonium acetate aqueous (pH = 6.4)/methanol (95/5, v/v) at a flow rate of 0.1 mL/min and a temperature of 30°C. Under these conditions, six of eight chiral drugs were baseline separated.     

Enantioseparation of Tröger's Base Derivatives by Capillary Electrophoresis Using Cyclodextrins as Chiral Selectors

The enantioseparation of seven Tröger's base derivatives (TBs) was carried out by capillary electrophoresis using α‐, β‐, and γ‐cyclodextrins as chiral selectors and phosphate at 20 mmol/l concentration, pH 2.5, as background electrolyte. The method was optimized with respect to the concentration of chosen chiral selectors (0–50 mmol/l) and the amount of organic solvent (acetonitrile, 0–25 % (v/v)) in the electrolyte. The results indicate that all the studied variables, i.e., type of chiral selector, its concentration, and the amount of the added organic solvent, have a significant impact on the enantioseparation of the studied TBs. The best results for the majority of the separated TBs were obtained utilizing β‐cyclodextrin at 5 mmol/l concentration and with various amounts of acetonitrile added ranging from 5 to 15% (v/v) in the background electrolyte. For the two smallest studied TBs, γ‐cyclodextrin with 10% (v/v) acetonitrile also provided good resolution. Chirality 25:379–383, 2013. © 2013 Wiley Periodicals, Inc.