Azithromycin as a new chiral selector in capillary electrophoresis

In capillary electrophoresis (CE), separation of enantiomers of a chiral compound can be achieved through the chiral interactions and/or complex formation between the chiral selector and the enantiomeric analytes on leaving their diastereomeric forms with different stability constants and hence different mobilities. A great number of chiral selectors have been employed in CE and among them macrocyclic antibiotics exhibited excellent enantioselective properties towards a wide number of racemic compounds. The use of azithromycin (AZM) as a chiral selector has not been reported previously. This work reports the use of AZM as a chiral selector for the enantiomeric separations of five chiral drugs and one amino acid (tryptophan) in CE. The enantioseparation is carried out using polar organic mixtures of acetonitrile (ACN), methanol (MeOH), acetic acid and triethylamine as run buffer. The influences of the chiral selector concentration, ACN/MeOH ratio, applied voltage and capillary temperature on enantioseparation are investigated. The results show that AZM is a viable chiral selector in CE for the enantioseparation of the type of chiral drugs investigated.     

Enantiomeric separation of chiral peptide nucleic acid monomers by capillary electrophoresis with charged cyclodextrins

Direct chiral separation of chiral peptide nucleic acid (PNA) monomers has been achieved for the first time by capillary electrophoresis (CE) with charged cyclodextrins as chiral selectors added to the electrophoretic buffer. Selectively modified 6-deoxy-6-N-histamino-beta-cyclodextrin and sulfobutyl ether-beta-CD were successfully used as chiral selectors for the enantiomeric separation of chiral monomers based on different aminoethylamino acids bearing thymine or adenine as nucleobases. Chiral separations were obtained at low selector concentrations (1-3 mM) with good enantioselectivity and resolution factors. Separations were optimized as a function of pH in order to exploit the effect of the electrostatic interactions between the oppositely charged selector and selectand. The method has been applied to the analysis of the enantiomeric excess of chiral monomers used for the solid phase synthesis of chiral PNA oligomers. CE chiral analysis showed that a very high enantiomeric purity was generally achieved in the synthesis of all monomers, except for histidine and aspartic acid based monomers in which ca. 10% of the "wrong" enantiomer was always present.     

Enantiomeric separation of alcohols and amines on a proline chiral stationary phase by gas chromatography

A new chiral stationary phase for gas chromatography was prepared by covalently attaching a diproline chiral selector that has proven to be effective in liquid chromatography to a methylhydrosiloxane-dimethylsiloxane copolymer. With this new chiral stationary phase for GC, racemic aromatic alcohols could be resolved without derivatization. Racemic aromatic and aliphatic amines could also be resolved after derivatization of the amino groups with trifluoroacetic anhydride or isopropyl isocyanate. On this stationary phase, the isopropyl isocyanate derivatives of amines showed higher enantioselectivity than the trifluoroacetic anhydride derivatives. In both the enantiomeric separations of alcohols and derivatized amines, the aromatic racemic analytes showed higher enantioselectivities than their aliphatic analogs. Some of the alpha-amino and alpha-hydroxy aromatic acids could also be separated after derivatization to N-trifluoroacetyl methyl esters for amino acids or O-trifluoroacetyl methyl esters for hydroxyl acids.     

Evaluation of the enantioselectivity of glycogen-based dual chiral selector systems towards basic drugs in capillary electrophoresis

Several chiral reagents including cyclodextrins (CDs) and derivatives, crown ethers, proteins, chiral surfactants and polymers have been involved in dual selector systems for enantioseparation of a series of chiral compounds by capillary electrophoresis (CE). In comparison to the chiral reagents above-mentioned, there is no report concerning the use of polysaccharides in dual chiral CE system. In this paper we first investigate the enantioselectivity of polysaccharide-based dual selector systems towards some chiral drugs. During our recent work, glycogen belonging to the class of branched polysaccharides has been used as a novel chiral selector in CE. In this study, three glycogen-based dual chiral CE systems have been established for enantiomeric separations of several racemic basic drugs consisting of duloxetine, cetirizine, citalopram, sulconazole, laudanosine, amlodipine, propranolol, atenolol and nefopam. These three dual systems combined glycogen (neutral polysaccharide) with chondroitin sulfate A (CSA, ionic polysaccharide), β-CD and HP-β-CD, respectively. It was found that the dual system of glycogen/CSA exhibited good enantioselective properties toward the tested drugs. More importantly, compared to the single selector systems, synergistic effect was observed when glycogen was used with CSA for most of the analytes. This indicated the enhancement of enantioseparation observed for these analytes in glycogen/CSA system might be due to some favorable interaction effects between glycogen and CSA. Moreover, in order to evaluate the stereoselectivity of glycogen/CSA, the influences of buffer pH and selector concentration on enantioseparation of the studied drugs were also investigated.     

Enantiomeric separations of dansyl amino acids using the macrocyclic antibiotic A35512B as a chiral selector in capillary electrophoresis.

The macrocyclic antibiotic A35512B was examined as a chiral selector for capillary electrophoresis (CE) using thirteen racemic dansyl amino acids as test analytes. The chiral selectivity of A35512B was evaluated as a function of the run buffer pH, antibiotic concentration, and organic modifier composition. After optimizing these parameters, the macrocylic antibiotic A35512B provided high resolutions of all the enantiomers for the thirteen dansyl amino acids tested in this study.     

Application of capillary electrophoresis technique for the enantioseparation of bioactive ferrocene-based compounds versus DFT calculated data

Herein, a series of bioactive ferrocene-modified N-heterocycles with alkyl linkers was prepared in good to quantitative yields starting from easy accessible ferrocene alcohols and heterocycles under acidic or neutral (for imidazole) conditions in racemic forms. The analytical resolution of a number of bioactive racemic ferrocene azoles 1 – 6 (where azole = imidazole, pyrazole, and benzotriazole derivatives) into enantiomers was first carried out by CE using sulfobuthylether-β-CD (captisol) as a chiral selector. The analytical approaches to highly enantiomeric-enriched ferrocene derivatives are based on the formation of their inclusion complexes. The best chiral separation was achieved using zone CE in a quartz capillary. The ACE was used to evaluate the stability constants of captisol complexes with enantiomeric forms of two ferrocene derivatives 1 , FcCHMe-imidazole, and 6 , FcCHMe-benzotriazole. The optimal conditions for the resolution of the studied (R, S)-ferrocene compounds 1 , 2 , and 6 were predicted on the basis of the performed quantum chemical calculations and then implemented by the electrophoretic method. A high correlation between density functional theory calculation results and experimental electrophoresis data were obtained. Successful enantioseparation of racemic mixtures is of great importance for the characterization and further applications of drug candidates in enantiopure forms and in the development of clinical treatment. The advantages of the CE procedure make it possible to have important practical value and significance for determining the purity and enantiomeric excess of other ferrocene-containing compounds.     

Chiral separation of amino acids by capillary electrophoresis with 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate as chiral selector

Summary 3-[(3-cholamidopropyl)-dimethylammoniol-1-propane sulfonate (CHAPS) can be used as an effective chiral selector for the separation of dansyl-amino acids by capillary electrophoresis (CE). While CHAPS can serve as an chiral selector, better enantiomeric separation can be performed by using CHAPS not as the sole chiral selector but as one of a [CHAPS-SDS-cyclodextrin] three-component system. In this CHAPS-SDS-CD system, enantiomeric separations of the amino acids can be readily accomplished by judiciously adjusting the pH of the solution, concentrations of CHAPS and SDS, and the concentration and type of CD. All amino acids can be baseline resolved in less than 15 minutes with resolution as high as 2.01 at pH 6.5 with 50 mM of CHAPS and 75 mM of SDS. The resolution is also dependent on the size of the CD. Substantial increase in the resolution can be readily achieved by replacing β-CD with γ-CD. For example, theR _s for Leu was increased by four-folds (from 1.65 to 6.29) while the elution time still remains as short as 20 min when β-CD was replaced by γ-CD.     

Enantiomeric separation of hydroxy acids and carboxylic acids by diamino-beta-cyclodextrins (AB,AC,AD) in capillary electrophoresis

Selectively modified 6,6'-dideoxy-6,6'-L-diamino-beta-cyclodextrins (AB, AC, AD) were successfully used as chiral selectors for the enantiomeric separation of hydroxy acids and carboxylic acids (in particular, phenoxyalkanoic acid herbicides) in capillary electrophoresis (CE). Chiral separations were obtained at a low selector concentration (1 mM) with good enantioselectivity and resolution factors. Separations were optimized as a function of pH. The different position of the charged groups on the upper rim greatly influenced the separation, accounting for electrostatic interactions between the protonated amino groups of the cyclodextrins (CDs) and the carboxylate of the selectands. The best enantiomeric separation of hydroxy acids was obtained with the AC regioisomer, whereas carboxylic acids were well resolved only by the AB regioisomer. A recognition model is proposed, based on two-dimensional nuclear magnetic resonance (2-D NMR) experiments, in which the orientation of the guest in the inclusion complex is determined by the electrostatic interactions between the selectand and the CD upper rim.     

New particle-loaded monoliths for chiral capillary electrochromatographic separation

Fritless particle-loaded monoliths for chiral capillary electrochromatographic (CEC) separation were prepared. Silica particles containing a chiral selector are suspended in a monomer solution, which is drawn into the capillary followed by in situ polymerization. Thereby the silica-based particles containing the chiral selector are embedded in a nonchiral continuous bed. This kind of chiral stationary phase is inexpensive, easy, and reproducible to prepare and circumvents the preparation of frits. As a model, teicoplanin aglycone as chiral selector bonded to 3 microm silica particles was used. The applicability of this approach is demonstrated by means of the chiral separation of aliphatic and aromatic amino acids and dipeptides. As a further application, the chiral selector ristocetin A bonded to 3 microm silica particles was used for the enantiomeric separation of chiral alpha-hydroxy acids. Since alpha-hydroxy acids migrate toward the anode, a cationic charge-providing agent was copolymerized with the matrix. This served to reverse the direction of the electroosmatic flow (EOF).     

Evaluation of A Chiral Crown Etherlc Column For the Separation of Racemic Amines

Abstract

Enantiomeric resolution of more than fifty racemic primary amines can be achieved on a column that utilizes a crown ether as a chiral selector. the racemic solute is solubilized in an acidic solvent, forming an ammonium ion from the primary amine functional group. an interaction between the lone pair electrons on the oxygens of the crown ether and the positive charge of the ammonium group leads to the formation of an inclusion complex. Due to the chirality of the crown ether there is stereoselective interaction resulting in enantiomeric separation. Excellent resolution is possible for amino acids, amino alcohols, amino esters and amines. Compounds are separated that were poorly resolved by conventional ligand exchange columns and by other means.     

Delta-cyclodextrin as novel chiral probe for enantiomeric separation by electromigration methods

Native delta-CD has been employed as chiral selector in CE and MEKC. To investigate the potential of the enantiodiscriminating properties of delta-CD, negatively charged 5-dimethylamino-1-naphthalene-sulfonyl (dansyl)-, 2,4-dinitrophenyl (DNP)- and FMOC-derivatives of several amino acids, 1,1'-binaphthyl-2,2'-diylhydrogenphosphate, flavanones and three positively charged drugs have been selected as testing samples. Enantioresolution factors up to 4.82 have been observed. The results were compared with those achieved by the conventional running buffer additives alpha-, beta- and gamma-CDs. For several examples a steady increase of enantioresolution with increasing degree of oligomerization has been detected.     

Functional amino acid ionic liquids as solvent and selector in chiral extraction

Amino acid ionic liquids (AAILs) have received great attention due to their potentials in catalysis and separations. In this work, functional AAILs were used as solvent and selector in chiral liquid–liquid extraction for the first time. The AAILs have shown distinct enantioselectivity in amino acid extraction. Using these functional AAILs as acceptor phase and ethylacetate as donor phase, more L-enantiomer of amino acid was extracted into the ionic liquid phase than that of D-enantiomer. The influencing factors, including AAILs structure, copper ion concentration, organic phase and amino acid concentration, were investigated. We found that the enantioselective enrichment of racemic amino acids was achieved through a chiral ligand-exchange mechanism. The enantioselectivity of single-step extraction was up to enantiomeric excess value of 50.6%. Moreover, the functional AAILs were found to be efficient extraction solvents for amino acids. The logarithm of distribution coefficient for L-Phe was in the range of 3.4–3.6 in the ionic liquid–ethylacetate two-phase system. This liquid–liquid extraction approach may extend the application of ionic liquids in chiral separations.     

Enantioseparation of Some Clinically Used Drugs by Capillary Electrophoresis Using Sulfated β-Cyclodextrin as a Chiral Selector

The enantiomeric separation of 37 clinically used racemic basic drugs among 50 drugs was achieved using sulfated β-cyclodextrin (S-β-CD) as chiral selector at pH2.5 and in the reversed polarity mode. The results obtained in this study were different from the one obtained using neutral β-CD and its derivatives as chiral selectors. Using S-β-CD as chiral selector did not require the presence of the substructure 4H to achieve chiral separation as observed with β-Cyclodextrin (β-CD) and its derivatives since among the 37 separated drugs only 7 possess the 4H substructure. The chiral discrimination depends on the appropriate interaction between the analyte and the sulfated β-cyclodextrin.     

Synthesis and application of clindamycin succinate as a novel chiral selector for capillary electrophoresis

A novel chiral selector, clindamycin succinate, was synthesized and first used as a chiral selector in capillary electrophoresis (CE). The chiral resolution ability of this kind of clindamycin derivation was studied by CE using some racemic drugs as model analytes. From the experimental results, it was found that both resolution and selectivity of the selector were dependent on the following parameters: concentration of chiral selectors, pH of the running buffer, temperature of the capillary column, applied voltage and organic modifier used. The results show that the chiral selector possesses high resolution toward some racemic drugs, including ofloxacin, chlorphenamine, tryptophan, propranolol, sotalol and metoprolol. Excellent chiral resolution of these tested drugs was achieved under the optimal conditions of 50 mM clindamycin succinate, 10% MeOH v/v, 50 mM Tris buffer, pH 4.0, at 22 kV and 20 °C within 25 min.     

Highly sensitive chiral analysis of amino acids by in-line single drop microextraction and capillary electrophoresis with laser-induced fluorescence detection

A highly sensitive method for chiral analysis of amino acids by in-line single drop microextraction (SDME) and chiral capillary electrophoresis (CE) with laser-induced fluorescence (LIF) detection was developed. In SDME, a drop of a basic aqueous acceptor phase covered with a thin organic layer was formed at the tip of a capillary by simple combination of sample-handling sequences of a CE apparatus. Then fluorescein isothiocyanate (FITC)-derivatized amino acids in an acidic donor solution were enriched into the drop through the organic layer. The enriched enantiomers were then resolved using a dual chiral selector of β-cyclodextrin (β-CD) and sodium taurodeoxycholate (STC). Here, in addition to serving as a labeling reagent providing high fluorescence signal, hydrophobic FITC was primarily used as a modifier aiding the extraction of zwitterionic amino acids by blocking the amino groups and increasing the hydrophobicity, yielding 220 times increase in extraction efficiency. Several hundred-fold enrichments were achieved with 10 min SDME, yielding LODs of 30-60 pM and enabling direct analysis of d-AAs in a 99% enantiomeric excess mixture. In view of no additional modification of the existing commercial CE instrument, this method without stirring can be easily realized using known operations. When a microstirrer was customized to the CE instrument several thousand-fold enrichments could be obtained with LODs in the low picomolar range of 1-3 pM.     

New triazine spectroscopic reagent for the separation of DL-amino acids by micellar electrokinetic chromatography

An approach to the chiral separation of racemic mixtures of amino acids by means of micellar electrokinetic chromatography after derivatization with a new triazine spectroscopic reagent, 3-(4,6-dichloro-1,3,5-triazinylamino)-7-dimethylamino-2-methylphenazine (DTDP), has been evaluated. It was found that the derivatives of the aliphatic amino acids such as serine, valine and arginine, could produce a strong UV absorption at 282 nm, whose apparent molar absorptivities are of 10(-4) M(-1) cm(-1), and thus the concentration of the amino acids down to 3 x 10(-7) M can still give a detectable signal (S/N = 3). Beta-Cyclodextrin (beta-CD) added to the buffer system was used as a chiral selector, and separation conditions were optimized. The presence of an organic modifier (2-propanol) was also a prerequisite for the chiral separation. The best results for the chiral separation of DTDP-amino acids were achieved in a mixed sodium dodecylsulfate-beta-CD-borate-2-propanol medium at pH 9.0. Compared to some of the commonly used derivatization methods, the present one offers a relatively stable derivative and strong UV absorption for the spectroscopically inert amino acids, thus enabling amino acids to be separated and detected by CE even with a simpler UV detector.     

Fast enantioseparation of arylglycine amides by capillary electrophoresis with highly sulfated-beta-cyclodextrin as a chiral selector

Nine racemic arylglycine amides were synthesized and successfully enantioseparated by capillary electrophoresis (CE) using highly sulfated beta-cyclodextrin (HS-beta-CD) as a chiral selector. Baseline enantioseparation of the analytes was obtained around neutral pH but not in the acidic conditions that are commonly used. HS-beta-CD content, buffer pH, type and concentration, and organic modifier concentration were studied and optimized for fast and efficient separation. A chiral CE separation system composed of 1.5% (w/v) HS-beta-CD, 0 to 10% (v/v) methanol and 20 mM 3-(N-morpholino)propanesulfonic acid at pH 6.5 was shown suitable for baseline enantioseparation of the mentioned amides within 6 min, including simultaneous enantioseparation of three positional isomer series (methyl-, methoxyl or chloro-substituted). By using this system, D-enantiomers migrated ahead of the L-enantiomers and the enantiomeric resolution order of arylglycine amides was more or less parallel to the pK(a), order of the analytes.     

Enantiomeric separation of racemic neolignans on chiralcel OD and determination of their absolute configuration with online circular dichroism

Effective enantiomeric separations of erythro- and threo-8.O.4'-neolignans with different aromatic substitution pattern (1a-i, 2a-i) are achieved on the commercially available chiral stationary phase cellulose tris(3,5-dimethylphenylcarbamate) (Chiralcel OD). It is shown that the chiral recognition of the stationary phase is significantly dependent on the substitution pattern of the racemic compounds. Online liquid chromatography (LC)-circular dichroism (CD) analysis allows for the establishment of a correlation between the absolute configuration of the separated erythro-8.O.4'-neolignans and their characteristic CD transitions, which could be used to determine or revise the configuration of previously isolated erythro-8.O.4'-neolignans. Although the absolute configurations of threo-isomers is not determined unambiguously from the LC-CD analysis, it is proven that both their elution order and chiroptical properties are significantly influenced by the substitution pattern of the aromatic rings.     

Separation of aminoalkanephosphonic acid enantiomers by indirect UV detection capillary electrophoresis with application of cyclodextrins

Indirect UV detection capillary electrophoresis (CE) was used for the separation of aminoalkanephosphonic acid (AP) enantiomers by applying commercially available cyclodextrins as chiral discriminators. The results show that the separation of the enantiomers depends on pH of the background electrolyte, the molar ratio of cyclodextrin to aminophosphonic acid, and on the type of the applied chiral selector. Optimization of process conditions allowed enantiomeric baseline separation or partial separation of 12 out of 14 alpha-aminophosphonic acids studied. This type of CE might therefore be successfully used for routine determination of enantiomeric purity of aminophosphonic acids.     

Enantioseparation of organic acids of pharmaceutical interest using eremomycin as a chiral selector

Strong adsorption of eremomycin on the fused-silica capillary wall was used for separation of enantiomers by CE. The capillary with adsorbed chiral selector was shown to be easily prepared and has reproducible properties. The effect of the chiral selector concentration, pH and composition of the BGE, and applied voltage on enantioseparation of acidic compounds, such as profens and aromatic carboxylic acids, was investigated. Two native α-amino acids, aspartic acid and glutamic acid, were enantioseparated. Fourteen tested compounds (including amino acids) were baseline resolved. Good selectivity of separation (α>1.09) was achieved. The migration order of ibuprofen and ketoprofen enantiomers was determined. The procedures were proposed for the analysis of flurbiprofen and warfarin in pharmaceuticals. Linearity was achieved in the concentration range of 4.0×10(-5)-2.0×10(-3) M for flurbiprofen and 3.2×10(-6)-4.9×10(-6) M for warfarin. The detection limits were found to be about 1×10(-5) M for flurbiprofen and 1×10(-6) M for warfarin.