Facile preparation of ethanediamine-ß-cyclodextrin modified capillary column for electrochromatographic enantioseparation of Dansyl amino acids.

Herein, the fabrication of a fascinating multifunctional cyclodextrin (CD) chiral stationary phase and its chiral separation performance in capillary electrochromatography are proposed. A facile interfacial polymerization was used to anchor ethanediamine-ß-cyclodextrin (EDA-ß-CD) polymerized with trimesoyl chloride (TMC) and to form the chiral stationary phase (CSP) composite onto the surface wall of the capillary. The characters of prepared columns were confirmed by Fourier transform infrared spectroscopy (FT-IR), X-ray Photoelectron Spectrometer (XPS), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). This novel CSP offers multi-typical interactions including hydrogen bonding, π-interaction, hydrophobic and electrostatic interaction as well as steric effects which contribute to prominent chiral recognition for Dansyl-DL-amino acids in CEC modes. The EDA-ß-CD modified column showed eminent enantioseparation performance towards five Dansyl-DL-amino acids (the DL-forms of valine, threonine, leucine, phenylalanine, serine). Besides, the prepared columns were perfectly reproducible and stable. The relative standard deviations of the enantiomer retention times for intra-day (n = 5), inter-day (n = 3) runs and column-to-columns (n = 3) are below 0.54%, 1.35% and 4.89%, individually. This innovative chiral stationary phase shows a broader application view and scope in chiral recognition domain.

Open-tubular capillary electrochromatography with ß-cyclodextrin-functionalized magnetic nanoparticles as stationary phase for enantioseparation of dansylated amino acids.

Magnetic nanoparticles (MNPs) modified with ß-cyclodextrin and mono-6-deoxy-6-(1-methylimidazolium)-ß-cyclodextrin tosylate (an ionic liquid), which called MNP-ß-CD and MNP-ß-CD-IL, were coated into the capillary inner wall. Compared to an uncoated capillary, the new systems show good reproducibility and durability. The systems based on the use of MNP-ß-CD or MNP-ß-CD-IL as stationary phases were established for enantioseparation of Dns-modified amino acids. Improved resolutions were obtained for both CEC systems. Primary parameters such as running buffer pH value and applied voltage were systematically optimized in order to obtain optimal enantioseparations. Under the optimized conditions, the capillaries exhibited excellent chiral recognition ability for six Dns-amino acids (the DL-forms of alanine, leucine, lsoleucine, valine, methionine, glutamic acid) and provided a promising way for the preparation of chiral column. Graphical Abstract Schematic presentation of the open-tubular capillary electrochromatography systems with MNP-ß-CD and MNP-ß-CD-IL as stationary phases for enantioseparation of dansylated amino acids.

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.

Cationic permethylated 6-monoamino-6-monodeoxy-ß-cyclodextrin as chiral selector of dansylated amino acids in capillary electrophoresis.

The resolution power of permethylated 6-monoamino-6-monodeoxy-ßCD (PMMABCD) - a single isomer, cationic CD derivative - developed previously for chiral analyses in capillary electrophoresis was further studied here. Dansylated amino acids (Dns-AA) were chosen as amphoteric chiral model compounds. Changes in the resolutions of Dns-AAs by varying pH and selector concentrations were investigated and correlated with their structures and chemical properties (isoelectric point and lipophilicity). Maximal resolutions could be achieved at pH 6 or pH 4. The separations improved with increasing concentration of the selector. Baseline or substantially better resolution for 8 pairs of these Dns-AAs could be achieved. Low CD concentration was enough for the separation of the most apolar Dns-AAs. Chiral discrimination ability of PMMABCD was demonstrated by the separation of an artificial mixture of 8 Dns-AA pairs.

Enantioseparation of dansyl amino acids by HPLC on a monolithic column dynamically coated with a vancomycin derivative.

In this work a chiral stationary phase was prepared by dynamically coating a monolithic reversed-phase HPLC column with a vancomycin-derivative as chiral selector. A hydrophobic alkyl-chain was attached to the vancomycin molecule, providing the immobilization of the chiral selector on the reversed-phase material. Dansyl amino acids were chosen as model analytes for testing the separation power of the dynamically coated phase. All investigated compounds were separated into their enantiomers. Compared with a conventionally packed vancomycin-CSP, a reversal of the enantiomer elution order was obtained.

CEC enantioseparations of carboxylic acids on silica-based monoliths modified with ergot alkaloid derivative.

The enantioseparation of carboxylic acids, including amino acid dansyl-derivatives, 2-arylpropionic acids and 2-aryloxypropionic acids, was tested by CEC on a porous silica sol-gel monolithic column that was prepared by polycondensation of tetramethoxysilane in acidic conditions and post-gelation heat treatment (120 degrees C for 3 h) in the presence of urea, and successively, by anchoring to the silica (+)-1-allyl-(5R,8S,10R)-terguride as the chiral selector. The bimodal structure of the sorbent showed through pores with a median value of 1.39 mum and a mesopore size distribution ranging between 6 and 12 nm (average pore size of 9.9 nm). To attain optimum separation conditions, the influence of the pH and the concentration of the buffer solution in the mobile phase on resolution were investigated. The monolithic column showed: (i) for the compounds studied resolution values two or three times higher in comparison with previously developed separation systems where the same chiral selector was used. For example, on the monolithic column Dns-serine enantiomers were much better separated (8 min with a selectivity factor of 1.34) than by HPLC (20 min, alpha=1.17); (ii) high chemical and mechanical stability as demonstrated by the use of such column for hundreds of analysis along about 1 year without significant variations of the resolution and the retention parameters.

Dodecyl thioglycopyranoside sulfates: novel sugar-based surfactants for enantiomeric separations by micellar electrokinetic capillary chromatography.

Four novel chiral anionic surfactants having carbohydrate hydrophilic heads, sodium n-dodecyl 1-thio-beta-D-glucopyranoside 6-hydrogen sulfate (6-betaGlcD), sodium n-dodecyl 1-thio-beta-L-glucopyranoside 6-hydrogen sulfate (6-betaGlcL), sodium n-dodecyl 1-thio-beta-L-fucopyranoside 3-hydrogen sulfate (3-betaFucL), and sodium n-dodecyl 1-thio-alpha-L-rhamnopyranoside 3-hydrogen sulfate (3-alphaRhaL), were synthesized by selective sulfation of the corresponding thioglycosides. Their CMC determined by fluorescence using pyrene as a probe in water was 1.3-2.7 mM. These surfactants found to be useful as chiral selectors for enantiomeric separation by MEKC. The enantiomeric separation was optimized with respect to pH, buffer concentration, and surfactant concentration. Under the optimized conditions (50 mM phosphate buffer at pH 6.5, 30 mM surfactant, 20 kV), the enantiomeric separations of five dansylated amino acids (Dns-AAs) were achieved within approximately 20 min with the migration order of Val

Preparation of monolithic silica columns for high-performance liquid chromatography.

Preparation methods of monolithic silica columns for HPLC including the surface modification were reviewed. Chemical modification methods recently reported to obtain stationary phases for reversed-phase (RP), chiral, ion-exchange, and hydrophilic interaction chromatography (HILIC) separations were discussed. Recent results related to preparation methods of monolithic silica were also covered. The characteristics and properties of silica monoliths and some applications of monolithic silica columns for different analytical and bioanalytical fields will be commented.

Synthesis and chromatographic properties of a chiral stationary phase derived from bovine serum albumin immobilized on magnesia-zirconia using phosphonate spacers.

A novel bovine serum albumin (BSA)-modified magnesia-zirconia stationary phase was prepared using the sodium salt of cis-(3-methyloxiranyl)phosphonic acid (fosfomycin) as spacer and glutaraldehyde as coupler. Baseline separation of six derivatized amino acids (DNB-Leu, Dansyl-Val, etc.) was achieved on this column using ammonium acetate buffer-isopropanol mobile phase at a flow rate of 1.0 mL/min. The effects of mobile phase composition, eluent pH value, column temperature, and flow rate on the retention and separation of chiral compounds were also investigated. The BSA chiral stationary phase (BSA-CSP) was relatively stable under experimental conditions. The coupling reaction in this method was mild, reliable, and reproducible; thus it was also suitable for the immobilization of various biopolymers with amino groups in the preparation of chromatography stationary phases.

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.

Synthesis and application of single-isomer 6-mono(alkylimidazolium)-beta-cyclodextrins as chiral selectors in chiral capillary electrophoresis.

Novel single isomers of positively charged beta-CDs were prepared via nucleophilic substitution of 6-monotosyl-beta-CD with alkylimidazoles to afford 6-mono(alkylimidazolium)-beta-CD tosylates and then 6-mono(alkylimidazolium)-beta-CD chlorides by anion exchange. The chiral resolution abilities of these cationic CDs were studied by CE using dansyl (Dns)-amino acids as model analytes. From the experimental results, it was found that both resolution and selectivity of these cationic CDs were dependent on the following parameters: concentration of chiral selectors, pH of the running buffer, counteranions of the CDs, side chain length of the n-alkyl-imidazolium cation, temperature of the capillary column, and organic modifier used. The concentration of chiral selectors required for enantioseparation varied from 3 to 30 mM. The BGE pH played an important role in the resolution of Dns-amino acids. For acidic BGEs, chiral resolution increased with pH (4.0-6.0) and reached a local maximum at pH 6.0. However, better resolutions were obtained with basic phosphate buffer at pH 9.6. Methanol was found to be an effective organic modifier for the resolution of Dns-amino acids by CE.

Chiral separation based on ligand-exchange capillary electrophoresis using a copper(II)-L-ornithine ternary complex as selector.

A ligand-exchange capillary electrophoresis was explored, with L-ornithine as the ligand and copper(II) as the central ion. Its applicability was demonstrated with underivatized and dansyl amino acids, a dipeptide, and drugs with amino alcohol structure. The enantioselectivity was found to be strongly dependent on pH and copper(II)-L-Orn complex concentration. Due to the adsorption of the positively charged species onto the capillary inner walls, the chiral separation selectivity is very high while the efficiency is relatively low. Permanent 1,3-propanediamine-coated capillaries show an improved separation efficiency and theoretical plate numbers increasing from 10(4) to 10(5). Similar phenomena were observed when sodium dodecyl sulfate (SDS) micelles were added to the copper(II) complex solution. The poor separation efficiency of chiral compounds in uncoated capillaries may result from the low rate of the ligand-exchange reactions, and the high enantioselectivity may derive from the complexing process in the adsorbed phase.

Cyclodextrin-modified monolithic columns for resolving dansyl amino acid enantiomers and positional isomers by capillary electrochromatography.

We describe beta- and gamma-cyclodextrins (beta- and gamma-CD)-modified monolithic columns prepared by sol-gel process and chemical modifications. The monolithic silica column was fabricated inside a fused-silica capillary with 100 microm inner diameter by sol-gel process. The monolithic silica matrix was chemically modified chiral selectors of beta- or gamma-CDs with a spacer of 3-glycidoxypropyltrimethoxysilane by on-column reactions. Gamma-CD-modified monolithic column has successfully been applied for the separation of dansyl amino acid enantiomers. Beta-CD-modified monolithic column has been used for the separation of the positional isomers of o-, m-, and p-cresols and the enantioseparation of racemates of benzoin and several dansyl amino acids by capillary electrochromatography, respectively. For the separation of neutral positional isomers, a positive electric field was applied. However, for the separation of negatively charged analytes, a negative electric field was applied at the inlet of column. The separation efficiency of 5.0 x 10(4) theoretical plates/m for dansyl-L-threonine was obtained at electric field strength of -300 V/cm in the mobile phase of 50 mM 2-(N-morpholino)ethanesulfonic acid (MES)-Tris/methanol (70/30) buffer at pH 7.0. L-enantiomers were eluted as the first peak. Scanning electron micrograph showed that monolithic columns have the morphology of continuous skeleton and large through-pores.

Fast enantiomeric separation with vancomycin as chiral additive by co-electroosmotic flow capillary electrophoresis: increase of the detection sensitivity by the partial filling technique.

A fast and sensitive method is described by using vancomycin as a chiral additive for enantiomeric separation by capillary electrophoresis (CE). In order to overcome disadvantages associated with use of vancomycin as chiral additive in CE, several strategies including the dynamic coating technique, the co-electroosmotic flow technique, and the partial filling technique were employed sequentially in this method. Using the polycationic polymer hexadimethrine bromide (HDB) as a buffer additive, the capillary wall was dynamically coated with a thin film formed by the adsorbed HDB. Consequently, the adsorption of vancomycin onto the capillary wall was minimized via electrostatic repulsion between the coating of the capillary wall and the vancomycin molecule. In addition, the reversed electroosmotic flow (from cathode to anode) produced by the positively charged capillary wall migrates in the same direction of negatively charged analytes (co-electroosmotic flow electrophoresis). Thereby the electrophoretic mobility of negatively charged analytes were drastically accelerated leading to a short separation time of less than 3.4 min. The separation time was further reduced by the use of a short-end-injection technique. For example, the analysis time was achieved by as short as 55 s for a baseline separation of dansyl-alpha-amino-n-butyric acid. Concurrently, the partial filling technique was used to avoid the loss of detection sensitivity caused by the presence of vancomycin in the running buffer. The effect of several parameters, such as HDB concentration, buffer pH, plug length of the chiral selector, concentration of the chiral selector and applied voltage, on enantioselectivity were investigated toward optimization. Besides the advantage of a very short separation time, the method is characterized by high detection sensitivity, high selectivity, and high efficiency.

[Fractal art in separative sciences]. / L'art des fractales en sciences séparatives.

Fractal geometry has provided a mathematical formalism for describing complex and dynamical structures. It has been applied successfully in a variety of areas such as astronomy, economics and biology. Because of its success in such a variety of areas it is natural to develop fractal application in separative sciences.

Chemically L-prolinamide-modified monolithic silica column for enantiomeric separation of dansyl amino acids and hydroxy acids by capillary electrochromatography and mu-high performance liquid chromatography.

A silica-based chiral monolithic column prepared by sol-gel process and chemical modification of chiral selector was used for enantioseparation of dansyl amino acids and hydroxy acids by capillary electrochromatography (CEC) and mu-high-performance liquid chromatography (mu-HPLC). L-Prolinamide was modified as a chiral selector. The chiral stationary phase (CSP), the chiral complex of Cu(II) with L-prolinamide, provides an anodic electroosmotic flow (EOF) in CEC. The EOF was found to be dependent on applied electric field strength, the pH, and the composition of mobile phases. Scanning electron micrograph showed that monolithic columns have the morphology of continuous skeleton and large through-pore. D-Enantiomers migrated before L-enantiomers except for dansyl-(Dns)-DL-Ser. The separation efficiencies of up to 17600 (D) and 13,200 plates m(-1) (L) were achieved for the separation of DL-indole-3-lactic acid.

Dansyl amino acid enantiomer separation on a teicoplanin chiral stationary phase: effect of eluent pH.

The retention and separation of a series of D,L dansyl amino acids (used as test solutes) on a teicoplanin stationary phase were investigated over a wide range of mobile phase (citrate buffer-methanol, 90:10, v/v) pH. An approach based on the development of various equilibria was carried out in order to describe the retention behavior of the solute in the chromatographic system. The equilibrium constants corresponding to the transfer of the anionic and zwitterionic forms of the dansyl amino acids from the mobile to the stationary phase were determined. These values allowed one to explain the decrease in the retention factor and the associated increase in the separation factor as the eluent pH was increased. Thermodynamic parameter variations were calculated so that the driving forces of the solute association with the teicoplanin phase were derived. This approach indicated that the chiral discrimination was principally controlled by the interaction between the anionic form of the solute and the stationary phase.

Enantiomer separation by nonaqueous and aqueous capillary electrochromatography on cyclodextrin stationary phases.

Native beta- and gamma-cyclodextrin bound to silica (ChiraDex-beta and ChiraDex-gamma) were packed into capillaries and used for enantiomer separation by capillary electrochromatography (CEC) under aqueous and nonaqueous conditions. Negatively charged analytes (dansyl-amino acids) were resolved into their enantiomers by nonaqueous CEC (NA-CEC). The addition of a small amount of water to the nonaqueous mobile phase enhanced the enantioselectivity but increased the elution time. The choice of the background electrolyte (BGE) determined the direction of the electroosmotic flow (EOF). With 2-(N-morpholino) ethanesulfonic acid (MES) or triethylammonium acetate (TEAA) as BGE an inverse EOF (anodic EOF) was observed while with phosphate a cathodic EOF was found. The apparent pH (pH*), the concentration of the BGE, and the nature of the mobile phase strongly influenced the elution time, the theoretical plate number and the chiral separation factor of racemic analytes.

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

A new type of chiral monolithic column was successfully developed for the enantioseparation of dansyl amino acids by ligand exchange-capillary electrochromatography (LE-CEC) in this work. The monolithic column matrix was prepared by a sol-gel process and then chemically modified with the spacer (3-glycidoxypropyl)trimethoxysilane and the chiral selector L-phenylalaninamide. After being conditioned with Cu(II) aqueous solution, the ligand exchange-chiral stationary phase (LE-CSP) possesses positive charges. When the external electric field was applied in CEC, electroosmotic flow (EOF) was generated on the surface of LE-CSP in the direction from the cathode to the anode. The EOF was found to be dependent on the applied electric field strength and the composition of the mobile phase. With the increase of pH of the mobile phase, the EOF showed a tendency to decrease. Scanning electron microscopy showed that the chiral monolithic column has a continuous skeleton and large through-pore structure. The separation efficiency (theoretic plate numbers) for the separation of Dns-DL-Leu reached up to 9.0 x 10(4) plates m(-1) for the D-enantiomer and 6.6 x 10(4) plates m(-1) for the L-enantiomer, by using pH 5.5, acetonitrile/0.50 mM Cu(Ac)2-50 mM NH4Ac (7:3) as mobile phase. The reproducibility and lifetime were satisfactory. CEC was carried out with conventional capillary electrophoresis equipment without pressurizing the ends of the capillary. No bubble was formed during the operation, after degassing the mobile phase and conditioning the column.

Separation of amino acids, their derivatives and enantiomers by impregnated TLC.

The present state of TLC with respect to separation of amino acids, their different derivatives and their enantiomers by the technique of impregnation is discussed. The main approaches to impregnation viz. mixing of a suitable reagent with the adsorbent prior to plate-making, immersion of the untreated plate in the solution of impregnating reagent prior to development, and modification of the adsorbent, have been identified and discussed for each class of these compounds. The role of impregnation in resolving enantiomers or in improving the separation of mixtures of amino acids or their derivatives in terms of ion pairing, complex formation, ligand exchange or other steric interactions has been elaborated in each category.