Sodium maleopimaric acid as pseudostationary phase for chiral separations of amino acid derivatives by capillary micellar electrokinetic chromatography

A new type of chiral surfactant, sodium maleopimaric acid (SMA), was synthesized, and employed for the enantioselective micellar electrokinetic chromatographic (MEKC) separation of amino acid enantiomers derivatized with naphthalene-2,3-dicarboxaldehyde (NDA-D/L-AAs). The effect of the surfactant concentration, type and concentration of the BGE, and buffer pH on the resolution was studied, and optimized conditions were used to evaluate the ability of this new surfactant to perform chiral separations toward NDA-D/L-AAs by MEKC. Enantiomeric separations of NDA-D/L-AAs were achieved with a running buffer consisting of 100 mM borate (pH 9.5) and 20 mM SMA in a 58.5 cm length x 50 microm id capillary. Under the conditions selected, two pairs of tested amino acid enantiomers including NDA-D/L-trptophan (Trp) and NDA-D/L-kynurenine (Kyn) were resolved.     

Ligand exchange chiral separations by cyclodextrin derivatives in capillary electrophoresis

A beta-cyclodextrin substituted by an imidazole-bound histamine (CDmh) was successfully used to separate underivatised tryptophan racemate in capillary electrophoresis in the presence of copper(II) ion by the ligand exchange mechanism. To the best of our knowledge, this is the first example of a cyclodextrin derivative behaving as the first coordination sphere ligand in the complex added to the background electrolyte (BGE).     

Separation of neutral and basic enantiomers by cyclodextrin electrokinetic chromatography using anionic cyclodextrin derivatives as chiral pseudo-stationary phases

Separations of neutral and basic racemates were performed using five different anionic cyclodextrin (CD) derivatives as chiral selectors, viz. carboxymethylated β-CD, β-CD phosphate sodium salt, sulfobutyl ether β-CD sodium salt, carboxymethylated γ-CD, and γ-CD phosphate sodium salt. For the separation of neutral racemates, an untreated fused silica capillary was employed and various neutral racemates were successfully separated. Since the pH of the buffer affected the electroosmotic flow (EOF), the resolution was improved by changing the buffer pH. A polyacrylamide coated capillary was employed for the separation of basic racemates to suppress EOF and to prevent adsorption of cationic analyte on the capillary surface. By choosing an appropriate type and concentration of anionic CD, about 40 basic racemates were successfully separated. Some rough binding constants of basic analytes with an anionic β-CD were measured to discuss the optimum concentration of the CD. The migration direction was dependent on the binding constants and the concentration of the CD. The analyte strongly bound to the anionic CD migrated towards the anode but the weakly bound one moved towards the cathode. Anionic γ-CDs were also very useful for the separation of basic enantiomers. Five neutral CDs were employed as chiral selectors to compare selectivity between charged and neutral CDs, and eleven racemates could only be resolved using anionic CDs. The separation of some basic racemates in human plasma was also described. The direct injection of plasma samples was possible for some enantiomers that did not interact strongly with plasma proteins.     

Thin layer chromatography of dansyl and dinitrophenyl derivatives of amino acids. A review

The dinitrophenyl (DNP) derivatives of amino acids have found continual application in protein sequencing since Sanger used them for the first time for the sequencing of insulin. Dansyl derivatives of amino acids have been widely used in protein sequencing because of their fluorescent nature. The success of protein sequencing largely depends upon correct identification of such derivatives. The choice for the method of identification is related to cost, the availability of instrumentation and to the sensitivity needed for the analysis. Thin layer chromatography (TLC) is simple and has several advantages over other chromatographic methods. Therefore the literature after 1972 is reviewed for TLC analysis of dansyl- and DNP-amino acids, the two important amino acid derivatives required for identifying protein sequences. Additionally, the literature on the TLC resolution of enantiomeric mixtures of dansyl amino acids is reviewed. Application of various adsorbents, composition of solvent systems and other experimental conditions together with successful resolution data have been discussed. TLC provides a direct and inexpensive method for the resolution of enantiomers, and is fast becoming a sensitive instrumentalized quantitative analytical technique.     

Screening for amino acid disorders by thin-layer chromatography of the dansyl amino acids

Summary A procedure is described for the screening of disorders of amino acid metabolism or tranpsort. The amino acids and other reactive constituents present in a small volume of deproteinized plasma or urine are derivatized with dansyl chloride. Desalting or concentrating of urine is not required. The fluorescent derivatives are separated by two-dimensional thin-layer chromatography and visualized by ultraviolet radiation. The time of development is less than 1 hr. The derivatives of thirty-seven amino acids and related constituents found in physiological fluids have been mapped in the system. A standard, reflective of normal plasma, is used to aid in identification of those derivatives generally observed on plasma and urine chromatograms and in detection of aminoacidemias. Aminoacidurias are detected by observing derivatives that in relation to others are present in greater than normal amounts.

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Analytischer Nachweis von Störungen des Aminosäurestoffwechsels durch Dünnschicht-Chromatographie der Dansylverbindungen

Zusammenfassung Dansylderivate von Aminosäuren und anderen reaktionsfähigen Stoffen in kleinen Volumina enteiweißten Plasmas oder Harns wurden hergestellt. Entsalzung oder Einengung des Harns ist nicht notwendig. Die fluoreszierenden Derivate werden durch zweidimensionale Dünnschichtchromatographie getrennt und im UV nachgewiesen. Die dazu erforderliche Zeit ist kürzer als 1 Stunde. Die Derivate von 37 Aminosäuren und verwandten Verbindungen in physiologischen Flüssigkeiten wurden erfaßt. In üblicherweise zur Beobachtung gelangenden Plasma- bzw. Harnchromatogrammen feststellbare Aminosäuren dienen zum Vergleich mit abnormalen Ergebnissen.

     

Determination of amine-containing phosphonic acids and amino acids as dansyl derivatives

Conditions are selected for the analytical separation of (N-phosphonomethyl glycine), products of its microbiological conversion, glutamic acid, and alanine as dansyl derivatives using reversed-phase HPLC: column (250 × 4.6 mm) ReproSil-PAH EPA; mobile phase, methanol + 20 mM CH₃COONa (pH 5.1) (20: 80); rate of mobile phase, 1 mL/min; working detector wavelength, 330 nm. The duration of separation is 35 min. The lower limits of the analytical range (in ng) for dansyl derivatives are as follows: glyphosate, 8.2; aminomethyl phosphonic acid, 24.2; glutamic acid, 9.4; alanine, 12.6: glycine, 17.7; and sarcosine, 19.3. The TLC study of dansyl derivatives of amino acids was performed on sorbfil plates PTSH-P-V using two-dimensional chromatography in the systems ethyl acetate-isopropanol-24% aqueous ammonia (45: 35: 20) in the first direction and chloroform-methanol-acetic acid (25: 5: 1) in the second one. For determining phosphonic acids (glyphosate and aminomethyl phosphonic acid), a version of one-dimensional chromatography with the sequential use of two systems, chloroform-methanol-acetic acid (25: 5: 0.2) and ethanol-24% aqueous ammonia (7: 3), was proposed.     

Micelles for signal enhancement and novel selectivity of dansyl amino acids

Fluorescence intensity of various chemical species is enhanced in the microenvironment provided by micelles. Parameters which affect fluorescence intensities are examined by using dansyl (Dns) amino acids as the probe. The retention behavior of Dns-amino acids in micellar LC is examined by using ion-exchange-induced stationary phases. The type and concentration of micellar agent and modifier ion as well as concentration of acetonitrile in the mobile phase affect the retention and signal intensity of Dns-amino acids. The order of elution of Dns-amino acids obtained with the micellar mobile phase is very different from that observed in conventional reversed-phase LC. Fluorescence intensities of Dns-amino acids are enhanced by the micellar mobile phase.     

Enantioseparation of chiral benzimidazole derivatives by electrokinetic chromatography using sulfated cyclodextrins

Baseline separation of 18 new substituted benzimidazole derivatives, potent AMP‐activated protein kinase (AMPK) activators, with one chiral center, was achieved by CD‐EKC using sulfated and highly sulfated CDs (SCDs and HS‐CDs) as chiral selectors. The influence of the type and concentration of the chiral selectors on the enantioseparations was investigated. The SCDs exhibit a very high enantioselectivity power since they allow excellent enantiomeric resolutions compared to those obtained with the neutral CDs. The enantiomers were resolved with analysis times around 6 min using 25 mM phosphate buffer at pH 2.5 containing either β‐S‐CD, HS‐β‐CD, HS‐γ‐CD (3 or 4% w/v) at 25°C, with a voltage of 20 kV. The apparent association constants of the inclusion complexes were calculated. The study of the solute structure‐enantioseparation relationships seems to show the high contribution of the interactions between the solutes phenyl ring and the CDs to the enantiorecognition process. The optimized method was briefly validated (LOD less than 1%) and the purity of enantiomers of compound 3 was determined. The enantiomer migration shows reversal order depending on the kind of CD.     

Development of dinitrophenylated cyclodextrin derivatives for enhanced enantiomeric separations by high-performance liquid chromatography

The synthesis and evaluation of new dinitrophenyl (DNP) substituted beta-cyclodextrin (beta-CD) chiral stationary phases (CSPs) for the enantioseparation of various classes of chiral analytes by HPLC are presented. The dinitrophenyl substituted beta-CD derivatives are synthesized and covalently bonded to functionalized 5 microm spherical porous silica gel. These are the first reported derivatized cyclodextrin which contains pi-electron deficient substituents (i.e., pi-acidic moieties). The column performance in terms of their ability to separate enantiomers is evaluated. A variety of different dinitro-substituted aryl groups are investigated and compared. The pH of the mobile phase buffers, the buffer composition, the number and position of the dinitro groups on the phenyl ring substituent, the degree of substitution, and the bonding strategy all greatly affect the performance of the CSPs. A large variety of racemic compounds have been separated successfully on these CSPs. The bonded dinitrophenyl-derivatized cyclodextrins are stable in all three mobile phase modes, namely, the reversed-phase, polar organic, and normal phase modes. No degradation in column performance was observed in any mode of operation even after more than 1000 injections. The analytical applicability of these types of CSPs for enantiomeric separations is discussed in detail.     

Enantioseparation of chiral allenic acids by micellar electrokinetic chromatography with cyclodextrins as chiral selector

Enantioseparation of chiral aryl allenic acids by micellar electrokinetic chromatography (MEKC) with cyclodextrins (CDs) as chiral selectors was described. The screen of chiral selectors (beta-CD, gamma-CD, and hydroxypropyl (HP)-gamma-CD) showed that the enantioseparation was not only dependent on the type of CD but also the presence of 2-propanol in the buffer. In order to optimize the operational parameters, the effect of the concentration of CDs, sodium dodecyl sulfate (SDS), and 2-propanol, as well as the buffer ionic strength and pH on enantioseparation were studied. It was proved that the concentration of CDs, 2-propanol, and the buffer ionic strength were the critical parameters. Under optimal conditions, baseline separations of all seven allenic acid enantiomers were achieved. Furthermore, the method validation in terms of repeatability, linearity, limit of detection (LOD), and limit of quantitation (LOQ) were performed. Using the present method, the optical purity of a nonracemic sample with the enantiomeric excess (e.e.%) value of 99.65% was determined.     

Evaluation of the enantiomeric selectivity in the chiral ligand-exchange chromatography of amino acids by a computational model

The chromatographic resolution of enantiomeric amino acids is accomplished on a reversed phase column using aqueous mobile phase containing the chiral reagent N,N-dimethyl-S-phenylalanine-Cu(II). The separation is a result of the whole interaction between the diastereomeric complex surface and the mixed stationary phase realized by the dynamic coating of the RP-18 carbon chains layer. The elution order seems to be related to the different water coordination capability on copper ion in the formation of the mixed ternary complexes.     

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.     

Enhanced chemiluminescence detection of some dansyl amino acids in liquid chromatography

Summary The dansylated amino acids alanine, glutamic acid, methionine and norleucine were separated by reversedphase HPLC and detected via chemical excitation using the post-column, TCPO-peroxyoxalate reaction system. Enhancement of the chemiluminescence emission was achieved by including the surfactant Triton X-100 in the eluent.     

Resolution of sulphur-containing amino acids by chiral phase gas chromatography

Summary Methyl esters of the pentafluoropropionyl-amino acid derivatives of the tetrafunctional, sulphur-bridged, stereoisomeric lanthionines, cystathionines and -methyl-lanthionines were resolved on glass capillaries coated with the chiral stationary phase N-propionyl-L-valine-N-tert-butylamide-polysiloxane (Chirasil-Val) within 35min. Interestingly, L-cystathionine elutes before its D-enantiomer in contrast to the usual order of emergence on an L-phase. The method was applied to the polypeptide antibiotic nisin, which contains mesolanthionine and 2S,3S,6R-3-methyl-lanthionine.N-Pentafluoropropionyl-S-alkylthiocysteine methyl esters (R=methyl, ethyl, n- and iso-propyl, n- and sec-butyl, n-octyl, neo-pentyl, cyclohexyl-, benzyl-, tolyl-) were separated on Chirasil-Val within 30min. The identity of all derivatives was shown by combined gas chromatography-mass spectrometry.     

Enantiomeric separations by use of polymeric surfactant electrokinetic chromatography

This review surveys the enantiomeric separation of drugs by electrokinetic chromatography using polymeric chiral surfactant pseudostationary phases. These phases have recently been shown to provide better mass transfer and increased rigidity and stability than regular micelles in micellar capillary electrophoresis. Characterization of the polymeric chiral surfactants is presented. Solution interactions of the pseudostationary phases via thermodynamics and fluorescence probe studies are evaluated. Also, case studies of enantiomeric separation of drugs using a single amino acid surfactant and the synergistic effect of the addition of gamma-cyclodextrin to the buffer is discussed. The use of dipeptide surfactants for chiral drug separations is described as well.     

2-Acyl-dimedones as UV-active protective agents for chiral amino acids: enantiomer separations of the derivatives on chiral anion exchangers

2-Acetyldimedone and 12 related compounds were employed as UV-active pre-column derivatizing agents for amino acids. Direct enantioseparation of the products was achieved using chiral anion exchanger stationary phases in polar-organic mobile phase mode. Under basic conditions, the reagents´ cyclic β-tricarbonyl motifs can give rise to exo- and endocyclic enols through tautomerization. However, with primary amines (proteinogenic and unusual amino acids, aminosulfonic and aminophosphonic acids), we exclusively observed the formation of exocyclic enamine-type products. Reaction yields depended strongly on the 2-acyl modification of the reagent; in particular, we observed a significant decrease when electronegative or sterically demanding substituents were present in α-position to the exocyclic carbonyl group. In addition to improving UV detectability of the products, the introduction of this protective group facilitated successful enantiomer separations of the amino acid derivatives on Cinchona-based chiral anion exchangers. Particularly high enantiomer selectivity was observed in combination with stationary phases bearing a new variation of selectors with π-acidic (electron-poor) bis(trifluoromethyl)phenyl groups. No racemization of the analytes occurred at any stage of the analytical method including the deprotection, which was achieved with hydrazine.

胶束电动力学毛细管电泳分离和测定牡蛎中的氨基酸

采用胶束电动力学毛细管电泳,在25mmol／L Na2HPO4-NaH2PO4(pH7．0),40mmol／L十二烷基硫酸钠和3mol／L尿素的实验条件下,在28min内实现了牡蛎中8种游离丹酰化氨基酸的分离和测定。在一定浓度范围内,氨基酸的峰面积与其浓度呈良好的线性关系,相关系数0．9955～0．9984之间。对于8种氨基酸,标准加入回收率在99．5％～100．7％之间。     

Precolumn diastereomerization and micellar electrokinetic chromatography on a plastic microchip: rapid chiral analysis of amino acids

Precolumn derivatization and chiral separation of DL-amino acids based on diastereomerization have been performed on an integrated poly(dimethylsiloxane) microchip. Diastereomeric derivatives were formed in a microfabricated precolumn reactor by the reaction of amino acid enantiomers with o-phthaldialdehyde/2,3,4,6-tetra-O-acetyl-1-thio-beta-D-glucopyranose (OPA/TATG), and separated by MEKC in an achiral environment without chiral selectors in the running buffer. Optimized precolumn reactions and chiral separations of amino acids were achieved within 2.5 min. Resolutions of diastereomers of OPA/TATG-amino acids were in the range of 2.5-6.1 at optimized separation conditions. Simultaneous separation of a mixture of five chiral amino acids was successfully performed in a single run in less than 100 s.     

Determination of 27 dansyl amino acid derivatives in biological fluids by reversed-phase high-performance liquid chromatography

The concentrations of free amino acids in plasma and in ascitic liquid of mice with Ehrlich ascitic tumours were determined by reversed-phase high-performance liquid chromatography using pre-column derivatization with Dns chloride and UV detection at 254 nm. Sample preparation is simple, and the Dns derivatives are stable. Complete separation of 27 amino acids, including proline and cysteine, was achieved in 70 min with detection limits of less than 25 pmol. There was no interference from Dns-Cl, Dns-OH and Dns-NH2. Retention time reproducibility was better than 1%. The described method enables a rapid, economical and reproducible quantification of free amino acids in biological fluids.