Direct chiral resolution of tartaric acid by ion-pair capillary electrophoresis using an aqueous background electrolyte with (1R,2R)-(-)-1,2-diaminocyclohexane as a chiral counterion

Chiral resolution of native DL-tartaric acid was achieved by ion-pair capillary electrophoresis (CE) using an aqueous-ethanol background electrolyte with (1R,2R)-(-)-1,2-diaminocyclohexane (R-DACH) as a chiral counterion. Factors affecting chiral resolution and migration time of tartaric acid were studied. By increasing the viscosity of the background electrolyte and the ion-pair formation, using organic solvents with a lower relative dielectric constant, resulted in a longer migration time. The optimum conditions for both high resolution and short migration time of tartaric acid were found to be a mixture of 65% v/v ethanol and 35% v/v aqueous solution containing 30 mM R-DACH and 75 mM phosphoric acid (pH 5.1) with an applied voltage of -30 kV at 25 degrees C, using direct detection at 200 nm. By using this system, the resolution (Rs) of racemic tartaric acid was approximately 1. The electrophoretic patterns of tartaric and malic acids suggest that two carboxyl groups and two hydroxyl groups of tartaric acid are associated with the enantioseparation of tartaric acid by the proposed CE method.     

Direct chiral resolution of malic acid in apple juice by ligand-exchange capillary electrophoresis using copper(II)-L-tartaric acid as a chiral selector

Chiral resolution of native DL-malic acid was achieved by ligand-exchange capillary electrophoresis using copper(II)-L-tartrate as a chiral selector. Factors affecting chiral resolution, migration time, and peak area of malic acid were studied. The running conditions for optimum separation of malic acid were found to be 1 mM copper(II) sulfate-1 mM L-tartrate (pH 5.1) with an effective voltage of -20 kV at 30 degrees C, using direct detection at 280 nm, and resolution (Rs) of racemic malic acid was approximately 4. With this system, D- and L-malic acids in apple juice were analyzed successfully.     

Enantioseparation of tartaric acid by ligand‐exchange capillary electrophoresis using contactless conductivity detection

A method for the determination of tartaric acid enantiomers using CE with contactless conductivity detection has been developed. Cu(II) as a central metal ion together with l ‐hydroxyproline were used as a chiral selector, the BGE was composed of 7 mM CuCl_2, 14 mM trans‐4‐hydroxy‐l ‐proline, and 100 mM ε‐aminocaproic acid; the pH was adjusted to 5 by hydrochloric acid. Separation with a resolution of 1.9 was achieved in 9 min in a polyacrylamide‐coated capillary to suppress the EOF. Various counterions of the BGE were studied, and migration order reversal was achieved when switching from ε‐aminocaproic acid to l ‐histidine. With detection limits of about 20 μM, the method was applied to the analysis of wine and grape samples; only l ‐tartaric acid was found.     

Direct chiral resolution of lactic acid in food products by capillary electrophoresis

Chiral resolution of native DL-lactic acid was performed by capillary electrophoresis using 2-hydroxypropyl-beta-cyclodextrin as a chiral selector. Various factors affecting chiral resolution, migration time, and peak area of lactic acid were studied. The running conditions for optimum separation of lactic acid were found to be 90 mM phosphate buffer (pH 6.0) containing 240 mM 2-hydroxypropyl-beta-cyclodextrin with an effective voltage of -30 kV at 16 degrees C, using direct detection at 200 nm. In order to enhance the sensitivity, sample injection was done under a pressure of 50 mbar for 200 s. On-line sample concentration was accomplished by sample stacking. With this system, D- and L-lactic acids in food products were analyzed successfully.     

Chiral separation with ligand-exchange micellar electrokinetic chromatography using a D-penicillamine-copper(II) ternary complex as chiral selector

D-Penicillamine is demonstrated for the first time as a chiral ligand for the enantioseparation of dansyl amino acids based on ligand-exchange micellar electrokinetic chromatography (LE-MEKC). Copper(II) was used as the central ion in the ternary complex. The effect of surfactant on the resolution was significant. A concentration of 20 mM sodium dodecyl sulfate (SDS) was shown to be necessary for the separation. Other important parameters, such as the concentration ratio of D-penicillamine (D-PEN) to Cu2+, the kind of metal central ion, the type and pH value of buffer, were also investigated. N-Acetyl-D-penicillamine and L-valine (Val), with similar structure to D-penicillamine, were applied as their copper(II) complexes as chiral selector and the chiral recognition mechanism is briefly discussed. Under optimum experimental conditions, i.e., 20 mM NH4OAc, pH 6.5, a 2:1 concentration ratio of D-penicillamine to Cu(II), 4 mM CuSO4 and 8 mM D-penicillamine, the chiral separation of eight pairs of different dansyl amino acid enantiomers was accomplished with resolution ranging from 1.1 to 5.9. When L-PEN was used instead of D-PEN, reversal of the migration order was observed.     

Methodology for predicting the separation of proteins by hydrophobic interaction chromatography and its application to a cell extract

Ligand-exchange micellar electrokinetic capillary chromatography was used for the chiral resolution of underivatized and dansyl amino acid enantiomers simultaneously. The separation was achieved by chiral copper(II)-L-valine complexes incorporated in micelles of sodium dodecyl sulfate (SDS). The enantioresolution was strongly affected by SDS and a concentration of 20 mM SDS was shown to be necessary for the separation. Other impacting factors were investigated including pH, the molar ratio of copper(II) to L-valine and the total concentration of complex. Using the proposed method, 11 different dansyl amino acids and two underivatized amino acids were separated successfully with a running electrolyte of 20 mM NH4OAc, 4 mM CuSO4, 8 mM L-valine and 20 mM SDS at pH 9.0 in less than 25 min. Experiments were also performed with other amino acid ligands in order to vary the stability and the sterical arrangement of the copper(II) complexes and the possible chiral recognition mechanism was also discussed briefly.     

Enantioseparation of α‐hydroxy acids by chiral ligand exchange CE with a dual central metal ion system

Using two kinds of central metal ions in a background electrolyte, ligand exchange CE was investigated for the simultaneous enantioseparation of dl ‐malic, dl ‐tartaric, and dl ‐isocitric acids. Ligand exchange CE with 100 mM d ‐quinic acid as a chiral selector ligand and 10 mM Cu(II) ion as a central metal ion could enantioseparate dl ‐tartaric acid but not dl ‐malic acid or dl ‐isocitric acid. A dual central metal ion system containing 0.5 mM Al(III) ion in addition to 10 mM Cu(II) ion in the background electrolyte enabled the simultaneous enantioseparation of the three α‐hydroxy acids. These results suggest that the use of a dual central metal ion system can be useful for enantioseparation by ligand exchange CE.     

Study of the behaviour of chiral alcohols by gas chromatography on chiral phases

Summary In order to study the resolution of chiral alcohols by gas chromatography twelve chiral phases derived from (+) tartaric acid were synthetized. The different factors whichcould modify the resolution were studied: importance of trapped fractions of the peak, structures of the chiral phases and racemic compounds.     

High-performance ligand-exchange chromatography of amino acids on chiral stationary phases

Three chiral stationary phases, obtained by grafting silica gel with (-)-trans-1,2-cyclohexanediamine, were studied for the resolution of α-amino acids by ligand-exchange chromatography. The packings were prepared by bonding the chiral ligand to silica gel via different hydrocarbon spacers. Separation of the optical isomers was accomplished by eluents containing a constant concentration of copper(II) acetate (0.05mM). The elution sequence of amino acids was found to be dependent on the grafting reaction selected to prepare the chiral packings.     

Preparation and evaluation of a novel chiral stationary phase based on covalently bonded chitosan for ligand-exchange chromatography

A novel chiral stationary phase based on chitosan covalently bonded onto silica gels has been prepared and used for the separation of various alpha-amino acid enantiomers as well as alpha-hydroxycarboxylic acid enantiomers by chiral ligand-exchange chromatography with copper(II) as a complexing ion. The methanol content and copper(II) ion concentration in the eluent affected retentivity and enantioselectivity. Furthermore, a plausible chiral recognition mechanism for resolution of alpha-amino acids was proposed.     

Enantioseparation of DL-isocitric acid by a chiral ligand exchange CE with Ni(II)-D-quinic acid system

The ratio of citric acid to D ‐isocitric acid can be used to distinguish authentic and adulterated fruit juices. To separate DL ‐isocitric acid enantiomers, we used ligand exchange CE. D ‐Quinic acid was used as a chiral selector ligand and Mn(II), Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) ions were used as the central ions of the chiral selector in the BGE. DL ‐Isocitric acid was found to be enantioseparated with the above metal ions except for Mn(II) ion. The optimum running conditions for the analysis of D ‐ and L ‐isocitric acids along with citric acid, an isomer of isocitric acid, were found to be a BGE (pH 5.0) containing 30% ACN, 20 mM acetic acid, 20 mM NiSO₄, and 80 mM D ‐quinic acid. Under these conditions, DL ‐isocitric and citric acids in fruit juices were analyzed successfully.     

Use of chiral amino acid ester‐based ionic liquids as chiral selectors in CE

In this study, the applicability of a chiral ionic liquid (CIL) as the sole chiral selector in CE was investigated for the first time. In particular, five amino acid ester‐based CILs were synthesized and used as additives in the BGE in order to evaluate their chiral recognition ability. The performance of these CILs as the sole chiral selectors was evaluated by using 1,1′‐binaphthyl‐2,2‐diylhydrogenphosphate (BNP) as the analyte and by comparing the resolution values. Different parameters were examined, such as the alkyl group bulkiness and the configuration of the cation, the anion type of the CIL and its concentration, and the pH of the BGE, in order to optimize the separation of the enantiomers and to demonstrate the effect that each parameter has on the chiral‐recognition ability of the CIL. Baseline separation of BNP within 13 min was achieved by using a BGE of 100 mM Tris/10 mM sodium tetraboratedecahydrate (pH 8) and a chiral selector of 60 mM l ‐alanine tert butyl ester lactate. The run‐to‐run and batch‐to‐batch reproducibilities were also evaluated by computing the %RSD values of the EOF and the two enantiomer peaks. In both cases, very good reproducibilities were observed, since all %RSD values were below 1%.     

The resolution of α-amino acid by chiral polymer-copper complex

The (N-benzyl-l-leucinato) copper(II) complex was shown pH titration to coordinate l-amino acids more strongly than d enantiomers. A chiral polymer complex, containing N-alkylated amino acid residue and copper(II) ion, was used partially to resolve some optically active amino acids. Unlike the (N-benzyl-l-amino acidate)-copper(II) complex, the polymer—copper(II) complex coordinates d-amino acids more strongly than l-enantiomers; the effect was explained by formation of (N,N-dialkylated-amino acidate) copper(II) complex in the polymer.     

Separation of dansylated amino acid enantiomers by chiral ligand-exchange CE with a zinc(II) L-arginine complex as the selecting system

A facile chiral ligand-exchange capillary electrophoretic method has been explored for the enantioseparation and UV detection of dansyl-amino acids with Zn(II) L-arginine complex as a chiral selecting system. Successful enantioseparation of 17 pairs of amino acid enantiomers has been achieved with a buffer of 100 mM boric acid, 5 mM ammonium acetate, 3 mM ZnSO4 and 6 mM L-Arg at pH 8.0, of which 10 pairs were fully resolved with resolution in between 1.59 and 4.21. This new method was shown to be applicable to the separation of some mixed pairs of amino acids and to the quantitative analysis of some real samples such as rice vinegars, with a linear range between 0.8 and 150 microg/mL, correlation coefficient above 0.99 and recovery in between 90.1 and 112.4%. It was found that amino acids with low resistance side chain(s), low tendency to form intramolecular hydrogen bond or high tendency to form intermolecular hydrogen bonds are more easily enantioseparated than those with extra carboxyl and/or phenyl groups. By the use of the suggested buffer, the running pH should be selected at 7.4-9.0 to compromise the resolution and elution speed.     

Preparation of two new liquid chromatographic chiral stationary phases based on diastereomeric chiral crown ethers incorporating two different chiral units and their applications

Two new liquid chromatographic chiral stationary phases based on diastereomeric chiral crown ethers incorporating two different chiral units such as optically active 3,3'-diphenyl-1,1'-binaphthyl and tartaric acid unit were prepared. Between the two CSPs, one was much superior to the other especially in the resolution of tocainide and its analogues (for example, in the resolution of tocainide the separation factor, alpha, was 4.26 vs. 1.00 on the two CSPs). From these results, the two chiral units composing the two diastereomeric chiral crown ether moieties of the stationary phases were expected to show "matched" or "mismatched" effect on the chiral recognition according to their stereochemistry. The different chiral recognition abilities of the two CSPs were rationalized by the different three-dimensional structures of the two diastereomeric chiral crown ethers.     

A Systematic Study of the Use of DL-Tartaric Acid in the Synthesis of Silica Materials Obtained by the Sol-Gel Method

DL-tartaric acid was used as a template for the formation of silica nanotubes and spheres by the sol-gel method from tetraethylorthosilicate (TEOS) as silica source. The reactions were carried out in ethanol/water mixtures in the presence of aqueous ammonia, between 0°C and 75°C, using both stirred and non-stirred conditions. TEM and SEM images show that the yield and microstructure of the silica is influenced by the synthetic conditions (temperature, ammonia (aq) concentration, gelation time, solvent mixture). It was observed that the chiral form of the tartaric acid used and the diffusion of TEOS to the template determines the eventual silica structure.     

天然酒石酸在合成手性含膦配体中的应用

本文综述了以天然酒石酸为光活性原料的手性含膦配体合成, 对三十三种手性含膦配体进行归类,同时也简要地介绍了部分配体的Rh(I)配合物催化剂对(Z)-α-乙酰氨基肉桂的催化氢反应结果。     

Synthesis and characterization of novel chiral imidazolium and pyridinium ionic liquids derived from tartaric acid and 2-oxazolidinone

Abstract

Novel chiral imidazolium and pyridinium ionic liquids based on tartaric acid and 2-oxazolidinone were designed. Symmetrical dicationic ionic liquids based on tartaric acid have been synthesized and characterized. These chiral ionic liquids were designed by employing very short and simple methods. Incorporation of alkyl halide over tartaric acid and 2-oxazolidinone is an important step. N-methyl imidazole and pyridine were used for preparation of quaternary salts. These ionic liquids have been evaluated for the asymmetric sulfide oxidation. Chiral ionic liquids based on tartaric acid showed superior chiral inducing property as compare to 2-oxazolidinone based chiral ionic liquids.     

Reversed phase liquid chromatographic determination of organic acids using on-line complexation with copper(II) ion

We describe a simple and sensitive liquid chromatographic method for the analysis of organic acids using on-line complexation with copper(II) ion. Organic acids complexed with copper(II) ion were separated on a reversed-phase C18 column and detected by UV absorption at 240 nm. The copper(II) ion concentration in the mobile phase had a great influence on separation and sensitivity. A mobile phase consisting of 10 mM copper(II) sulfate in 5 mM sulfuric acid (pH 2.3) was used to separate nine organic acids (tartaric, malic, malonic, lactic, acetic, citric, maleic, succinic and fumaric acids). The detection limits of the examined organic acids calculated at S/N = 3 ranged from 0.6 to 100 μM. The detector signal was linear over three orders of magnitude of organic acid concentration. The method successfully measured organic acids in juice and vinegar samples.     

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.