Chiral recognition of amino acids and dipeptides by a water-soluble zinc porphyrin

A chiral water-soluble zinc porphyrin was optically resolved on a chiral HPLC column, and the binding of chiral amino acids and peptides to each of the enantiomers was examined spectrophotometrically in basic aqueous solution. The binding data apparently indicated that the zinc porphyrin has chiral selectivity for amino acids and dipeptides. This was reasonably explained in terms of the triple cooperation of coordination, Coulomb, and steric interactions of the chiral amino carboxylates with the porphyrin. A compensatory relationship among the thermodynamic parameters for chiral recognition was also shown.     

Degradingdehydroabietylisothiocyanate as a chiral derivatizing reagent for enantiomeric separations by capillary electrophoresis

Abietic acid is a naturally occurring enantiomeric diterpenic acid. Its absolute optical purity and very stable stereochemistry structure makes it an excellent starting material for preparing chiral derivatizing reagents for chromatographic or electrophoretic applications. This paper describes the synthesis and evaluation of a novel chiral derivatization reagent, i.e., degradingdehydroabietylisothiocyanate (DDHAIC) derived from dehydroabietic acid. Its applicability for the enantioseparation of racemic amino acids by CE was demonstrated. DDHAIC reacted readily with amino acids at an elevated temperature (70 degrees C). The resulting derivatives were highly stable and separable by MEKC. Separation of amino acid-DDHAIC diastereomers was achieved with a running buffer consisting of 50 mM Na(2)HPO(4) (pH 9.0), 18 mM SDS, and 25% v/v ACN. Under the conditions selected, diastereomers formed from ten pairs of tested amino acid enantiomers including D/L-Asn, D/L-Met, D/L-Leu, D/L-Phe, D/L-Trp, D/L-Ser, D/L-Val, D/L-Ala, D/L-Thr, and R/S-vigabatrin were well resolved. The resolution values were in the range of 0.95-8.9.     

3-[(3-Dehydroabietamidopropyl)dimethylammonio]-1-propane-sulfonate as a new type of chiral surfactant for enantiomer separation in micellar electrokinetic chromatography

A new type of chiral surfactant, 3-[(3-dehydroabietamidopropyl) dimethylammonio]-1-propanesulfonate (DHAMAP) has been synthesized. The ability of this compound to perform chiral separation of D/L-amino acids derivatized with naphthalene-2, 3-dicarboxaldehyde (NDA-D/L-amino acids) has been investigated by capillary electrophoresis (CE). Enantiomeric separation of NDA-D/L-amino acids was achieved with a running buffer consisting of 50 mM borate (pH 9.75) and 25 mM DHADMP. Under the conditions selected, 6 pairs of tested amino acids enantiomers including NDA-D/L-tryptophan, NDA-D/L-phenylalanine, D/L-D/L-kynurenine, NDA-D/L-beta-phenylalanine, NDA-D/L-4-methylphenylalanine and NDA-D/L-arginine were well resolved. The resolution values were in the range of 1.56-5.40.     

Comparison of enantiomer separation on two chiral stationary phases derived from (+)-18-crown-6-2,3,11,12-tetracarboxylic acid of the same chiral selector

Liquid chromatographic comparisons for enantiomer resolution of α-amino acids and chiral primary amino compounds were made using chiral stationary phases (CSPs) prepared by covalently bonding (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18-C-6-TA) of the same chiral selector. The resolution of all α-amino acids on CSP 1 developed in our group was found to be better than that on CSP 2 reported by Machida et al. All α-amino acids examined in this study were well enantioseparated on CSP 1 (α=1.22–2.47), while four analytes were not resolved or all the other analytes were poorly resolved on CSP 2 than on CSP 1. However, in resolving the primary amino compounds without a carbonyl group, CSP 1 was comparable with CSP 2. Although (+)-18-C-6-TA of the same chiral selector was used to prepare CSP 1 and CSP 2, this study showed that different connecting methods for the CSPs might influence their ability to resolve the analytes depending on their structures related to the chiral recognition mechanism.     

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.     

Highly Stereoselective Recognition and Deracemization of Amino Acids by Supramolecular Self‐Assembly

The highly stereoselective supramolecular self‐assembly of α‐amino acids with a chiral aldehyde derived from binol and a chiral guanidine derived from diphenylethylenediamine (dpen) to form the imino acid salt is reported. This system can be used to cleanly convert D ‐amino acids into L ‐amino acids or vice versa at ambient temperature. It can also be used to synthesize α‐deuterated D ‐ or L ‐amino acids. A crystal structure of the ternary complex together with DFT computation provided detailed insight into the origin of the stereoselective recognition of amino acids.     

Chiral separation of fluorescamine-labeled amino acids using microfabricated capillary electrophoresis devices for extraterrestrial exploration

Chiral separations of fluorescamine-labeled amino acids are characterized and optimized on a microfabricated capillary electrophoresis (CE) device. A standard mixture of acidic and neutral amino acids is labeled with fluorescamine in less than 5 min and the hydroxypropyl-beta-cyclodextrin (HPbetaCD) concentration, temperature, and pH are optimized (15 mM HPbetaCD, 6 degrees C, pH < 9) to achieve high-quality and low background chiral separations in less than 200 s. All four stereoisomers formed in the labeling reaction of the chiral dye with the chiral amino acids are typically resolved. At pH > 9, isomerization of the dye chiral center is observed that occurs on the time scale of the chip separation. Typical limits of detection are approximately 50 nM. These results demonstrate the feasibility of combining fluorescamine labeling of amino acids with microfabricated CE devices to develop low-volume, high-sensitivity apparatus and methods for extraterrestrial exploration.     

L-α-三氟乙酰基氧基丙酰基氯对外消旋胺及氨基酸的气相色谱拆分

L-乳酸与三氟乙酸酐反应, 生成L-α-三氟乙酰氧基乳酸, 再与二氯亚砜作用, 合成新的手性试剂----L-α-三氟乙酰氧基丙酰氯. 它与DL-α-苯乙胺及三种DL-α-氨基酸反应, 生成相应的非对映异构体酰胺, 在以Carbowax为固定相的毛细管柱上进行气相色谱拆分. 以相应的L-胺及L-氨基酸在相同条件下进行比较, 发现D-异构体的保留时间较短 .     

Capillary electrophoresis and column chromatography in biomedical chiral amino acid analysis

Free amino acids are typically quantified as the sum of their enantiomers, because in terrestrial organisms they mainly exist in the left-handed form. However, with increasing understanding of the biological significance of right-handed amino acids interest in enantioselective quantification of amino acids has steadily increased. Initially, electrophoretic and chromatographic methods using chiral (pseudo)-stationary phases or chiral eluents were applied to the separation of amino acid enantiomers. Later, derivatization of amino acids prior to chromatography with chiral reagents gained in popularity, because the diastereomers formed can be resolved on conventional reversed-phase columns. Novel multi-interaction chiral columns turned attention back to direct chiral chromatographic methods. Hyphenation to mass spectrometry has increasingly replaced optical detection because of superior selectivity, although this has not obviated the need for baseline resolution of amino acid enantiomers. Despite the progress made, enantioselective separation and quantification of amino acids remains an analytical challenge owing to frequently incomplete resolution of all naturally occurring enantiomers and insufficient sensitivity for the determination of the trace amounts of d-amino acids typically found in biological fluids and tissues. Chiral GC-MS analysis of heptafluorobutanol/pentafluoropropionanhydride amino acid derivatives on an Rt-gDEXsa column     

2D separations on a 1D chip: gradient elution moving boundary electrophoresis-chiral capillary zone electrophoresis

A new method is described for two-dimensional (2D) separations using a microfluidic chip normally employed for single dimension electrophoresis. The method employs a combination of gradient elution moving boundary electrophoresis (GEMBE) and chiral capillary zone electrophoresis (CZE). The simplicity of the first dimension GEMBE method enables its implementation in the injection channel of a conventional electrophoresis chip, simplifying the design and operation of the device. The method was used for high resolution 2D chiral separations of a mixture of amino acids considered as possible signatures of extant or extinct life for solar system exploration. The enantiomers of aspartic acid, glutamic acid, serine, alanine, and valine were all resolved as well as glycine (achiral) and several unidentified impurities, giving an estimated peak capacity of 35 for the region between valine and glycine. The results highlight the need for high peak capacity separations for chiral amino acid analysis if accurate enantiomeric ratios are to be determined.     

Low-molecular-weight chiral cation exchangers: novel chiral stationary phases and their application for enantioseparation of chiral bases by nonaqueous capillary electrochromatography

Cation exchange type chiral stationary phases (CSPs) based on 3,5-dichlorobenzoyl amino acid and amino phosphonic acid derivatives as chiral selectors (SOs) and silica as chromatographic support were developed and applied to enantiomer separations of chiral bases by nonaqueous capillary electrochromatography (NA-CEC). As a rationale for efficient CSP development we adopted the combined use of the "reciprocity principle of chiral recognition" and nonaqueous ion-pair CE as screening assay. Thus, (S)-atenolol was employed as chiral counter-ion added to the BGE in CE and a series of N-derivatized amino acids and amino phosphonic acids were screened to derive reciprocally information on their chiral recognition abilities for atenolol enantiomers. Two SO candidates, namely N-(3,5-dichlorobenzoyl)-O-allyl-tyrosine and N-(4-allyloxy-3,5-dichlorobenzoyl)-1-amino-3-methylbutane phosphonic acid that have been identified as potential SOs in the CE screening were, after immobilization on thiol-modified silica, evaluated in cation-exchange NA-CEC. The strong chiral cation exchanger with the free phosphonic acid group exhibited enhanced enantioselectivity compared to the weak chiral cation exchanger with the carboxylic acid group. A wide variety of chiral bases could be successfully resolved on the strong chiral cation exchanger with alpha-values up to 2.2 and efficiencies up to 375000 m-1 including beta-blockers and other amino alcohols, local anesthetics like etidocaine, antimalarial agents like mefloquine, Tr?ger's base, phenothiazines like promethazine, and antihistaminics. The influence of several experimental parameters (electrolyte concentration, acid-base ratio and acetonitrile-methanol ratio) was evaluated.     

Amino acid recognition of pyridine bis(oxazoline)-copper(II) complex in aqueous solvent

Enantioselective recognition of amino acids has been studied with C₂-symmetric chiral pyridine bis(oxazoline)-copper(II) complexes at physiological pH condition. UV-visible titration revealed strong binding of submillimolar dissociation constant between pyridine bis(oxazoline)-copper(II) complex and amino acids in aqueous solution. Moderate selectivity of up to 2:1 between d- and l-amino acids was achieved. The enantiomers were baseline resolved by capillary electrophoresis, using the bis(l-lysine)-copper(II) complex as a chiral selector.     

Stereoselective analysis of D and L dansyl amino acids as the mixed chelate copper(II) complexes by HPLC

This paper reviews the mixed chelation approach to resolution of the optical isomers of D and L dansyl amino acids by high performance liquid chromatography. The use of eluants containing Cu(II) complexes of L-proline, L-arginine, L-histidine, and L-histidine methyl ester effected the separation of many D and L amino acids, including those with aliphatic, polar, and aromatic substituents. The mechanism of separation, which is based on the preferential ternary complex formation of the analyte amino acid and the chiral chelate with Cu(II) in the mobile phase, is discussed. The stereoselectivity depends mainly on the different steric interactions between the alkyl side chains of the amino acid analytes and the chiral ligands coordinating around Cu(II), although such parameters as pH, temperature, organic modifier, and concentration of the chiral additive also affect the chromatographic separation. Among the chiral ligands studied, L-histidine methyl ester is unique in that it possesses both achiral selectivity for the dansyl amino acids and chiral selectivity for the respective D and L enantiomers. With a mobile phase gradient of acetonitrile in a buffer containing Cu(II) L-histidine methyl ester complex, a stereoselective procedure was devised for the analysis of D and L amino acid enantiomers, achieving the separation that the current amino acid analyzer could not perform. Finally, the use of the mixed chelation approach in two biomedical studies is described. In the first application, the histidine methyl ester gradient was adapted for analyzing amino acids in cerebrospinal fluid; in the second, an L-aspartame Cu(II) complex eluant was developed for measuring the urine concentration of D and L pipecolic acid (piperidine-2-carboxylic acid), a metabolite of lysine.     

Liquid chromatographic resolution of proline and pipecolic acid derivatives on chiral stationary phases based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid

Two liquid chromatographic chiral stationary phases based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid were applied to the resolution of the amide derivatives of cyclic α‐amino acids including proline and pipecolic acid. Among the five amide derivatives of proline, aniline amide was resolved best on the first chiral stationary phase, which contains two N–H tethering amide groups, with the separation factor of 1.31 and the resolution of 2.60, and on the second chiral stationary phase, which contains two N–CH₃ tethering amide groups, with the separation factor of 1.57 and the resolution of 5.50. Among the five amide derivatives of pipecolic acid, 2‐naphthyl amide was resolved best on the first chiral stationary phase with the separation factor of 1.30 and the resolution of 1.75, but 1‐naphthylmethyl amide was resolved best on the second chiral stationary phase with the separation factor of 1.30 and the resolution of 2.26. In general, the second chiral stationary phase was found to be better than the first chiral stationary phase in the resolution of the amide derivatives of cyclic α‐amino acids. In this study, the second chiral stationary phase was first demonstrated to be useful for the resolution of secondary amino compounds.     

HPLC separation of amino acid enantiomers and small peptides on macrocyclic antibiotic-based chiral stationary phases: a review

The search for new and effective chiral selectors capable of separating a wide variety of enantiomeric compounds is an ongoing process. In the past decade, macrocyclic antibiotics have proved to be an exceptionally useful class of chiral selectors for the separation of enantiomers of biological and pharmacological importance by means of HPLC, TLC and electrophoresis. More chiral analytes have been resolved through the use of glycopeptides than with all the other macrocyclic antibiotics combined (ansamycins, thiostrepton, aminoglycosides, etc.). The glycopeptides avoparcin, teicoplanin, ristocetin A and vancomycin have been extensively used as chiral selectors in the form of chiral bonded phases in HPLC, and HPLC stationary phases based on these glycopeptides have been commercialized. Teicoplanin, vancomycin, their analogs and ristocetin A seem to be the most useful glycopeptide HPLC bonded phases for the enantioseparation of proteins and unusal native and derivatized amino acids. In fact, the macrocyclic glycopeptides are to some extent complementary to one another: where partial enantioresolution is obtained with one glycopeptide, there is a high probability that baseline or better separation can be obtained with another. This review sets out to characterize the physicochemical properties of these antibiotics and their application in the enantioseparations of amino acids. The mechanism of separation, the sequence of elution of the stereoisomers and the relation to the absolute configuration are also discussed.     

Enantiomeric recognition of amino acids by amphiphilic crown ethers in Langmuir monolayers

Four new chiral, amphiphilic crown ethers differing by the hydrophobic tailgroups were synthesized, and their capacity to recognize enantiomeric amino acids was examined using Langmuir films. Surface pressure and surface potential measurements performed on the subphases containing L or D enantiomers of alanine, valine, phenylglycine, and tryptophane indicate that the crown ethers forming the monolayer interact with the amino acids. The effects observed are ascribed to the formation of host-guest complexes. The differences in the magnitude of the effects measured show that the crown ethers are capable of discriminating between different amino acids as well as the enantiomers. Our results demonstrate that the structure of the monolayer plays a decisive role in the molecular recognition process including chiral recognition.     

Chiral separation of halogenated amino acids by ligand-exchange capillary electrophoresis

The chiral separation of halogenated amino acids by ligand-exchange CE is described. Halogenated amino acids attracted increasing interest in recent years because of their physiological activities. Different chiral selectors, as there are L-4-hydroxyproline, L-histidine, and N-alkyl derivatives of L-4-hydroxyproline in form of their copper(II) complexes, are compared for their chiral recognition ability for halogenated amino acids. The influence of various parameters, such as selector concentration, pH, organic modifier, and field strength, on the resolution was investigated. All halogenated amino acids investigated were baseline-separated under optimized conditions.     

Amino acids as chiral selectors in enantioresolution by liquid chromatography

Amino acids are unique in terms of their structural features and multidimensional uses. With their simple structures and the ready availability of both enantiomers, amino acids not only serve as a chiral pool for synthesis but also provide an inexpensive pool for resolution studies. There has been no attempt to review the application of amino acids as chiral selectors for chromatographic enantioresolution of pharmaceuticals and other compounds. The present paper deals with application of l-amino acids and complexes of l-amino acids with a metal ion, particularly Cu(II), as an impregnating reagent in thin-layer chromatography or as a chiral ligand exchange reagent or a chiral mobile phase additive in both thin-layer chromatography and high-performance liquid chromatography. Enantiomeric resolution of β-blockers, nonsteroidal anti-inflammatories, amino acids (and their derivatives) and certain other compounds is discussed.     

Zwitterionic chiral stationary phases based on cinchona and chiral sulfonic acids for the direct stereoselective separation of amino acids and other amphoteric compounds

An extensive series of free amino acids and analogs were directly resolved into enantiomers (and stereoisomers where appropriate) by HPLC on zwitterionic chiral stationary phases (Chiralpak ZWIX(+) and Chiralpak ZWIX(?)). The interaction and chiral recognition mechanisms were based on the synergistic double ion‐paring process between the analyte and the chiral selectors. The chiral separation and elution order were found to be predictable for primary α‐amino acids with apolar aliphatic side chains. A systematic investigation was undertaken to gain an insight into the influence of the structural features on the enantiorecognition. The presence of polar and/or aromatic groups in the analyte structure is believed to tune the double ion‐paring equilibrium by the involvement of the secondary interaction forces such as hydrogen bonding, Van der Waals forces and π–π stacking in concert with steric parameters. The ZWIX chiral columns were able to separate enantiomers and stereoisomers of various amphoteric compounds with no need for precolumn derivatization. Column switching between ZWIX(+) and ZWIX(?) is believed to be an instrumental tool to reverse or control the enantiomers elution order, due to the complementarity of the applied chiral selectors.     

Determination of absolute configurations of amino acids by asymmetric autocatalysis of 2-alkynylpyrimidyl alkanol as a chiral sensor

Asymmetric autocatalysis of 2-alkynyl-5-pyrimidyl alkanol is employed as a chiral sensor of 20 amino acids. Asymmetric autocatalysis using amino acids as chiral initiators gave pyrimidyl alkanols of the absolute configurations that were correlated with those of the amino acids. The enantiomeric excesses of pyrimidyl alkanol are invariably high even when the enantiomeric excess of amino acids is as low as 0.1%. Thus, by determining the absolute configuration of pyrimidyl alkanol with high enantiomeric excess, one can determine the absolute configuration of amino acids even when their enantiomeric excess is low.