Application of a new chiral derivatizing agent to the enantioseparation of secondary amino acids

Chiral secondary amino acids, their importance, and methods of analysis

Naturally occurring secondary amino acids, with proline as the main representative, contain an alpha-imino group in a cycle that is typically four-, five-, and six-membered. The unique ring structure exhibits exceptional properties-conformational rigidity, chemical stability, and specific roles in protein structure and folding. Many proline analogues have been used as valuable compounds for the study of metabolism of both prokaryotic and eukaryotic cells and for the synthesis of compounds with desired biological, pharmaceutical, or industrial properties. The D-forms of secondary amino acids play different roles in living organisms than the L-forms. They have different metabolic pathways, biological, physiological, and pharmacological effects, they can be indicators of changes and also serve as biomarkers of diseases. In the scientific literature, the number of articles examining D-amino acids in biological samples is increasing. The review summarises information on the occurrence and importance of D- and L-secondary amino acids-azetidic acid, proline, hydroxyprolines, pipecolic, nipecotic, hydroxypipecolic acids and related peptides containing these D-AAs, as well as the main analytical methods (mostly chromatographic) used for their enantiomeric determination in different matrices (biological samples, plants, food, water, and soil).

Chiral derivatizations applied for the separation of unusual amino acid enantiomers by liquid chromatography and related techniques

Amino acids are essential for life, and have many functions in metabolism. One particularly important function is to serve as the building blocks of peptides and proteins, giving rise complex three dimensional structures through disulfide bonds or crosslinked amino acids. Peptides are frequently cyclic and contain proteinogenic as well as nonproteinogenic amino acids in many instances. Since most of the proteinogenic α-amino acids contain at least one stereogenic center (with the exception of glycine), the stereoisomers of all these amino acids and the peptides in which they are to be found may possess differences in biological activity in living systems. The impetus for advances in chiral separation has been highest in the past 25 years and this still continues to be an area of high focus. The important analytical task of the separation of isomers is achieved mainly by chromatographic and electrophoretic methods. This paper reviews indirect separation approaches, i.e. derivatization reactions aimed at creating the basis for the chromatographic resolution of biologically and pharmaceutically important enantiomers of unusual amino acids and related compounds, with emphasis on the literature published from 1980s. The main aspects of the chiral derivatization of amino acids are discussed, i.e. derivatization on the amino group, transforming the molecules into covalently bonded diastereomeric derivatives through the use of homochiral derivatizing agents. The diastereomers formed (amides, urethanes, urea and thiourea derivatives, etc.) can be separated on achiral stationary phases. The applications are considered, and in some cases different derivatizing agents for the resolution of complex mixtures of proteinogenic d,l-amino acids, non-proteinogenic amino acids and peptides/amino acids from peptide syntheses or microorganisms are compared.

Stereoselective Synthesis of Quaternary Proline Analogues

This review describes available methods for the diastereoselective and asymmetric synthesis of quaternary prolines. The focus is on the preparation of alpha-functionalized prolines with the pyrrolidine moiety not embedded in a polycyclic frame. The diverse synthetic approaches are classified according to the bond which is formed to complete the quaternary skeleton.

High-performance liquid chromatographic enantioseparation of unusual secondary amino acids on a D-penicillamine-based chiral ligand exchange column

The application of a chiral ligand-exchange column (CLEC) for the direct high-performance liquid chromatographic enantioseparation of unusual secondary amino acids using D-penicillamine-Cu(II) complex as chiral selector is reported. The amino acids investigated were pyrrolidine-2-carboxylic acid, piperidine-2-carboxylic acid, piperazine-2-carboxylic acid, morpholine-3-carboxylic acid, and thiomorpholine-3-carboxylic acid analogs. Chromatographic results are given as the retention, separation, and resolution factors. The chromatographic conditions were varied to achieve optimal separation. The elution sequence of the enantiomers was determined and in most cases the S isomer eluted before R.

New Amperometric Biosensors Based on Diamond Paste for the Assay ofL‐ andD‐Pipecolic Acids in Serum Samples

Monocrystalline natural diamond, L-amino acid oxidase (L-AAOD), D-amino acid oxidase (D-AAOD), and paraffin oil were used for the design of the modified diamond paste. The technique used for the direct voltammetric assay was differential pulse voltammetry (DPV) with applied potential pulse amplitude of 25 mV vs. Ag/AgCl. Using the new amperometric biosensors L-pipecolic acid (L-PA) and D-pipecolic acid (D-PA) were determined reliably from serum samples at 700 and 200 mV vs. Ag/AgCl, respectively, with low limits of detection.

Chirality determination of unusual amino acids using precolumn derivatization and liquid chromatography-electrospray ionization mass spectrometry

Unusual amino acids such as beta-methoxytyrosine (beta-MeOTyr), allo-threonine (allo-Thr) and allo-isoleucine (allo-Ile) were derivatized with N-alpha-(2,4-dinitro-5-fluorophenyl)-L-alaninamide (FDAA), 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate (GITC), (S)-N-(4-nitrophenoxycarbonyl)phenylalanine methoxyethyl ester (S-NIFE), or o-phthalaldehyde/isobutyryl-L-cysteine (OPA-IBLC), and then separated via reversed-phase high-performance chromatography followed by UV and electrospray ionization mass spectrometry detection. FDAA generally showed the highest enantioselectivity but the lowest sensitivity among the chiral derivatizing agents (CDAs) investigated. The detection limit of FDAA-derivatized amino acids was in the low picomolar range. Although the enantioselectivity of FDAA derivatives was generally quite high, its selectivity among beta-MeOTyr isomers was poor. The best separation of beta-MeOTyr stereoisomers was achieved with S-NIFE. Due to the complex relationships between the investigated CDAs, stereochemical analyses using a combination of two or more of the CDAs gave the most reliable results for a given separation problem. In general, the methods described are selective and reliable, and are being applied to the analysis of unusual amino acids as they occur in marine peptides.

Direct and indirect high-performance liquid chromatographic enantioseparation of β-amino acids

Direct and indirect reversed-phase (RP) high-performance liquid chromatographic methods were developed for the separation of enantiomers of 18 unnatural beta-amino acids, including several beta-3-homo amino acids. The direct separations of the underivatized analytes were performed on chiral stationary phases (CSPs) containing macrocyclic glycopeptide antibiotic teicoplanin (Chirobiotic T column) and teicoplanin aglycone (Chirobiotic TAG column). The indirect method involved pre-column derivatization with a new chiral derivatizing agent (CDA), (S)-N-(4-nitrophenoxycarbonyl)phenylalanine methoxyethyl ester ((S)-NIFE), and subsequent separation of diastereomers on Discovery C18 and Hyperpep 300 C18 columns. The different methods were compared in systematic chromatographic examinations. The effects of organic modifier, mobile phase composition, pH and flow rate on the separation were investigated.

Indirect high-performance liquid chromatographic separation of stereoisomers of beta-alkyl-substituted amino acids by the application of (S)-N-(4-nitrophenoxycarbonyl)phenylalanine methoxyethyl ester as chiral derivatizing agent

The indirect high-performance liquid chromatographic enantioresolution of beta-alkyl-substituted analogues of tyrosine, phenylalanine, 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid and tryptophan is reported. (S)-N-(4-Nitrophenoxycarbonyl)phenylalanine methoxyethyl ester, a recently developed chiral derivatizing agent, was used for pre-column derivatization of the investigated analytes. The diastereoisomers formed were analysed under reversed-phase conditions. The effects of parameters such as the amount and type of the organic modifier and the type of the stationary phase on the resolution and retention of the derivatives were investigated. Chromatographic conditions were found for the separation of all four stereoisomers of each analyte.