HPLC enantioseparation of phenylalanine analogs by application of (S)-N-(4-nitrophenoxycarbonyl)phenylalanine methoxyethyl ester as a new chiral derivatizing agent

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.

A sensitive and simple ultra-high-performance-liquid chromatography-tandem mass spectrometry based method for the quantification of D-amino acids in body fluids

D-Amino acids are increasingly being recognized as important signaling molecules in mammals, including humans. D-Serine and D-aspartate are believed to act as signaling molecules in the central nervous system. Interestingly, several other D-amino acids also occur in human plasma, but very little is currently known regarding their function and origin. Abnormal levels of D-amino acids have been implicated in the pathogenesis of different diseases, including schizophrenia and amyotrophic lateral sclerosis (ALS), indicating that D-amino acid levels hold potential as diagnostic markers. Research into the biological functions of D-amino acids is hindered, however, by the lack of sufficiently sensitive, high-throughput analytical methods. In particular, the interference of large amounts of L-amino acids in biological samples and the low concentrations of D-amino acids are challenging. In this paper, we compared 7 different chiral derivatization agents for the analysis of D-amino acids and show that the chiral reagent (S)-NIFE offers outstanding performance in terms of sensitivity and enantioselectivity. An UPLC-MS/MS based method for the quantification of D-amino acids human biological fluids was then developed using (S)-NIFE. Baseline separation (R(s)>2.45) was achieved for the isomers of all 19 chiral proteinogenic amino acids. The limit of detection was <1 nM for all amino acids except d-alanine (1.98 nM), d-methionine (1.18 nM) and d-asparagine (5.15 nM). For measurements in human plasma, cerebrospinal fluid and urine, the accuracy ranged between 85% and 107%. The intra-assay and inter-assay were both <16% RSD for these three different matrices. Importantly, the method does not suffer from spontaneous racemization during sample preparation and derivatization. Using the described method, D-amino acid levels in human cerebrospinal fluid, plasma and urine were measured.

Chromatographic separation of enantiomers of non-protein alpha-amino acids after derivatization with Marfey's reagent and its four variants

Some non-protein alpha-amino acids were derivatized with 1-fluoro-2,4-dinitrophenyl-5-L-alaninamide (Marfey's reagent, MR, FDNP-L-Ala-NH(2),) and four of its structural variants FDNP-L-Phe-NH(2), FDNP-L-Val-NH(2), FDNP-L-Leu-NH(2) and FDNP-L-Pro-NH(2). The resultant diastereomers were separated by normal and reversed phase thin layer chromatography (TLC) and reversed phase HPLC. In normal phase TLC, best resolution was obtained with solvent combination of phenol-water (3:1) while in reversed phase TLC mixtures of acetonitrile with triethylammonium phosphate buffer were found successful for resolution of diastereomers. The separation behavior of diastereomers prepared with different reagents was compared. The diastereomers of most of the amino acids prepared with FDNP-L-Leu-NH(2) were best separated while those prepared with FDNP-L-Pro-NH(2) failed to separate in most of the cases. The diastereomers were also separated on a reversed phase C(8) column with gradient elution using mixture of aqueous-trifluoroacetic acid (TFA) and acetonitrile and with detection at 340 nm. The effects of TFA concentration, flow rate and run time on HPLC separation were studied.

Synthesis and antihyperglycemic evaluation of various protoberberine derivatives

Various berberine derivatives (2-17) were synthesized and their antihyperglycemic activities were evaluated in a model of beta-cell-membrane chromatography and a model of alloxan-induced diabetes mice. The results indicated that compounds 5 and 14 exhibited antihyperglycemic activity. Their structure-activity relationships were discussed.

Increasing UV detection sensitivity in the supercritical fluid chromatographic analysis of alcohol polyethers

Alcohol ethoxylates (AEOs) that contain a wide distribution of oligomers pose a challenge for ultraviolet (UV) absorbance detection due to the fact that the AEOs absorb strongly only in the range of commercial UV detectors between 190 and 200 nm. Most mobile phase components, with the exceptions of water and carbon dioxide, also absorb in this region. Ethoxylated hexadecanol and octadecanol were derivatized with disilazane-chlorosilane mixtures for the formation of phenyl containing silylethers. Derivatized samples were analyzed by supercritical fluid chromatography (SFC) coupled with both electrospray ionization mass spectrometry and UV absorbance detection. An increase in the number of phenyl groups incorporated into the derivatives increased the number of oligomers observed by UV detection. An increase in the number of oligomers detected increased the calculated average molar ethoxylate values. The average molar oligomer values calculated by SFC-UV for these alcohols were consistent with the nominal reported values.

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.

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

A new chiral derivatizing agent, (S)-N-(4-nitrophenoxycarbonyl)phenylalanine methoxyethyl ester, (S)-NIFE, was applied for the high-performance liquid chromatographic separation of enantiomers of 19 unnatural secondary amino acids: proline, pipecolic acid analogues, piperazine-2-carboxylic acid, morpholine-3-carboxylic acid, thiomorpholine-3-carboxylic acid and analogues containing the 1,2,3,4-tetrahydroisoquinoline, 1,2,3,4-tetrahydronorharmane, 1,2,3,4-tetrahydro-2-carboline and 2-benzazepine skeletons. Excellent resolutions were achieved for most of the investigated compounds by using a reversed-phase mobile phase system. The conditions of separation were optimized by variation of the mobile phase composition.