A new strategy for the selective determination of d-amino acids: Enzymatic and chemical modifications for pre-column derivatization

Application of gas-diffusion microextraction to solid samples using the chromatographic determination of α-diketones in bread as a case study

For the first time, gas-diffusion microextraction was used for the direct analysis of solid samples (vicinal diketones in bread).

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.

Enzymatic detection of D-amino acids

D: -Amino acids play several key roles and are widely diffused in living organisms, from bacteria (in which D-alanine is a component of the cell wall) to mammals (where D-serine is involved in glutamatergic neurotransmission in the central nervous system). The study of the biological processes involving D-amino acids and their use as clinical or biotechnological biomarkers requires reliable methods of quantifying them. Although "traditional" analytical techniques have been (and still are) employed for such tasks, enzymatic assays based on enzymes which possess a strict stereospecificity (i.e., that are only active on the D-enantiomers of amino acids) allowed the set-up of low-cost protocols with a high sensitivity and selectivity and suitable for determining the D-amino acid content of complex biological samples. The most exploited enzyme in these assays is D-amino acid oxidase, a flavoenzyme that exclusively oxidizes D-amino acids and possesses with a broad substrate specificity and a high kinetic efficiency.

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.

Simultaneous determination of D-amino acids by the coupling method of D-amino acid oxidase with high-performance liquid chromatography

An enzymatic assay system of D-amino acids was established using the D-amino acid oxidase of Schizosaccharomyces pombe. In this method, the enzyme converts the D-amino acids to the corresponding α-keto acids, which are then reacted with 1,2-diamino-4,5-methylenedioxybenzene (DMB) in an organic solvent. The resultant fluorescent compounds are separated and quantified by high-performance liquid chromatography (HPLC). Use of an organic solvent following the α-keto acid modification with DMB prevents the non-enzymatic deamination of L-amino acids, which are generally present at much higher concentrations than D-amino acids in biological samples. With this method, D-Glu, D-Asn, D-Gln, D-Ala, D-Val, D-Leu, D-Phe, and D-Ile can be quantified in the order of micromolar, and other D-amino acids except D-Asp can be assayed within a sensitivity range of 50-100 μM. The established enzymatic method was used to analyze the d-amino acid contents in human urine. The concentration of D-Ser obtained using this enzymatic method (223 μM) was in good agreement with that obtained using the conventional HPLC method (198 μM). The enzymatic method also demonstrated that the human urine contained 5.45 μM of d-Ala and 0.91 μM of D-Asn. Both D-amino acids were difficult to be identified using the conventional method, because the large signals from L-amino acids masked those from d-amino acids. The enzymatic method that we have developed can circumvent this problem.

Micro-magnetic particles frit for capillary electrochromatography

This paper presents a new method for making frit using soft-ferrite-based micro-magnetic particles (MMPs) in a micro-space, such as in a capillary tube. The MMPs-frit was made by injecting an aliquot of 10 microm (outer diameter; o.d.)-MMPs-suspension in methanol (ca. 1mg/ml) into a capillary tube (75 microm inner diameter (i.d.) x 375 microm o.d. x ca. 35 cm length) that was already sandwiched between a pair of cylindrical Neodium (Nd-Fe-B) magnets (1.5 mm o.d. x 1.5 mm height, 280 mT) at a position where the frit was made. The MMPs were trapped in the capillary tube as a frit due to the attraction of the magnets placed at surface on the capillary tube. With regard to durability, the frit was stable for methanol flow with a flow rate of 400 microl/min at room temperature. Using such a frit, a capillary column (20 cm long) was prepared by injecting a 5 microm (o.d.)-ODS-particle suspension in methanol (ca. 0.4 mg/microl) into the capillary tube. The MMPs-frits-ODS-packed column was stable for methanol for a flow pressure less than 20MPa. When comparing the present column with a conventional sintered-frits-ODS-packed column for the purposes of separating five kinds of biogenic amines by means of an on-column derivatization capillary electrochromatography (CEC), the performance of the MMPs-frits capillary column was almost equivalent to that of the sintered-frits-ODS-packed column.

Plasma methionine determination by capillary electrophoresis-UV assay: application on patients affected by retinal venous occlusive disease

Methionine is an important amino acid involved in protein synthesis and transmethylation reactions. It is also the precursor of homocysteine and cysteine, two important risk factors for cardiovascular diseases. As homocysteine research has gained impulsion, the evaluation of plasma methionine concentrations has acquired importance. Methionine measurement generally has been performed by HPLC after o-phthalaldehyde derivatization. Its separation from other amino acids is time-consuming. We set up a new specific capillary electrophoresis method in which analyte derivatization was avoided by sample concentration before analysis. Methionine was detected by UV absorbance at 204 nm with a detection limit of 0.5 micromol/L. By a capillary with an effective length of 50 cm filled with 125 mmol/L Tris phosphate buffer at pH 2.3, the separation occurred in less than 14 min. Precision tests indicated a good test repeatability for both migration times (coefficient of variation [CV]<0.3%) and areas (CV<2.0%). Moreover, a good reproducibility of intraassay and interassay tests was obtained (CV<2.9% and CV<3.5%, respectively). The Passing-Bablok regression and the Bland-Altman test for methods comparison suggest that the data obtained by our method and by a reference HPLC assay are similar. Assay performance was evaluated measuring methionine concentrations in retinal venous occlusive disease.

Assay of Trace d-Amino Acids in Neural Tissue Samples by Capillary Liquid Chromatography/Tandem Mass Spectrometry

A sensitive chiral capillary HPLC-MS/MS method well suited for the determination of amino acid enantiomers in biological samples was developed. The method involved precolumn derivatization of the sample with 7-fluoro-4-nitrobenzoxadiazole (NBD-F). After derivatization, NBD-amino acids were stacked on a C18 reversed-phase extraction microcolumn, thus enriching and cleaning up the analytes. Various chiral stationary phases (CSPs) including cyclodextrin-bonded silica, Pirkle-type, vancomycin, and teicoplanin-bonded silica particles were evaluated for resolving NBD-F tagged amino acid enantiomers with mobile phases compatible with MS detection. It was found that only teicoplanin aglycon CSP provided sufficient resolution of NBD-Asp and NBD-Ser enantiomers to quantify trace levels of D-Asp and D-Ser in tissue samples. MS/MS detection of NBD-amino acid derivatives was very sensitive and selective. The high selectivity allowed the use of a stable isotope-labeled analyte analogue (i.e., L-aspartic acid-2,3,3-d3) as internal standard for the quantitation to improve assay reproducibility and reliability. Neural tissue samples dissected from rat brain and the central nervous system (CNS) of Aplysia californica, a widely used neuronal model, were analyzed to determine the chirality of glutamic acid (Glu), aspartic acid (Asp), and serine (Ser). The former two are major excitatory amino acids in the brain, and the last one has been recently identified as a neuromodulator. Both D-Ser and D-Asp were detected in rat brain. While the D-Asp level decreased rapidly through the developmental stages of the rat, the D-Ser level increased steadily from 82.3 microg/g of wet tissue in 3-day prenatal rats to 241.3 microg/g of wet tissue in 90-day-old rats. Interestingly, no D-Ser was detected in the CNS of Aplysia, a "primitive" invertebrate. However, the D-Asp level in this animal was found to be high. In a particular connective nerve sample, D-Asp was at 323.2 microg/g of wet tissue and constituted 60.2% of total Asp. D-Glu was not detected either in rat brain or in Aplysia's CNS.