Novel high-throughput and widely-targeted liquid chromatography–time of flight mass spectrometry method for d-amino acids in foods

Development of a three-dimensional HPLC system for the determination of serine, threonine and allo-threonine enantiomers in the plasma of patients with chronic kidney disease

A highly-selective three-dimensional high-performance liquid chromatographic (3D-HPLC) system was developed for the determination of serine (Ser), threonine (Thr) and allo-threonine (aThr) enantiomers in human plasma to screen the new biomarker of chronic kidney disease (CKD). d-Ser has been reported to be the candidate biomarker of CKD, however, multiple biomarkers are still required. Therefore, Ser analogs of hydroxy amino acids are the focus in the present study. For the sensitive analysis, the amino acids were derivatized with 4-fluoro-7-nitro-2,1,3-benzoxadiazole and detected by their fluorescence. The 3D-HPLC system consisted of a reversed-phase column (Singularity RP18, 1.0 × 250 mm), an anion-exchange column (Singularity AX, 1.0 × 150 mm) and a Pirkle-type chiral stationary phase (Singularity CSP-013S, 1.5 × 250 mm). The developed method was validated and applied to the human plasma samples obtained from 15 healthy volunteers and 165 CKD patients. The concentrations of the d-forms were 1.13-2.26 (Ser), 0.01-0.03 (Thr) and 0.04-0.10 μM (aThr) for the healthy volunteers and 0.95-19.0 (Ser), 0-0.57 (Thr) and 0.04-1.02 μM (aThr) for the CKD patients. The concentrations and the %d values of all the target d-amino acids were increased along with the decreasing of renal function and further investigation for clinical applications are expected.

Simultaneous separation and identification of all structural isomers and enantiomers of aminobutyric acid using a highly sensitive chiral resolution labeling reagent

Aminobutyric acid has structural isomers (α-, β-, and γ-aminobutyric acids) and enantiomers (D/L-forms) with various unique functions. Therefore, a quantitative method for determining the content of each aminobutyric acid must be developed. In general, quantitative simultaneous analysis of multiple compounds is conducted via high-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS). However, simultaneous separation and highly sensitive detection of all aminobutyric acids are complicated, so highly sensitive analytical methods for the separation and identification of each compound have not yet been established. We previously developed highly sensitive chiral resolution labeling reagents. Herein, we propose a highly sensitive analytical method for the simultaneous separation and identification of all aminobutyric acids via LC-MS and labeling with our original highly sensitive chiral resolution labeling reagent, 1-fluoro-2,4-dinitrophenyl-5-L-valine-N,N-dimethylethylenediamine amide (L-FDVDA). The labeling reagent was completely bound to all aminobutyric acids through incubation overnight (>15 h) at 50 °C. Additionally, the labeled aminobutyric acids could be stored for at least 1 week at 4 °C. Furthermore, we demonstrated simultaneous separation and identification of aminobutyric acids in biological samples and foods through LC-MS using a C18 column after labeling with L-FDVDA. Our method is expected to be adopted for the analysis of the contents of all aminobutyric acids in biological and clinical samples as well as various foods.

Separation and Identification of Isoleucine Enantiomers and Diastereomers Using an Original Chiral Resolution Labeling Reagent

D-Amino acids, which are present in small amounts in living organisms, are responsible for a variety of physiological functions. Some bioactive/biomolecular peptides also contain D-amino acids in their sequences; such peptides express different functions than peptides composed only of L-form amino acids. Among the 20 amino acids that make up proteins, threonine (Thr) and isoleucine (Ile) have two chiral carbons and thus have two enantiomers and diastereomers. These stereoisomers have been previously analyzed through HPLC using chiral columns or chiral resolution labeling reagents. However, the separation and identification of these stereoisomers are highly laborious and complicated. Herein, we propose an analytical method for the separation and identification of Ile stereoisomers through LC-MS using our original chiral resolution labeling reagent, 1-fluoro-2,4-dinitrophenyl-5-L-valine-N,N-dimethylethylenediamine-amide (L-FDVDA) and a PBr column packed with pentabromobenzyl-modified silica gel. Twenty DL-amino acids including Thr stereoisomers (41 amino acids including glycine) were separated and identified using C18 column. Ile stereoisomers could be separated using not a C18 column but a PBr column. Additionally, we showed that peptides containing Thr and Ile stereoisomers can be accurately detected through labeling with L-FDVDA.

Enantiomeric purity analysis of synthetic peptide therapeutics by direct chiral high-performance liquid chromatography-electrospray ionization tandem mass spectrometry

The determination of chiral purity is critical to the evaluation of the quality of peptide pharmaceutical products. For synthetic peptides, the undesirable d-isomers can be introduced as impurities in amino acid starting materials and can also be formed during peptide synthesis and in some cases during product shelf life. A chiral high-performance liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS) method is described that facilitates rapid and accurate determination of amino acid chiral purity of a peptide. The peptide is hydrolyzed in deuterated acid to facilitate correction for any racemization occurring during this step of sample preparation, and the amino acids are subsequently separated by chiral chromatography interfaced with ESI-MS/MS for quantitation. The amino acid samples are analyzed directly following hydrolysis using high-low chromatography and extraction of selected ion response, providing efficiency and simplicity by avoiding the derivatization steps and multiple external standards required by traditional methodologies. GMP method validation feasibility is described for all nineteen chiral proteogenic amino acids. The practical application of the chiral HPLC-ESI-MS/MS method was demonstrated through the recovery of d-amino acid substitutions at each residue of an octapeptide across the 0.1-1.0 % range of interest. The method was applied to the analysis of four model peptides, each consisting of 8-14 amino acid residues, and the results were comparable to those provided by traditional testing methods.

Determination of d- and l-Amino Acids in Garlic Foodstuffs by Liquid Chromatography-Tandem Mass Spectrometry

Black garlic is currently attracting interest as a health food and constituent of commercial supplements; however, no data regarding the d-amino acids within black garlic have been reported. Therefore, the amino acid compositions of methanol extracts from fresh and black garlic were compared herein. We investigated the contents of the d- and l-forms of amino acids in commercial fresh, black, and freeze-dried garlic foodstuffs by liquid chromatography-tandem mass spectrometry (LC-MS/MS) using a pre-column chiral derivatization reagent, succinimidyl 2-(3-((benzyloxy)carbonyl)-1-methyl-5-oxoimidazolidin-4-yl) acetate. Several d-amino acids, namely, the d-forms of Asn, Ala, Ser, Thr, Glu, Asp, Pro, Arg, Phe, Orn, Lys, and Tyr, were observed in the methanol extract of black garlic, whereas only d-Ala was detected in that of fresh garlic foodstuffs. These data suggest that several d-amino acids can be produced during fermentation for preparing black garlic.

Enantiodiscriminating Lipophilic Liquid Membrane-Based Assay for High-Throughput Nanomolar Enantioenrichment of Chiral Building Blocks

The reported optical resolution method was designed to support high-throughput enantioseparation of molecular building blocks obtained by automated small-scale synthetic methods. Lipophilic esters of common resolving agents were prepared and used as liquid membranes on the indifferent polymer surface of a microtiter assay. Chiral model compounds were enriched in one of the enantiomers starting from the aqueous solutions of their racemic mixture. Enantiodiscrimination was provided by forming diastereomeric coordination complexes of lipophilic enantiopure esters with the enantiomers of the chiral building blocks inside the liquid membranes. This enantiomeric recognition resulted in a greater distribution ratio of the preferred isomer in the membrane phase, thus the process enables a simultaneous enantioenrichment of the solutions outside the membrane. This paper reports a novel microplate-integrated stereoselective membrane enrichment technique satisfying the need for automatable enantioseparation on a subpreparative scale.

Metabolome analysis to investigate the effect of controlled fermentation on taste-related metabolites in terasi

INTRODUCTION

Terasi is a fermented shrimp paste unique to Indonesia and is used in dishes to add umami and saltiness. In a previous study, the controlled fermentation of terasi was optimized using starters containing three bacterial isolates: Staphylococcus saprophyticus, Bacillus subtilis, and Lactobacillus murinus. However, the influence of controlled fermentation using these starters on the metabolites in terasi has not yet been studied.

OBJECTIVES

Therefore, this study aimed to investigate the effect of controlled fermentation on taste-related metabolites in terasi using a metabolomics approach.

RESULTS

Non-targeted analysis indicated that amino acids contributed to variations during fermentation. Subsequently, targeted analysis of amino acids revealed that terasi subjected to controlled fermentation using a starter with a 2:1:2 ratio of S. saprophyticus, B. subtilis, and L. murinus, respectively, resulted in a product containing D-amino acids, such as D-Asp, D-Gln, and D-Leu that was unique when compared to other terasi products prepared using controlled fermentation. Genetic analysis of isolates from the terasi produced using controlled fermentation was also carried out, and this is the first study to suggest that Staphylococcus spp. has the potential to produce D-amino acids.

CONCLUSION

In conclusion, the ratio of bacterial species in starter cultures used in controlled fermentation influenced the amino acid profile of the product and starters with a higher ratio of Staphylococcus spp. may result in the production of D-amino acids.

Separation and identification of the DL-forms of short-chain peptides using a new chiral resolution labeling reagent

There are several reports of D-amino acids being the causative molecules of serious diseases, resulting in the formation of, for example, prion protein and amyloid β. D-Amino acids in peptides and proteins are typically identified by sequencing each residue by Edman degradation or by hydrolysis with hydrochloric acid for amino acid analysis. However, these approaches can result in racemization of the L-form to the D-form by hydrolysis and long pre-treatment for hydrolysis. To address these problems, we aimed to identify the DL-forms of amino acids in peptides without hydrolysis. Here, we showed that the DL-forms in peptides which are difficult to separate on a chiral column can be precisely separated by labeling with 1-fluoro-2,4-dinitrophenyl-5-D-leucine-N,N-dimethylethylenediamine-amide (D-FDLDA). Additionally, the peptides could be quantitatively analyzed using the same labeling method as for amino acids. Furthermore, the detection sensitivity of a sample labeled with D-FDLDA was higher than that of the conventional reagents Nα-(5-fluoro-2,4-dinitrophenyl)-L-alaninamide (L-FDAA) and Nα-(5-fluoro-2,4-dinitrophenyl)-L-leucinamide (L-FDLA) used in Marfey's method. The proposed method for identifying DL-forms of amino acids in peptides is a powerful tool for use in organic chemistry, biochemistry, and medical science.

Based on Multi-Activity Integrated Strategy to Screening, Characterization and Quantification of Bioactive Compounds from Red Wine

According to French Paradox, red wine was famous for the potential effects on coronary heart disease (CHD), but the specific compounds against CHD were unclear. Therefore, screening and characterization of bioactive compounds from red wine was extremely necessary. In this paper, the multi-activity integrated strategy was developed and validated to screen, identify and quantify active compounds from red wine by using ultra high performance liquid chromatography-fraction collector (UHPLC-FC), ultra fast liquid chromatography-quadrupole-time-of-flight/mass spectrometry (UFLC-Q-TOF/MS) and bioactive analysis. UHPLC-FC was employed to separate and collect the components from red wine, which was further identified by UFLC-Q-TOF/MS to acquire their structural information. Furthermore, the active fractions were tested for antioxidant activity, inhibitory activity against thrombin and lipase activities in vitro by the activity screening kit. As the results, there were 37 fractions had antioxidant activity, 22 fractions had thrombin inhibitory activity and 28 fractions had lipase inhibitory activity. Finally, 77 active components from red wine were screened and 12 ingredients out of them were selected for quantification based on the integration of multi-activity. Collectively, the multi-activity integrated strategy was helpful for the rapid and effective discovery of bioactive components, which provided reference for exploring the health care function of food.

Preparation of racemic α-amino acid standards for accurate mass spectrometric analysis via racemization catalyzed by a hydrophobic pyridoxal derivative

Racemic α-amino acid standards for chiral metabolomics were prepared from l-α-amino acids using a hydrophobic pyridoxal derivative, namely 3-hydroxy-2-methyl-5-((octyloxy)methyl)isonicotinaldehyde (OPy), as the racemization catalyst. Among the 19 tested proteinogenic amino acids, 13 (including the generally unstable asparagine, glutamine, and tryptophan) underwent efficient racemization/epimerization under mildly basic conditions at room temperature, while solid-phase extraction allowed for effective and simple catalyst removal and amino acid recovery, obviating the need for chromatographic separation and recrystallization. Isotopically labeled racemic amino acids are commonly employed as internal standards for highly accurate mass spectrometric analysis. However, as isotopically labeled d-amino acids are often unavailable or highly expensive, the developed method was used to prepare racemic labeled amino acids, which were shown to enhance the repeatability and accuracy of d,l-amino acid quantitation in human urine by liquid chromatography-mass spectrometry (LC-MS). Given that our method should also be applicable to non-proteinogenic α-amino acids and the N-termini of peptides, the present study is expected to accelerate the development of LC-MS-based chiral metabolomics.

Development of a novel method for polar metabolite profiling by supercritical fluid chromatography/tandem mass spectrometry

Supercritical carbon dioxide (scCO₂), the main fluid in the mobile phase for supercritical fluid chromatography (SFC), is non-polar. The majority of polar compounds are little soluble in scCO₂, thereby rendering them poor candidates for achieving separation by carbon dioxide-based SFC. There is no reported method for the comprehensive analysis of hydrophilic metabolites by SFC with mobile phases comprising a high CO₂ ratio. In this study, we investigated the effect of additives in the modifier for enabling the application of SFC to profile diverse polar compounds for metabolomics. Eleven types of columns were screened by using proteinogenic amino acids as the model compounds. The addition of water and acids (formic acid and trifluoroacetic acid (TFA)) to the modifier was also investigated to improve the solubility of the polar compounds and mitigate the unfavorable interaction between the stationary phase and the polar compounds. A significant improvement in the peak shapes of the amino acids was observed upon addition of TFA. The CO₂/modifier ratio and TFA concentration in the mobile phases were investigated using the CROWNPAK CR-I (+) column, which showed the best performance during the column-screening. The CO₂/methanol/water/TFA ratio of 70/27/3/0.15 (v/v/v/v) was determined as the optimized mobile phase composition. Furthermore, the applicability of the optimized analytical method to other polar compounds was examined; 100 cationic and amphoteric compounds with predicted logP_ow values that ranged from -5.9 to 1.7 could be simultaneously analyzed without derivatization. Anionic compounds such as organic acids, phosphates, and sugars were excluded from the target analytes. Most of the previously reported SFC methods for analyzing polar compounds employ a gradient elution and require the use of high modifier ratios at 40% or more. In the proposed method, the use of water and TFA enabled the rapid and simultaneous analysis under isocratic elution within 10 min, even with a high CO₂ ratio of 70%. Additionally, a rat serum extract was analyzed using the optimized conditions, and 43 polar metabolites were successfully detected. This result demonstrates the applicability of the SFC/tandem mass spectrometry method to real samples.

Fast enantiomeric separation of amino acids using liquid chromatography/mass spectrometry on a chiral crown ether stationary phase

Fast enantiomeric separation of amino acids was studied by liquid chromatography/mass spectrometry (LC/MS) on a chiral crown ether stationary phase. A chiral crown ether bonded silica column (3 mm internal diameter (i.d.), 5 cm long) packed with 3 μm particles was employed instead of a 15 cm column packed with 5 μm particles used in our previous study. In addition, the extra-column variance, becoming more serious for smaller columns, was reduced by replacing 0.127 mm i.d. post-column tubes with shorter, smaller-diameter (0.0635 mm i.d.) tubes. The results demonstrated the benefits of using shorter columns packed with smaller particles and the reduction of the extra-column band broadening for fast enantiomeric separation. Finally, the enantiomeric separation of 18 pairs of proteinogenic amino acids was achieved within 2 min with a resolution (Rs) > 1.5 for each pair using an isocratic mobile phase of acetonitrile/water/trifluoroacetic acid (ACN/W/TFA) = 96/4/0.5, and a flow rate 1.2 mL/min at 30°C. This is the highest throughput method for simultaneous chiral separation of all proteinogenic amino acids except proline to date.

Determination of Kynurenine Enantiomers by Alpha-Cyclodextrin, Cationic-βeta-Cyclodextrin and Their Synergy Complemented with Stacking Enrichment in Capillary Electrophoresis

We present high resolution fast, cost-effective and sensitive Capillary zone electrophoresis (CZE) methods for determination of enantiomeric compounds of Kynurenine pathway, i.e. D, L-Kynurenine (KYN), in human serum and urine samples by cationic-β-CD and its synergistic dual chiral selector system (SD-CSs) with α-CD in 50 mM borax borate buffer (pH 9.0) as BGE. Acid-mediated stacking enrichment by HCl delivered 15 nM limit of detection (LOD) and 50 nM limit of quantification (LOQ). The methods gave advantages of linearity in the concentration range of 50 nM-1000 nM, reproducibility (RSD ≤ 3.35), selectivity against interfering amino acids, and remarkable recoveries. SD-CSs delivered resolution of D, L-KYN twice that of individual chiral selectors (CSs) under similar conditions. The binding constants (Kb) and electrophoretic mobilities (µeff) of D, L-KYN with different concentrations of CSs were calculated to find the migration order of enantiomers. The chiral recognition mechanism was investigated by molecular docking and molecular mechanics, which revealed strong hydrogen bonding between Kynurenine enantiomers and the SD-CSs as compared to individual CS as the key player in binding, formation of stable complexes which led to the ultimate separation.

Profiling of taste-related compounds during the fermentation of Japanese sake brewed with or without a traditional seed mash (kimoto)

Japanese sake production involves three processes: rice koji fermentation, seed mash fermentation, and main mash fermentation. Traditional seed mash (kimoto) production utilizes natural lactic acid produced by lactic acid bacteria for pure cultures of only sake yeast, preventing the growth of wild yeast and other unwanted bacteria. Recently, because kimoto production requires substantial time and labor, sake yeast mass-cultured in usual liquid medium has been used as a seed mash alternative. Sake quality is highly similar to that of kimoto, suggesting that they share similar component profiles. However, comparative component analyses of sake brewed with kimoto and sake brewed with cultured yeast are lacking. In this study, a time-course analysis of hydrophilic compounds in the main mash brewed with kimoto and with cultured yeast as well as a sensory evaluation of the products were performed. As a result, differences in various compounds and in umami taste level between sake brewed with kimoto and cultured yeast were detected. This is the first comparative analysis of changes in the component profile during sake main mash brewing using kimoto seed mash and cultured sake yeast; our results clarify the effects of kimoto seed mash on main mash brewing and sake quality.

Chiral separation of underivatized amino acids in supercritical fluid chromatography with chiral crown ether derived column

The supercritical fluid chromatographic separation of underivatized amino acids was explored using immobilized chiral crown ether column CROWNPAK CR-I (+) and mass spectrometric detection. The type of modifier, acidic additives, and the role of water were investigated. Enantioseparation was achieved for all 18 amino acids investigated with short retention times (less than 3 minutes) and average resolution of greater than 5.0. Analysis of enantiomerically pure standards demonstrated the D enantiomer eluted first for all amino acids using a CROWNPAK CR-I (+) column.

Serine racemase enhances growth of colorectal cancer by producing pyruvate from serine

Serine racemase (SRR) catalyses not only the racemization but also the dehydration of L-serine and D-serine, resulting in the formation of pyruvate and ammonia. Although SRR activity is important in the central nervous system, SRR has not been linked to cancer metabolism before. Here we show that SRR supports proliferation of colorectal-cancer cells. We find that SRR expression is upregulated in colorectal adenoma and adenocarcinoma lesions compared with non-neoplastic mucosa in human colorectal-cancer specimens. SRR-mediated dehydration of serine contributes to the pyruvate pool in colon-cancer cells, enhances proliferation, maintains mitochondrial mass and increases basal reactive oxygen species production, which has anti-apoptotic effects. Moreover, SRR promotes acetylation of histone H3 by maintaining intracellular acetyl-CoA levels. Inhibition of SRR suppresses growth of colorectal tumours in mice and augments the efficacy of 5-fluorouracil treatment. Our findings highlight a previously unknown mechanism through which a racemase supports cancer-cell growth and suggest that SRR might be a molecular target for colorectal-cancer therapy.

D-amino acids in foods

With the only exception of glycine, all amino acids exist in two specular structures which are mirror images of each other, called D-(dextro) and L-(levo) enantiomers. During evolution, L-amino acids were preferred for protein synthesis and main metabolism; however, the D-amino acids (D-AAs) acquired different and specific functions in different organisms (from playing a structural role in the peptidoglycan of the bacterial cell wall to modulating neurotransmission in mammalian brain). With the advent of sophisticated and sensitive analytical techniques, it was established during the past few decades that many foods contain considerable amounts of D-AAs: we consume more than 100 mg of D-AAs every day. D-AAs are present in a variety of foodstuffs, where they fulfill a relevant role in producing differences in taste and flavor and in their antimicrobial and antiaging properties from the corresponding L-enantiomers. In this review, we report on the derivation of D-AAs in foods, mainly originating from the starting materials, fermentation processes, racemization during food processing, or contamination. We then focus on leading-edge methods to identify and quantify D-AAs in foods. Finally, current knowledge concerning the effect of D-AAs on the nutritional state and human health is summarized, highlighting some positive and negative effects. Notwithstanding recent progress in D-AA research, the relationships between presence and nutritional value of D-AAs in foods represent a main scientific issue with interesting economic impact in the near future.

Analysis of Enantiomers in Products of Food Interest

The separation of enantiomers has been started in the past and continues to be a topic of great interest in various fields of research, mainly because these compounds could be involved in biological processes such as, for example, those related to human health. Great attention has been devoted to studies for the analysis of enantiomers present in food products in order to assess authenticity and safety. The separation of these compounds can be carried out utilizing analytical techniques such as gas chromatography, high-performance liquid chromatography, supercritical fluid chromatography, and other methods. The separation is performed mainly employing chromatographic columns containing particles modified with chiral selectors (CS). Among the CS used, modified polysaccharides, glycopeptide antibiotics, and cyclodextrins are currently applied.

Mechanistic study on the high-selectivity enantioseparation of amino acids using a chiral crown ether-bonded stationary phase and acidic, highly organic mobile phase by liquid chromatography/time-of-flight mass spectrometry

The separation mechanism of amino acid enantiomers using a chiral crown ether-bonded stationary phase, CROWNPAK CR-I(+), and acetonitrile (ACN)-rich mobile phases (MPs) was studied. The retention factors of proteinogenic l-amino acids (except proline) formed U-shaped plots against the ACN content in the MP with a sharp increase at a high ACN content, while d-amino acids showed much smaller increases or monotonous decreases in retention within the same range. The use of an acidic, highly organic MP with trifluoroacetic acid (TFA) provided a high enantioselectivity with a short separation time from the contribution of the increased binding of the ammonium group of the analytes to the crown ether functionality of the stationary phase and electrostatic repulsion counteracting the hydrophilic partition mechanism. Optimizing the sample diluent and MP alleviated the peak distortion caused by a moving water band that accompanied the hydrophilic interaction liquid chromatography-like elution conditions. The liquid chromatography/time-of-flight mass spectrometry method with the optimized MP - ACN/ethanol/water/TFA = 80/15/5/0.5 (v/v/v/v) - enabled the determination of eighteen pairs of proteinogenic amino acid enantiomers within 10 min. The conditions also provided the following advantages: (i) fast and highly reproducible separations under isocratic conditions, (ii) high sensitivity and low backpressure using the MP with a high organic content, and (iii) highly reliable peak identification by combining two columns (CR-I(+) and CR-I(-)), reversing the elution orders of the enantiomers.

Development and validation of a rapid, selective, and sensitive LC-MS/MS method for simultaneous determination of D- and L-amino acids in human serum: application to the study of hepatocellular carcinoma

A validated liquid chromatography-tandem mass spectrometry method was developed for the simultaneous determination of D- and L-amino acids in human serum. Under the optimum conditions, except for DL-proline, L-glutamine, and D-lysine, the enantioseparation of the other 19 enantiomeric pairs of proteinogenic amino acids and nonchiral glycine was achieved with a CROWNPAK CR-I(+) chiral column within 13 min. The lower limits of quantitation for L-amino acids (including glycine) and D-amino acids were 5-56.25 μM and 0.625-500 nM, respectively, in human serum. The intraday precision and interday precision for all the analytes were less than 15%, and the accuracy ranged from -12.84% to 12.37% at three quality control levels. The proposed method, exhibiting high rapidity, enantioresolution, and sensitivity, was successfully applied to the quantification of D- and L-amino acid levels in serum from hepatocellular carcinoma patients and healthy individuals. The serum concentrations of L-arginine, L-isoleucine, L-aspartate, L-tryptophan, L-alanine, L-methionine, L-serine, glycine, L-valine, L-leucine, L-phenylalanine, L-threonine, D-isoleucine, D-alanine, D-glutamate, D-glutamine, D-methionine, and D-threonine were significantly reduced in the hepatocellular carcinoma patients compared with the healthy individuals (P < 0.01). D-Glutamate and D-glutamine were identified as the most downregulated serum markers (fold change greater than 1.5), which deserves further attention in hepatocellular carcinoma research. Graphical abstract Simultaneous determination of D- and L-amino acids in human serum from hepatocellular carcinoma patients and healthy individuals. AA amino acid, HCC hepatocellular carcinoma, LC liquid chromatography, MS/MS tandem mass spectrometry, NC normal control, TIC total ion chromatogram.

A water-soluble pillar[5]arene-based chemosensor for highly selective and sensitive fluorescence detection of l-methionine

Cationic pillar[5]arene (AWP5) was employed as a water-soluble chemosensor for recognition of amino acids. AWP5 could fluorescently detect l-methionine in water with high selectivity and sensitivity.