Enantiomeric separation of nonproteinogenic amino acids by high-performance liquid chromatography

Enantioselective metabolomics by liquid chromatography-mass spectrometry

Metabolomics strives to capture the entirety of the metabolites in a biological system by comprehensive analysis, often by liquid chromatography hyphenated to mass spectrometry. A particular challenge thereby is the differentiation of structural isomers. Common achiral targeted and untargeted assays do not distinguish between enantiomers. This may lead to information loss. An increasing number of publications demonstrate that the enantiomeric ratio of certain metabolites can be meaningful biomarkers of certain diseases emphasizing the importance of introducing enantioselective analytical procedures in metabolomics. In this work, the state-of-the-art in the field of LC-MS based metabolomics is summarized with focus on developments in the recent decade. Methodologies, tagging strategies, workflows and general concepts are outlined. Selected biological applications in which enantioselective metabolomics has documented its usefulness are briefly discussed. In general, targeted enantioselective metabolomics assays are often based on a direct approach using chiral stationary phases (CSP) with polysaccharide derivatives, macrocyclic antibiotics, chiral crown ethers, chiral ion exchangers, donor-acceptor phases as chiral selectors. Rarely, these targeted assays focus on more than 20 analytes and usually are restricted to a certain metabolite class. In a variety of cases, pre-column derivatization of metabolites has been performed, especially for amino acids, to improve separation and detection sensitivity. Triple quadrupole instruments are the detection methods of first choice in targeted assays. Here, issues like matrix effect, absence of blank matrix impair accuracy of results. In selected applications, multiple heart cutting 2D-LC (RP followed by chiral separation) has been pursued to overcome this problem and alleviate bias due to interferences. Non-targeted assays, on the other hand, are based on indirect approach involving tagging with a chiral derivatizing agent (CDA). Besides classical CDAs numerous innovative reagents and workflows have been proposed and are discussed. Thereby, a critical issue for the accuracy is often neglected, viz. the validation of the enantiomeric impurity in the CDA. The majority of applications focus on amino acids, hydroxy acids, oxidized fatty acids and oxylipins. Some potential clinical applications are highlighted.

Unlocking the potential of Escherichia coli modified magnetic particles for chiral discrimination of racemic tryptophan

Enzymes possess a highly specific affinity toward their substrates. In this study, an enzyme-based biological method was established for chiral discrimination of D/L-tryptophan (Trp). The polydopamine modified magnetic particles (PDA@Fe₃O₄) were prepared for immobilization of the genetically engineered bacterium Escherichia coli (E. coli) DH5α. The bacteria-magnetic particles conjugates (bacteria@PDA@Fe₃O₄) demonstrate excellent chiral discrimination performance toward D/L-Trp at pH 7.0 and 45 °C. The investigation for the principle exhibits that the immobilized E. coli DH5α can produce tryptophanase, and the enzyme can selectively recognize and degrade L-Trp. The Michaelis constant of tryptophanase produced by bacteria@PDA@Fe₃O₄ was measured to be 25.7 µg mL^-1. This method avoids the purification of tryptophanase and unlocks the potential of bacteria modified magnetic particles for chiral discrimination of racemic tryptophan.

D-cysteine is an endogenous regulator of neural progenitor cell dynamics in the mammalian brain

d-amino acids are increasingly recognized as important signaling molecules in the mammalian central nervous system. However, the d-stereoisomer of the amino acid with the fastest spontaneous racemization ratein vitro in vitro, cysteine, has not been examined in mammals. Using chiral high-performance liquid chromatography and a stereospecific luciferase assay, we identify endogenous d-cysteine in the mammalian brain. We identify serine racemase (SR), which generates the N-methyl-d-aspartate (NMDA) glutamate receptor coagonist d-serine, as a candidate biosynthetic enzyme for d-cysteine. d-cysteine is enriched more than 20-fold in the embryonic mouse brain compared with the adult brain. d-cysteine reduces the proliferation of cultured mouse embryonic neural progenitor cells (NPCs) by ∼50%, effects not shared with d-serine or l-cysteine. The antiproliferative effect of d-cysteine is mediated by the transcription factors FoxO1 and FoxO3a. The selective influence of d-cysteine on NPC proliferation is reflected in overgrowth and aberrant lamination of the cerebral cortex in neonatal SR knockout mice. Finally, we perform an unbiased screen for d-cysteine-binding proteins in NPCs by immunoprecipitation with a d-cysteine-specific antibody followed by mass spectrometry. This approach identifies myristoylated alanine-rich C-kinase substrate (MARCKS) as a putative d-cysteine-binding protein. Together, these results establish endogenous mammalian d-cysteine and implicate it as a physiologic regulator of NPC homeostasis in the developing brain.

Polysaccharides as separation media for the separation of proteins, peptides and stereoisomers of amino acids

Reliable separation of peptides, amino acids and proteins as accurate as possible with the maximum conformation and biological activity is crucial and essential for drug discovery. Polysaccharide, as one of the most abundant natural biopolymers with optical activity on earth, is easy to be functionalized due to lots of hydroxyl groups on glucose units. Over the last few decades, polysaccharide derivatives are gradually employed as effective separation media. The highly-ordered helical structure contributes to complex, diverse molecular recognition ability, allowing polysaccharide derivatives to selectively interact with different analytes. This article reviews the development, application and prospects of polysaccharides as separation media in the separation of proteins, peptides and amino acids in recent years. The chiral molecules mechanism, advantages, limitations, development status and challenges faced by polysaccharides as separation media in molecular recognition are summarized. Meanwhile, the direction of its continued development and future prospects are also discussed.

Speeding up Viedma Deracemization through Water-catalyzed and Reactant Self-catalyzed Racemization

Viedma deracemization is based on solution phase racemization, dissolution of racemic or scalemic conglomerates and crystal growth through autocatalytic cluster formation. With rate limiting racemization, its acceleration by appropriate catalysts may result in speeding up deracemization. A conglomerate-forming chiral compound may principally racemize directly, or via reverse of its formation reaction. For a hydrazine derivative, we investigated available racemization pathways in presence of pyrrolidine or thiourea amine as base catalysts: via Mannich or aza-Michael reaction steps and their reverse, or by enolization. Racemization by enolization was computationally found to dominate, both under water-free conditions and in presence of water, involving a multitude of different pathways. Faster racemization in presence of water resulted indeed in more rapid deracemization, when the base was pyrrolidine. Under water-free conditions, the role of water as enolization catalyst is assumed by chiral hydrazine itself - in autocatalytic racemization and in which both reactant and product are catalysts.

Application of an (18-Crown-6)-2,3,11,12-Tetracarboxylic Acid-Based Chiral Stationary Phase in Capillary Electrochromatography

Capillary electrochromatography is employed for the enantioseparation of α-amino acids and their derivatives. The synthesis and application of a covalently bonded chiral stationary phase containing (-)-(18-crown-6)-2,3,11,12-tetracarboxylic acid as chiral selector is described. Enantioseparations are performed using methanol/Tris-citric acid (20 mM, pH 3.0-4.5) (20:80, v/v) as mobile phase.

Nanoparticle-based Chemiluminescence for Chiral Discrimination of Thiol-Containing Amino Acids

The ability to recognize the molecular chirality of enantiomers is extremely important owing to their critical role in drug development and biochemistry. Convenient discrimination of enantiomers has remained a challenge due to lack of unsophisticated methods. In this work, we have reported a simple strategy for chiral recognition of thiol-containing amino acids including penicillamine (PA), and cysteine (Cys). We have successfully designed a nanoparticle-based chemiluminescence (CL) system based on the reaction between cadmium telluride quantum dots (CdTe QDs) and the enantiomers. The different interactions of CdTe QDs with PA enantiomers or Cys enantiomers led to different CL intensities, resulting in the chiral recognition of these enantiomers. The developed method showed the ability for determination of enantiomeric excess of PA and Cys. It has also obtained an enantioselective concentration range from 1.15 to 9.2 mM for PA. To demonstrate the potential application of this method, the designed platform was applied for the quantification of PA in urine and tablet samples. For the first time, we presented a novel practical application of nanoparticle-based CL system for chiral discrimination.

High-performance liquid chromatographic enantioseparation of N-aryl-thioureidoalkylphosphonates and thiourylenedi(alkylphosphonates) on polysaccharide-based chiral stationary phases

The first successful enantioseparation of representative O,O-diphenyl-N-arylthioureidoalkylphosphonates, (±)-Ptc-Val^P (OPh)₂ & (±)-Ptc-Leu^P (OPh)₂ and thiourylenedi(isobutyl phosphonate), Tcm[Val^P (OPh)₂ ]₂ on analytical and semipreparative scale was achieved by high-performance liquid chromatography using polysaccharide-based chiral stationary phases (CPs). Atc-AA^P (OPh)₂ was obtained using modified tricomponent condensations of the corresponding aldehydes, N-arylthiourea and triphenyl phosphite whereas Tcm[Val^P (OPh)₂ ]₂ by the condensations of aldehydes, thiourea, and triphenyl phosphite. The prepared, racemic (±)-Atc-AA^P (OPh)₂ [(±)-Ptc-Val^P (OPh)₂ , (±)-Ptc-Leu^P (OPh)₂ , (±)-Ptc-Pgly^P (OPh)₂ and (±)-Ntc-Pgly^P (OPh)₂ ] and racemic (±)-Tcm[AA^P (OPh)₂ ]₂ [(±)-Tcm[Nva^P (OPh)₂ ]₂ & (±)-Tcm[Val^P (OPh)₂ ]₂ ] were adequately characterized and used for chromatographic separations on high-performance liquid chromatography-chiral stationary phases. The best results were obtained for (±)-Ptc-Val^P (OPh)₂ , (±)-Ptc-Leu^P (OPh)₂ and (±)-Tcm[Val^P (OPh)₂ ]₂ .

(+) or (-)-1-(9-fluorenyl)ethyl chloroformate as chiral derivatizing agent: A review

Over the last 30years, (±)-1-(9-fluorenyl)ethyl chloroformate ((±)-FLEC) was used as a chiral derivatizing agent in various analytical applications involving a wide range of endogenous, pharmaceutical and environmentally relevant molecules. This comprehensive review aims to present all the significant aspects related to the state of the art in FLEC labeling and subsequent chiral separation of the resulting diastereomers using LC, SFC and CE techniques.

Time-Resolved Visual Chiral Discrimination of Cysteine Using Unmodified CdTe Quantum Dots

Herein, we demonstrate a simple yet novel luminescence assay for visual chiral discrimination of cysteine. Thioglycolic acid (TGA)-capped cadmium-telluride (CdTe) quantum dots (QDs) exposing green emission were directly synthesized in aqueous solution. The interaction between cysteine molecules and CdTe QDs induced the aggregation of QDs via hydrogen bonding. As a result of electronic coupling within these aggregates, a redshift both in the absorption and emission spectra of QDs occured. The difference in the kinetics of the interactions between L- and D-cysteine with CdTe QDs led to chiral recognition of these enantiomers. Addition of D-cysteine to CdTe QDs in a basic media caused a green-to-yellow color change, while no color alteration in QDs emission was observed in the presence of L-cysteine after 2 hours. Notably, the QDs used in the proposed assay are free from any labling/modification, which makes the present strategy highly attractive for sensing applications. Furthermore, the presented chiral assay is able to determine the enantiomeric excess (ee) of D-cysteine in the whole range of ee values (from -100% to 100%).

Purification of organic acids by chromatography with strong anionic resins: Investigation of uptake mechanisms

Bio-based organic acids are promising renewable carbon sources for the chemical industry. However energy-consuming purification processes are used, like distillation or crystallization, to reach high purities required in some applications. That is why preparative chromatography was studied as an alternative separation technique. In a previous work dealing with the purification of lactic, succinic and citric acids, the Langmuir model was insufficient to explain the elution profiles obtained with a strong anionic resin. Consequently the Langmuir model was coupled with a usual ion-exchange model to take into account the retention of their conjugate bases (<2%), which are commonly neglected at low pH (<1.5). Elution simulations with both uptake mechanisms fitted very well with experimental pulse tests. Only two parameters were optimized (equilibrium constant of acid uptake and ion-exchange selectivity coefficient of conjugate base) and their value were coherent with experimental and resin suppliers' data. These results confirmed that the singular tailing and apparent delay observed with succinic and citric acids can be explained by the high affinity of succinate and citrate for resin cationic sites. The model was implemented in a preparative chromatography simulation program in order to optimize operating parameters of our pilot-scale ISMB unit (Improved Simulated Moving Bed). The comparison with experimental ISMB profiles was conclusive.

Preparation of imprinted cryogel cartridge for chiral separation of l-phenylalanine

l-Phe-imprinted cryogel cartridge was prepared for the chiral separation of l-Phe. N-Methacryloyl l-phenylalanine (MAPA) was used as a functional monomer for complexing with l-Phe. The selectivity of the membranes was investigated by using d-Phe, l-Trp, and d-Trp as competitor molecules. The PHEMAPA-l-Trp membranes were 6.4, 4.3, and 5.5 times more selective for l-Phe than d-Phe, l-Trp, and d-Trp, respectively. The PHEMAPA-l-Phe cryogel cartridge was incorporated into the fast protein liquid chromatography (FPLC) equipment and was able to separate D,l-Phe racemic mixture efficiently. The PHEMAPA-l-Phe membranes were shown to be reusable many times without significant loss of the adsorption capacity.

Teicoplanin bonded sub-2 μm superficially porous particles for enantioseparation of native amino acids

Superficially porous particles (SPPs) demonstrate superior efficiency than totally porous particles in chiral separations. In order to obtain high efficiency and fast separation, sub-2 μm SPPs with high surface area are synthesized, and with teicoplanin bonded, such materials are successfully applied into the rapid enantioseparation of native amino acids. In brief, 1.27 ± 0.06 μm nonporous silica particles are prepared by a modified seeded growth method, followed by mesoporous shell fabrication via one-pot templated dissolution and redeposition strategy, and pore size expansion via acid-refluxing. The diameter of the formed SPPs is 1.49 ± 0.04 μm, with the shell thickness as 206 nm. Nitrogen physisorption experiments show that the Brunauer-Emmett-Teller (BET) specific surface area is 213.6 m(2)/g and pore size is 9 nm. After teicoplanin derivatization with bonding capacity as 83.3 μmol/g, the prepared chiral stationary phase is packed into a stainless steel tube with the geometry of 50 mm × 2.1 mm i.d.. In less than 6.4 min, six native amino acids (norleucine, alanine, valine, methionine, leucine, norvaline) are enantioseparated with resolution factors ranging from 1.9 to 5.0. Besides, the resolution for chiral separation is improved with ethanol-water instead of methanol-water as the mobile phase. Moreover, the low temperature gives higher resolution, but longer retention time and higher backpressure. Finally, the effect of flow rate on enantiomeric separation is studied and fast chiral separation within 1 min is obtained with flow rate of 0.4 mL/min. All these results show that the synthesized teicoplanin bonded sub-2 μm SPPs have great potential to achieve the enantioseparation of native amino acids with high resolution and rapid speed.

Proteomic analysis of peptides tagged with dimedone and related probes

Owing to its labile nature, a new role for cysteine sulfenic acid (-SOH) modification has emerged. This oxidative modification modulates protein function by acting as a redox switch during cellular signaling. The identification of proteins that undergo this modification represents a methodological challenge, and its resolution remains a matter of current interest. The development of strategies to chemically modify cysteinyl-containing peptides for liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis has increased significantly within the past decade. The method of choice to selectively label sulfenic acid is based on the use of dimedone or its derivatives. For these chemical probes to be effective on a proteome-wide level, their reactivity toward -SOH must be high to ensure reaction completion. In addition, the presence of an adduct should not interfere with electrospray ionization, the efficiency of induced dissociation in MS/MS experiments or with the identification of Cys-modified peptides by automated database searching algorithms. Herein, we employ a targeted proteomics approach to study the electrospray ionization and fragmentation effects of different -SOH specific probes and compared them to commonly used alkylating agents. We then extend our study to a whole proteome extract using shotgun proteomic approaches. These experiments enable us to demonstrate that dimedone adducts do not interfere with electrospray by suppressing the ionization nor impede product ion assignment by automated search engines, which detect a + 138 Da increase from unmodified peptides. Collectively, these results suggest that dimedone can be a powerful tool to identify sulfenic acid modifications by high-throughput shotgun proteomics of a whole proteome.

Simultaneous analysis of the non-canonical amino acids norleucine and norvaline in biopharmaceutical-related fermentation processes by a new ultra-high performance liquid chromatography approach

In this study, a precise and reliable ultra-high performance liquid chromatography (UHPLC) method for the simultaneous determination of non-canonical (norvaline and norleucine) and standard amino acids (aspartic acid, glutamic acid, serine, histidine, glycine, threonine, arginine, tyrosine, methionine, valine, phenylalanine, isoleucine, leucine) in biopharmaceutical-related fermentation processes was established. After pre-column derivatization with ortho-phthaldialdehyde and 2-mercaptoethanol, the derivatives were separated on a sub-2 μm particle C18 reverse-phase column. Identification and quantification of amino acids were carried out by fluorescence detection. To test method feasibility on standard HPLC instruments, the assay was properly transferred to a core-shell particle C18 reverse-phase column. The limits of detection showed excellent sensitivity by values from 0.06 to 0.17 pmol per injection and limits of quantification between 0.19 and 0.89 pmol. In the present study, the newly established UHPLC method was applied to a recombinant antibody Escherichia coli fermentation process for the analysis of total free amino acids. We were able to specifically detect and quantify the unfavorable amino acids in such complex samples. Since we observed trace amounts of norvaline and norleucine during all fermentation phases, an obligatory process monitoring should be considered to improve quality of recombinant protein drugs in future.