Simple Resolution of Enantiomeric NMR Signals of α‐Amino Acids by Using Samarium(III) Nitrate With L‐Tartarate

Readily available L‐tartaric acid, which is a bidentate ligand with two chiral centers forming a seven‐membered chelate ring, was applied to the chiral ligand for the chiral nuclear magnetic resonance (NMR) shift reagent of samarium(III) formed in situ. This simple method does not cause serious signal broadening in the high magnetic field. Enantiomeric ¹³C and ¹H NMR signals and enantiotopic ¹H NMR signals of α‐amino acids were successfully resolved at pH 8.0 and the 1:3 molar ratio of Sm(NO₃)₃:L‐tartaric acid. It is elucidated that the enantiomeric signal resolution is attributed to the anisotropic magnetic environment for the enantiomers induced by the chiral L‐tartarato samarium(III) complex rather than differences in stability of the diastereomeric substrate adducts. The present ¹³C NMR signal resolution was also effective for the practical simultaneous analysis of plural kinds of DL‐amino acids. Chirality 27:353–357, 2015.© 2015 Wiley Periodicals, Inc.     

Utilization of (18‐Crown‐6)‐2,3,11,12‐tetracarboxylic Acid as a Chiral NMR Solvating Agent for Diamines and β‐Amino Acids

The compound (18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid was evaluated as a chiral nuclear magnetic resonance (NMR) solvating agent for a series of diamines and bicyclic β‐amino acids. The amine must be protonated for strong association with the crown ether. An advantage of (18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid over many other crown ethers is that it undergoes a neutralization reaction with neutral amines to form the protonated species needed for binding. Twelve primary diamines in neutral and protonated forms were evaluated. Diamines with aryl and aliphatic groups were examined. Some are atropisomers with equivalent amine groups. Others have two nonequivalent amine groups. Association equilibria for these systems are complex, given the potential formation of 2:1, 1:1, and 1:2 crown‐amine complexes and given the various charged species in solution for mixtures of the crown ether with the neutral amine. The crown ether produced enantiomeric differentiation in the ¹H NMR spectrum of one or more resonances for every diamine substrate. Also, a series of five bicyclic β‐amino acids were examined and (18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid caused enantiomeric differentiation in the ¹H NMR spectrum of three or more resonances of each compound. Chirality 27:708–715, 2015. © 2015 Wiley Periodicals, Inc.     

A Simple 13C NMR Method for the Discrimination of Complex Mixtures of Stereoisomers: All Eight Stereoisomers of α‐Tocopherol Resolved

A simple one‐dimensional ¹³C NMR method is presented to discriminate between stereoisomers of organic compounds with more than one chiral center. By means of this method it is possible to discriminate between all eight stereoisomers of α‐tocopherol. To achieve this the chiral solvating agent (S)‐(+)‐1‐(9‐anthryl)‐2,2,2‐trifluoroethanol and the compound of interest were dissolved in high concentrations in chloroform‐d, and the nuclear magnetic resonance (NMR) spectrum was recorded at a low temperature. The individual stereoisomers of α‐tocopherol were assigned by spikes of the reference compounds. The method was also applied to six other representative examples. Chirality 27:850–855, 2015. © 2015 Wiley Periodicals, Inc.     

Metabolic adaptation of Chlamydia trachomatis to mammalian host cells

Metabolic adaptation is a key feature for the virulence of pathogenic intracellular bacteria. Nevertheless, little is known about the pathways in adapting the bacterial metabolism to multiple carbon sources available from the host cell. To analyze the metabolic adaptation of the obligate intracellular human pathogen Chlamydia trachomatis, we labeled infected HeLa or Caco‐2 cells with ¹³C‐marked glucose, glutamine, malate or a mix of amino acids as tracers. Comparative GC‐MS‐based isotopologue analysis of protein‐derived amino acids from the host cell and the bacterial fraction showed that C. trachomatis efficiently imported amino acids from the host cell for protein biosynthesis. FT‐ICR‐MS analyses also demonstrated that label from exogenous ¹³C‐glucose was efficiently shuffled into chlamydial lipopolysaccharide probably via glucose 6‐phosphate of the host cell. Minor fractions of bacterial Ala, Asp, and Glu were made de novo probably using dicarboxylates from the citrate cycle of the host cell. Indeed, exogenous ¹³C‐malate was efficiently taken up by C. trachomatis and metabolized into fumarate and succinate when the bacteria were kept in axenic medium containing the malate tracer. Together, the data indicate co‐substrate usage of intracellular C. trachomatis in a stream‐lined bipartite metabolism with host cell‐supplied amino acids for protein biosynthesis, host cell‐provided glucose 6‐phosphate for cell wall biosynthesis, and, to some extent, one or more host cell‐derived dicarboxylates, e.g. malate, feeding the partial TCA cycle of the bacterium. The latter flux could also support the biosynthesis of meso‐2,6‐diaminopimelate required for the formation of chlamydial peptidoglycan.     

Mutations in the major gas vesicle protein GvpA and impacts on gas vesicle formation in Haloferax volcanii

Gas vesicles are proteinaceous, gas‐filled nanostructures produced by some bacteria and archaea. The hydrophobic major structural protein GvpA forms the ribbed gas vesicle wall. An in‐silico 3D‐model of GvpA of the predicted coil‐α1‐β1‐β2‐α2‐coil structure is available and implies that the two β‐chains constitute the hydrophobic interior surface of the gas vesicle wall. To test the importance of individual amino acids in GvpA we performed 85 single substitutions and analyzed these variants in Haloferax volcanii ΔA + A_mut transformants for their ability to form gas vesicles (Vac⁺ phenotype). In most cases, an alanine substitution of a non‐polar residue did not abolish gas vesicle formation, but the replacement of single non‐polar by charged residues in β1 or β2 resulted in Vac^– transformants. A replacement of residues near the β‐turn altered the spindle‐shape to a cylindrical morphology of the gas vesicles. Vac^– transformants were also obtained with alanine substitutions of charged residues of helix α1 suggesting that these amino acids form salt‐bridges with another GvpA monomer. In helix α2, only the alanine substitution of His53 or Tyr54, led to Vac^– transformants, whereas most other substitutions had no effect. We discuss our results in respect to the GvpA structure and data available from solid‐state NMR.     

Comparison of Chiral Separations of Aminophosphonic Acids and Their Aminocarboxylic Acid Analogs Using a Crown Ether Column

Crown ethers are capable of complexing with primary amines and have been utilized in chromatography to separate amino acid racemates. This application has been extended to resolve (1‐amino‐1‐phenylmethyl)phosphonic acid and (1‐aminoethyl)phosphonic acid racemates, along with their aminocarboxylic acid analogs (2‐phenylglycine and alanine, respectively), via a ChiroSil RCA crown ether based chiral stationary phase. Effects of the organic modifier, temperature, and acid type and concentration on retention and selectivity were also investigated. Trends in retention and selectivity varied between aminophosponic acids and their aminocarboxylic analogs. Computer modeling and ¹H NMR analyses were performed to potentially gain a better understanding of interactions of the aforementioned molecules with the ChiroSil RCA chiral stationary phase. Theoretical predictions of the most stable conformations for (R)‐ and (S)‐enantiomers were compared to elution order; it was found that the elution order agreed with molecular modeling such that the longest retention correlated with the predicted most stable complex between the enantiomer and crown ether. ¹H NMR demonstrated interactions of aminophosphonic and aminocarboxylic racemates with (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid in solution and was utilized to determine enantiomeric excess of (1‐amino‐1‐phenylmethyl)phosphonic acid after its enantioenrichment via crystallization through diastereomeric salt formation with the crown ether followed by filtration. Chirality 25:369–378, 2013. © 2013 Wiley Periodicals, Inc.     

Enantioselective recognition of carboxylic acids by novel fluorescent triazine‐based thiazoles

Hydrogen bonding and π‐π interactions take special part in the enantioselectivity task. In this regard, because of having both hydrogen acceptor and hydrogen donor groups, melamine derivatives become more of an issue for enantioselectivity. In the light of such information, triazine‐based chiral, fluorescence active novel thiazole derivatives L1 and L2 were designed and synthesized from (S)‐(?)‐2‐amino‐1‐butanol and (1S,2R)‐(+)‐2‐amino‐1,2‐diphenylethanol. The structural establishment of these compounds was made by spectroscopic methods such as FTIR, ¹H, and ¹³C NMR. While the solution of these compounds in DMSO did not show any fluorescence emission, it was observed that the emission increased 44‐fold for L1 and 55‐fold for L2 in 95% water, similar to the aggregation‐induced emission (AIE) characterized compounds. In this regard, enantioselective capabilities of these compounds against carboxylic acids were tested, and in experiments carried out at a ratio of 40/60 DMSO/H₂O, it was determined that R‐2ClMA increased the fluorescence emission of L1 chiral receptor by 2.59 times compared to S‐isomer.     

Chiral 1,2‐Dialkenyl Diaziridines: Synthesis,Enantioselective Separation,and Nitrogen Inversion Barriers

trans‐1,2‐Disubstituted diaziridines form stable enantiomers at ambient conditions because of the two stereogenic pyramidal nitrogen atoms. Functionalized trans‐1,2‐disubstituted diaziridines can be utilized as a chiral switching moiety between two enantiomeric states in more complex molecular structures. However, the synthesis of functionalized diaziridines is quite challenging, because of the limited tolerance of reaction conditions that can be applied. Here we present a strategy to make trans‐1,2‐disubstituted diaziridines accessible as versatile building blocks in C‐C‐bond formations, i.e., the Heck reaction, and therefore introducing aryl substituents. The synthesis of trans‐1,2‐dialkenyl diaziridines with terminal alkenyl substituents and their stereodynamic properties are described. Chirality 27:156–162, 2015. © 2014 Wiley Periodicals, Inc.     

Separation of Ofloxacin and Its Six Related Substances Enantiomers by Chiral Ligand‐Exchange Chromatography

A chiral ligand‐exchange high‐performance liquid chromatography method was developed for the enantioseparation of ofloxacin and its six related substances termed impurities A, B, C, D, E, and F. The separation was performed on a conventional C₁₈ column. Different organic modifiers, copper salts, amino acids, the ratio of Cu²⁺ to amino acid, pH of aqueous phase, and column temperature were optimized. The optimal mobile phase conditions were methanol‐water systems consisting of 5 mmol/L copper sulfate and 10 mmol/L L‐isoleucine (L‐Ile). Under such conditions, good enantioseparation of ofloxacin and impurities A, C, E, and F could be observed with resolutions (R_S) of 3.54, 1.97, 3.21, 3.50, and 2.12, respectively. On the relationship between the thermodynamic parameters and structures of analytes, the mechanism of chiral recognition was investigated. It was concluded that ofloxacin and impurities A, C, E, and F were all enthalpically driven enantioseparation and that low column temperature was beneficial to enantioseparation. Furthermore, the structure–separation relationship of these analytes is also discussed. Chirality 27:843–849, 2015. © 2015 Wiley Periodicals, Inc.     

Dynamics of amino acid redistribution in the carnivorous Venus flytrap (Dionaea muscipula) after digestion of 13C/15N‐labelled prey

• Amino acids represent an important component in the diet of the Venus flytrap (Dionaea muscipula), and supply plants with much needed nitrogen resources upon capture of insect prey. Little is known about the significance of prey‐derived carbon backbones of amino acids for the success of Dionaea's carnivorous life‐style.
• The present study aimed at characterizing the metabolic fate of ¹⁵N and ¹³C in amino acids acquired from double‐labeled insect powder. We tracked changes in plant amino acid pools and their δ¹³C‐ and δ¹⁵N‐signatures over a period of five weeks after feeding, as affected by contrasting feeding intensity and tissue type (i.e., fed and non‐fed traps and attached petioles of Dionaea).
• Isotope signatures (i.e., δ¹³C and δ¹⁵N) of plant amino acid pools were strongly correlated, explaining 60% of observed variation. Residual variation was related to contrasting effects of tissue type, feeding intensity and elapsed time since feeding. Synthesis of nitrogen‐rich transport compounds (i.e., amides) during peak time of prey digestion increased ¹⁵N‐ relative to ¹³C‐ abundances in amino acid pools. After completion of prey digestion, ¹³C in amino acid pools was progressively exchanged for newly fixed ¹²C. The latter process was most evident for non‐fed traps and attached petioles of plants that had received ample insect powder.
• We argue that prey‐derived amino acids contribute to respiratory energy gain and loss of ¹³CO₂ during conversion into transport compounds (i.e., 2 days after feeding), and that amino‐nitrogen helps boost photosynthetic carbon gain later on (i.e., 5 weeks after feeding).

     

Electrochromatographic enantioseparation of amino acids using polybutylmethacrylate-based chiral monolithic column by capillary electrochromatography

This article describes the development of a polybutylmethacrylate‐based monolithic capillary column as a chiral stationary phase. The chiral monolithic column was prepared by polymerization of butyl methacrylate (BMA), ethylene dimethacrylate (EDMA), and N‐methacryloyl‐l ‐glutamic acid (MAGA) in the presence of porogens. The porogen mixture included N,N‐dimethyl formamide and phosphate buffer. MAGA was used as a chiral selector. The effect of MAGA content was investigated on electrochromatographic enantioseparation of d,l ‐histidine, d,l ‐tyrosine, d,l ‐phenyl alanine, and d,l ‐glutamic acid. The effect of acetonitrile (ACN) content in mobile phase on electro‐osmotic flow was also investigated. It was demonstrated that the poly(BMA‐EDMA‐MAGA) monolithic chiral column can be used for the electrochromatographic enantioseparation of amino acids by capillary electrochromatography (CEC). The mobile phase was ACN/10 mM phosphate buffer (45:55%) adjusted to pH 2.7. It was observed that l ‐enantiomers of the amino acids migrated before d ‐enantiomers. The separation mechanism of electrochromatographic enantioseparation of amino acids in CEC is discussed. Chirality 24:606–609, 2012. © 2012 Wiley Periodicals, Inc.     

HIGH‐RESOLUTION MAGIC ANGLE SPINNING NMR ANALYSIS OF WHOLE CELLS OF CHAETOCEROS MUELLERI (BACILLARIOPHYCEAE) AND COMPARISON WITH 13C‐NMR AND DISTORTIONLESS ENHANCEMENT BY POLARIZATION TRANSFER 13C‐NMR ANALYSIS OF LIPOPHILIC EXTRACTS1

Lipid composition in extracted samples of Chaetoceros muelleri Lemmermann was studied with ¹³C‐NMR and distortionless enhancement by polarization transfer (DEPT) ¹³C‐NMR, resulting in well‐resolved ¹³C‐NMR spectra with characteristic resonance signals from carboxylic, olefinic, glyceryl, methylene, and methyl groups. The application of a DEPT pulse sequence aided in the assignment of methylene and methine groups. Resonance signals were compared with literature references, and signal assignment included important unsaturated fatty acids such as eicosapentaenoic and docosahexaenoic and also phospholipids and glycerols. Results from the extracted samples were used to assign resonance signals in a high‐resolution magic angle spinning (HR MAS) DEPT ¹³C spectrum from whole cells of C. muelleri. The NMR analysis on whole cells yielded equally good information on fatty acids and also revealed signals from carbohydrates and amino acids. Broad resonance signals and peak overlapping can be a problem in whole cell analysis, but we found that application of HR MAS gave a well‐resolved spectrum. The chemical shift of metabolites in an NMR spectrum depends on the actual environment of nuclei during analysis, and some differences could therefore be expected between extracted and whole cell samples. The shift differences were small, and assignment from analysis of lipophilic extract could be used to identify peaks in the whole cell spectrum. HR MAS ¹³C‐NMR therefore offers a possibility for broad‐range metabolic profiling directly on whole cells, simultaneously detecting metabolites that are otherwise not detected in the same analytical set up and avoiding tedious extraction procedures.     

The Syntheses of new Camphorsulfonylated Ligands Derived from 2-Amino-2'-Hydroxy-1,1'-Binaphthyl and Their Enantioselectivities in the Addition of Dialkylzinc Reagents to Aldehydes

A series of new camphorsulfonylated ligands derived from chiral 2‐amino‐2′‐hydroxy‐1,1′‐binaphthyl (NOBIN) and (+)‐camphorsulfonic acid were synthesized by a short and simple synthetic sequence, and their enantioselective catalytic activities were assessed in the nucleophilic addition reaction of dialkylzinc reagents to aldehydes in the presence of titanium tetraisopropoxide. The most efficient ligand, N‐hydroxycamphorsulfonylated (S)‐NOBIN, gave (S)‐addition products with good yields and up to 87% of ee value. The ¹H nuclear magnetic resonance (NMR) and ¹³C NMR results of the titanium titration experiments on this ligand indicate that the most likely catalytic reactive species involved in this catalytic asymmetric addition is a bimetallic titanium complex. A possible catalytic reaction mechanism is proposed. Chirality 24:825–832, 2012. © 2012 Wiley Periodicals, Inc.     

Synthesis and Quantitative Analysis of Diastereomeric Linked Ester Conjugates With Remote Stereocenters Using High‐Field NMR and Chiral HPLC

A stereochemically safe high‐yielding procedure for linking unprotected as well as protected hydroxycarboxylic acids to chiral secondary alcohols via glycolic acid linker is proposed. L‐menthol has been linked with both enantiomers of mandelic, malic, and methoxyphenylacetic acid using bromo‐ or iodoacetyl group as a precursor of the glycolic acid linker. High‐field nuclear magnetic resonance (NMR) and chiral high‐performance liquid chromatography (HPLC) determination of high diastereomeric ratio (dr) (>99%) of the products bearing remote stereocenters was explored. Chiral HPLC allowed quantitation of the diastereomers up to dr 99.9/0.1. High‐field NMR quantitation of the diastereomeric and parent alcoholic impurities in esters was demonstrated at the molar 0.3% and 0.03% levels, respectively. These analyses were done via comparison of integral intensities from major component ¹³C satellites in ¹H or even in ¹³C spectra to the ¹H or ¹³C signals of impurities. Despite lower sensitivity, the last option generally has much better selectivity. In this way the dynamic resolution is brought down by two orders. Chirality 25:793–798, 2013. © 2013 Wiley Periodicals, Inc.     

Chiral Pool‐Based Synthesis of Naphtho‐Fused Isocoumarins

A variety of chiral derivatives of benzo[d]naphtho[1,2‐b]pyran‐6‐one were prepared in a single step by Et₃N‐mediated condensation of homophthalic anhydride with different derivatives of (S)‐amino acid chlorides at –5 °C by employing a chiral pool methodology. Chirality 27:951–957, 2015. © 2015 Wiley Periodicals, Inc.     

Chiral Resolution and Absolute Configuration of the Enantiomers of the Psychoactive “Designer Drug” 3,4‐Methylenedioxypyrovalerone

Illicit rac‐MDPV (3,4‐methylenedioxypyrovalerone), manufactured in clandestine labs, has become widely abused for its cocaine‐like stimulant properties. It has recently been found as one of the toxic materials in the so‐called “bath salts,” producing, among other effects, psychosis and tachycardia in humans when introduced by any of the several routes of administration (e.g., intravenous, oral, etc.). The considerable toxicity of this “designer drug” probably resides in one of the enantiomers of the racemate. In order to obtain a sufficient amount of the enantiomers of rac‐MDPV to determine their activity, we improved the known synthesis of rac‐MDPV and found chemical resolving agents, (+)‐ and (–)‐2’‐bromotetranilic acid, that gave the MDPV enantiomers in >96% enantiomeric excess as determined by ¹H nuclear magnetic resonance and chiral high‐performance liquid chromatography. The absolute stereochemistry of these enantiomers was determined by single‐crystal X‐ray diffraction studies. Chirality 27:287‐293, 2015. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

Handedness preference and switching of peptide helices. Part I: Helices based on protein amino acids

In this article, we review the relevant results obtained during almost 60 years of research on a specific aspect of stereochemistry, namely handedness preference and switches between right‐handed and left‐handed helical peptide structures generated by protein amino acids or appropriately designed, side‐chain modified analogs. In particular, we present and discuss here experimental and theoretical data on three categories of those screw‐sense issues: (i) right‐handed/left‐handed α‐helix transitions underwent by peptides rich in Asp, specific Asp β‐esters, and Asn; (ii) comparison of the preferred conformations adopted by helical host–guest peptide series, each characterized by an amino acid residue (e.g. Ile or its diastereomer aIle) endowed with two chiral centers in its chemical structure; and (iii) right‐handed (type I)/left‐handed (type II) poly‐(Pro)_n helix transitions monitored for peptides rich in Pro itself or its analogs with a pyrrolidine ring substitution, particularly at the biologically important position 4. The unique modular and chiral properties of peptides, combined with their relatively easy synthesis, the chance to shape them into the desired conformation, and the enormous chemical diversity of their coded and non‐coded α‐amino acid building blocks, offer a huge opportunity to structural chemists for applications to bioscience and nanoscience problems. Copyright © 2014 European Peptide Society and John Wiley & Sons, Ltd.