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Enantiomeric discrimination plays a crucial role in the fields of analytical science and chemical biology. Herein, we achieve chiral recognition via angle-resolved polarized Raman scattering using a composite material consisting of an anisotropic two-dimensional material, ReSe2, and chiral Au nanoparticles (CAuNPs). A near-field effect is induced through the attachment of CAuNPs to ReSe2, in conjunction with modulation of polarized light, resulting in simultaneous angular polarization of ReSe2. This angular polarization was utilized for the chiral detection of tyrosine. Varying enantiomeric excess (ee) leads to inverse nonreciprocal angular deflections, which we propose arise from interactions between the chiral near-field and optical vectors modified by chiral molecules with distinct ee values. This phenomenon is primarily attributed to the optical spin Hall effect within the field, generating distinct vector deflections that can be utilized to discriminate between different enantiomers. Ultimately, we demonstrate selective identification of chiral substances by establishing a correlation between the optical inversion angle and ee value. This investigation presents an efficient and insightful approach for achieving rapid detection of chiral substances using rational optical devices. [ABSTRACT FROM AUTHOR]

(R,R)- and (S,S)-enantiomers of anthracene-containing pyridino-18-crown-6 ether having te/t-butyl groups at the stereogenic centers were prepared with the aim of achieving higher enantioselectivity than for the reported (S,S)-analogue having isobutyl groups. The enantiomeric recognition abilities of the new sensor molecules toward chiral protonated primary amines and amino acid esters were studied in acetonitrile by UV-vis and fluorescence spectroscopies. The pKa values of these pyridino-crown ethers and their reported (S,S)-analogues having methyl or isobutyl groups have also been determined in acetonitrile. [ABSTRACT FROM AUTHOR]

Herein, the chiral nanostructured bimetallic Au–Ag films (CNAAFs) with both photomagnetic‐chiral anisotropy to enantiomers (PM‐ChAC‐E) and surface‐enhanced Raman scattering‐chiral anisotropy to enantiomers (SERS‐ChAC‐E) for efficient enantiomeric discrimination are reported. The CNAAFs are synthesized by electrodeposition on n‐type Si (n‐Si) substrates with the assistance of N‐acetyl‐L/D‐cysteine (S/R‐NAC) as symmetry‐breaking agents, which are consisted of polycrystalline Boerdijk–Coxeter–Bernal (BCB) tetrahelical structured Au–Ag alloy nanowires grown on n‐Si substrates perpendicularly. The anisotropic factors of PM‐ChAC‐E (gPM‐ChA) and SERS‐ChAC‐E (gSERS‐ChA) of CNAAFs in the range of 0.68–1.12 and 1.45–1.89 are obtained, respectively. Considering the increase of magnetic fields and SERS intensities with enantiomeric excess (ee) values of adsorbed enantiomers on the CNAAFs are higher than that of pure chiral nanostructure Au films, the enhancement of PM‐ChAC‐E and SERS‐ChAC‐E effects of CNAAFs can be attributed to the enhanced selective spin polarization coupling between CNAAFs and enantiomers. It is predicted to be due to the strong surface plasmon resonance generated by the active electrons of Ag under laser irradiation. [ABSTRACT FROM AUTHOR]

Simple Summary: Insects use olfaction, i.e., their sense of smell, to detect odors that elicit behavioral responses, with structurally similar compounds eliciting different responses. The roles of specific recognition proteins, i.e., odorant-binding proteins (OBPs) and odorant receptors (ORs), located in insect antennae, in discriminating between structurally similar compounds are not fully understood. Here, we explore current research in understanding the role of OBPs and ORs in discriminating between enantiomers—mirror image structures—in insect chemical ecology and chemoperception. Olfaction is a complex recognition process that is critical for chemical communication in insects. Though some insect species are capable of discrimination between compounds that are structurally similar, little is understood about how this high level of discrimination arises. Some insects rely on discriminating between enantiomers of a compound, demonstrating an ability for highly selective recognition. The role of two major peripheral olfactory proteins in insect olfaction, i.e., odorant-binding proteins (OBPs) and odorant receptors (ORs) has been extensively studied. OBPs and ORs have variable discrimination capabilities, with some found to display highly specialized binding capability, whilst others exhibit promiscuous binding activity. A deeper understanding of how odorant-protein interactions induce a response in an insect relies on further analysis such as structural studies. In this review, we explore the potential role of OBPs and ORs in highly specific recognition, specifically enantiomeric discrimination. We summarize the state of research into OBP and OR function and focus on reported examples in the literature of clear enantiomeric discrimination by these proteins. [ABSTRACT FROM AUTHOR]

Tip‐enhanced Raman scattering (TERS) has received significant interest as a new nano‐vibrational spectroscopy technique, which can be used to obtain site‐specific molecular information. TERS instruments have made remarkable progress, and the application of TERS has been expanding to nanomaterials, liquid samples, biological materials, and electrochemistry. Furthermore, molecular structures with imaging can be explored even at subnanometer resolution. This review consists of two parts: basic studies on the reproducibility of TERS and novel applications of TERS. In the former part, we report the electric field analysis, polarization, and excitation wavelength dependence of TERS. These studies are very important for understanding the characteristic features of TERS. In the latter part, we introduce two examples of TERS applications to nanomaterials, epitaxial graphene, and graphene oxide and two examples of liquid TERS that use chemically modified tips: nanoscale pH measurement and enantiomeric discrimination using TERS. Finally, we discuss the perspective of TERS. [ABSTRACT FROM AUTHOR]

A C2-symmetric host (host 1) was synthesized by derivatization of chiral diphenylethylenediamine with phenylisocyanate. High-resolution proton nuclear magnetic resonance (1H NMR) was used to investigate whether the chiral host could be used for enantiomeric discrimination of its structural analogues:α-phenylethylamine (guest 2), α-(p-methoxyphenyl)ethylamine (guest 9) and their derivatives (guests 3~8 and 10~13), and hydrogen-bonding interactions underlying the enantiomeric discrimination. It was observed that the host was able to recognize the urea and amide derivatives of the two primary amines, except for guests 7 and 12 bearing two NO2 groups. The host had stronger hydrogen-bonding interactions with the urea derivatives of guests 2 and 9 than with the corresponding amide derivatives, and high enantiomeric discrimination ability for the CHCH3 groups of the (R)- and (S)-urea derivatives.

A surface‐enhanced Raman scattering‐chiral anisotropy (SERS‐ChA) effect is reported that combines chiral discrimination and surface Raman scattering enhancement on chiral nanostructured Au films (CNAFs) equipped in the normal Raman scattering Spectrometer. The CNAFs provided remarkably higher enhancement factors of Raman scattering (EFs) for particular enantiomers, and the SERS intensity was proportional to the enantiomeric excesses (ee) values. Except for molecules with mesomeric species, all of the tested enantiomers exhibited high SERS‐ChA asymmetry factors (g), ranging between 1.34 and 1.99 regardless of polarities, sizes, chromophores, concentrations and ee. The effect might be attributed to selective resonance coupling between the induced electric and magnetic dipoles associated with enantiomers and chiral plasmonic modes of CNAFs. [ABSTRACT FROM AUTHOR]

The enantioselectivity of β-cyclodextrin (β-CD) towards L- and D-N-acetyltryptophan (NAcTrp) has been studied in aqueous solution and the crystalline state. NMR studies in solution show that β-CD forms complexes of very similar but not identical geometry with both L- and D-NAcTrp and exhibits stronger binding with L-NAcTrp. In the crystalline state, only β-CD–L-NAcTrp crystallizes readily from aqueous solutions as a dimeric complex (two hosts enclosing two guest molecules). In contrast, crystals of the complex β-CD–D-NAcTrp were never obtained, although numerous conditions were tried. In aqueous solution, the orientation of the guest in both complexes is different than in the β-CD–L-NAcTrp complex in the crystal. Overall, the study shows that subtle differences observed between the β-CD–L,D-NAcTrp complexes in aqueous solution are magnified at the onset of crystallization, as a consequence of accumulation of many soft host–guest interactions and of the imposed crystallographic order, thus resulting in very dissimilar propensity of each enantiomer to produce crystals with β-CD.

This article describes the enantiomeric discrimination properties of new chiral calix[4]arene derivatives bearing (S)-/(R)-1-phenylethylamine moieties (5a and 5b, respectively) towards the 1-phenylethylamine enantiomers on QCM surface. Initial experiments demonstrated that the 5b coated QCM sensor was the most effective sensing material for enantiomeric discrimination of (R)-/(S)-1-phenylethylamine by exhibiting much more sensing ability towards (R)-enantiomer than (S)-enantiomer. Sensitivity, detection limit and time constant of the 5b coated QCM sensor has been were calculated as 0.082 Hz/µM, 2.7 µM, and 319.2 s, respectively. Additionally, effects of calixarene content and different coating technique on enantiomeric discrimination, and Langmuir and Freundlich isotherms of the sensing results also were studied. As a result, it has been demonstrated that the coating of QCM sensor with a chiral calix[4]arene (5b) having (S)-1-phenylethylamine moieties provides substantially enantiomeric discrimination of 1-phenylethylamine enantiomers. [ABSTRACT FROM AUTHOR]

The identification of enantiomers is of great importance in chiral separations and medicinal chemistry. While Surface-enhanced Raman spectroscopy (SERS) is a technique that provides vibrational fingerprints of analytes. The enantiomers identification relies on the SERS difference between left and right-handed circularly polarized light or additional selectors for indirect distinction. In this work, Au–Ag core shell nanobipyramid (L/D-Au@Ag BPs) were synthesized guiding by chiral encoder of L/D-cysteine. L/D-Au@Ag BPs produced plasmon-induced circular dichroism signals in the plasmon resonance absorption band, which can be tuned by modulation the amount of cysteine. Moreover, the chiral anisotropy factor of L/D-Au@Ag BPs at 532 nm can reach 5.11 × 10−3. Due to the selective resonance coupling between L/D-Au@Ag BPs and different enantiomers, L/D-Au@Ag BPs were further used as SERS substrates for efficient discrimination of biologically relevant small molecules. Chiral Au@Ag BPs display the potential for chiral drug identification. [Display omitted] • Au–Ag core shell nanobipyramid (L/D-Au@AgBPs) were synthesized guiding by chiral encoder of L/D-cysteine. • L/D-Au@AgBPs produced plasmon-induced circular dichroism signals in the plasmon resonance absorption band. • The chiral asymmetry factor of L/D-Au@AgBPs at 532 nm can reach 5.11 × 10−3. • L/D-Au@AgBPs were used as a SERS substrate for efficient discrimination of chiral biomolecular. [ABSTRACT FROM AUTHOR]

In this work, the inclusion complexes of leucine as the guest and a series of cyclic peptides with four to nine alanine as building block (host molecules) were investigated by M062X/6-31 + G (d) level of theory. The calculations predicted the stability of the enantiomers in the cavity of cyclic peptides and the possibility of distinction between the enantiomers for the used level of theory. The results indicated that the most stable aggregate is formed between CyclAla9 and R and S- leucine enantiomers and CyclALA4 have the least interaction with R and S- leucine enantiomers. Geometrical changes of leucine due to complexation and hydrogen bonding were considered. The NBO calculations for analyzing the nature of the intermolecular hydrogen bonds were performed. In another part of this work, the UV absorption spectra of complexes of R, and S-leucine with CyclAla9 calculated using ONIOM method in the gas phase and it was showed that the UV absorption spectroscopy could be used as a chirality discriminator tool in recognition of enantiomers in the gas phase. [ABSTRACT FROM AUTHOR]

A new kind of chiral compounds with multiple amino, amido and phenolic hydroxy groups has been synthesized from d -phenylalanine and d -phenylglycine, respectively. The enantiomeric discriminations of α-hydroxy acids and N -Ts-α-amino acids have been finished in the presence of the above chiral compounds as chiral solvating agents by 1 H NMR spectroscopy. The results show that the chiral compounds are highly effective and practical chiral solvating agents towards α-hydroxy acids and N -Ts-α-amino acids. [ABSTRACT FROM AUTHOR]

Chiral carboxylic acids are ubiquitous in nature and are of pivotal importance in practical applications in the field of food science and pharmaceuticals. Simultaneous enantiomeric analysis of carboxylic acids mixtures is a significant research goal. Herein, we demonstrate a new strategy involving the use of the highly selective chiral derivatizing agent (R)-2-amino-1,1,1-trifluoropropane ((R)-TFPA) for the enantiomeric discrimination and selective identification of carboxylic acids in mixtures, even structurally similar drugs. The key success of this sensing approach lies in the distinguishable 19F NMR signals of diastereoisomers formed via derivatization of the substrates by (R)-TFPA. The utility of this approach was demonstrated by simultaneously differentiating 20 chiral carboxylic acids in a complicated mixture with accurate determination of the enantiomeric excess (ee). Most importantly, our approach can be applied to food analysis, where the diverse carboxylic acids in real samples without separation were unambiguously identified, such as vinegar, yogurt, and grape. [Display omitted] • Simultaneous analysis of multiple carboxylic acids in complex mixtures based on 19F NMR spectroscopy. • Enantiomeric discrimination and selective identification of carboxylic acids. • Accurate determination of the enantiomeric excess. • Identification of carboxylic acids in real samples without separation. • In - situ selective identification of carboxylic acids in various foods. [ABSTRACT FROM AUTHOR]

The work involves an experimental (1H NMR) and theoretical (MD, MM-PBSA and DFT) investigation of the molecular recognition and discrimination properties of a chiral aza-15-crown-5 against methyl esters of alanine, phenylalanine and valine hydrochloride salts. The results indicate that the receptor binds enantiomers with moderate binding constants (88–1,389 M−1), with phenylalanine being more discriminated. The difference in experimental binding free energies (ΔG R − ΔG S) for alanine, phenylalanine and valine enantiomers were calculated as −0.36, −1.58 and 0.80 kcal mol−1, respectively. The differences in theoretical binding energies were calculated by MM-PBSA (ΔE R PB − ΔE S PB=) as −0.30, −1.45 and 0.88, by B3LYP/6-31+G(d) as −1.17, −0.84 and 0.74 and by M06-2X/6-31+G(d) as −1.40, −3.26 and 1.66 kcal mol−1. The data obtained give valuable information regarding the molecular recognition mode of the organoammonium complexes of chiral aza-crown ether with C 1 symmetry, which may be relevant to biological systems. [ABSTRACT FROM AUTHOR]

Resonant plasmonic-molecular chiral interactions are a promising route to enhanced biosensing. However, biomolecular optical activity primarily exists in the far-ultraviolet regime, posing significant challenges for spectral overlap with current nano-optical platforms. We demonstrate experimentally and computationally the enhanced chiral sensing of a resonant plasmonic-biomolecular system operating in the far-UV. We develop a full-wave model of biomolecular films on Al gammadion arrays using experimentally derived chirality parameters. Our calculations show that detectable enhancements in the chiroptical signals from small amounts of biomolecules are possible only when tight spectral overlap exists between the plasmonic and biomolecular chiral responses. We support this conclusion experimentally by using Al gammadion arrays to enantiomerically discriminate ultrathin (<10 nm thick) films of tyrosine. Notably, the chiroptical signals of the bare films were within instrumental noise. Our results demonstrate the importance of using far-UV active metasurfaces for enhancing natural optical activity.

l-Tryptophan containing anion receptors 1 and 2 have shown interesting features. Receptor 1 behaves as a hetero-ditopic dicompartimental receptor for fluoride and chloride with a certain selectivity for fluoride as supported by 1H NMR titrations and molecular modeling, while receptor 2 forms only 1:1 complexes. Moreover, receptor 1 reveals a certain affinity towards the l enantiomers of amino acids alanine and serine. [ABSTRACT FROM PUBLISHER]

Abstract: The application of the G-SERFph pulse sequence is presented on enantiomeric mixtures dissolved in a chiral liquid crystal. It aims at editing, within one single 2D spectrum, every proton coupling which is experienced by a given proton site in the molecule, and leads to real phased T-edited spectroscopy (T = J +2D). This NMR experiment is based on the combination of homonuclear semi-selective refocusing techniques with a spatial frequency encoding of the sample. This approach, which consists in handling selectively each coupling in separate cross sections of the sample, is applied to the visualization of enantiomers dissolved in a chiral liquid crystalline phase. Advantages and limits of this methodology are widely discussed. [Copyright &y& Elsevier]

Abstract: The kinetic method has been applied for determination of d-Phe/l-Phe enantiomeric ratio. Discrimination of enantiomers was inferred from product ion mass spectra of trimeric cluster ions containing the analyte (l,d-Phe), Cu2+ as a central metal and l-Trp as a chiral reference ligand. Unsatisfactory quantitative results achieved on an ion trap were rationalized by high-resolution mass spectrometry. The formation of Mg2+-containing cluster isobaric to trimeric cluster [Cu(l-Trp)2Phe]+ was observed. Interference like this was identified as a possible reason for deterioration of quantitative low-resolution mass spectrometric analyses of real-world samples based on the kinetic method. Cation-exchanger was used for easy removal of magnesium from a sample and improvement of quantitation. Chiral dependence of formation of the Cu2+-containing trimeric cluster was also observed. Heterochiral diastereoisomeric ions were created less effectively. [Copyright &y& Elsevier]

Abstract: The chiral recognition mechanism of Penicillin G Acylase (PGA) was investigated with a set of 18 new chiral acidic compounds. A series of 2-aryloxyalkyl- and 2-arylalkyl-2-aryloxyacetic acids in which the absolute configuration has been reported to exert a strong influence on pharmacological activity, were synthesized and analysed on PGA-based chiral stationary phase (CSP) and 11 racemates were completely resolved with a mobile phase composed of 50mM phosphate buffer (pH 7.0). The influence of structural variations of analytes on retention and enantioselectivity was investigated by application of molecular modelling studies. Docking experiments were also carried out to rationalize the observed enantioselective behaviour. The computation approach revealed to be helpful in elucidating the molecular basis of the enantioselectivity observed on PGA-CSP. [Copyright &y& Elsevier]

Abstract: Spectroscopic evidence was used to demonstrate the formation of molecular associates in an aqueous solution of phthalimido tryptophan. These molecular associates are loosely formed through π–π aromatic stacking, properties that are not sufficient to cause NMR spectroscopic enantiomeric discrimination. A cyclomaltooligosaccharide with a larger cavity, such as cyclomaltooctaose (γ-cyclodextrin), is capable of forming a ternary complex with these molecular associates and enhances π–π aromatic stacking interactions, resulting in NMR enantiomeric discrimination. Electrospray-ionization mass spectroscopy (ESIMS) and NOESY two-dimensional NMR spectroscopic methods were used to study these complexes. Association constants and thermodynamic data for these cyclomaltooligosaccharide complexes were also estimated. [Copyright &y& Elsevier]

Plants emit complex blends of volatiles, including chiral compounds that might be detected by vertebrates and invertebrates. Insects are ideal model organisms for studying the underlying receptor neuron mechanisms involved in olfactory discrimination of enantiomers. In the present study, we have employed two-column gas chromatography linked to recordings from single olfactory receptor neurons of Mamestra brassicae, in which separation of volatiles in a polar and a chiral column was performed. We here present the response properties of olfactory receptor neurons tuned to linalool. The narrow tuning of these receptor neurons was demonstrated by their strong responses to (R)-(−)-linalool, the weaker responses to the (+)-enantiomer as well as a few structurally related compounds, and no responses to the other numerous plant released volatiles. The enantioselectivity was verified by parallel dose-response curves, that of (R)-(−)-linalool shifted 1 log unit to the left of the (S)-(+)-linalool curve. A complete overlap of the temporal response pattern was found when comparing the responses of the same strength. Analysis of the spike amplitude and waveform indicated that the responses to the two enantiomers originated from the same neuron. [ABSTRACT FROM AUTHOR]

Chiral crown ethers 1 and 5 are useful enantiomeric discriminating agents in 1H NMR spectroscopy for neutral and protonated primary amines, amino acids, and amino alcohols. The presence of the carboxylic acid groups in 1 and 5 provide sites at which ytterbium(III) can bind. Adding ytterbium(III) nitrate to crown–substrate mixtures in methanol-d4 causes shifts in the spectra of substrates and often enhances the chiral discrimination in the 1H NMR spectrum. The enhancement in enantiomeric discrimination that occurs in the presence of ytterbium(III) allows lower concentrations of the crown ether to be used in chiral recognition studies. Several amide derivatives of 1 were prepared and evaluated as chiral NMR discriminating agents, although except for 1e, these were less effective than 1. [ABSTRACT FROM AUTHOR]

We report here an unexpected difference in the solubilities of D- and L-tyrosine in water, which could be discerned by their rate of crystallization and the resulting concentrations of their saturated solutions. A supersaturated solution of 10 mM L-tyrosine at 20 °C crystallized much more slowly than that of D-tyrosine under the same conditions, and the saturated solution of L-tyrosine was more concentrated than that of D-tyrosine. Supersaturated solutions of 10 mM DL-tyrosine in water formed precipitates of predominantly D-tyrosine and DL-tyrosine, resulting in an excess of L-tyrosine in the saturated solution. The experimental setups were monitored independently by UV-absorption, radioactivity tracing, optical rotation and X-ray diffraction. The process of nucleation and crystallization of D- and L-tyrosine is characterized by an exceptionally high cooperativity. It is possible that minute energy differences between D- and L-tyrosine, originating from parity violation or other non-conservative chiral discriminatory rules, could account for the observations. The physical process that initiated chiral selection in biological systems remains a challenging problem in understanding the origin of life, and it is possible that chiral compounds were concentrated from supersaturated racemic mixtures by preferential crystallization. [ABSTRACT FROM AUTHOR]

The sex pheromone for Anomala octiescostata has been previously identified as a 4:1 mixture of ( R)-buibuilactone and ( R)-japonilure. Field experiments showed that although not active per se, the secondary component increased the activity of major component, buibuilactone. The enantiomers of these semiochemicals, i.e., ( S)-buibuilactone and ( S)-japonilure are neither attractants nor behavioral antagonists to the pheromone system of A. octiescostata. Chiral GC-EAD experiments demonstrated that, in marked contrast to the Japanese beetle and Osaka beetle, which can detect a stereoisomer not produced by their conspecific females, the male antennae of A. octiescostata lack olfactory receptor neurons (ORNs) tuned to ( S)-buibuilactone. The same enantiomeric anosmia (inability to smell a compound) was also observed in Anomala cuprea male antennae. Both in a Y-olfactometer and field experiments, it was clarified that ( S)-buibuilactone was not a behavioral antagonist for A. cuprea. Contrary to male antennae of both A. octiescostata and A. cuprea, which lack ORNs tuned to the nonnatural ( S)-buibuilactone, fragrance evaluation by skilled perfumers revealed that the human nose can discriminate (both in intensity and quality) enantiomerically pure samples of ( R)- and ( S)-buibuilactone and japonilure. [ABSTRACT FROM AUTHOR]

Several derivatives of tyrosine or its methyl ester have been synthesized in which the para hydroxyl group on the aromatic side chain has been converted to a long chain alkyl ether or urethane. The surface behavior of these compounds is discussed. Enantiomeric discrimination, the ability of a chiral molecule to distinguish between mirror-image stereoisomers, has been investigated for the urethane derivates by comparing the surface pressure-area isotherms of the enantiomer with that of the racemic compound. Enantiomeric discrimination was demonstrated for the methyl ester analog, but was not conclusively observed for the zwitterionic compounds. Possible structural requirements for enantiomeric discrimination in a chiral molecule are discussed. [ABSTRACT FROM AUTHOR]

The enantioselectivity of β-cyclodextrin (β-CD) towards L- and D- N -acetyltryptophan (NAcTrp) has been studied in aqueous solution and the crystalline state. NMR studies in solution show that β-CD forms complexes of very similar but not identical geometry with both L- and D-NAcTrp and exhibits stronger binding with L-NAcTrp. In the crystalline state, only β-CD-L-NAcTrp crystallizes readily from aqueous solutions as a dimeric complex (two hosts enclosing two guest molecules). In contrast, crystals of the complex β-CD-D-NAcTrp were never obtained, although numerous conditions were tried. In aqueous solution, the orientation of the guest in both complexes is different than in the β-CD-L-NAcTrp complex in the crystal. Overall, the study shows that subtle differences observed between the β-CD-L,D-NAcTrp complexes in aqueous solution are magnified at the onset of crystallization, as a consequence of accumulation of many soft host-guest interactions and of the imposed crystallographic order, thus resulting in very dissimilar propensity of each enantiomer to produce crystals with β-CD.