Recent advances of optically active helical polymers as adsorbents and chiral stationary phases for chiral resolution

Chiral resolution is very important and still a big challenge due to different biological activity and same physicochemical property of one pair (R)- and (S)-isomer. There is no doubt that chiral selectors are essentially needed for chiral resolution, which can stereoselectively interact with a pair of isomers. To date, a large amount of optically active helical polymers as chiral selectors have been synthesized via two strategies. First, the target helical polymers are derived from natural polysaccharide such as cellulose and amylose. Second, they can be synthesized by polymerization of chiral monomers. Alternatively, an achiral polymer is prepared first followed by static or dynamic chiral induction. Furthermore, a part of them is harnessed as chiral stationary phases for chromatographic chiral separation and as chiral adsorbents for enantioselective adsorption/crystallization, resulting in good enantioseparation efficiency. In summary, the present review will focus on recent progress of the polymers with optical activity for chiral resolution, especially the literature published in the past 10 years. In addition, development prospects and future challenges of optically active helical polymers will be discussed in detail.

Chiral Recognition of Amino Acids Using CC2 Porous Organic Cages

Enantiomers have the same physical properties but different chemical properties due to the difference in the orientation of groups in space and thus Chiral discrimination is quite necessary, as an enantiomer of drug can have lethal effects. In this study, we used the CC2 cage for chiral discrimination of amino acids using density functional theory. The results indicated the physisorption of amino acids in the central cavity of the cage. Among the four selected amino acids, proline showed maximum interactions with the cage and maximum chiral discrimination energy is also observed in the case of proline that is 2.78 kcal/mol. Quantum theory of atoms in molecules and noncovalent interaction index analyses showed that the S enantiomer in each case has maximum interactions. The charge transfer between the analyte and surface is further studied through natural bond orbital analysis. It showed sensitivity of cage for both enantiomers, but a more pronounced effect is seen for S enantiomers. In frontier molecular orbital analysis, the least E_H-L gap is observed in the case of R proline with a maximum charge transfer of -0.24 e^-. Electron density difference analysis is carried out to analyze the pattern of the charge distribution. The partial density of state analysis is computed to understand the contribution of each enantiomer in overall density of the complexes. Our results show that S-CC2 porous organic cages have a good ability to differentiate between two enantiomers. S-CC2 porous organic cages efficiently differentiated the S enantiomer from the R enantiomers of selected amino acids.

Recognition and Sensing of Chiral Organic Molecules by Chiral Porphyrinoids: A Review

Porphyrinoids are extremely attractive for their electronic, optical, and coordination properties as well as for their versatile substitution at meso/β-positions. All these features allow porphyrinoids to behave as chiroptical hosts for chiral recognition by means of non-covalent interactions towards chiral guests. Over the years, chiral discrimination of chiral molecules such as amino acids, alcohols, amines, hydroxy-carboxylic acids, etc. has aroused the interest of the scientific community. Hence, this review aims to report on the progress to date by illustrating some relevant research regarding the chiral recognition of a multitude of chiral organic guests through several chiral mono- and bis-porphyrins via different spectroscopic techniques.

Electrochemical Polymerization Induced Chirality Fixation of Crystalline Pillararene-Based Polymer and Its Application in Interfacial Chiral Sensing

A new strategy has been developed for the direct chirality fixation, which is induced by electrochemical polymerization, of macrocyclic hosts pillar[5]arene. Taking advantage of electrochemical polymerization, thiophene-modified pillar[5]arene monomers (Th-P[5]A) have been regularly arranged under the action of an electric field to form chiral nanofiber-like crystalline pillar[5]arene-based polymers (poly-Th-P[5]A), showing a significant circular dichroism (CD) signal. With the active photochemical properties, poly-Th-P[5]A is first used as a photoelectrochemical (PEC) chiral sensor for the identification and determination of l- and d-ascorbic acid (l-AA, d-AA) without adding any extra photoactive probes. Importantly, the chiral recognition between poly-Th-P[5]A and l-AA also triggers a polarity conversion for the photocurrent of the polymer, and it greatly results in a broad chiral detection range for l-AA, crossing 6 orders of magnitude. This work provides a promotional strategy for building a PEC chiral recognition platform based on pillararenes.

Recent advances of the ionic chiral selectors for chiral resolution by chromatography, spectroscopy and electrochemistry

Ionic chiral selectors have been received much attention in the field of asymmetric catalysis, chiral recognition, and preparative separation. It has been shown that the addition of ionic chiral selectors can enhance the recognition efficiency dramatically due to the presence of multiple intermolecular interactions, including hydrogen bond, π-π interaction, van der Waals force, electrostatic ion-pairing interaction, and ionic-hydrogen bond. In the initial research stage of the ionic chiral selectors, most of work center on the application in chromatographic separation (capillary electrophoresis, high-performance liquid chromatography, and gas chromatography). Differently, more and more attention has been paid on the spectroscopy (nuclear magnetic resonance, fluorescence, ultraviolet and visible absorption spectrum, and circular dichroism spectrum) and electrochemistry in recent years. In this tutorial review as regards the ionic chiral selectors, we discuss in detail the structural features, properties, and their application in chromatography, spectroscopy, and electrochemistry.

Electrochemical enantioselective sensor for effective recognition of tryptophan isomers based on chiral polyaniline twisted nanoribbon

Effective enantioselective recognition with chiral nanomaterials remains a challenge in the field of chemistry and biology. In this paper, a pair of left- and right-handed polyaniline (defined as S-PANI and R-PANI) were synthesized by chemical oxidation of aniline to form a specially twisted nanoribbon, which was induced by enantiomeric camphorsulfonic acid. Both S-PANI and R-PANI were used to construct electrochemical chiral sensors for the discrimination of tryptophan isomers (D- and L-Trp). Owing to the formation of efficient chiral nanospace with special nanoribbon morphology and enormous amounts of oxygen-containing functional groups of S-PANI or R-PANI, the high enantioselectivity was obtained with the recognition efficiency of 4.90 (D-Trp) on S-PANI and 4.20 (L-Trp) on R-PANI, respectively. The obtained chiral electrodes were also used for the determination of the enantiomeric excess (ee) for Trp, and a good linear relationship between peak currents and ee% of Trp was obtained. Furthermore, the strategy we proposed has tremendous potential in enantiomer recognition field.

Ionic Current Rectification Triggered Photoelectrochemical Chiral Sensing Platform for Recognition of Amino Acid Enantiomers on Self-Standing Nanochannel Arrays

Chirality is an intrinsic and essential property of nature, and the enantiomeric discrimination of chiral molecules can provide important information leading to a better understanding of chiral recognition in biological systems and furthering the development of useful molecular devices in biochemical and pharmaceutical studies. Therefore, the exploration of new detection techniques with high accuracy and reliability for high-throughput enantioselective detection is still highly desirable. Herein, a chiral enantiomers selective recognition platform based on self-standing titanium dioxide nanochannel arrays (TiO₂ NCAs) is proposed to implement the ionic current rectification triggered photoelectrochemical (PEC) detection, which provides a novel sensing platform to discriminate chiral amino acid through synchronous output dual response signals of ionic current and photocurrent. The utilization of nanochannel arrays guaranteed the high-throughput detection, and the dual signal model avoided a "false positive" detection. In addition, based on the fabricated detection platform, various chiral substances can be facilely detected through integrating different chiral regulation units, which shed light on the construction of reliable chiral sensors for practical applications in a biological environment.

Chiral electrochemical recognition of tryptophan enantiomers at a multi-walled carbon nanotube–N-carboxymethyl chitosan composite-modified glassy carbon electrode

A simple chiral electrochemical sensor based on N-carboxymethyl chitosan covalently binding with ethylenediamine-carboxylic multiwalled carbon nanotubes was developed for recognition of tryptophan enantiomers.

A new nanosensor for the chiral recognition of cysteine enantiomers based on gold nanorods

A schematic illustration of the chiral recognition of d-Cys and l-Cys using GNRs in the presence of Cu²⁺ (0.125 mM).

A highly selective and sensitive chemosensor forl-tryptophan by employing a Schiff based Cu(ii) complex

Construction of a new Cu(ii) complex (1) based modified glassy carbon electrode (1-GCE) for highly selective and sensitive detection ofl-tryptophan (l-Trp).

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%).

Ternary system based on fluorophore-surfactant assemblies--Cu²⁺ for highly sensitive and selective detection of arginine in aqueous solution

A new cationic dansyl derivative-based (DIlSD) fluorescence probe was designed and synthesized. Its combination with anionic surfactant SDS assemblies shows enhanced fluorescence intensity and blue-shifted maximum wavelength. Its fluorescence can be slightly quenched by Cu(2+); however, the fluorescence quenching efficiency by Cu(2+) is highly increased upon titration of arginine (Arg). As a result, the ternary system containing the cationic fluorophore, anionic surfactant, and Cu(2+) functions as a highly sensitive and selective sensor to Arg. The optimized sensor system displays a detection limit of 170 nM, representing the highest sensitivity to Arg in total aqueous solution by a fluorescent sensor. Control experiments reveal that the imidazolium groups in the fluorophore, the anionic surfactant, and Cu(2+) all play important roles in the process of sensing Arg. The electrostatic interaction between the cationic fluorophore and anionic surfactants facilitates the binding of imidazolium rings with Cu(2+), the surfactant surface-anchored Cu(2+) is responsible for further binding of Arg, and the electrostatic interaction between anionic surfactants and positively charged amino acids accounts for the selective responses to Arg.

Visual chiral recognition of tryptophan enantiomers using unmodified gold nanoparticles as colorimetric probes

A simple protocol to distinguish enantiomers is extremely intriguing and useful. In this study, we propose a low-cost, facile, sensitive method for visual chiral recognition of enantimers. It is based on the inherent chirality of gold nanoparticles (AuNPs), and the unmodified AuNPs are used as chiral selector for D- and L-Tryptophan (Trp). In the presence of D-Trp, an appreciable red-to-blue color change of AuNPs solution can be observed, whereas no color change is found in the presence of L-Trp. The method can be used to detect D-Trp in the range of 0.2-10 μM, and the limit of detection is 0.1 μM. The chiral assay described in this work is easily readout with the naked eye or using a UV-vis spectrometer. Furthermore, the AuNPs can selectively adsorb D-Trp, and simple centrifugation can allow the precipitation of D-Trp with AuNPs and leave a net excess of the other enantiomer in solution, thus resulting in enantioseparation. In this method, AuNPs do not need any labeling or modifying with chiral molecules. The method is more attractive because of its high sensitivity, low cost, ready availability and simple manipulation.