Optical Chirality Sensing with an Auxiliary‐Free Earth‐Abundant Cobalt Probe

A bifunctional coumarin-based CD probe for chiral analysis of amino acids in aqueous solution

Amino acids play important roles in human pathology and physiology and the qualitative and quantitative determination of chiral amino acids in humans and mammals also has important impacts on the life sciences. Therefore, the introduction of artificial probes to assess the concentrations and enantiomeric compositions [ee = ([D] - [L])/([D] + [L])] of amino acids in aqueous solution is necessary in understanding certain biological processes and diagnosing and treating diseases. Herein, a bifunctional achiral coumarin probe (Br-Coumarin) is reported to determine the absolute configuration, ee value, and concentration of 16 amino acids in THF/H2O = 1/4 solution at micromolar concentrations. The effectiveness and practicability of the sensing methods are illustrated through the evaluation of various ee values and concentrations of Cys, Pro, and Phe samples with good accuracy. Besides, the reactions of the probe with various amino acids exhibit different colors under daylight or UV light (365 nm). Particularly, Br-Coumarin can be used to visually detect Cys, Lys, Arg, and Pro with high selectivity under both daylight and UV light (365 nm) and exhibits excellent selectivity and sensitivity for the fluorescent recognition of Cys.

Chiral Optical Sensing of Amino Acids with 2-Trifluoromethyl Benzaldehyde for Ophiopogon japonicus Authentication

The detection and analysis of chiral molecules have long been challenging in analytical chemistry. This study introduces a novel approach that utilizes 2-trifluoromethyl benzaldehyde as a small-molecule probe capable of forming a stable Schiff base with chiral amino acids in aqueous solution under alkaline conditions. The amino acid Schiff bases present a strong Cotton effect and UV absorption at wavelengths exceeding 260 nm, enabling chiral analysis, including assignment of absolute configuration, enantiomeric composition, and total concentration. An application of this method was the authentication of the herbal medicine Ophiopogon japonicus. Using principal component analysis and orthogonal partial least squares discriminant analysis, we successfully differentiated O. japonicus samples collected in two distinct locations with 20 samples. This rapid and convenient method offers a new approach to quality control of herbal medicine.

A stereodynamic probe of Pt(II) molecular hinge for chiroptical sensing of cryptochiral compounds

Cryptochiral compounds are indispensable in biology systems and medical applications, yet the development of high-performance detection method remains challenging. Here, we report a hinge-like dinuclear Pt(II) complex as a stereodynamic probe for cryptochirality sensing. This probe features a constrained, closed conformation achieved through synergistic intramolecular π - π stacking and metal-metal interactions between cyclometalated Pt(II) wings. The probe shows a robust chiroptical response to center-to-axial chiral induction during the rapid condensation with cryptochiral compounds that adopts a more energetically favorable conformation. The resulting intense Cotton effects at long wavelengths enable in-situ circular dichroism analysis to determinate the absolute configuration and enantiometric composition of the cryptochiral molecules. Furthermore, the near-infrared phosphorescence characteristics of the hinge have been explored to develop it as an optical sensor for the accurate quantification of cryptochiral molecules. With its dual CD and phosphorescence capabilities, this probe shows great potential for high-throughput screening analysis.

Molecular Sensing of Chiral Carboxylic Acid Enantiomers Using CD Inductions in the Visible Light Region

The reaction between a chiral carboxylic acid molecule and 1,1'-bis(diphenylphosphino)ferrocenepalladium dichloride in the presence of a mild base generates a chiroptically active metal complex displaying strong circular dichroism (CD) signals in the visible light region, a highly sought-after goal in the optical sensing realm. The molecular recognition process is complete within a few minutes and can be used for fast chiroptical determination of the enantiomeric composition and concentration of carboxylic acid samples. This method is operationally simple and broadly applicable to a large variety of structures including important drugs, natural products, amino acids, and hydroxy acids. All components needed are commercially available, and this optical sensing assay can be readily adapted by any laboratory interested in chirality analysis and high-throughput experimentation.

Optical Relay Sensing of Cryptochiral Alcohols Displaying α-, β-, γ- and δ-Stereocenters or Chirality by Virtue of Isotopic Substitution

A reaction-based optical relay sensing strategy that enables accurate determination of the concentration and enantiomeric ratio (er) of challenging chiral alcohols exhibiting stereocenters at the α-, β-, γ- or even δ-position or hard-to-detect cryptochirality arising from H/D substitution is described. This unmatched application scope is achieved with a conceptually new sensing approach by which the alcohol moiety is replaced with an optimized achiral sulfonamide chromophore to minimize the distance between the covalently attached chiroptical reporter unit and the stereogenic center in the substrate. The result is a remarkably strong, red-shifted CD induction that increases linearly with the sample er. The CD sensing part of the tandem assay is seamlessly coupled to a redox reaction with a quinone molecule to generate a characteristic UV response that is independent of the enantiopurity of the alcohol and thus allows determination of the total analyte concentration. The robustness and utility of the CD/UV relay are further verified by chromatography-free asymmetric reaction analysis with small aliquots of crude product mixtures, paving the way toward high-throughput chiral compound screening workflows which is a highly sought-after goal in the pharmaceutical industry.

Circular Dichroism Sensing: Strategies and Applications

The analysis of the absolute configuration, enantiomeric composition, and concentration of chiral compounds are frequently encountered tasks across the chemical and health sciences. Chiroptical sensing methods can streamline this work and allow high-throughput screening with remarkable reduction of operational time and cost. During the last few years, significant methodological advances with innovative chirality sensing systems, the use of computer-generated calibration curves, machine learning assistance, and chemometric data processing, to name a few, have emerged and are now matched with commercially available multi-well plate CD readers. These developments have reframed the chirality sensing space and provide new opportunities that are of interest to a large group of chemists. This review will discuss chirality sensing strategies and applications with representative small-molecule CD sensors. Emphasis will be given to important milestones and recent advances that accelerate chiral compound analysis by outperforming traditional methods, conquer new directions, and pioneering efforts that lie at the forefront of chiroptical high-throughput screening developments. The goal is to provide the reader with a thorough understanding of the current state and a perspective of future directions of this rapidly emerging field.

Chiroptical Recognition of Carboxylates with Charge-Neutral Double-Stranded Zinc(II) Helicates

Chirality analysis of small molecules for the determination of their enantiopurity is nowadays ruled by streamlined chromatographic methods which utilize chiral stationary phases. Chiroptical probes which rely on host-guest interactions are so far overshadowed by the latter but have the benefit of depending only on common spectroscopic techniques such as CD spectroscopy to distinguish enantiomers and to quantify their ratio. Interest into this receptor-based approach is constantly rising because non-invasive high-throughput screenings with a minimal waste production can be performed. In this study we investigate the possibility to utilize metal-based containers in form of charge-neutral helicates able to recognize anions for this purpose. Key building block of the helicates are triazole units which show rotational freedom and give rise to either a meso-structure or a racemic mixture of the right- and left-handed complex. A chiroptical response of the probe is observed upon recognition of chiral mono- or dicarboxylates and chirality analysis of tartrate is conducted by CD spectroscopy.

Quinine-Fabricated Surface-Enhanced Raman Spectroscopy Chiral Sensing Platform Enables Simultaneous Enantioselective Discrimination and Identification of Aliphatic Amino Acids

Due to low optical activity and structural simplicity, synchronous chiral discrimination and identification of aliphatic amino acids (AAs) are still challenging yet demanding. Herein, we developed a novel surface-enhanced Raman spectroscopy (SERS)-based chiral discrimination-sensing platform for aliphatic AAs, in which l- and d-enantiomers are able to discriminately bind with quinine to generate distinct differences in the SERS vibrational modes. Meanwhile, the plasmonic sub-nanometer gaps supported by the rigid quinine enable the maximization of SERS signal enhancement to reveal feeble signals, allowing for simultaneously acquiring the structural specificity and enantioselectivity of aliphatic amino acid enantiomers in a single SERS spectrum. Different kinds of chiral aliphatic AAs were successfully identified by using this sensing platform, demonstrating its potential and practicality in recognizing chiral aliphatic molecules.

Fluorine-based Zn salan complexes

We synthesised and characterised the racemic and chiral versions of two Zn salan fluorine-based complexes from commercially available materials. The complexes are susceptible to absorbing H₂O from the atmosphere. In solution (DMSO-H₂O) and at the millimolar level, experimental and theoretical studies identify that these complexes exist in a dimeric-monomeric equilibrium. We also investigated their ability to sense amines via¹⁹F NMR. In CDCl₃ or d⁶-DMSO, strongly coordinating molecules (H₂O or DMSO) are the limiting factor in using these easy-to-make complexes as chemosensory platforms since their exchange with analytes requires an extreme excess of the latter.

Chiral Co3Y Propeller-Shaped Chemosensory Platforms Based on 19F-NMR

Two propeller-shaped chiral Co^III₃Y^III complexes built from fluorinated ligands are synthesized and characterized by single-crystal X-ray diffraction (SXRD), IR, UV-vis, circular dichroism (CD), elemental analysis, thermogravimetric analysis (TGA), electron spray ionization mass spectroscopy (ESI-MS), and NMR (¹H, ¹³C, and ¹⁹F). This work explores the sensing and discrimination abilities of these complexes, thus providing an innovative sensing method using a ¹⁹F NMR chemosensory system and opening new directions in 3d/4f chemistry. Control experiments and theoretical studies shed light on the sensing mechanism, while the scope and limitations of this method are discussed and presented.

Chiral Recognition of Hydantoin Derivatives Enabled by Tetraaza Macrocyclic Chiral Solvating Agents Using 1H NMR Spectroscopy

Enantiomers of a series of hydantoin derivatives were prepared from d- and l-amino acids with p-tolyl isocyanate and 3,5-bis(trifluoromethyl)phenyl isocyanate as guests for chiral recognition by ¹H NMR spectroscopy. Meanwhile, several tetraaza macrocyclic compounds were synthesized as chiral solvating agents from d-phenylalanine and (1S,2S)-(+)-1,2-diaminocyclohexane. An uncommon enantiomeric discrimination has been successfully established for hydantoin derivatives, representatives of five-membered N,N-heterocycles, in the presence of tetraaza macrocyclic chiral solvating agents (TAMCSAs) 1a-1c by means of ¹H NMR spectroscopy. Several unprecedented nonequivalent chemical shifts (up to 1.309 ppm) were observed in the split ¹H NMR spectra. To evaluate practical applications in the determination of enantiomeric excess (ee), the ee values of samples with different optical purities (up to 95% ee) were accurately calculated by the integration of relevant proton peaks. To better understand the chiral discriminating behavior, Job plots of (±)-G1 with TAMCSA 1a were investigated. Furthermore, in order to further explore any underlying intermolecular hydrogen bonding interactions, theoretical calculations of the enantiomers of (S)-G1 and (R)-G1 with TAMCSA 1a were performed by means of the hybrid density functional theory (B3LYP/6-31G*) of the Gaussian 16 program.

Four-Component Ugi Reaction for Optical Chirality Sensing and Surface Nanoengineering of Chiral Self-Assemblies

Using green chemistry to control chirality at hierarchical levels as well as chiroptical activities endows with new opportunities to the development of multiple functions. Here, the four-component Ugi reaction is introduced for the general and precise optical chirality sensing of amines as well as the surface nanoengineering of chiral soft self-assemblies. To overcome the relatively weak Cotton effects, direct synthesis of a folded peptide structure on a rotatable ferrocene core with axial chirality was accomplished from chiral amine, 1,1'-ferrocenyl dicarboxylic acid, formaldehyde and isocyanide. Enhanced Cotton effects benefiting from the folded structure allow for the precise and quantitative sensing of natural and synthetic chiral amines covering alkyl, aromatic amines and amino acid derivatives. In addition, aqueous reaction enables the modification of amine-bearing dye to microfibers self-assembled from π-conjugated amino acids. Surface dye-modification via Ugi reaction barely changes the pristine morphology, showing non-invasive properties in contrast to dye staining, which is applicable in soft nano/microarchitectures from self-assembly. This work which combines the four-component Ugi reaction to enable precise ee% detection and surface nanoengineering of soft chiral assemblies sheds light on the advanced application of green chemistry to chirality science.

Wide-Range and Highly Sensitive Chiral Sensing by Discrete 2D Chirality Transfer on Confined Surfaces of Au(I)-Thiolate Nanosheets

Circular dichroism (CD) chiral sensing is very promising to meet the ever-increasing demands for high-throughput chiral analysis in asymmetric synthesis. However, it is still very challenging to sensitively quantify the composition of enantiomers in a wide concentration range because the existing sensing systems show either linear CD response resultant from stoichiometric chiral transfer or nonlinear CD response resultant from amplified chiral transfer and thus have the drawbacks of low sensitivity and narrow quantification range, respectively. Herein, we propose a sensing system of two-dimensional (2D) Au(I)-thiolate nanosheets. The disordered interligand interactions on the confined surfaces of nanosheets enable the formation of discrete amplified chiral domains around the adsorbed chiral analytes, resulting in a linearly amplified chiral transfer behavior, which provides a solution for highly sensitive and wide-range quantification of enantiomer compositions. Taking (1R, 2R)-(-)- and (1S, 2S)-(+)-1,2-diamino cyclohexanes as example analytes, the concentration and full-range enantiomeric excess (ee) values have been quickly determined by adsorbing them on the surface of Au(I)-MPA (MPA: 3-mercaptopropionic acid) nanosheets in the concentration range of 1.0 × 10^-6 to 4.0 × 10^-5 M. By engineering the surface functional groups, Au(I)-thiolate nanosheets can be extended to sense other types of analytes, and several polyols with multiple chiral centers have been sensed by boronic acid functionalized nanosheets at the 10^-7 M level. The high performances, good extendibility, and one-pot high-yield aqueous synthesis ensure these Au(I)-thiolate nanosheets can be developed as a practical and powerful chiral sensing platform.

Concentration and pH effect on the electronic circular dichroism and anisotropy spectra of aqueous solutions of glyceric acid calcium salt

Electronic circular dichroism (ECD) and anisotropy spectra carry information on differential absorption of left- and right-circularly polarized light (LCPL and RCPL) by optically active compounds. This makes them powerful tools for the rapid determination of enantiomeric excesses (ee) in asymmetric synthetic and pharmaceutical chemistry, as well as for predicting the ee inducible by ultraviolet (UV) CPL. The ECD response of a chiral molecule is, however, critically dependent on the properties of the surrounding medium. Here, we report on the first ECD/anisotropy spectra of aqueous solutions of the calcium salt dihydrate of glyceric acid. A systematic study of the effect of the salt concentration and pH on the chiroptical response revealed significant changes and the appearance of a new ECD band of opposite sign. Based on the literature, this can be rationalized by the increase in the relative proportion of free glyceric acid/glycerate to Ca²⁺ complexes with glycerate with decreasing salt concentration or pH. Glyceric acid can be readily produced under astrophysical conditions. The anisotropy spectra of the solution containing prevalently the free form of this dihydroxy carboxylic acid resemble the ones of previously investigated aliphatic chain hydroxycarboxylic acids and proteinogenic amino acids. This indicates possible common handedness of stellar CPL-induced asymmetry in the potential comonomers of primitive proto-peptides.

Chiroptical Switching and Quantitative Chirality Sensing with (Pseudo)halogenated Quinones

(Pseudo)halogenated quinones react smoothly with chiral amines, amino alcohols, and amino acids toward push-pull conjugates with optical sensing and switching applications. The chiroptically active conjugates serve as redox switches between two reversibly interconverting states with remarkably different UV and CD signatures. Addition of sodium borohydride generates a hydroquinone derivative that is quantitatively re-oxidized to the original quinone upon exposure to air. This chiroptical quinone/hydroquinone redox switch system combines several attractive features such as simple set-up, use of inexpensive chemicals, short response time, and thermal and photochemical stability. A conceptually new sensing approach that is based on integrated chiroptical amplification and redox switching enables on-the-fly deconvolution of otherwise overlapping CD spectra and is used for quantitative er analysis of challenging samples containing constitutional isomers in varying enantiomeric compositions.

Broadband Optical Activity Spectroscopy with Interferometric Fourier-Transform Balanced Detection

Spectrally resolved measurements of optical activity, such as circular dichroism (CD) and optical rotatory dispersion (ORD), are powerful tools to study chiroptical properties of (bio)molecular and nanoplasmonic systems. The wider utilization of these techniques, however, has been impeded by the bulky and slow design of conventional spectropolarimeters, which have been limited to a narrowband scanning approach for more than 50 years. In this work, we demonstrate broadband measurements of optical activity by combining a balanced detection scheme with interferometric Fourier-transform spectroscopy. The setup utilizes a linearly polarized light field that creates an orthogonally polarized weak chiral free-induction-decay field, along with a phase-locked achiral transmitted signal, which serves as the local oscillator for heterodyne amplification. By scanning the delay between the two fields with a birefringent common-path interferometer and recording their interferogram with a balanced detector that measures polarization rotation, broadband CD and ORD spectra are retrieved simultaneously with a Fourier transform. Using an incoherent thermal light source, we achieve state-of-the-art sensitivity for CD and ORD across a broad wavelength range in a remarkably simple setup. We further demonstrate the potential of our technique for highly sensitive measurements of glucose concentration and the real-time monitoring of ground-state chemical reactions. The setup also accepts broadband pulses and will be suitable for broadband transient optical activity spectroscopy and broadband optical activity imaging.

Discrimination of enantiomers of amides with two stereogenic centers enabled by chiral bisthiourea derivatives using 1H NMR spectroscopy

Enantiomers of a few new amides containing two stereogenic centers have been derived from d- and l-α-amino acids as guests for chiral recognition by 1H NMR spectroscopy. A variety of chiral amides with two or more stereogenic centers often exist in the products of catalytic asymmetric synthesis, natural products or their total synthetic products, and chiral drugs. It would be a challenging and meaningful work to explore their chiral recognition. For this purpose, a class of novel chiral bisthiourea derivatives 1-9 has been synthesized from (1S,2S)-(+)-1,2-diaminocyclohexane, d-α-amino acids, and isothiocyanates as chiral solvating agents (CSAs). CSAs 1-9 proved to afford better chiral discriminating results towards most amides with two stereogenic centers, which have been rarely studied as chiral substrates by 1H NMR spectroscopy. In particular, CSAs 7, 8 and 9, featuring 3,5-bis(trifluoromethyl)benzene residues, exhibit outstanding chiral discriminating capabilities towards all amides, providing well-separated 1H NMR signals and sufficiently large nonequivalent chemical shifts. To test their practical application in the determination of enantiomeric excess, 1H NMR spectra of chiral amides (G16) with different optical purities were measured in the presence of CSAs 7 and 8, respectively. Their ee values (up to 90%) were accurately calculated by the integration of the NH proton of the CONHPh group of G16. To better understand the chiral discriminating behavior, Job plots of (±)-G16 with CSA 7 and (±)-G17 with CSA 8 and the association constants (Ka) of (S,R)-G16 and (R,S)-G16 with CSA 7 were evaluated, respectively. In order to further reveal any underlying intermolecular hydrogen bonding interactions, theoretical calculations of the enantiomers of (S,R)-G16 and (R,S)-G16 with CSA 7 were performed by means of the hybrid density functional theory (B3LYP) with the standard basis sets of 3-21G of the Gaussian 03 program, respectively.

The origin of supramolecular chirality in 1-ferrocenyl amino acids

π-Conjugated amino acids are widely applied in chiroptical materials, in which chiroptical activities are believed to originate from supramolecular packing. However, the intramolecular contribution has been largely ignored. In this work, we report that intramolecular chirality transfer behaviors in ferrocene-conjugated amino acids depend on the substituent groups, which influence the modality of multiple intramolecular interactions, as well as the molecular geometry. The structural basis and structure-property relationships of chirality and chiroptical activities were unveiled in this work. Based on single crystal structure and density functional theory calculations, we demonstrate that intramolecular weak forces, including hydrogen bonds, CHπ interactions and van der Waals interactions, affect the molecular geometry and contribute to diverse Cotton effects. This work provides evidence for the ignored intramolecular factors in self-assembled systems and paves the way for the fabrication of functional chiroptical systems.

Integrating amino acid oxidase with photoresponsive probe: A fast quantitative readout platform of amino acid enantiomers

Low-cost, high-throughput, broadly useful photoresponsive enantiomeric excess (ee) sensing of amino acids remains challenging to date. Herein, based on the selective oxidation reaction of amino acid oxidase (AAO) to amino acid enantiomers (D/L-AA) and the oxidation reaction of substrate (H₂O₂) with aromatic boronic ester, we put forward a photoresponsive strategy for the determination of D/L-AA at a certain concentration. In this scheme, the substrate H₂O₂ produced by the enzyme-catalyzed reaction was determined by sensitive fluorescent and colorimetric response of ethyl-3-(3-(benzothiazol-2-yl)-5-methyl-2-((4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl)oxy)phenyl)-2-cyanoacrylate (HBT-PB) to reflect the enantiomeric content at a certain concentration. The photoresponsive probe HBT-PB was readily available and inexpensive with sensitive long-wavelength red fluorescence and colorimetric light response to H₂O₂, the detection limit (LOD) was estimated as 2.91 μM. The operation of the sensing method was simple and data collection and processing are straightforward. The practicability of the scheme was favorably confirmed by accurate and scientific analysis of methionine and Dopa samples. As a result, the scheme was not only suitable for high-throughput screening but also adaptable to low-cost and sensitive RGB colorimetric analysis platform (LOD of methionine and Dopa was calculated as 9.23 μM and 8.34 μM respectively) with modern plate readers, and possessed extremely high enantioselectivity and wide applicability which benefited from the specificity and efficiency of enzyme catalytic reaction.

Enhancing Optical Activities of Benzimidazole Derivatives through Coassembly for High-Efficiency Synthesis of Chiroptical Nanomaterials and Accurate ee % Detection of Natural Acids

Developing efficient protocols to enhance the optical activities of chiral self-assemblies is a key to realizing their chiroptical functions such as chiral sensing and displays. Here, we have reported a coassembly protocol to efficiently boost the chiroptical responses, whereby the synthesis of chiroptical nanomaterials and highly accurate detection of enantiomeric excess (ee %) were achieved. A series of benzimidazole derivatives with different topologies underwent spontaneous aggregation and symmetry breaking in solution, generating silent Cotton effects, yet exclusive weak left-handed circularly polarized luminescence (CPL). The coassembly with natural hydroxyl acids via complementary H bonds afforded chiral nanostructures with emerged Cotton effects and enhanced CPL. Dissymmetry g-factors were dramatically boosted (g_lum from 1 × 10^-3 to 5.5 × 10^-2, g_abs from 0 to 6.7 × 10^-3). In addition, proof of concept of recognition and detection of natural chiral molecules was realized with high accuracy.

Rapid chiral analysis based on liquid-phase cyclic chemiluminescence

Rapid chiral analysis has become one of the important aspects of academic and industrial research. Here we describe a new strategy based on liquid-phase cyclic chemiluminescence (CCL) for rapid resolution of enantiomers and determination of enantiomeric excess (ee). A single CCL measurement can acquire multistage signals that provide a unique way to examine the intermolecular interactions between chiral hosts and chiral guests, because the lifetime (τ) of the multistage signals is a concentration-independent and distinguishable constant for a given chiral host-guest system. According to the τ values, CCL allows discrimination between a wide range of enantiomeric pairs including chiral alcohols, amines and acids by using only one chiral host. Even the chiral systems hardly distinguished by nuclear magnetic resonance and fluorescence methods can be distinguished easily by CCL. Additionally, the τ value of a mixture of two enantiomers is equal to the weighted average of each enantiomer, which can be used for the direct determination of ee without the need to separate the chiral mixture and create calibration curves. This is extremely crucial for the cases without readily available enantiomerically pure samples. This strategy was successfully applied to monitoring of the Walden inversion reaction and analysis of chiral drugs. The results were in good agreement with those obtained by high-performance liquid chromatography, indicating the utility of CCL for routine quick ee analysis. Mechanism study revealed that the τ value is possibly related to the activity of the chiral substance to catalyze a luminol-H2O2 reaction. Our research provides an unprecedented and general protocol for chirality differentiation and ee determination, which is anticipated to be a useful technology that will find wide application in chirality-related fields, particularly in asymmetric synthesis and the pharmaceutical industry.

Stereochemical analysis of chiral amines, diamines, and amino alcohols: Practical chiroptical sensing based on dynamic covalent chemistry

Practical chiroptical sensing with a small group of commercially available aromatic aldehydes is demonstrated. Schiff base formation between the electron-deficient 2,4-dinitrobenzaldehyde probe and either primary amines, diamines, or amino alcohols proceeds smoothly in chloroform at room temperature and is completed in the presence of molecular sieves within 2.5 hours. The substrate binding coincides with a distinct circular dichroism signal induction at approximately 330 nm, which can be correlated to the absolute configuration and enantiomeric composition of the analyte. The usefulness of this sensing method is highlighted with the successful sensing of 18 aliphatic and aromatic amines and amino alcohols and five examples showing quantitative %ee determination with good accuracy.

Circularly Polarized Luminescence of Aluminum Complexes for Chiral Sensing of Amino Acid and Amino Alcohol

Determination of the absolute configuration (AC) of chiral molecules is a key issue in many fields related to chirality such as drug development, the asymmetric reaction screening, and the structure determination of natural compounds. Although various methods, such as X-ray crystallography and NMR spectroscopy, are used to determine the AC, a simple and cheap alternative method is always anticipated. So far, electronic circular dichroism (ECD) spectroscopy has been widely used to ascertain the AC and enantiomeric excess (ee) values by applying appropriate organic probes. Here, circularly polarized luminescence (CPL) spectroscopy was applied to determine the AC and ee values of a series of amino acid and amino alcohol. The measurements were conducted by mixing the amino acids or amino alcohols with an achiral 1-hydroxy-2-naphthaldehyde. Upon in situ formation of the Schiff base complexes, the system showed emission enhancement and CPL in the presence of Al³⁺ , whose intensity and sign can be used to assign the chiral sense of the amino acids and amino alcohols. The authenticity of the method was further compared with the established CD spectroscopy, revealing that CPL spectra of formed Al³⁺ complex were effective to determine the AC of chiral species.

Tandem Use of Optical Sensing and Machine Learning for the Determination of Absolute Configuration, Enantiomeric and Diastereomeric Ratios, and Concentration of Chiral Samples

We have developed an optical method for accurate concentration, er, and dr analysis of amino alcohols based on a simple mix-and-measure workflow that is fully adaptable to multiwell plate technology and microscale analysis. The conversion of the four aminoindanol stereoisomers with salicylaldehyde to the corresponding Schiff base allows analysis of the dr based on a change in the UV maximum at 420 nm that is very different for the homo- and heterochiral diastereomers and of the concentration of the sample using a hypsochromic shift of another absorption band around 340 nm that is independent of the analyte stereochemistry. Subsequent in situ formation of Cu^II assemblies in the absence and presence of base enables quantification of the er values for each diastereomeric pair by CD analysis. Applying a linear programming method and a parameter sweep algorithm, we determined the concentration and relative amounts of each of the four stereoisomers in 20 samples of vastly different stereoisomeric compositions with an averaged absolute percent error of 1.7 %.

Chiral Discrimination by a Binuclear Pd Complex Sensor Using 31P{1H} NMR

An axially chiral binuclear μ-hydroxo Pd complex (BPHP) first served as an excellent chiral sensor for discriminating a variety of analytes including amino alcohol, amino amide, amino acid, mandelic acid, diol, diamine, and monoamine by ³¹P{¹H} NMR. A detailed recognition mechanism was proposed based on the single crystal and mass spectrum of Pd-complexes. In general, BPHP sensor, through extracting the acidic hydrogen of an analyte by its Pd-OH group, forms stable diastereomeric complexes with two enantiomers of the analyte giving well distinguishable split ³¹P{¹H} NMR signals for chiral discrimination.

Quantitative Chiroptical Sensing of Free Amino Acids, Biothiols, Amines, and Amino Alcohols with an Aryl Fluoride Probe

The comprehensive determination of the absolute configuration, enantiomeric ratio, and total amount of standard amino acids by optical methods adaptable to high-throughput screening with modern plate readers has remained a major challenge to date. We now present a small-molecular probe that smoothly reacts with amino acids and biothiols in aqueous solution and thereby generates distinct chiroptical responses to accomplish this task. The achiral sensor is readily available, inexpensive, and suitable for chiroptical analysis of each of the 19 standard amino acids, biothiols, aliphatic, and aromatic amines and amino alcohols. The sensing method is operationally simple, and data collection and processing are straightforward. The utility and practicality of the assay are demonstrated with the accurate analysis of 10 aspartic acid samples covering a wide concentration range and largely varying enantiomeric compositions. Accurate er sensing of 85 scalemic samples of Pro, Met, Cys, Ala, methylpyrrolidine, 1-(2-naphthyl)amine, and mixtures thereof is also presented.

Chiroptical sensing of unprotected amino acids, hydroxy acids, amino alcohols, amines and carboxylic acids with metal salts

Optical chirality sensing of unprotected amino acids, hydroxy acids, amino alcohols, amines and carboxylic acids based on a practical mix-and-measure protocol with readily available copper, iron, palladium, manganese, cerium or rhodium salts is demonstrated. The generation of strong cotton effects allows quantitative ee analysis of small sample amounts with high speed. In contrast to previously reported assays the use of chromophoric reporter ligands and the control of metal coordination kinetics and redox chemistry are not necessary which greatly simplifies the sensing procedure with the benefit of reduced waste production and cost.

The Chirality Induction and Modulation of Polymers by Circularly Polarized Light

Chirality is a natural attribute nature of living matter and plays an important role in maintaining the metabolism, evolution and functional activities of living organisms. Asymmetric conformation represents the chiral structure of biomacromolecules in living organisms on earth, such as the L-amino acids of proteins and enzymes, and the D-sugars of DNA or RNA, which exist preferentially as one enantiomer. Circularly polarized light (CPL), observed in the formation regions of the Orion constellation, has long been proposed as one of the origins of single chirality. Herein, the CPL triggered asymmetric polymerization, photo-modulation of chirality based on polymers are described. The mechanisms between CPL and polymers (including polydiacetylene, azobenzene polymers, chiral coordination polymers, and polyfluorene) are described in detail. This minireview provides a promising flexible asymmetric synthesis method for the fabrication of chiral polymer via CPL irradiation, with the hope of obtaining a better understanding of the origin of homochirality on earth.

Reaction-based chiroptical sensing of ClO− using circularly polarized luminescence via self-assembly organogel

A chiral amino acid functionalized probe, PTZ-D, could self-assemble into a chiral organogel displaying unprecedented chiroptical monitoring of ClO⁻ with switchable CPL signals.

Spiro[pyrrol-benzopyran]-based probe with high asymmetry for chiroptical sensing via circular dichroism

We proposed an efficient approach to construct a novel spiro[pyrrol-benzopyran] scaffold with high asymmetry for reaction-based chiroptical sensing via circular dichroism.