Toward Fluorescence‐Based High‐Throughput Screening for Enantiomeric Excess in Amines and Amino Acid Derivatives

Enantiomer Recognition Based on Chirality Transfer from Chiral Amines to Ternary Dynamic Covalent Systems

Enantiomer recognition is usually required in organic synthesis and materials and life sciences. This paper describes an enantiomer recognition method based on ternary dynamic covalent systems constructed via the complexation of chiral amines with a chiral boronate derived from 1,4-phenylenediboric acid and an L-DOPA-modified naphthalenediimide. The ternary systems aggregate into chiral assemblies driven by π-π interactions, and the chirality is transferred from the chiral amines to assemblies with high stereospecificity. Consequently, the enantiomer composition of chiral amines and the absolute configuration of the major enantiomer can be determined according to the sign of the Cotton effect of the ternary system by using circular dichroism (CD) spectroscopy. This method offers the advantage of using the long wavelength CD signals of the boronate at around 520 nm, thereby avoiding interference with those of the carbon skeleton. This ternary system provides a novel approach to the design of enantiomer recognition systems.

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 with Broad Selective Sensor Arrays

The detection and discrimination of chiral analytes has always been a topical theme in food and pharmaceutical industries and environmental monitoring, especially when dealing with chiral drugs and pesticides, whose enantiomeric nature assessment is of crucial importance. The typical approach matches novel chiral receptors designed ad hoc for the discrimination of a target enantiomer with emerging nanotechnologies. The massive synthetic efforts requested and the difficulty of analyzing complex matrices warrant the ever-growing exploitation of sensor array as an alternative route, using a limited number of chiral or both chiral and achiral sensors for the stereoselective identification and dosing of chiral compounds. This review aims to illustrate a little-explored winning strategy in chiral sensing based on sensor arrays. This strategy mimics the functioning of natural olfactory systems that perceive some couples of enantiomeric compounds as distinctive odors (i.e., using an array of a considerable number of broad selective receptors). Thus, fundamental concepts related to the working principle of sensor arrays and the role of data analysis techniques and models have been briefly presented. After the discussion of existing examples in the literature using arrays for discriminating enantiomers and, in some cases, determining the enantiomeric excess, the remaining challenges and future directions are outlined for researchers interested in chiral sensing applications.

A Real-Time Strategy for Chiroptical Sensing and Enantiomeric Excess Determination of Primary Amines via an Acid-Base Reaction

Two achiral aromatic carboxylic acids that included the 1,8-naphthalimide group and an imidazolium cation were synthesized and exploited as chiroptical sensors. These compounds showed the real-time discrimination and enantiomeric excess determination of chiral amines and amino alcohols via an acid-base interaction, especially for UV-silent chiral compounds.

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.

A Fluorescence Sensor Array Based on Zinc(II)-Carboxyamidoquinolines: Toward Quantitative Detection of ATP*

The newly prepared fluorescent carboxyamidoquinolines (1-3) and their Zn(II) complexes (Zn@1-Zn@3) were used to bind and sense various phosphate anions utilizing a relay mechanism, in which the Zn(II) ion migrates from the Zn@1-Zn@3 complexes to the phosphate, namely adenosine 5'-triphosphate (ATP) and pyrophosphate (PPi), a process accompanied by a dramatic change in fluorescence. Zn@1-Zn@3 assemblies interact with adenine nucleotide phosphates while displaying an analyte-specific response. This process was investigated using UV-vis, fluorescence, and NMR spectroscopy. It is shown that the different binding selectivity and the corresponding fluorescence response enable differentiation of adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), pyrophosphate (PPi), and phosphate (Pi). The cross-reactive nature of the carboxyamidoquinolines-Zn(II) sensors in conjunction with linear discriminant analysis (LDA) was utilized in a simple fluorescence chemosensor array that allows for the identification of ATP, ADP, PPi, and Pi from 8 other anions including adenosine 5'-monophosphate (AMP) with 100 % correct classification. Furthermore, the support vector machine algorithm, a machine learning method, allowed for highly accurate quantitation of ATP in the range of 5-100 μM concentration in unknown samples with error <2.5 %.

A rapid and sensitive method for chiroptical sensing of α-amino acids via click-like labeling with o-phthalaldehyde and p-toluenethiol

A highly practical method for comprehensive chiroptical sensing of free α amino acids with streamlined operation and high sensitivity via dual CD/UV measurements is developed. The assay takes advantage of an efficient and selective three-component labeling reaction of primary amines with o-phthalaldehyde and p-toluenethiol reagents to derivatize the NH₂ group of analytes into an isoindole. The covalent labeling generates sensitive UV and CD readouts, both of which show an excellent linear relationship with the concentration of analytes. The high reactivity and the novel optical reporting mechanism allow fast and accurate measurement without background interference. The sensing assay works well for a remarkably broad range of analyte concentrations, with an unprecedented lower limit of 10 micromolar concentration.

A highly practical method for comprehensive chiroptical sensing of free α amino acids with streamlined operation and high sensitivity via dual CD/UV measurements is developed.

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.

Predictive chirality sensing via Schiff base formation

Among the large number of chiroptical sensors that have been developed to date, few allow rational determination of the absolute configuration of chiral substrates together with quantitative ee analysis. We have prepared and tested stereodynamic N-aryl aminobenzaldehyde sensors that bind chiral amines via Schiff base formation. The covalent binding of the amine substrate generates a conformational bias in the chromophoric sensor moiety which results in characteristic CD signals. Computational analysis revealed that CD prediction of the sign of the Cotton effect and thus determination of the absolute configuration of the substrate becomes practical with a sterically crowded sensor design because the number of conformations to be considered is largely reduced and the chiroptical sensor response is less sensitive to conformational equilibria. The amplitude of the measured CD signal can be used for quantitative ee analysis of nonracemic amine samples with the help of a calibration curve.

An indicator displacement assay recognizes enantiomers of chiral carboxylates

Analyte chirality induces changes in fluorescence.

A dual chromophore sensor for the detection of amines, diols, hydroxy acids, and amino alcohols

The determination of enantiomeric excess (ee) in various groups of chiral compounds, namely amines, amino alcohols, diols, and hydroxy acids is performed using a dual chromophore FRET/PET based sensor ensemble. The sensing ensemble utilizes fluorescence changes from two chromophores (indicators) to classify 13 pairs of enantiomers as well as allows for the qualitative and quantitative determination of ee of various classes of chiral compounds with high accuracy (<2% error).

Chiral Receptors for Lysine Based on Covalently Linked Bis- and Tris-binaphthylphosphoric Acids

The synthesis and application of three chiral receptors based on the covalent linkage of 1,1'-binaphthylphosphoric acids is reported. The binding of the lysine enantiomers to the chiral receptors was investigated by DOSY-NMR and NMR titrations, revealing that the bisphosphoric acid 1d acts as a highly stereoselective receptor for binding of d-lysine.

Array-Based Protein Sensing Using an Aggregation-Induced Emission (AIE) Light-Up Probe

Protein detection and identification are important for the diagnosis of diseases; however, the development of facile sensing probes still remains challenging. Here, we present an array-based "turn on" protein-sensing platform capable of detecting and identifying proteins using aggregation-induced emission luminogens (AIEgens). The water-soluble AIEgens in which fluorescence was initially turned off showed strong fluorescence in the presence of nanomolar concentrations of proteins via restriction of the intramolecular rotation of the AIEgens. The binding affinities between the AIEgens and proteins were associated with various chemical functional groups on AIEgens, resulting in distinct fluorescent-signal outcomes for each protein. The combined fluorescence outputs provided sufficient information to detect and discriminate proteins of interest by linear discriminant analysis. Furthermore, the array-based sensor enabled classification of different concentrations of specific proteins. These results provide novel insight into the use of the AIEgens as a new type of sensing probe in array-based systems.

Post-Assembly Reactivity of N-Aryl Iminoboronates: Reversible Radical Coupling and Unusual B-N Dynamic Covalent Chemistry

Post-assembly reaction of a dynamic covalent iminoboronate system following addition of Cp2 Co resulted in the formation of a series of new reductively coupled dianionic dimers via C-C bond formation. The dimers formed as a mixture of BN-containing isomeric products: diastereomers rac5 and meso5, with coupled five-membered rings, and enantiomeric rac6, with a fused six-membered ring bicyclic system from C-C bond formation and rearrangement of the B-N bonds. Each isomer was identified using 1 H NMR spectroscopy in combination with single crystal X-ray structure determination. Interestingly, interconversion between the coupled five-membered rings (rac5 ) and fused bicyclic systems (rac6 ) was found to occur through an unprecedented breaking and reforming of the B-N covalent bond. Further, the coupled products could be converted quantitatively back to their iminoboronate precursors with addition of the electron abstractor Ph3 C+ .

Optical Analysis of Reaction Yield and Enantiomeric Excess: A New Paradigm Ready for Prime Time

This Perspective highlights the advances of optical methods for asymmetric reaction discovery. Optical analysis allows for the determination of absolute configuration, enantiomeric excess and reaction yield that is amenable to high-throughput experimentation. Thus, the synthetic organic community is encouraged to incorporate the methods discussed to expedite the development of high-yielding, enantioselective transformations.

Diasteroselective multi-component assemblies from dynamic covalent imine condensation and metal-coordination chemistry: mechanism and narcissistic stereochemistry self-sorting

Self-assembly of a modified tris(2-pyridylmethyl)amine TPMA ligand, zinc(ii) or cobalt(ii) ions, and amino acids have been used effectively as stereo dynamic optical probes for the determination of the enantiomeric excess of free amino acids either using Electronic or Vibrational Circular Dichroism (CD and VCD). Herein, we report the mechanistic and stereochemical study of the self-assembly process which reveals a complex equilibrium in solution where even small variations in the experimental conditions can profoundly affect the final products of the reaction. In particular, variation on the metal stoichiometry switch give rises to an entirely enantio narcissistic self-assembly of the structure.

Enantio narcissistic self-assembly is observed in a multi-metallic structure used as stereodynamic probe for CD measurements.

Concentration-Independent Stereodynamic g-Probe for Chiroptical Enantiomeric Excess Determination

Enantiomeric excess (ee) determination is crucial in many aspects of science, from synthesis to materials. Within this subject, coupling molecular sensors with chiroptical techniques is a straightforward approach to the stereochemical analysis of chiral molecules, especially in terms of process immediacy and labor. Stereodynamic probes typically consist of racemic mixtures of rapidly interconverting enantiomeric conformers able to recognize a chiral analyte and greatly amplify its chiroptical readout. A great number of sensors have been developed, but their activity is generally restricted to one or a few classes of chemicals, and the analysis outcome relies on precise knowledge of the probe and analyte concentrations. This aspect in particular limits the potential practical applications. Here we report an oxo-vanadium(V) aminotriphenolate complex that was found to act as a concentration-independent stereodynamic sensor for a wide range of compounds. The bare complex is CD-silent, but coordination of an enantioenriched substrate immediately gives rise to intense Cotton effects in the visible region. Furthermore, a geometry change during the substrate-complex interaction leads to a marked optical response, as witnessed by a strong red-shift of the probe absorption bands, thus allowing the generation of dichroic signals in an "interference-free" area of the spectrum. This peculiarity allows for a linear correlation at high wavelengths between the ee of the analyte and anisotropy g-factor. This parameter derives from the differential circularly polarized light absorption of the sample but is independent of concentration. The newly developed sensor based on a simple coordination process has an unprecedented general character in terms of substrate scope and employment.

High-Throughput Assay for Enantiomeric Excess Determination in 1,2- and 1,3-Diols and Direct Asymmetric Reaction Screening

A simple and efficient method for determination of the yield, enantiomeric/diasteriomeric excess (ee/de), and absolute configuration of crude chiral diols without the need of work-up and product isolation in a high throughput setting is described. This approach utilizes a self-assembled iminoboronate ester formed as a product by dynamic covalent self-assembly of a chiral diol with an enantiopure fluorescent amine such as tryptophan methyl ester or tryptophanol and 2-formylphenylboronic acid. The resulting diastereomeric boronates display different photophysical properties and allow for fluorescence-based ee determination of molecules containing a 1,2- or 1,3-diol moiety. This method has been utilized for the screening of ee in a number of chiral diols including atorvastatin, a statin used for the treatment of hypercholesterolemia. Noyori asymmetric hydrogenation of benzil was performed in a highly parallel fashion with errors <1 % ee confirming the feasibility of the systematic examination of crude products from the parallel asymmetric synthesis in real time and in a high-throughput screening (HTS) fashion.

Molecular Recognition and Chirality Sensing of Epoxides in Water Using Endo-Functionalized Molecular Tubes

Chiral epoxides are important intermediates in chemistry and biology. The high-throughput screening of asymmetric epoxidation conditions requires fast determination of the absolute configurations and ee values of chiral epoxides. Herein, we report molecular recognition and chiroptical sensing of epoxides in water using endo-functionalized molecular tubes. The absolute configurations and ee values were simultaneously determined by circular dichroism spectroscopy. In addition, real-time monitoring as well as the application to real asymmetric epoxidation was demonstrated. The method is simple, environmentally friendly, and amenable to high-throughput screening.

Recent advances in boronic acid-based optical chemosensors

This minireview highlights the developments in optical chemosensors from 2014 to 2016 that utilise the boronic acid interaction with polyols or Lewis bases.