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

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 Enantiodifferentiation of Chiral Nitriles

Chiroptical sensing of nitriles is achieved with excellent functional group tolerance by hydrozirconation and subsequent transmetalation of the corresponding iminate to a chromophoric palladium complex. A one-pot workflow that uses the Schwartz reagent and [(η3-1-tert-butylindenyl)(μ-Cl)Pd]2 as sensor generates a palladium complex displaying red-shifted CD inductions and characteristic UV changes. These chiroptical responses are accurately correlated to the enantiomeric ratio and total concentration of the original nitrile.

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.

Organometallic Chirality Sensing via "Click"-Like η6-Arene Coordination with an Achiral Cp*Ru(II) Piano Stool Complex

Piano stool complexes have been studied over many years and found widespread applications in organic synthesis, catalysis, materials and drug development. We now report the first examples of quantitative chiroptical molecular recognition of chiral compounds through click-like η6-arene coordination with readily available half sandwich complexes. This conceptually new approach to chirality sensing is based on irreversible acetonitrile displacement of [Cp*Ru(CH3CN)3]PF6 by an aromatic target molecule, a process that is fast and complete within a few minutes at room temperature. The metal coordination coincides with characteristic circular dichroism inductions that can be easily correlated to the absolute configuration and enantiomeric ratio of the bound molecule. A relay assay that decouples the determination of the enantiomeric composition and of the total sample amount by a practical CD/UV measurement protocol was developed and successfully tested. The introduction of piano stool complexes to the chiroptical sensing realm is mechanistically unique and extends the scope of currently known methods with small-molecule probes that require the presence of amino, alcohol, carboxylate or other privileged functional groups for binding of the target compound. A broad application range including pharmaceutically relevant multifunctional molecules and the use in chromatography-free asymmetric reaction analysis are also demonstrated.

Carborane-Cluster-Wrapped Copper Cluster with Cyclodextrin-like Cavities for Chiral Recognition

Chiral atomically precise metal clusters, known for their remarkable chiroptical properties, hold great potential for applications in chirality recognition. However, advancements in this field have been constrained by the limited exploration of host-guest chemistry, involving metal clusters. This study reports the synthesis of a chiral Cu16(C2B10H10S2)8 (denoted as Cu16@CB8, where C2B10H12S2H2 = 9,12-(HS)2-1,2-closo-carborane) cluster by an achiral carboranylthiolate ligand. The chiral R-/S-Cu16@CB8 cluster features chiral cavities reminiscent of cyclodextrins, which are surrounded by carborane clusters, yet they crystallize in a racemate. These cyclodextrin-like cavities demonstrated the specific recognition of amino acids, as indicated by the responsive output of circular dichroism and circularly polarized luminescence signals of Cu16 moieties of the Cu16@CB8 cluster. Notably, a quantitative chiroptical analysis of amino acids in a short time and a concomitant deracemization of Cu16@CB8 were achieved. Density functional tight-binding molecular dynamics simulation and noncovalent interaction analysis further unraveled the great importance of the cavities and binding sites for chiral recognition. Dipeptide, tripeptide, and polypeptide containing the corresponding amino acids (Cys, Arg, or His residues) display the same chiral recognition, showing the generality of this approach. The functional synergy of dual clusters, comprising carborane and metal clusters, is for the first time demonstrated in the Cu16@CB8 cluster, resulting in the valuable quantification of the enantiomeric excess (ee) value of amino acids. This work opens a new avenue for chirality sensors based on chiral metal clusters with unique chiroptical properties and inspires the development of carborane clusters in host-guest chemistry.

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.

High-throughput determination of enantiopurity in atroposelective synthesis of aryl triazoles

Atropisomeric scaffolds are a common design element found in pharmaceuticals, many deriving from an N-C axis of chirality. The handedness associated with atropisomeric drugs is oftentimes crucial for their efficacy and/or safety. With the increased use of high-throughput screening (HTS) for drug discovery, the need for rapid enantiomeric excess (ee) analysis is needed to keep up with the fast workflow. Here, we describe a circular dichroism (CD) based assay that could be applied to the ee determination of N-C axially chiral triazole derivatives. Analytical samples for CD were prepared from crude mixtures by three sequential steps: liquid-liquid extraction (LLE), a wash-elute, and complexation with Cu(ii) triflate. The initial ee measurement of five samples of atropisomer 2 was conducted by the use of a CD spectropolarimeter with a 6-position cell changer, resulting in errors of less than 1% ee. High-throughput ee determination was performed on a CD plate reader using a 96-well plate. A total of 28 atropisomeric samples (14 for 2 and 14 for 3) were screened for ee. The CD readings were completed in 60 seconds with average absolute errors of ±7.2% and 5.7% ee for 2 and 3, respectively.

Bioconjugation via Hetero-Selective Clamping of Two Different Amines with ortho-Phthalaldehyde

Amino groups are common in both natural and synthetic compounds and offer a very attractive class of endogenous handles for bioconjugation. However, the ability to differentiate two types of amino groups and join them with high hetero-selectivity and efficiency in a complex setting remains elusive. Herein, we report a new method for bioconjugation via one-pot chemoselective clamping of two different amine nucleophiles using a simple ortho-phthalaldehyde (OPA) reagent. Various α-amino acids, aryl amines, and secondary amines can be crosslinked to the ϵ-amino side chain of lysine on peptides or proteins with high efficiency and hetero-selectivity. This method offers a simple and powerful means to crosslink small molecule drugs, imaging probes, peptides, proteins, carbohydrates, and even virus particles without any pre-functionalization.

Analysis of the Amine Submetabolome Using Novel Isotope-Coded Pyrylium Salt Derivatization and LC-MS: Herbs and Cancer Tissues as Cases

The amine submetabolome, including amino acids (AAs) and biogenic amines (BAs), is a class of small molecular compounds exhibiting important physiological activities. Here, a new pyrylium salt named 6,7-dimethoxy-3-methyl isochromenylium tetrafluoroborate ([d₀]-DMMIC) with stable isotope-labeled reagents ([d₃]-/[d₆]-DMMIC) was designed and synthesized for amino compounds. [d₀]-/[d₃]-/[d₆]-DMMIC-derivatized had a charged tag and formed a set of molecular ions with an increase of 3.02 m/z and the characteristic fragment ions of m/z 204.1:207.1:210.1. When DMMIC coupled with liquid chromatography-mass spectrometry (LC-MS), a systematic methodology evaluation for quantitation proved to have good linearity (R² between 0.9904 and 0.9998), precision (interday: 2.2-21.9%; intraday: 1.0-19.7%), and accuracy (recovery: 71.8-108.8%) through the test AAs. Finally, the methods based on DMMIC and LC-MS demonstrated the advantaged application by the nontargeted screening of BAs in a common medicinal herb Senecio scandens and an analysis of metabolic differences among the amine submetabolomes between the carcinoma and paracarcinoma tissues of esophageal squamous cell carcinoma (ESCC). A total of 20 BA candidates were discovered in S. scandens as well as the finding of 13 amine metabolites might be the highest-potential differential metabolites in ESCC. The results showed the ability of DMMIC coupled with LC-MS to analyze the amine submetabolome in herbs and clinical tissues.

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.

Extendable stapling of unprotected peptides by crosslinking two amines with o-phthalaldehyde

Peptide modification methods that do not rely on the cysteine residue are underdeveloped, and their development could greatly expand the current toolbox for peptide chemistry. During the course of preliminary investigations into the classical ortho-phthalaldehyde (OPA)-amine-thiol condensation reaction, we found that in the absence of thiol, OPA readily condenses with two primary alkyl amines to form a class of underexplored isoindolin-1-imine compounds under mild aqueous conditions. From the intramolecular version of this OPA-2amines reaction, an efficient and selective methodology using mild reaction conditions has been developed for stapling unprotected peptides via crosslinking of two amino groups in both an end-to-side and side-to-side fashion. The stapling method is superfast and broadly applicable for various peptide substrates with the reacting amino groups separated by a wide range of different amino acid units. The macrocyclization reactions of selected substrates are completed within 10 seconds at 5 mM concentration and within 2 minutes at 50 μM concentration. Importantly, the resulting cyclized peptides with an isoindolinimine linkage can be extended in a one-pot sequential addition manner with several different electron-deficient π electrophiles, thereby generating more complex structures.

Methods for peptide stapling, or covalently linking amino acid residues to create a non-linear structure, mostly rely on cysteine residues, which imposes a significant practical limitation. Here the authors disclose a method to chemoselectively macrocyclize two free-amine-containing residues in mild, peptide-relevant conditions, using a commercially available reagent.

Simultaneous identification and quantification of amino acids in biofluids by reactive 19F-tags

A robust method to identify and quantify amino acids close to physiological conditions by 1D ¹⁹F NMR was established. Each ¹⁹F-derivatized amino acid has its characteristic chemical-shift profile that is readily identified in the mixture of amino acids or in biofluids including fetal bovine serum and cell lysates. The method shows great potential in metabolomics and biochemical analysis.

Iodine-Mediated Tryptathionine Formation Facilitates the Synthesis of Amanitins

Synthetic methods on the macrocyclization of peptides are of high interest since they facilitate the synthesis of various types of potentially bioactive compounds, e.g. addressing targets like protein-protein-interactions. Herein, we report on an efficient method to construct tryptathionine-cross-links in peptides between the amino acids Trp and Cys. This reaction not only is the basis for the total synthesis of the death cap toxin α-amanitin but also provides rapid access to various new amanitin analogues. This study for the first time presents a systematic compilation of structure-activity relations (SAR) of amatoxins with regard to RNA polymerase II inhibition and cytotoxicity with one amanitin derivative of superior RNAP II inhibition. The present approach paves the way for the synthesis of structurally diverse amatoxins as future payloads for antibody-toxin conjugates in cancer therapy.

Biomolecular labeling based on lysine-clickable 6π-azaelectrocyclization toward innovative cancer theranostics

An amino group at side chain of lysine residue can be targeted for protein modification because of the convenience for covalent bond formation. We have achieved an efficient protein modification by utilizing amine-clickable 6π-azaelectrocyclization, termed RIKEN click reaction recently, which enabled direct click labeling of protein without any introduction of specific functional groups such as alkynes and azides. On the basis of the RIKEN click reaction, we established the double click labeling method. The double click methods composed of copper-free strain-promoted [3 + 2] cyclization or tetrazine ligation and RIKEN click reaction were developed. The double click method realized highly effective proteins including radiolabeling of bioactive peptides and anti-tumor antibodies. In this personal review, the development of double click probes, practical radiolabeling of biological active molecules such as cyclic RGDyK peptides, proteins, and antibodies with α-emission or β-emission radionuclides, and their applications for PET imaging and α-emission cancer treatment are summarized.