High-Throughput Determination of Enantiopurity by Microplate Circular Dichroism

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.

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.

Review of predicting protein stability changes upon variations

Forecasting alterations in protein stability caused by variations holds immense importance. Improving the thermal stability of proteins is important for biomedical and industrial applications. This review discusses the latest methods for predicting the effects of mutations on protein stability, databases containing protein mutations and thermodynamic parameters, and experimental techniques for efficiently assessing protein stability in high-throughput settings. Various publicly available databases for protein stability prediction are introduced. Furthermore, state-of-the-art computational approaches for anticipating protein stability changes due to variants are reviewed. Each method's types of features, base algorithm, and prediction results are also detailed. Additionally, some experimental approaches for verifying the prediction results of computational methods are introduced. Finally, the review summarizes the progress and challenges of protein stability prediction and discusses potential models for future research directions.

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.

Chemometric sensing of stereoisomeric compound mixtures with a redox-responsive optical probe

The analysis of mixtures of chiral compounds is a common task in academic and industrial laboratories typically achieved by laborious and time-consuming physical separation of the individual stereoisomers to allow interference-free quantification, for example using chiral chromatography coupled with UV detection. Current practice thus impedes high-throughput and slows down progress in countless chiral compound development projects. Here we describe a chemometric solution to this problem using a redox-responsive naphthoquinone that enables chromatography-free click chemistry sensing of challenging mixtures. The achiral probe covalently binds amino alcohols within a few minutes at room temperature and generates characteristic UVA and CDA spectra that are intentionally altered via sodium borohydride reduction to provide a second, strikingly different chiroptical data set (UVB and CDB). Chemometric partial least squares processing of the chiroptical outputs then enables spectral deconvolution and accurate determination of individual analyte concentrations. The success of this approach is demonstrated with 35 samples covering considerably varied total analyte amounts and stereoisomeric ratios. All chemicals and machine learning algorithms are readily available and can be immediately adapted by any laboratory.

Employing a chiroptical sensor for the absolute stereochemical determination of α-amino and α-hydroxyphosphonates

The absolute stereochemistry of the α-amino and α-hydroxyphosphonates is determined using a chiroptical sensor. The induced helicity of the host-guest complex is correlated to the chirality of the guest molecule via a simple binding model. The relative size of the substituents dictates the predominant helical population, leading to an easy circular dichroic readout.

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.

Determination of Enantiomeric Excess and Diastereomeric Excess via Optical Methods. Application to α-methyl-β-hydroxy-carboxylic acids

Characterization of chiral molecules in solution is paramount for measuring reaction success. However, techniques to distinguish between chiral molecules containing more than one stereocenter through the use of optical techniques remains a challenge. Herein, we report a techique using a series of circular dichroism spectra to train multivariate regression models that are capable of predicting the complete speciation of 3-hydroxy-2-methylbutanoic acid stereoisomers. From this, it is possible to rapidly and accurately determine the enantiomeric excess and diastereomeric excess of the solution without the need for chiral chromatography.

Data-Driven Prediction of Circular Dichroism-Based Calibration Curves for the Rapid Screening of Chiral Primary Amine Enantiomeric Excess Values

Here, we describe the prediction of the circular dichroism (CD) response of a three-component chiroptical sensor for enantiomeric excess (ee) determination of chiral amines using a multivariate fit to electronic and steric parameters. These computationally derived parameters can be computed for nearly any amine and correlate well with the CD response of the 12 amines comprising the training set. The resulting model was used to accurately predict the CD response of a test set of chiral amines. Theoretical calibration curves were then created and used to determine the ee of solutions of unknown ee. Using this method, the error in ee determination differed by less than 10% compared to experimentally generated calibration curves.

Biomacromolecule-Assisted Screening for Reaction Discovery and Catalyst Optimization

Reaction discovery and catalyst screening lie at the heart of synthetic organic chemistry. While there are efforts at de novo catalyst design using computation/artificial intelligence, at its core, synthetic chemistry is an experimental science. This review overviews biomacromolecule-assisted screening methods and the follow-on elaboration of chemistry so discovered. All three types of biomacromolecules discussed─enzymes, antibodies, and nucleic acids─have been used as "sensors" to provide a readout on product chirality exploiting their native chirality. Enzymatic sensing methods yield both UV-spectrophotometric and visible, colorimetric readouts. Antibody sensors provide direct fluorescent readout upon analyte binding in some cases or provide for cat-ELISA (Enzyme-Linked ImmunoSorbent Assay)-type readouts. DNA biomacromolecule-assisted screening allows for templation to facilitate reaction discovery, driving bimolecular reactions into a pseudo-unimolecular format. In addition, the ability to use DNA-encoded libraries permits the barcoding of reactants. All three types of biomacromolecule-based screens afford high sensitivity and selectivity. Among the chemical transformations discovered by enzymatic screening methods are the first Ni(0)-mediated asymmetric allylic amination and a new thiocyanopalladation/carbocyclization transformation in which both C-SCN and C-C bonds are fashioned sequentially. Cat-ELISA screening has identified new classes of sydnone-alkyne cycloadditions, and DNA-encoded screening has been exploited to uncover interesting oxidative Pd-mediated amido-alkyne/alkene coupling reactions.

Rapid chiral assay of amino compounds using diethyl squarate

The versatility and importance of chiral compounds make it urgent to develop fast and efficient methods to detect the absolute configuration, enantiomeric excess(ee), and concentration of chiral compounds. In this study, we demonstrate that commercially available diethyl squarate can rapidly react with various types of chiral amino compounds and exhibit characteristic ultraviolet (UV) and circular dichroism (CD) signals. The UV and CD signals can determine the total concentration of the two enantiomers and ee value of the sample, respectively. The probe showed a broad substrate scope, applicable to 39 tested chiral amino compounds, including chiral amino acids, amino alcohols, and amines. Additionally, the probe accurately detected 10 samples of phenylalanine, phenylglycinol, and phenethylamine with the error range less than 8%, demonstrating the practicability of this method.

Selective chiroptical sensing of D/L-cysteine

A chromophoric bifunctional probe design that allows selective chiroptical sensing of cysteine in aqueous solution is introduced. The common need for chiral HPLC separation is eliminated which expedites and simplifies the sample analysis while reducing solvent waste. Screening of the reaction between six phenacyl bromides and the enantiomers of cysteine showed that cyclization to an unsaturated thiomorpholine scaffold coincides with characteristic UV and CD effects, in particular when the reagent carries a proximate auxochromic nitro group. The UV changes and CD inductions were successfully used for determination of the absolute configuration, enantiomeric composition and total concentration of 18 test samples. This assay is highly selective for free cysteine while other amino acids, cysteine derived small peptides and biothiols do not interfere with the chiroptical signal generation.

Accelerated Asymmetric Reaction Screening with Optical Assays

Asymmetric reaction development often involves optimization of several mutually dependent parameters that affect the product yield and enantiomeric excess. Widely available high-throughput experimentation equipment and optical sensing assays can drastically streamline comprehensive optimization efforts and speed up the discovery process at reduced cost, workload, and waste production. A variety of chiroptical assays that utilize fluorescence, UV, and circular dichroism measurements to determine reaction yields and ee values are now available, enabling the screening of numerous small-scale reaction samples in parallel with multi-well plate technology. Many of these optical methods considerably shorten work-up protocols typically required for traditional asymmetric reaction analysis and some can be directly applied to crude mixtures thus eliminating cumbersome separation and purification steps altogether.

1 Introduction

2 Fluorescence Assays

3 UV Sensing Methods

4 Sensing with Circular Dichroism Probes

5 Hybrid Approaches

6 Optical Analysis with Intrinsically CD-Active Reaction Products

7 Conclusion

Application of aromatic amide-derived atropisomers as chiral solvating agents for discrimination of optically active mandelic acid derivatives in 1 H nuclear magnetic resonance spectroscopy

Non-biaryl atropisomers and their stereochemistry have attracted much attentions in the past years. However, application of the non-biaryl atropisomers as chiral solvating agents is yet to be explored. In this work, four aromatic amide-derived atropisomeric phosphine ligands (hosts) were used as chiral solvating agents to recognize various mandelic acid derivatives (guests) in ¹ H nuclear magnetic resonance (NMR) spectroscopy. It is found that chiral center configurations of the four hosts have different effects on the enantiorecognition to the used guests. In addition, the host and guest interaction was further investigated by determination of the host-guest complex stoichiometry using the Job's method and density functional theory calculation, respectively. Moreover, chiral analysis accuracy of these hosts was evaluated through relationship between enantiomeric excess values of 4-chloromandelic acid provided by NMR and gravimetry, respectively.

Chemoselective and Enantioselective Fluorescent Identification of Specific Amino Acid Enantiomers

The enantiomers of chiral amino acids play versatile roles in biological systems including humans. They are also very useful in the asymmetric synthesis of diverse chiral organic compounds. Therefore, identification...

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.

Optical deciphering of multinary chiral compound mixtures through organic reaction based chemometric chirality sensing

The advances of high-throughput experimentation technology and chemometrics have revolutionized the pace of scientific progress and enabled previously inconceivable discoveries, in particular when used in tandem. Here we show that the combination of chirality sensing based on small-molecule optical probes that bind to amines and amino alcohols via dynamic covalent or click chemistries and powerful chemometric tools that achieve orthogonal data fusion and spectral deconvolution yields a streamlined multi-modal sensing protocol that allows analysis of the absolute configuration, enantiomeric composition and concentration of structurally analogous—and therefore particularly challenging—chiral target compounds without laborious and time-consuming physical separation. The practicality, high accuracy, and speed of this approach are demonstrated with complicated quaternary and octonary mixtures of varying chemical and chiral compositions. The advantages over chiral chromatography and other classical methods include operational simplicity, increased speed, reduced waste production, low cost, and compatibility with multiwell plate technology if high-throughput analysis of hundreds of samples is desired.

The stereoselective analysis of mixtures of chiral compounds typically requires time-consuming chromatography. Here, the authors combine reaction based chiroptical sensing and chemometric tools to directly determine the absolute configuration, enantiomeric composition and concentration of convoluted samples without physical separation.

Chiral recognition via a stereodynamic vanadium probe using the electronic circular dichroism effect in differential Raman scattering

Intermolecular interactions sensitive to chirality occur in many biological events. We report a complex formation between a versatile vanadium-based probe and a chiral co-ligand monitored via the combination of electronic circular dichroism (ECD) and Raman scattering. This "ECD-Raman" effect was discovered relatively recently and can be measured using a Raman optical activity (ROA) spectrometer. Simulated spectra based on experimental ECD and degree of circularity (DOC) values agree with the observed ones. Sensitive recognition of the chiral enantiopure co-ligand is thus enabled by a combination of resonance of the excitation light with the diastereoisomeric complex, co-ligand complexation, circular dichroism, and polarized Raman scattering from the achiral solvent. Relatively dilute solutions could be detected (10^-4 mol dm^-3), about 1000× less than is necessary for conventional ROA detection of the pure co-ligand and comparable to concentrations needed for conventional ECD spectroscopy. The results thus show that differential ECD-Raman measurements can be conveniently used to monitor molecular interactions and molecular spectroscopic properties.

High-throughput screening of α-chiral-primary amines to determine yield and enantiomeric excess

A novel screening protocol was developed using a combination of a fluorescent indicator displacement assay and a circular dichroism (CD) active Fe(II) complex to determine concentration and enantiomeric excess (ee) of α-chiral amines, respectively. The analyte concentration is quantified with a pre-formed non-fluorescent imine, where transimination with the chiral amine results in displacement of the fluorophore 2-naphthylamine. After discerning the concentration of amine via fluorescence in a wellplate reader, the analyte is then incorporated into a three-component octahedral Fe(II) assembly for ee determination using an EKKO CD plate-reader. With these two assays, both the ee and yield of asymmetric transformations of 192 samples could be determined with acceptable errors in under fifteen minutes (not counting the preparation time). This combined speed and accuracy provides an attractive solution to overcoming analytical bottlenecks when creating α-chiral amines.

Fundamentals to function: Quantitative and scalable approaches for measuring protein stability

Folding a linear chain of amino acids into a three-dimensional protein is a complex physical process that ultimately confers an impressive range of diverse functions. Although recent advances have driven significant progress in predicting three-dimensional protein structures from sequence, proteins are not static molecules. Rather, they exist as complex conformational ensembles defined by energy landscapes spanning the space of sequence and conditions. Quantitatively mapping the physical parameters that dictate these landscapes and protein stability is therefore critical to develop models that are capable of predicting how mutations alter function of proteins in disease and informing the design of proteins with desired functions. Here, we review the approaches that are used to quantify protein stability at a variety of scales, from returning multiple thermodynamic and kinetic measurements for a single protein sequence to yielding indirect insights into folding across a vast sequence space. The physical parameters derived from these approaches will provide a foundation for models that extend beyond the structural prediction to capture the complexity of conformational ensembles and, ultimately, their function.

Rapid organocatalytic chirality analysis of amines, amino acids, alcohols, amino alcohols and diols with achiral iso(thio)cyanate probes

The widespread occurrence and significance of chiral compounds does not only require new methods for their enantioselective synthesis but also efficient tools that allow rapid determination of the absolute configuration, enantiomeric composition and overall concentration of nonracemic mixtures. Although chiral analysis is a frequently encountered challenge in the chemical, environmental, materials and health sciences it is typically addressed with slow and laborious chromatographic or NMR spectroscopic techniques. We now show with almost 40 analytes representing 5 different compound classes, including mono-alcohols which are particularly challenging sensing targets, that this task can be solved very quickly by chiroptical sensing with a single, readily available arylisocyanate probe. The probe reacts smoothly and irreversibly with amino and alcohol groups when an organocatalyst is used at room temperature toward urea or carbamate products exhibiting characteristic UV and CD signals above 300 nm. The UV signal induction is not enantioselective and correlated to the total concentration of both enantiomers, the concomitant generation of a CD band allows determination of the enantiomeric composition from the same sample, and the sense of the induced Cotton effect reveals the absolute configuration by comparison with a reference. This approach eliminates complications that can arise when enantiomerically impure NMR derivatizing agents are used and it outperforms time-consuming HPLC protocols. The generation of distinct UV and CD signals at high wavelengths overcomes issues with insufficient resolution of overlapping signals often encountered with chiral NMR solvating agents that rely on weak binding forces. The broad solvent compatibility is another noteworthy and important characteristic of this assay. It addresses frequently encountered problems with insufficient solubility of polar analytes, for example pharmaceuticals, in standard mobile phase mixtures required for chiral HPLC analysis. We anticipate that the broad application spectrum, ruggedness and practicality of organocatalytic chiroptical sensing with aryliso(thio)cyanate probes together with the availability of automated CD multi-well plate readers carry exceptional promise to accelerate chiral compound development projects at reduced cost and with less waste production.

Organocatalysis with a simple arylisocyanate probe enables accelerated optical concentration and enantiomeric ratio determination of a large variety of chiral compounds based on straightforward UV/CD analysis.

Chiroptical sensing of amino acids, amines, amino alcohols, alcohols and terpenes with π-extended acyclic cucurbiturils

The efficiency and scope of two acyclic π-wall extended cucurbiturils, M2 and M3, exhibiting rapidly interconverting helical conformers for chiroptical sensing of amines, amino acids, alcohols, and terpenes at micromolar concentrations in water is evaluated. The formation of 1 : 1 host-guest complexes results in spontaneous induction of circular dichroism signals that can be used for accurate determination of the absolute configuration and enantiomeric composition of the analyte based on a simple mix-and-measure protocol.

Chemoselective bioconjugation based on modular click chemistry with 4-halocoumarins and aryl sulfonates

We report chemoselective and modular peptide bioconjugation using stoichiometric amounts of 4-halocoumarin and arylsulfonate agents that undergo metal-free C(sp²)-heteroatom bond formation at micromolar concentrations. The underlying ipso-substitution click chemistry is irreversible and generates stable and inherently fluorescent bioconjugates, and the broad selection of coumarin tags offers high labeling flexibility and versatility. Different coumarins and arylsulfonates can be selectively attached to amino and thiol groups in the small peptides glutathione and ornipressin, and both free as well as latent thiols captured in disulfide bridges can be targeted if desired. The broad utility, ease of use, storage, and preparation of 4-halocoumarins and arylsulfonates are very attractive features that extend currently available dual bioconjugation capabilities.

We report chemoselective and modular peptide bioconjugation using stoichiometric amounts of 4-halocoumarin and arylsulfonate agents that undergo metal-free C(sp²)-heteroatom bond formation at micromolar concentrations.