Pattern-based recognition for the rapid determination of identity, concentration, and enantiomeric excess of subtly different threo diols

Recognition of Amino Acid Salts by Temperature-Dependent Allosteric Binding with Stereodynamic Urea Receptors

Positive homotropic artificial allosteric systems are important for the regulation of cooperativity, selectivity and nonlinear amplification. Stereodynamic homotropic allosteric receptors can transmit and amplify induced chirality by the first ligand binding to axial chirality between two chromophores. We herein report stereodynamic allosteric urea receptors consisting of a rotational shaft as the axial chirality unit, terphenyl units as structural transmission sites and four urea units as binding sites. NMR titration experiments revealed that the receptor can bind two carboxylate guests in a positive homotropic allosteric manner attributed to the inactivation by intramolecular hydrogen-bonding between urea units within the receptor. In addition, the VT-CD spectra observed upon binding of the urea receptor with l- or D-amino acid salts in MeCN showed interesting temperature-dependent Cotton effects, based on the differences of the receptor shaft unit and the guest structure. The successful discrimination of hydrocarbon-based side chains of amino acid salts indicated that the input of chiral and steric information for the guest was amplified as outputs of the Cotton effect and the temperature-dependence of VT-CD spectra through cooperativity of positive allosteric binding.

Enantioselective and Chemoselective Optical Detection of Chiral Organic Compounds without Resorting to Chromatography

Enantiorecognition and resolution are of essential importance in many diverse areas of science. Whenever there arises a need to analyze/investigate enantiomers in different situations chromatography stands up in our minds immediately. Nevertheless, chemoselective and enantioselective recognition/discrimination (without going for separation) constitutes a different perception and requirement. The techniques using chiroptical sensing cause detection based on molecular interactions induced in different manners. Enantioselective sensing of monosaccharides in γ-cyclodextrin assembly and by diboronic acid based fluorescent sensors, application of bi-naphthol and H8 BINOL based sensors and dendrimers, metal-to-ligand charge transfer transitions in CD, exciton-coupled circular dichroism, surface enhanced Raman spectroscopy, and enantioselective indicator displacement sensor arrays for enantioselective recognition/detection of chiral organic compounds, such as amines, amino acids/alcohols, and hydroxycarboxylic acids have been discussed in progressive manner with mechanistic explanations, wherever available. Besides, the chiroptical vs LC approach has been discussed. The present paper is focused on certain different non-chromatographic optical techniques and aims to extend an understanding and a view to consider such techniques which have been successful in selective detection, and determination of absolute configuration and enantiomeric excess, (without resorting to separation vis-à-vis LC) and that have potential use in high-throughput chiral assay and combinatorial search for asymmetric catalysts and reagents.

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.

Bis-Boronic Acid Liposomes for Carbohydrate Recognition and Cellular Delivery

Liposomes are effective therapeutic delivery nanocarriers due to their ability to encapsulate and enhance the pharmacokinetic properties of a wide range of drugs and diagnostic agents. A primary area in which improvement is needed for liposomal drug delivery is to enhance the delivery of these nanocarriers to cells. Cell membrane glycans provide exciting targets for liposomal delivery since they are often densely clustered on cell membranes and glycan overabundance and aberrant glycosylation patterns are a common feature of diseased cells. Herein, we report a liposome platform incorporating bis-boronic acid lipids (BBALs) to increase valency in order to achieve selective saccharide sensing and enhance cell surface binding interactions based on carbohydrate binding interactions. In order to vary properties, multiple BBALs ( 1a-d ) with variable linkers in between the binding units were designed and synthesized. Fluorescence-based microplate screening of carbohydrate binding showed that these compounds exhibit varying binding properties depending on their structures. Additionally, fluorescence microscopy experiments indicated enhancements in cellular association when BBALs were incorporated in liposomes. These results demonstrate that multivalent BBALs serve as an exciting glycan binding liposome system for targeted liposome delivery.

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.

Nanogel receptors for high isoelectric point protein detection: influence of electrostatic and covalent polymer-protein interactions

An aldehyde acrylate-based functional monomer was incorporated into poly(N-isopropylacrylamide-co-methacrylic acid) nanogels for use as protein receptors. The aldehyde component forms dynamic imines with surface exposed lysine residues, while carboxylic acid/carboxylate moieties form electrostatic interactions with high isoelectric point proteins. Together, these interactions effect protein adsorption and recognition.

A molecular self-assembled colourimetric chemosensor array for simultaneous detection of metal ions in water

We propose a novel strategy for a high-throughput sensing of metal ions using a molecular self-assembled colourimetric chemosensor array. The proposed colourimetric assay has been achieved by only using the combination of commercially available materials. Importantly, the easy-to-prepare assay can be utilised to quantitatively detect metal ions under competitive conditions.

The In Situ Enzymatic Screening (ISES) Approach to Reaction Discovery and Catalyst Identification

The importance of discovering new chemical transformations and/or optimizing catalytic combinations has led to a flurry of activity in reaction screening. The in situ enzymatic screening (ISES) approach described here utilizes biological tools (enzymes/cofactors) to advance chemistry. The protocol interfaces an organic reaction layer with an adjacent aqueous layer containing reporting enzymes that act upon the organic reaction product, giving rise to a spectroscopic signal. ISES allows the experimentalist to rapidly glean information on the relative rates of a set of parallel organic/organometallic reactions under investigation, without the need to quench the reactions or draw aliquots. In certain cases, the real-time enzymatic readout also provides information on sense and magnitude of enantioselectivity and substrate specificity. This article contains protocols for single-well (relative rate) and double-well (relative rate/enantiomeric excess) ISES, in addition to a colorimetric ISES protocol and a miniaturized double-well procedure. © 2017 by John Wiley & Sons, Inc.

Fluorinated Boronic Acid-Appended Pyridinium Salts and 19F NMR Spectroscopy for Diol Sensing

The identification and discrimination of diols is of fundamental importance in medical diagnostics, such as measuring the contents of glucose in the urine of diabetes patients. Diol sensors are often based on fluorophore-appended boronic acids, but these severely lack discriminatory power and their response is one-dimensional. As an alternative strategy, we present the use of fluorinated boronic acid-appended pyridinium salts in combination with ¹⁹F NMR spectroscopy. A pool of 59 (bio)analytes was screened, containing monosaccharides, phosphorylated and N-acetylated sugars, polyols, carboxylic acids, nucleotides, and amines. The majority of analytes could be clearly detected and discriminated. In addition, glucose and fructose could be distinguished up to 1:9 molar ratio in mixtures. Crucially, the receptors feature high sensitivity and selectivity and are water-soluble, and their ¹⁹F-NMR analyte fingerprint is pH-robust, thereby making them particularly well-suited for medical application. Finally, to demonstrate this applicability, glucose could be detected in synthetic urine samples down to 1 mM using merely a 188 MHz NMR spectrometer.

Biomimetic Chirality Sensing with Pyridoxal-5'-phosphate

Pyridoxal-5'-phosphate (PLP) is introduced to a biomimetic indicator displacement assay for simultaneous determination of the absolute configuration, enantiomeric composition and concentration of unprotected amino acids, amino alcohols and amines. The chiroptical assay is based on fast imine metathesis with a PLP aryl imine probe to capture the target compound for circular dichroism and fluorescence sensing analysis. The substrate binding yields characteristic Cotton effects that provide information about the target compound ee and the synchronous release of the indicator results in a nonenantioselective off-on fluorescence response that is independent of the enantiomeric sample composition and readily correlated to the total analyte concentration.

Fluorescent chirality recognition by simple boronate ensembles with aggregation-induced emission capability

Chiral boronate ensembles showed enantioselective aggregation behaviors for chiral diamines and cinchona alkaloids, enabling the fluorescent recognition of their chirality.

From Substituent Effects to Applications: Enhancing the Optical Response of a Four-Component Assembly for Reporting EE Values

High-throughput screening for asymmetric catalysts has stimulated an interest in optically-based enantiomeric-excess (ee) sensors, primarily for their improved time and cost efficiency when compared to the standard HPLC analysis. We present herein substituent-effect studies on a recently reported Zn(II) multicomponent assembly that is used for chiral, secondary alcohol ee detemination. The systematic altering of assemblies formed from select substituted pyridyl ligands pointed to the conclusion that steric effects dominate the mode of interaction at the pyridyl 3- and 6- positions. From these results we identified a new Zn(II)-centered multicomponent assembly with a higher dynamic range than previously reported. Calibration curves of the CD signals resulting from the new assembly led to an ee assay with a 1.7% error. To further the utility of the new assembly, a correlation was developed between alcohol substituent size to the respective enantiopure CD value.

Application of a High-Throughput Enantiomeric Excess Optical Assay Involving a Dynamic Covalent Assembly: Parallel Asymmetric Allylation and Ee Sensing of Homoallylic Alcohols

Parallel synthesis and high-throughput ee screening.

Asymmetric Ir-catalyzed C–C coupling of primary alcohols with allyl-acetates, as described by Krische, to form chiral secondary homo-allylic alcohols were performed in parallel as a means to optimize the ee values thereof. Specifically, approximately 400 examples of this reaction were performed by varying the catalyst, added acids and bases, and starting reactants, to form 4-phenyl-1-butene-4-ol (1). The ee values for the transformations were determined in a high-throughput fashion using a 4-component assembly that creates a circular dichroism signal indicative of the extent of asymmetric induction. Further, a parallel and rapid quantitative TLC method measures the yield of each reaction, revealing which reactions give reliable ee values in the CD-based assay. Overall, the nearly 200 reactions whose ee values were determined could be quantitated in under two hours. Using a combination of the TLC method to measure yield with the CD-assay to measure ee values, several trends in reaction conditions were revealed. For example, it was found that the cyclometalated iridium catalyst modified by BINAP and m-nitro-p-cyano-benzoic acid delivered adduct 1 with the highest levels of enantiomeric enrichment (94%), whereas the corresponding SEGPHOS-modified catalyst gave a comparable yield but lower ee (91%). Most importantly, this study shows that supramolecular assemblies can report hundreds of ee values in a rapid and reliable fashion to analyze parallel synthesis routines.

Array-based detection of persistent organic pollutants via cyclodextrin promoted energy transfer

We report herein the selective array-based detection of 30 persistent organic pollutants via cyclodextrin-promoted energy transfer. The use of three fluorophores enabled the development of an array that classified 30 analytes with 100% accuracy and identified unknown analytes with 96% accuracy, as well as identifying 92% of analytes in urine.

Reactivity-based dynamic covalent chemistry: reversible binding and chirality discrimination of monoalcohols

In an effort to develop reactivity-based dynamic covalent bonding and to expand the scope and application of the dynamic covalent chemistry, in situ-generated simple generic iminium ions were utilized for the dynamic covalent binding of monoalcohols with high affinity. Hammett analysis was conducted to manipulate the equilibrium and correlate with the reactivity of reactants. The structural features of aldehydes and secondary amines were identified, and both polar and steric effects have significant impact on the binding. In particular, the substrates which can participate in π-π and polar-π interactions are able to afford apparent equilibrium constants in the magnitude of 10(4) M(-2), demonstrating the power of weak supramolecular forces to stabilize the dynamic covalent assembly. The generality of the assembly was validated with a series of mono secondary alcohols. To showcase the practicality of our system, chirality discrimination and ee measurement of chiral secondary alcohols were achieved.

A cross reactive sensor array to probe divalent metal ions

A simple sensing ensemble was designed to discriminate structurally similar divalent metal chlorides utilizing multivariate data analysis.

Single-nucleotide-resolution DNA differentiation by pattern generation in lipid bilayer membranes

Pattern generation/recognition in lipid bilayers is introduced for the differentiation of anionic biological relevant polymers. The amplification of the polymer differences during transport events allows the straightforward identification of a wide range collection of anionic polymers. The introduced approach displays excellent resolution even for single mutations in short single-stranded oligonuclotides.

Rapid optical methods for enantiomeric excess analysis: from enantioselective indicator displacement assays to exciton-coupled circular dichroism

CONSPECTUS: The advent of high-throughput screening (HTS) for chiral catalysts has encouraged the development of fast methods for determining enantiomeric excess (ee). Traditionally, chromatographic methods such as chiral HPLC have been used for ee determination in HTS. These methods, however, are not optimal because of high duty cycle. Their long analysis time results in a bottleneck in the HTS process. A more ideal method for HTS that requires less analysis time such as chiroptical methods are thus of interest. In this Account, we summarize our efforts to develop host-guest systems for ee determination. The first part includes our enantioselective indicator displacement assays (eIDAs), and the second part focuses on our circular dichroism based host-guest systems. Our first eIDA utilizes chiral boronic acid receptors, along with prescreened indicators, to determine ee for chiral α-hydroxyacids and vicinal diols with ±7% average error (AE). To further the practicality for this system, a HTS protocol was developed. Our second eIDA uses diamino chiral ligands and Cu(II) as the receptor for the ee determination of α-amino acids. The system reported ±12% AE, and a HTS protocol was developed for this system. Our first CD based host-guest system uses metal complexes composed of Cu(I) or Pd(II) with enantiopure 2,2'-diphenylphosphino-1,1'-binaphthyl (BINAP) as host to determine the ee of chiral vicinal diamines (±4% AE), primary amines (±17% AE), and cyclohexanones (±7% AE). Primary amines and cyclohexanones were derivatized to form chiral imines or chiral hydrazones to allow coordination with the metal complex. Upon coordination of chiral analytes, the metal-to-ligand (BINAP) charge transfer band was modulated, thus allowing the discrimination of chiral analytes. As an effort to improve the accuracy for chiral primary amine ee determination, a system with a host composed of o-formylphenyl boronic acid (FPBA) and enantiopure 1,1'-bi-2-naphthol (BINOL) was used to reduce the AE to ±5.8%. In the presence of amines, the FPBA-BINOL host forms an imine-coordinated boronic ester, thus affecting the CD signal of the boron complex. Another chiral primary amine ee determination system was developed with Fe(II) and 3-hydroxy-2-pyridinecarbaldehyde. The chiral imines, formed by the pyridinecarbaldehyde and chiral amines, would coordinate to the Fe(II) ion yielding exciton-coupled circular dichroism (ECCD) active metal complexes. This system was able to determine the ee of chiral amines with ±5% AE. Furthermore, this imine-Fe(II) complex system also successfully determined the ee of α-chiral aldehydes with ±5% AE. Other ECCD based hosts were subsequently developed; one with bisquinolylpyridylamine and Cu(II) for chiral carboxylates and amino acids and another multicomponent system with pyridine chromophores for chiral secondary alcohol ee determination. Both of the systems were able to determine ee of the chiral analytes with ±3% AE. Overall, our group has developed ee determining host-guest systems that target various functionalities. To date, we are able to determine the ee of vicinal diols, α-hydroxyacids, vicinal diamines, cyclohexanones, amines, α-chiral aldehydes, carboxylates, amino acids, and secondary alcohols with ±7% or lower average error. Future development will involve improving the average error and employing the current systems to analyze real-life samples resulting from parallel syntheses.

Imine-based chiroptical sensing for analysis of chiral amines: from method design to synthetic application

A robust chiroptical method for fast enantiopurity determination of chiral amines utilizing HPLC-CD, applied to monitoring crude transamination reactions.

The use of principal component analysis and discriminant analysis in differential sensing routines

Statistical analysis techniques such as principal component analysis (PCA) and discriminant analysis (DA) have become an integral part of data analysis for differential sensing. These multivariate statistical tools, while extremely versatile and useful, are sometimes used as "black boxes". Our aim in this paper is to improve the general understanding of how PCA and DA process and display differential sensing data, which should lead to the ability to better interpret the final results. With various sets of model data, we explore several topics, such as how to choose an appropriate number of hosts for an array, selectivity compared to cross-reactivity, when to add hosts, how to obtain the best visually representative plot of a data set, and when arrays are not necessary. We also include items at the end of the paper as general recommendations which readers can follow when using PCA or DA in a practical application. Through this paper we hope to present these statistical analysis methods in a manner such that chemists gain further insight into approaches that optimize the discriminatory power of their arrays.

Molecular recognition with boronic acids-applications in chemical biology

Small molecules have long been used for the selective recognition of a wide range of analytes. The ability of these chemical receptors to recognise and bind to specific targets mimics certain biological processes (such as protein-substrate interactions) and has therefore attracted recent interest. Due to the abundance of biological molecules possessing polyhydroxy motifs, boronic acids-which form five-membered boronate esters with diols-have become increasingly popular in the synthesis of small chemical receptors. Their targets include biological materials and natural products including phosphatidylinositol bisphosphate, saccharides and polysaccharides, nucleic acids, metal ions and the neurotransmitter dopamine. This review will focus on the many ways in which small chemical receptors based on boronic acids have been used as biochemical tools for various purposes, including sensing and detection of analytes, interference in signalling pathways, enzyme inhibition and cell delivery systems. The most recent developments in each area will be highlighted.

Correlating sterics parameters and diastereomeric ratio values for a multicomponent assembly to predict exciton-coupled circular dichroism intensity and thereby enantiomeric excess of chiral secondary alcohols

Linear free energy relationship (LFER) substituent parameters are commonly employed for exploring reaction mechanisms and very recently have been used to guide the design of asymmetric catalysts, but their usage in dynamic covalent chemistry is rare. Herein, the properties of an in situ-generated dynamic multicomponent covalent assembly that creates tris(pyridine) metal complexes incorporating chiral secondary alcohols were explored using LFER-based steric parameters. The diastereomeric ratio (dr) of the assembly was correlated with the magnitude of the exciton-coupled circular dichroism (ECCD) induced by chiral alcohols. Charton steric parameters were successfully correlated with the dr values. Through the combination of these correlations, both the dr and CD intensity were predicted for test alcohols. These correlations were also employed to measure a few new Charton parameters. Finally, the prediction of enantiomeric excess (ee) of test samples with various alcohol structures was also successfully achieved. The prediction of spectral properties in advance by using well-established steric parameters is shown to be useful for rapid ee screening because the need for calibration curves and enantiomerically enriched samples is avoided.