Bisboronic Acid Research Articles

Fluorescent Chemosensors in the Creation of a Commercially Available Continuous Glucose Monitor.

Fully automated insulin delivery (i.e., an artificial pancreas) would revolutionize diabetes disease management, minimize negative secondary disease outcomes, and simultaneously reduce health care costs and system burdens. Continuous glucose monitoring (CGM) is an essential aspect of the artificial pancreas. Abiotic fluorescent chemosensors play a key role in generating long-lived CGM sensors for this purpose. In this Perspective, we detail our initial discoveries of chemosensors for saccharides, as well as the development and advancement of bis((o-aminomethylphenyl)boronic acid)anthracene-based sensors for commercial use. While a few companies have sought to bring a copolymerized diboronic acid CGM sensor to the market, Senseonics is the only one, to date, to have done so. In this case, the system has been approved in the U.S. and Europe to provide accurate CGM for up to 365 days with a single sensor and can be integrated directly with an insulin pump, bringing an artificial pancreas one step closer to realization.

The use of multicomponent reactions in the development of bis-boronic acids for the detection of β-sialic acid.

Sialic acid (SA) is a naturally occurring monosaccharide found in glycoproteins and glycolipids. Changes in the expression of SA are associated with several diseases; thus, the detection of SA is of great significance for biological research, cancer diagnosis, and treatment. Boronic acid analogs have emerged as a promising tool for detecting sugars such as SA due to its reversible covalent bonding ability. In this study, 11 bis-boronic acid compounds and 2 mono-boronic acid compounds were synthesized via a highly efficient Ugi-4CR strategy. The synthesized compounds were subjected to affinity fluorescence binding experiments to evaluate their binding capability to SA. Compound A1 was shown to have a promising binding constant of 2602 ± 100 M-1 at pH = 6.0. Density Functional Theory (DFT) calculations examining the binding modes between A1 and SA indicated that the position of the boronic acid functional group was strongly correlated with its interaction with SA's α-hydroxy acid unit. The DFT calculations were consistent with the observations from the fluorescence experiments, demonstrating that the number and relative positions of the boronic acid functional groups are critical factors in enhancing the binding affinity to SA. DFT calculations of both S and R configuration of A1 indicated that the effect of the S/R configuration of A1 on its binding with β-sialic acid was insignificant as the Ugi-4CR generated racemic products. A fluorine atom was incorporated into the R2 substituent of A1 as an electron-withdrawing group to produce A5, which possessed a significantly higher capability to bind to SA (Keq = 7015 ± 5 M-1 at pH = 6.0). Finally, A1 and A5 were shown to possess exceptional binding selectivity toward β-sialic acid under pH of 6.0 and 6.5 while preferring to bind with glucose, fructose, and galactose under pH of 7.0 and 7.5.

Rhenium(I) bromo tricarbonyl complexes from anthracenyl derivatized ligands

The ligand 2-(1H-pyrazol-1-yl)pyrazine (pypyr) was prepared by reaction of 2-bromopyrazine with N-lithium pyrazolate with 92.5% yield, while the anthracenyl derivatives 2-(anthracen-9-yl)-5-(1H-pyrazol-1-yl)pyrazine (pypyr-anthra) and 9,10-bis(5-(1H-pyrazol-1-yl)pyrazin-2-yl)anthracene (pypyr-anthra-pypyr) were prepared by reaction of the 9-anthraceneboronic or bis-boronic acid pinacol ester with 2-bromo-5-(1H-pyrazol-1-yl)pyrazine with medium yields (49.6 and 31.5 % respectively). The respective ReI(CO)3Br complexes: [(pypyr)Re(CO)3Br], [(pypyr-anthra)Re(CO)3Br] and [Br(CO)3Re(pypyr-anthra-pypyr)Re(CO)3Br] were prepared in high yields (94, 84 and 78 % respectively) by reaction with bromotricarbonyl(tetrahydrofuran)rhenium(I) dimer. DFT modelling suggests the anthracenyl moiety is not coplanar with the pyrazolyl-pyrazine fragment, defining a dihedral angle of 65° and 67° for [(pypyr-anthra)Re(CO)3Br] and [Br(CO)3Re(pypyr-anthra-pypyr)Re(CO)3Br] respectively. Each one of the three rhenium(I) molecules shows a quasi-reversible reduction wave around − 1.10 V, assigned by comparison with the uncoordinated ligand, the pypyr fragment, while additional reductions waves related to this core or the anthryl arms are present. UV–Vis spectra show absorption bands and/or shoulders around 400–450 nm for the series of compounds in solution. The respective extinction coefficients (∼4 × 103 M−1cm−1), the sensitivity to solvent polarity and DFT modelling suggest they corresponds to MLCT Redπ → π(pyr)* transitions. Despite their high emissivity, the uncoordinated ligands completely lose their emission upon coordinated to ReI(CO)3Br. The rhenium(I) complexes emit around 650 nm upon excitation, although [(pypyr)Re(CO)3Br] is a moderate emissive molecule, while [(pypyr-anthra)Re(CO)3Br] and [Br(CO)3Re(pypyr-anthra-pypyr)Re(CO)3Br] are very weak ones. In contrast these two last molecules showed a remarkably high yield as singlet oxygen sensitizers.

Dual-Boronic Acid Reagents That Combine Dynamic and Covalent Bioconjugation.

Boronic acids and boronate esters find appreciable use in chemical biology. Molecules containing orthogonal boronic acid pairs can be utilized for sequential metal-catalyzed cross-couplings for facile preparation of complex bioconjugates including protein-protein conjugates. In this paper, we expand bis-boronic acid reagents for tandem covalent and dynamic bioconjugation. Sequential cross-coupling of 2-nitroarylboronic acid with cysteine residues and condensation of phenylboronic acid with salicylhydroxamic acids (SHA) readily afforded bioconjugates under physiological conditions with dual covalent and dynamic linkages. Both small molecule- and macromolecule-protein conjugates were amenable with this approach and reversible upon addition of excess unfunctionalized SHA or reactive oxygen species. These investigations provide new insights into the kinetic stability of SHA adducts.

Bis-Boronic Acid Liposomes for Carbohydrate Recognition and Cellular Delivery.

Liposomes are effective therapeutic 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 maximize 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 recognition based on carbohydrate binding interactions. In order to vary properties, multiple BBALs (1 a-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 within liposomes. These results demonstrate that multivalent BBALs serve as an exciting glycan binding liposome system for targeted delivery.

Control of Glucose‐Induced Degradation and Cargo Release in Multi‐Responsive Polymer Hydrogels

Back Cover: In article number 2100121, Suchetan Pal and co-workers develop a family of multi-responsive polymer hydrogels crosslinked with bis boronic acid molecules. Hydrogels crosslinked with tetrahydroxydiboron show glucoseresponsive degradation in the presence of glucose oxidase (GOX) enzyme. Furthermore, insulin releases under high glucose conditions in the physiological milieu.

Transition-Metal-Free Borylation of Aryl Bromide Using a Simple Diboron Source.

In this study, we developed a simple transition-metal-free borylation reaction of aryl bromides. Bis-boronic acid (BBA), was used, and the borylation reaction was performed using a simple procedure at a mild temperature. Under mild conditions, aryl bromides were converted to arylboronic acids directly without any deprotection steps and purified by conversion to trifluoroborate salts. The functional group tolerance was considerably high. The mechanism study suggested that this borylation reaction proceeds via a radical pathway.

Determination of Dopamine Using 2-(4-Boronophenyl)quinoline-4-carboxylic Acids as Fluorescent Probes

The selective identification of dopamine is a significant issue because this compound is an important neurotransmitter closely related to Parkinson’s disease and other mental disorders. 2-(4-Boronophenyl)quinoline-4-carboxylic acid (PBAQA) has been previously reported as a water-soluble fluorescent probe for catechol. However, there are no significant differences in the binding constants between catechol and catecholamines, such as dopamine or levodopa. Here a series of bis-boronic acid compounds based on PBAQA were synthesized and the binding activities were characterized. As a representative compound, the binding constant of 4-(4-((3-(3-borono-4-chlorobenzamido)propyl)carbamoyl)quinolin-2-yl)boronic acid to dopamine is up to 104 L⋅mol−1 and much higher than previously reported boronic acid probes. Dopamine selectivity may be achieved by the variation of the substituents in the probe molecules. 4-(4-((3-(3-Borono-4-methoxybenzamido)propyl)carbamoyl)quinolin-2-yl)boronic acid has a stronger binding affinity to dopamine (Ka=5204 ± 106 L⋅mol−1) than catechol (Ka=2588 ± 273 L⋅mol−1) or levodopa (Ka=2383 ± 273 L⋅mol−1). This fluorescence response was used for determining dopamine in a range from 5 × 10−5 mol⋅L−1 to 5 × 10−4 mol⋅L−1 with a detection limit of 7.7 × 10−6 mol⋅L−1. This compound has been successfully used for the assay of dopamine in rabbit plasma, exhibiting excellent specificity. It is believed that synthesized compounds hold great promise as practical platforms to monitor dopamine levels.

2-step synthesis yields large 2-D COF crystals

Organic polymers don’t readily form large, two-dimensional single crystals. If they did, their properties could be tuned via organic chemistry and they could join the growing family of 2-D materials being studied for use in electronics and sensing. Some covalent organic frameworks (COFs) do take on a 2-D shape, but they typically form insoluble polycrystalline powders with tiny crystalline domains—less than 50 nm. Northwestern University scientists Austin M. Evans, William R. Dichtel, and coworkers attribute the problem to poorly controlled nucleation and growth of COF crystals. To bypass those problems, they devised a two-step synthesis strategy that separates the nucleation and growth processes (Science 2018, DOI: 10.1126/science.aar7883). For the nucleation step, the team reacted a bisboronic acid compound and a triphenylene derivative. The reaction formed a colloidal suspension of COF-5 nanoparticles that remained stable because of the presence of acetonitrile in a custom-blended solvent. Then they sl...

Anion and sugar recognition by 2,6-pyridinedicarboxamide bis-boronic acid derivatives

AbstractTwo 2,6-pyridinedicarboxamide derivatives containing arylboronic acid fragments were prepared and fully characterized including X-ray crystal diffraction analysis of a pinacol ester. These compounds are potential bifunctional receptors for sugars and anions. Acid dissociation and stability constants for complexation of both receptors with glucose and fructose were determined by potentiometric titrations in aqueous DMSO. Also, binding of alizarin red S indicator was studied spectrophotometrically and a highly sensitive detection of fructose by an indicator displacement assay was proposed. Complexation with anions was studied by1H NMR titrations in DMSO-d6. Binding of acetate anion occurs only via hydrogen bonding to OH groups of boronic acid fragments and does not affect signals of NH protons but chloride anion induces large shift of the signals of NH protons and small shifts of the signals of OH groups. This behavior makes possible anion discrimination based on preference in the type of binding site rather than simply on anion basicity as is typical for majority of neutral hydrogen bonding anion receptors.

Triazole-linked fluorescent bisboronic acid capable of selective recognition of the Lewis Y antigen

Cell surface carbohydrates of the Lewis blood group antigens, Lewis X (Lex), Lewis Y (Ley), Lewis A (Lea), and their sialylated derivatives, such as sialy Lewis X (sLex) and sialy Lewis A (sLea), play important roles in various recognition processes. These cell surface carbohydrates have also been associated with the development and progression of many types of cancers. Recently, we synthesized four anthracene-based fluorescent bisboronic acid sensors (compounds 2a–d) linked by ‘click’ chemistry with tethers of different lengths to match the epitope of various Lewis group of sugars. Among the four compounds, 2a appears to have both high sensitivity and selectivity for Ley among other carbohydrate antigens.

Synthesis of fluorescent bisboronic acid sensors and their recognition of mono-/oligo-saccharides

Sensors capable of recognizing cell surface carbohydrates, such as sialyl Lewis X (sLex), are invaluable research tools and for the diagnosis and early detection of many forms of cancer. In this paper, we report the design and synthesis of a series of bisboronic acids 6(a–f) as fluorescent sensors towards mono-/oligosaccharides. Among them, compounds 6d and 6e showed strong binding affinities with glucose and fructose, while compound 6c, in which two anthracene-based boronic acid units were linked by a hexamethylene spacer, was able to recognize sLex selectivity and stained HEPG2 cells at 1μmol/L.

Glucose selective bis-boronic acid click-fluor.

Four novel bis-boronic acid compounds were synthesised via copper catalysed azide-alkyne cycloaddition (CuAAC) reactions. Glucose selectivity was observed for a particular structural motif. Moreover, a new glucose selective fluorescent sensor was designed and synthesised as a result.

Bisboronic Acids for Selective, Physiologically Relevant Direct Glucose Sensing with Surface-Enhanced Raman Spectroscopy.

This paper demonstrates the direct sensing of glucose at physiologically relevant concentrations with surface-enhanced Raman spectroscopy (SERS) on gold film-over-nanosphere (AuFON) substrates functionalized with bisboronic acid receptors. The combination of selectivity in the bisboronic acid receptor and spectral resolution in the SERS data allow the sensors to resolve glucose in high backgrounds of fructose and, in combination with multivariate statistical analysis, detect glucose accurately in the 1-10 mM range. Computational modeling supports assignments of the normal modes and vibrational frequencies for the monoboronic acid base of our bisboronic acids, glucose and fructose. These results are promising for the use of bisboronic acids as receptors in SERS-based in vivo glucose monitoring sensors.

Integrated Microfluidic Isolation of Aptamers Using Electrophoretic Oligonucleotide Manipulation.

We present a microfluidic approach to integrated isolation of DNA aptamers via systematic evolution of ligands by exponential enrichment (SELEX). The approach employs a microbead-based protocol for the processes of affinity selection and amplification of target-binding oligonucleotides, and an electrophoretic DNA manipulation scheme for the coupling of these processes, which are required to occur in different buffers. This achieves the full microfluidic integration of SELEX, thereby enabling highly efficient isolation of aptamers in drastically reduced times and with minimized consumption of biological material. The approach as such also offers broad target applicability by allowing selection of aptamers with respect to targets that are either surface-immobilized or solution-borne, potentially allowing aptamers to be developed as readily available affinity reagents for a wide range of targets. We demonstrate the utility of this approach on two different procedures, respectively for isolating aptamers against a surface-immobilized protein (immunoglobulin E) and a solution-phase small molecule (bisboronic acid in the presence of glucose). In both cases aptamer candidates were isolated in three rounds of SELEX within a total process time of approximately 10 hours.

Boronic Acids and Derivatives—Probing the Structure–Activity Relationships for Mutagenicity

Despite a recent publication describing boronic acids as a “novel class of bacterial mutagen,” process and analytical chemists may not be aware of the potential worker exposure hazards, or of the need to assess boron compounds as potential genotoxic impurities (GTIs) per ICH M7. This publication provides new Ames data for 44 commercially available boronic acids, boronic acid derivatives, and boron-containing reagents. Trends in the Ames data are discussed from a structure–activity perspective. Common reagents such as bis(pinacolato)diboron and bis-boronic acid were shown to be mutagenic in the Ames assay. Currently available in silico computational models were found to provide little value in predicting the outcome of the Ames assay for boronic acids and derivatives. We propose oxygen-mediated oxidation of boron compounds to generate organic radicals as a potential mechanism for mutagenicity. It is hoped that this paper will result in increased awareness of this class of GTIs and prompt publication of add...

A fluorescent bisboronic acid compound that selectively labels cells expressing oligosaccharide Lewis X.

Two fluorescent diboronic acid compounds (6a and 6b) with a dipeptide linker were synthesized as potential sensors for cell surface saccharide Lewis X (Le(X)). Compound 6a with a dipeptide (H-Asp-Ala-) as the linker was found to selectively label CHOFUT4 cells, which express Le(x), at micromolar concentrations, while non-Le(x)-expressing control cells were not labeled.

A modified procedure for the palladium catalyzed borylation/Suzuki-Miyaura cross-coupling of aryl and heteroaryl halides utilizing bis-boronic acid

A modified Pd-catalyzed method of forming aryl- and heteroarylboron species and a two-step, one-pot borylation/Suzuki-Miyaura cross coupling using the atom economical tetrahydroxydiboron (bis-boronic acid, BBA) is reported. By using ethylene glycol as an additive, the new method results in increased yields, lower BBA loading, faster reaction times, and a broader reaction scope, including previously problematic substrates such as heterocycles.

A fluorescent bisboronic acid compound that selectively labels cells expressing oligosaccharide Lewis X

A fluorescent bisboronic acid compound that selectively labels cells expressing oligosaccharide Lewis X

Rapid small intestinal permeability assay based on riboflavin and lactulose detected by bis-boronic acid appended benzyl viologens

BackgroundAlthough organoboronic acids are efficient high-throughput sugar sensors, they have not been pursued for gut permeability studies. A modification of the lactulose/mannitol assay is described by which small intestinal permeability is assessed at the time of urine collection using a lactulose/riboflavin ratio. MethodsVolunteers ingested 50mg riboflavin and either 5g mannitol or 10g lactulose. Urine was collected for 6hrs. Riboflavin was assayed by autofluorescence. Riboflavin was removed by C18 solid phase extraction. Lactulose and mannitol were then assayed using 1,1’-bis(2-boronobenzyl)-4,4’-bipyridinium (4,4’oBBV) coupled to the fluorophore HPTS. ResultsThe temporal profile over 6hrs for riboflavin paralleled mannitol. Riboflavin recovery in urine was 11.1±1.9 % (mean±SEM, n=7), similar to mannitol. There was selective binding of 4,4’oBBV to lactulose, likely involving cooperativity between the fructose and galactose moieties. Lower limits of detection and quantification were 90 and 364μM. The lactulose assay was insensitive to other permeability probes (e.g., sucrose, sucralose) while tolerating glucose or lactose. This assay can be adapted to automated systems. Stability of 4,4’oBBV exceeds 4years. ConclusionsRiboflavin measured by autofluorescence combined with lactulose measured with 4,4’oBBV represents a useful new chemistry for rapid measurement of intestinal permeability with excellent stability, cost and throughput benefits.