Probing Intramolecular B−N Interactions in Ortho-Aminomethyl Arylboronic Acids

A massively parallel screening platform for converting aptamers into molecular switches

Aptamer-based molecular switches that undergo a binding-induced conformational change have proven valuable for a wide range of applications, such as imaging metabolites in cells, targeted drug delivery, and real-time detection of biomolecules. Since conventional aptamer selection methods do not typically produce aptamers with inherent structure-switching functionality, the aptamers must be converted to molecular switches in a post-selection process. Efforts to engineer such aptamer switches often use rational design approaches based on in silico secondary structure predictions. Unfortunately, existing software cannot accurately model three-dimensional oligonucleotide structures or non-canonical base-pairing, limiting the ability to identify appropriate sequence elements for targeted modification. Here, we describe a massively parallel screening-based strategy that enables the conversion of virtually any aptamer into a molecular switch without requiring any prior knowledge of aptamer structure. Using this approach, we generate multiple switches from a previously published ATP aptamer as well as a newly-selected boronic acid base-modified aptamer for glucose, which respectively undergo signal-on and signal-off switching upon binding their molecular targets with second-scale kinetics. Notably, our glucose-responsive switch achieves ~30-fold greater sensitivity than a previously-reported natural DNA-based switch. We believe our approach could offer a generalizable strategy for producing target-specific switches from a wide range of aptamers.

Probing the Role of the Bridging Nitrogen in the Signaling Mechanism of an Anthracene-Boronic Acid Sugar Sensor and a Different Version of the PET-Based Mechanism

The N-quaternized derivative 5 of the James-Shinkai anthracene-boronic acid fluorescence sugar sensor 1 was prepared to probe the role of the bridging nitrogen in the signaling mechanism of 1. Both 5 and 1 contain positively charged bridging groups NMe⁺ or NH⁺, respectively, but 5 lacks the ability to form the intramolecular ammonium-boronate doubly ionic hydrogen bond present in 1. Receptors 1 and 5 display opposite fluorescence vs pH profiles with a small turn-on effect of the sugar binding to the zwitterion of 5 in contrast to a large effect observed with 1. It is concluded that the ammonium-boronate hydrogen bond is essential for the signaling mechanism of 1. Its possible function is enabling the PET quenching effect by shifting the NH⁺ proton toward boronate anion inside the hydrogen bond, the degree of which is modulated by the ester formation with diols affecting the basicity of boronate anion. This mechanism agrees with observed signaling selectivity of 1 toward a series of di- and polyols of variable structures as well as with the behavior of 1 in buffered D₂O and methanol solvents at controlled pH and provides an addition to the established "loose bolt" mechanism signaling mode essential for receptors with nonpolar fluorophores.

Internal and External Catalysis in Boronic Ester Networks

In dynamic materials, the reversible condensation between boronic acids and diols provides adaptability, self-healing ability, and responsiveness to small molecules and pH. The thermodynamics and kinetics of bond exchange determine the mechanical properties of dynamic polymer networks. Here, we investigate the effects of diol structure and salt additives on the rate of boronic acid-diol bond exchange, binding affinity, and the mechanical properties of the corresponding polymer networks. We find that proximal amides used to conjugate diols to polymers and buffering anions induce significant rate acceleration, consistent with an internal and external catalysis, respectively. This rate acceleration is reflected in the stress relaxation of the gels. These findings contribute to the fundamental understanding of the boronic ester dynamic bond and offer molecular strategies to tune the macromolecular properties of dynamic materials.

Lysine-Targeting Reversible Covalent Inhibitors with Long Residence Time

We report a new reversible lysine conjugation that features a novel diazaborine product and much slowed dissociation kinetics in comparison to the previously known iminoboronate chemistry. Incorporating the diazaborine-forming warhead RMR1 to a peptide ligand gives potent and long-acting reversible covalent inhibitors of the staphylococcal sortase. The efficacy of sortase inhibition is demonstrated via biochemical and cell-based assays. A comparative study of RMR1 and an iminoboronate-forming warhead highlights the significance and potential of modulating bond dissociation kinetics in achieving long-acting reversible covalent inhibitors.

Molecular Boronic Acid-Based Saccharide Sensors

Boronic acids can reversibly bind diols, a molecular feature that is ubiquitous within saccharides, leading to their use in the design and implementation of sensors for numerous saccharide species. There is a growing understanding of the importance of saccharides in many biological processes and systems; while saccharide or carbohydrate sensing in medicine is most often associated with detection of glucose in diabetes patients, saccharides have proven to be relevant in a range of disease states. Herein the relevance of carbohydrate sensing for biomedical applications is explored, and this review seeks to outline how the complexity of saccharides presents a challenge for the development of selective sensors and describes efforts that have been made to understand the underpinning fluorescence and binding mechanisms of these systems, before outlining examples of how researchers have used this knowledge to develop ever more selective receptors.

A solvent-dependent chirality-switchable thia-Michael addition to α,β-unsaturated carboxylic acids using a chiral multifunctional thiourea catalyst

An asymmetric thia-Michael addition of arylthiols to α,β-unsaturated carboxylic acids using a thiourea catalyst that bears arylboronic acid and tertiary amine moieties is reported.

An asymmetric thia-Michael addition of arylthiols to α,β-unsaturated carboxylic acids using a thiourea catalyst that bears arylboronic acid and tertiary amine moieties is reported. Both enantiomers of the Michael adducts can be obtained in high enantioselectivity and good yield merely by changing the solvent. The origin of the chirality switch in the products was examined in each solvent via spectroscopic analyses.

Examination of pinanediol-boronic acid ester formation in aqueous media: relevance to the relative stability of trigonal and tetrahedral boronate esters

The interaction of pinanediol with 2-fluorophenylboronic acid and several other substituted phenylboronic acids was studied in 40% vol. aqueous acetonitrile by 1H and 11B NMR, potentiometric and spectrophotometric titrations at variable pH values. The experimental results reveal the formation of a very stable trigonal ester (Ktrig ≈ 2 × 104 M-1) and a significantly less stable tetrahedral hydroxocomplex (Ktet ≈ 5 × 103 M-1) in contrast to the traditionally observed inverted order of stabilities Ktrig < Ktet. Comparison of the crystal structure of the trigonal ester isolated from aqueous acetonitrile with the DFT simulated structure of the respective hydroxocomplex shows that an unusual order of stabilities Ktrig > Ktet is observed in spite of the existence of the usual strain release effect in the O-B-O angle considered responsible for the typically observed increased stability of the tetrahedral hydroxocomplex. A complementary study of the stability of the six-membered cyclic boronate esters of chromotropic acid demonstrated the order Ktrig ≪ Ktet although the strain was absent in these esters. The results for m-, p-substituted phenylboronic acids show that the stability of both five- and six-membered trigonal esters formed with pinanediol and chromotropic acid, respectively, is insensitive to electronic effects but the electron accepting substituents stabilize the hydroxocomplexes. It follows from the whole set of results that Ktet can be much larger than Ktrig in the absence of the strain, but with a sufficiently acidic diol, and that the presence of the strain does not necessarily make Ktet larger than Ktrig for a less acidic diol with a purely saturated hydrocarbon backbone. Thus, the electronic effects manifested in the acidity of the diol appear to be more significant than the strain release effect in determining the Ktet/Ktrig ratio.

Selective Binding of Dopamine and Epinephrine in Water by Molecularly Imprinted Fluorescent Receptors

Catecholamines play important roles in biology but their structural similarity makes it challenging to construct synthetic receptors with selective binding. A combination of covalent and noncovalent binding groups in the hydrophobic core of water-soluble nanoparticles enabled them to recognize dopamine and epinephrine with an association constant (Ka ) of 3-4×104  M-1 in water, an order of magnitude higher than those of previously reported synthetic hosts. In addition, minute structural changes among analogues were detected including the addition or removal of a single hydroxyl or methyl group.

Boron-Nitrogen Double Tweezers Comprising Arylboronic Esters and Diamines: Self-Assembly in Solution and Adaptability as Hosts for Aromatic Guests in the Solid State

The thermodynamic stability of 1 : 1 and 2 : 1 boron-nitrogen (B←N) adducts formed between aromatic boronic esters with mono- and diamines was studied in solution by NMR and UV-vis spectroscopy with association energies (ΔG°) ranging from -11 to -28 kJ mol^-1 . The effect of different substituents in the boronic ester, the nature of the diamine linker, and the effect of the solvent was explored. Stable 2 : 1 B←N adducts with diamines such as 1,3-diaminopropane were produced in solutions of hydrogen-bonding acceptor solvents (acetonitrile and ethyl acetate), which can be isolated in the solid state as crystalline solvates, whereas the use of noncoordinating solvents such as 1,2-dichloroethane afforded mainly 1 : 1 B←N adducts. In suitable combinations, aromatic bis-pyridyl diamines produced stable 2 : 1 B←N adducts that were isolated either as solvent-free solids, solvates, or cocrystals. In these crystalline forms, double-tweezer hosts were observed with an exceptional syn/anti conformational guest-adaptability driven by simultaneous donor-acceptor and C-H⋅⋅⋅π interactions in the tweezer cavities, resembling preorganized covalent tweezer hosts. Interestingly, cocrystals with electron-rich guests such as tetrathiafulvalene and pyrene showed non-centrosymmetric crystal lattices with infinite π-stacked donor-acceptor columns.

A fluorescent molecularly imprinted device for the on-line analysis of AFP in human serum

MIT is a promising strategy in antibody free analysis for tumour markers. Conventional nanosized MIPs with off-line analysis are beset by tedious operation and unsatisfactory analysis performance. In this work, an on-line analytical device to directly detect AFP, which is a typical tumour marker in cancer screening, was prepared for the first time. A microscope slide was chosen to be the basis of the device. APBA-PA, a polymerizable fluorescent boronic acid monomer, was synthesised and grafted on the surface of the microscope slide to act as the signal transduction pathway between the templates and the device. Along with the hydrolysis of TEOS and the elution of the templates, a portable, stable, easy to operate and low-cost analysis device for AFP with excellent repeatability was successfully prepared. Owing to the excellent selectivity and highly sensitive fluorescence response ability of the device towards the templates, the on-line detection of AFP in human serum was realized. A series of characterizations were applied to the device, and its analysis performance and possible detection mechanism were carefully studied. Furthermore, the device exhibited appropriate application prospects by comparing its analysis results with those of the commercially available ELISA. In our perception, this work is an important step towards MIPs for clinical applications.

β-Hydroxy-Stabilized Boron-Nitrogen Heterocycles Enable Rapid and Efficient C-Terminal Protein Modification

Bioorthogonal chemistry has enabled the development of bioconjugates in physiological environments while averting interference from endogenous biomolecules. Reactions between carbonyl-containing molecules and alkoxyamines or hydrazines have experienced a resurgence in popularity in bioorthogonal chemistry owing to advances that allow the reactions to occur under physiological conditions. In particular, ortho-carbonyl-substituted phenylboronic acids (CO-PBAs) exhibit greatly accelerated rates of hydrazone and oxime formation via intramolecular Lewis acid catalysis. Unfortunately, the rate of the reverse reaction is also increased, yielding a kinetically less stable bioconjugate. When the substrate is a hydrazine derivative, an intramolecular reaction between the boronic acid and the hydrazone can lead to the formation of a heterocycle containing a boron-nitrogen bond. We have shown previously that α-amino hydrazides undergo rapid reaction with CO-PBAs to form highly stable, tricyclic products, and that this reaction is orthogonal to the popular azide-alkyne and tetrazine-alkene reactions. In this work, we explore a series of heteroatom-substituted hydrazides for their ability to form tricyclic products with two CO-PBAs, 2-formylphenylboronic acid (2fPBA), and 2-acetylphenylboronic acid (AcPBA). In particular, highly stable products were formed using β-hydroxy hydrazides and 2fPBA. C-Terminal β-hydroxy hydrazide proteins are available using conventional biochemical methods, which alleviates one of the difficulties with applications of bioorthogonal chemical reactions: site-specific incorporation of a reactive group into the biomolecular target. Using sortase-mediated ligation (SML), C-terminal threonine and serine hydrazides were appended to a model eGFP protein in high yield. Subsequent labeling with 2fPBA functionalized probes could be performed quickly and quantitatively at neutral pH using micromolar concentrations of reactants. The SML process was applied directly to an expressed protein in cellular extract, and the C-terminal modified target protein was selectively immobilized using 2fPBA-agarose. Elution from the agarose yielded a highly pure protein that retained the hydrazide functionality. This strategy should be generally applicable for rapid, efficient site-specific protein labeling, protein immobilization, and preparation of highly pure functionalized proteins.

Charge reversal and swelling in saccharide binding polyzwitterionic phenylboronic acid-modified poly(4-vinylpyridine) nanoparticles

We present the synthesis and characterization of zwitterionic poly(4-vinylpyridine) nanoparticles quaternized with phenylboronic acid (QxPVP-PBA) whose size and surface charge can be tuned by varying the saccharide and the degree of quaternization.

Structure-Reactivity Relationships in Boronic Acid-Diol Complexation

Boronic acids have found widespread use in the field of biomaterials, primarily through their ability to bind with biologically relevant 1,2- and 1,3-diols, including saccharides and peptidoglycans, or with polyols to prepare hydrogels with dynamic covalent or responsive behavior. Despite a wide range of boronic acid architectures that have been previously considered, there is a need for greater understanding of the structure-reactivity relationships that govern binding affinity to diols. In this study, various boronic acids and other organoboron compounds were investigated to determine their pK a and their binding constants with the biologically relevant diols including sorbitol, fructose, and glucose. Boronic acid pK a values were determined through spectroscopic titration, whereas binding constants were determined by fluorescence spectroscopy during competitive binding studies. Key structure-reactivity relationships clearly indicated that both boronic acid structure and solution pH must be carefully considered. By considering a variety of boronic acids with systematically varied electronics and sterics, these results provide guidance during selection of organoboron compounds in sensing, delivery, and materials chemistry.

Versatile Bioconjugation Chemistries of ortho-Boronyl Aryl Ketones and Aldehydes

Biocompatible and bioorthogonal conjugation reactions have proven to be powerful tools in biological research and medicine. While the advent of bioorthogonal conjugation chemistries greatly expands our capacity to interrogate specific biomolecules in situ, biocompatible reactions that target endogenous reactive groups have given rise to a number of covalent drugs as well as a battery of powerful research tools. Despite the impressive progress, limitations do exist with the current conjugation chemistries. For example, most known bioorthogonal conjugations suffer from slow reaction rates and imperfect bioorthogonality. On the other hand, covalent drugs often display high toxicity due to off-target labeling and immunogenicity. These limitations demand continued pursuit of conjugation chemistries with optimal characteristics for biological applications. A spate of papers appearing in recent literature report the conjugation chemistries of 2-formyl and 2-acetyl phenylboronic acids (abbreviated as 2-FPBA and 2-APBA, respectively). These simple reactants are found to undergo fast conjugation with various nucleophiles under physiological conditions, showing great promise for biological applications. The versatile reactivity of 2-FPBA and 2-APBA manifests in dynamic conjugation with endogenous nucleophiles as well as conjugation with designer nucleophiles in a bioorthogonal manner. 2-FPBA/APBA conjugates with amines in biomolecules, such as lysine side chains and aminophospholipids, in a highly dynamic manner to give iminoboronates. In contrast to typical imines, iminoboronates enjoy much improved thermodynamic stability, yet are kinetically labile for hydrolysis due to imine activation by the boronic acid. Dynamic conjugations as such present a novel binding mechanism analogous to hydrogen bonding and electrostatic interactions. Implementation of this covalent binding mechanism has yielded reversible covalent probes of prevalent bacterial pathogens. It has also resulted in reversible covalent inhibitors of a therapeutically important protein Mcl-1. Such covalent probes/inhibitors with 2-FPBA/APBA warheads avoid permanent modification of their biological target, potentially able to mitigate off-target labeling and immunogenicity of covalent drugs. The dynamic conjugation of 2-FPBA/APBA has been recently extended to N-terminal cysteines, which can be selectively targeted via formation of a thiazolidino boronate (TzB) complex. The dynamic TzB formation expands the toolbox for site-specific protein labeling and the development of covalent drugs. On the front of bioorthogonal conjugation, 2-FPBA/APBA has been found to conjugate with α-nucleophiles under physiologic conditions with rate constant ( k₂) over 1000 M^-1 s^-1, which overcomes the slow kinetics problems and rekindles the interest of using the conjugation of α-nucleophiles for biological studies. With fast kinetics being a shared feature, this family of conjugation chemistries gives remarkably diverse product structures depending on the choice of nucleophile. Importantly, both dynamic and irreversible conjugations have been developed, which we believe will enable a wide array of applications in biological research. In this Account, we collectively examine this rapidly expanding family of conjugation reactions, seeking to elucidate the unifying principles that would guide further development of novel conjugation reactions, as well as their applications in biology.

Mechanistic Insight into Asymmetric Hetero-Michael Addition of α,β-Unsaturated Carboxylic Acids Catalyzed by Multifunctional Thioureas

Carboxylic acids and their corresponding carboxylate anions are generally utilized as Brønsted acids/bases and oxygen nucleophiles in organic synthesis. However, a few asymmetric reactions have used carboxylic acids as electrophiles. Although chiral thioureas bearing both arylboronic acid and tertiary amine were found to promote the aza-Michael addition of BnONH₂ to α,β-unsaturated carboxylic acids with moderate to good enantioselectivities, the reaction mechanism remains to be clarified. Detailed investigation of the reaction using spectroscopic analysis and kinetic studies identified tetrahedral borate complexes, comprising two carboxylate anions, as reaction intermediates. We realized a dramatic improvement in product enantioselectivity with the addition of 1 equiv of benzoic acid. In this aza-Michael reaction, the boronic acid not only activates the carboxylate ligand as a Lewis acid, together with the thiourea NH-protons, but also functions as a Brønsted base through a benzoyloxy anion to activate the nucleophile. Moreover, molecular sieves were found to play an important role in generating the ternary borate complexes, which were crucial for obtaining high enantioselectivity as demonstrated by DFT calculations. We also designed a new thiourea catalyst for the intramolecular oxa-Michael addition to suppress another catalytic pathway via a binary borate complex using steric hindrance between the catalyst and substrate. Finally, to demonstrate the synthetic versatility of both hetero-Michael additions, we used them to accomplish the asymmetric synthesis of key intermediates in pharmaceutically important molecules, including sitagliptin and α-tocopherol.

Boronic acid liposomes for cellular delivery and content release driven by carbohydrate binding

Boronic acid liposomes enable triggered content release and cell delivery driven by carbohydrate binding. Dye release assays using hydrophilic and hydrophobic fluorophores validate dose-dependent release upon carbohydrate treatment. Microscopy results indicate dramatic enhancements in cell delivery, showcasing the prospects of boronic acid lipids for drug delivery.

RNA-based boronate internucleosidic linkages: an entry into reversible templated ligation and loop formation

The synthesis of a 5′-boronoribonucleotidic phosphoramidite building block has been achieved and incorporated at the 5′ extremities of RNA sequences for the templated assembly of RNA shortmers.

Convergent Approach to Boronic Acid Functionalized Polycarbonates: Accessing New Dynamic Material Platforms

Polycarbonates are routinely utilized for diverse medicinal applications and are highly efficacious scaffolds for drug delivery and antimicrobial treatments. In order to provide for robust, dynamic platforms for biomedical applications, we have developed new routes for the incorporation of boronic acids into the polycarbonate backbone. These routes take advantage of straightforward postsynthesis modification of established polycarbonate backbones, enabling the preparation of a diverse array of boronic acid functionalized polycarbonates from readily accessible polycarbonates. In particular, this approach circumvents the need for de novo monomer synthesis, functional group incompatibilities, and deprotection steps that often limit other methods. This strategy has been demonstrated using a broad array of unprotected boronic acids to produce both neutral and cationic boronic acid functionalized polycarbonates.

An efficient methodology to introduce o-(aminomethyl)phenyl-boronic acids into peptides: alkylation of secondary amines

Current approaches for incorporating boronic acids into peptides require one of the following: the synthesis of commercially unavailable pinacol-protected boronate ester amino acid building blocks, amidation of small-molecule amine-containing boronic acids, or reductive amination of amine residues with 2-formylphenyl boronic acid. These methods have drawbacks, such as the use of excess starting materials, the lack of reactive-site specificity, or the inability to add multiple boronic acids in solution. In addition, several of these approaches do not allow for incorporation of the critical o-aminomethyl functionality that allows for binding of sacharrides under physiological conditions. In this work, we report three methods to functionalize synthetic peptides with boronic acids using solid-phase and solution-phase chemistries by alkylating a secondary amine with o-(bromomethyl)phenylboronic acid. Solution-phase chemistries afforded the highest yields, and were used to synthesize seven complex biotinylated multi-boronic acid peptides.

Art, auto-mechanics, and supramolecular chemistry. A merging of hobbies and career

While the strict definition of supramolecular chemistry is "chemistry beyond the molecule", meaning having a focus on non-covalent interactions, the field is primarily associated with the creation of synthetic receptors and self-assembly. For synthetic ease, the receptors and assemblies routinely possess a high degree of symmetry, which lends them an aspect of aesthetic beauty. Pictures of electron orbitals similarly can be seen as akin to works of art. This similarity was an early draw for me to the fields of supramolecular chemistry and molecular orbital theory, because I grew up in a household filled with art. In addition to art, my childhood was filled with repairing and constructing mechanical entities, such as internal combustion motors, where many components work together to achieve a function. Analogously, the field of supramolecular chemistry creates systems of high complexity that achieve functions or perform tasks. Therefore, in retrospect a career in supramolecular chemistry appears to be simply an extension of childhood hobbies involving art and auto-mechanics.

Comparison of boron-assisted oxime and hydrazone formations leads to the discovery of a fluorogenic variant

We use kinetic data, photophysical properties, and mechanistic analyses to compare recently developed high-rate constant oxime and hydrazone formations. We show that when Schiff base formation between aldehydes and arylhydrazines is carried out with an appropriately positioned boron atom, then aromatic B-N heterocycles form irreversibly. These consist of an extended aromatic structure amenable to the tailoring of specific properties such as reaction rate and fluorescence. The reactions work best in neutral aqueous buffer and can be designed to be fluorogenic - properties which are particularly interesting in bioconjugation.

Improved δ-valerolactam templates for the assembly of Aβ-miniamyloids by boronic ester formation

Numerically defined oligomers of amyloidogenic peptides were obtained by boronic ester formation with synthetic polyol templates.

“Click-fluors”: triazole-linked saccharide sensors

A series of boronic acid-containing saccharide receptors was synthesised via copper catalysed azide–alkyne cycloaddition (CuAAC) reactions.

Boronic acid and boronic ester containing polyoxometalates

Three organoboron functionalized polyoxometalates have been synthesized using Schiff base chemistry including a boronic acid, its methyl ester and its trimethylene glycol ester.

Boronic acids facilitate rapid oxime condensations at neutral pH

We report here the discovery and development of boron-assisted oxime formation as a powerful connective reaction for chemical biology.

We report here the discovery and development of boron-assisted oxime formation as a powerful connective reaction for chemical biology. Oximes proximal to boronic acids form in neutral aqueous buffer with rate constants of more than 10⁴ M^–1 s^–1, the largest to date for any oxime condensation. Boron's dynamic coordination chemistry confers an adaptability that seems to aid a number of elementary steps in the oxime condensation. In addition to applications in bioconjugation, the emerging importance of boronic acids in chemical biology as carbohydrate receptors or peroxide probes, and the growing list of drugs and drug candidates containing boronic acids suggest many potential applications.

Universal reaction mechanism of boronic acids with diols in aqueous solution: kinetics and the basic concept of a conditional formation constant

To establish a detailed reaction mechanism for the condensation between a boronic acid, RB(OH)2, and a diol, H2L, in aqueous solution, the acid dissociation constants (Ka(BL)) of boronic acid diol esters (HBLs) were determined based on the well-established concept of conditional formation constants of metal complexes. The pKa values of HBLs were 2.30, 2.77, and 2.00 for the reaction systems, 2,4-difluorophenylboronic acid and chromotropic acid, 3-nitrophenylboronic acid and alizarin red S, and phenylboronic acid and alizarin red S, respectively. A general and precise reaction mechanism of RB(OH)2 with H2L in aqueous solution, which can serve as a universal reaction mechanism for RB(OH)2 and H2L, was proposed on the basis of (a) the relative kinetic reactivities of the RB(OH)2 and its conjugate base, that is, the boronate ion, toward H2L, and (b) the determined pKa values of HBLs. The use of the conditional formation constant, K', based on the main reaction: RB(OH)2 + H2L (K1)⇌ RB(L)(OH)(-) + H3O(+) instead of the binding constant has been proposed for the general reaction of uncomplexed boronic acid species (B') with uncomplexed diol species (L') to form boronic acid diol complex species (esters, BL') in aqueous solution at pH 5-11: B' + L' (K')⇌ BL'. The proposed reaction mechanism explains perfectly the formation of boronic acid diol ester in aqueous solution.

Linear and fast hydrogel glucose sensor materials enabled by volume resetting agents

A volume resetting agent is incorporated into a boronic acid functionalized hydrogel containing an embedded photonic crystal, yielding a glucose sensor material with a linear and fast response, minimal hysteresis, and good stability under simulated physiological conditions. The hydrogel can be tuned to modulate both the sensitivity and kinetics.

Synthesis, reactivity and biological activity of N(4)-boronated derivatives of 2'-deoxycytidine

By seeking new stable boron-containing nucleoside derivatives, potential BNCT boron delivery agents, a novel synthetic approach was tested, aimed at a boron attachment via a single bond to an aliphatic carbon of sp(3) hybridization. The latter allowed successful modification of deoxycytidine in the reaction with 2-(iodomethyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane of the deoxynucleoside amino group. For new compounds, detailed NMR, LDI HRMS (Laser Desorption/Ionization High-Resolution Mass Spectrometry) analyses along with in vivo phosphorylation studies, toxicity assays and DFT modelling are presented.

Colorimetric Sugar Sensing Using Boronic Acid-Substituted Azobenzenes

In association with increasing diabetes prevalence, it is desirable to develop new glucose sensing systems with low cost, ease of use, high stability and good portability. Boronic acid is one of the potential candidates for a future alternative to enzyme-based glucose sensors. Boronic acid derivatives have been widely used for the sugar recognition motif, because boronic acids bind adjacent diols to form cyclic boronate esters. In order to develop colorimetric sugar sensors, boronic acid-conjugated azobenzenes have been synthesized. There are several types of boronic acid azobenzenes, and their characteristics tend to rely on the substitute position of the boronic acid moiety. For example, o-substitution of boronic acid to the azo group gives the advantage of a significant color change upon sugar addition. Nitrogen-15 Nuclear Magnetic Resonance (NMR) studies clearly show a signaling mechanism based on the formation and cleavage of the B-N dative bond between boronic acid and azo moieties in the dye. Some boronic acid-substituted azobenzenes were attached to a polymer or utilized for supramolecular chemistry to produce glucose-selective binding, in which two boronic acid moieties cooperatively bind one glucose molecule. In addition, boronic acid-substituted azobenzenes have been applied not only for glucose monitoring, but also for the sensing of glycated hemoglobin and dopamine.

Fluorescent boronic acid polymer grafted on silica particles for affinity separation of saccharides

Boronic acid affinity gels are important for effective separation of biological active cis-diols, and are finding applications both in biotech industry and in biomedical research areas. To increase the efficacy of boronate affinity separation, it is interesting to introduce repeating boronic acid units in flexible polymer chains attached on solid materials. In this work, we synthesize polymer brushes containing boronic acid repeating units on silica gels using surface-initiated atom transfer radical polymerization (ATRP). A fluorescent boronic acid monomer is first prepared from an azide-tagged fluorogenic boronic acid and an alkyne-containing acrylate by Cu(I)-catalyzed 1,3-dipolar cycloaddition reaction (the CuAAC click chemistry). The boronic acid monomer is then grafted to the surface of silica gel modified with an ATRP initiator. The obtained composite material contains boronic acid polymer brushes on surface and shows favorable saccharide binding capability under physiological pH conditions, and displays interesting fluorescence intensity change upon binding fructose and glucose. In addition to saccharide binding, the flexible polymer brushes on silica also enable fast separation of a model glycoprotein based on selective boronate affinity interaction. The synthetic approach and the composite functional material developed in this work should open new opportunities for high efficiency detection, separation, and analysis of not only simple saccharides, but also glycopeptides and large glycoproteins.

Recent advances in fluorescent arylboronic acids for glucose sensing

Continuous glucose monitoring (CGM) is crucial in order to avoid complications caused by change in blood glucose for patients suffering from diabetes mellitus. The long-term consequences of high blood glucose levels include damage to the heart, eyes, kidneys, nerves and other organs, among others, caused by malign glycation of vital protein structures. Fluorescent monitors based on arylboronic acids are promising candidates for optical CGM, since arylboronic acids are capable of forming arylboronate esters with 1,2-cis-diols or 1,3-diols fast and reversibly, even in aqueous solution. These properties enable arylboronic acid dyes to provide immediate information of glucose concentrations. Thus, the replacement of the commonly applied semi-invasive and non-invasive techniques relying on glucose binding proteins, such as concanavalin A, or enzymes, such as glucose oxidase, glucose dehydrogenase and hexokinases/glucokinases, might be possible. The recent progress in the development of fluorescent arylboronic acid dyes will be emphasized in this review.

Intramolecular interactions in ortho-methoxyalkylphenylboronic acids and their catechol esters

Catechol esters of ortho-methoxyalkylphenylboronic acids have been synthesized and characterized by (17)O NMR spectroscopy. The results were compared with the data for the parent acids. The influence of intramolecular and intermolecular hydrogen bonds on the properties of the boronic acids has been discussed. The (17)O NMR data for the boronic esters proved that there are no O → B interactions in the investigated compounds. This fact is connected with weak Lewis acidity of the parent acids and their low sugars' receptors activity. Crystal structure of ortho-methoxyphenylboronic acid catechol ester was determined.

End-to-end conformational communication through a synthetic purinergic receptor by ligand-induced helicity switching

The long-range communication of information, exemplified by signal transduction through membrane-bound receptors, is a central biochemical function. Reversible binding of a messenger ligand induces a local conformational change that is relayed through the receptor, inducing a chemical effect typically several nanometres from the binding site. We report a synthetic receptor mimic that transmits structural information from a boron-based ligand binding site to a spectroscopic reporter located more than 2 nm away. Reversible binding of a diol ligand to the N-terminal binding site induces a screw-sense preference in a helical oligo(aminoisobutyric acid) foldamer, which is relayed to a reporter group at the remote C-terminus, communicating information about the structure and stereochemistry of the ligand. The reversible nature of boronate esterification was exploited to switch the receptor sequentially between left- and right-handed helices, while the exquisite conformational sensitivity of the helical relay allowed the reporter to differentiate even between purine and pyrimidine nucleosides as ligands.

Study of boron-nitrogen dative bonds using azetidine inversion dynamics

A method for probing the strength of B-N dative bonds is reported. The activation parameters for nitrogen inversion in a series of azetidines tethered to boronate esters have been quantified by VT-NMR and the measured barriers correlated with data obtained by (11)B NMR, X-ray crystallography and MP2 calculations.