Boronic acid based dynamic click chemistry: recent advances and emergent applications

Boron enabled bioconjugation chemistries

Novel bioconjugation reactions have been heavily pursued for the past two decades. A myriad of conjugation reactions have been developed for labeling molecules of interest in their native context as well as for constructing multifunctional molecular entities or stimuli-responsive materials. A growing cluster of bioconjugation reactions were realized by tapping into the unique properties of boron. As a rare element in human biology, boronic acids and esters exhibit remarkable biocompatibility. A number of organoboron reagents have been evaluated for bioconjugation, targeting the reactivity of either native biomolecules or those incorporating bioorthogonal functional groups. Owing to the dynamic nature of B-O and B-N bond formation, a significant portion of the boron-enabled bioconjugations exhibit rapid reversibility and accordingly have found applications in the development of reversible covalent inhibitors. On the other hand, stable bioconjugations have been developed that display fast kinetics and significantly expand the repertoire of bioorthogonal chemistry. This contribution presents a summary and comparative analysis of the recently developed boron-mediated bioconjugations. Importantly, this article seeks to provide an in-depth discussion of the thermodynamic and kinetic profiles of these boron-enabled bioconjugations, which reveals structure-reactivity relationships and provides guidelines for bioapplications.

An innovative carbon dots polarity probe based on intramolecular charge-transfer for visual monitoring of the total polar materials in frying oil

The presence of Total Polar Materials (TPM) in edible oils is a crucial indicator for assessing oil quality. It is of paramount importance to develop a rapid and dependable technique for monitoring polarity in frying oil. Sensitive polarity responsive fluorescence carbon dots (F-CDs) were synthesized by using p-phenylenediamine as precursors and 2-formylphenylboronic acid pinacol ester (2-FAPE) as a post-modifier. The construction of the fluorescent probe F-CDs involved a strong intramolecular charge-transfer (ICT) mechanism, with 2-FAPE serving as the electron-withdrawing fluorophore and the π-conjugated structure acting as a potent electron-donating group. A strong linear relationship was observed between the emission wavelength and the TPM value of frying oil within a range of 11% to 30%. Notably, the fluorescence color of the probe transitioned from blue to yellow under UV light at 365 nm as the TMP value increased. This study expands the range of sensing applications for CDs in food safety.

pH-Responsive Degradable Electro-Spun Nanofibers Crosslinked via Boronic Ester Chemistry for Smart Wound Dressings

Recent advances in the treatment of chronic wounds have focused on the development of effective strategies for cutting-edge wound dressings based on nanostructured materials, particularly biocompatible poly(vinyl alcohol) (PVA)-based electro-spun (e-spun) nanofibers. However, PVA nanofibers need to be chemically crosslinked to ensure their dimensional stability in aqueous environment and their capability to encapsulate bioactive molecules. Herein, a robust approach for the fabrication of pH-degradable e-spun PVA nanofibers crosslinked with dynamic boronic ester (BE) linkages through a coupling reaction of PVA hydroxyl groups with the boronic acid groups of a phenyl diboronic acid crosslinker is reported. This comprehensive analysis reveals the importance of the mole ratio of boronic acid to hydroxyl group for the fabrication of well-defined BE-crosslinked fibrous mats with not only dimensional stability but also the ability to retain uniform fibrous form in aqueous solutions. These nanofibers degrade in both acidic and basic conditions that mimic wound environments, leading to controlled/enhanced release of encapsulated antimicrobial drug molecules. More importantly, drug-loaded BE-crosslinked fibers show excellent antimicrobial activities against both Gram-positive and Gram-negative bacteria, suggesting that this approach of exploring dynamic BE chemistry is amenable to the development of smart wound dressings with controlled/enhanced drug release.

Tuning the Stability and Kinetics of Dioxazaborocanes

We investigated the equilibrium reaction of boronic acid (BA), diethanolamines (DEA), and 1,3,6,2-dioxazaborocanes (DOAB) in aqueous solutions, both theoretically and experimentally. Our findings show that the association constant can be adjusted by substituting BA and DEA derivatives, ranging from 100 to 103 M-1, exhibiting a bell-shaped pH dependency. The highest stability was achieved when the pKa values of DEA and BA were closely matched. This approach enabled the preparation of a highly stable DOAB under physiological conditions. Furthermore, the hydrolysis kinetics of DOABs were controllable over a range of five orders of magnitude based on the substituent's steric effect. In the slowest case, this resulted in quasi-static stability with only 1 % cleavage in the first hour, followed by a week-long cleavage period to reach equilibrium. These insights could establish a unique chemistry platform for designing scheduled cleavability on a day-to-week timescale, relevant to protein engineering, immunotherapy, and other smart drug delivery applications.

Enzyme-Like Catalysis of Vinyl Copolymer Carrying Boron Directly Connected to Backbone: Catalytic Esterification through Cooperation of Boron with Neighboring Carboxylic Anhydride

Alternating-rich copolymer of vinylboronic acid pinacol ester (VBpin) and maleic anhydride (MAH) was found to catalyze direct dehydrative esterification of carboxylic acid and alcohol. The key to the catalytic function is the activation of the MAH unit by the neighboring Lewis acidic boron directly connected to the backbone through the formation of five-membered ring. The effects of the side-chain cooperation were clarified through comparisons with the polymers having similar structures and a conventional titanium catalyst as well as the analyses of reactions with carboxylic acid or alcohol. The catalytic activity was enhanced as the molecular weight was higher, which is owing to the structural feature that boron is directly attached to the backbone. The cooperative catalysis is of interest because of its conceptual similarity with enzyme.

Reversible Conjugation of Polypeptides and Proteins Utilizing a [3.3.1] Scaffold under Mild Conditions

An investigation of reversible protein conjugation and deconjugation is presented. Despite numerous available protein conjugation methods, there has been limited documentation of achieving protein conjugation in a controlled and reversible manner. This report introduces a protocol that enables protein modification in a multicomponent fashion under aqueous buffer and mild conditions. A readily available mercaptobenzaldehyde derivative can modify the primary amine of peptides and proteins with a distinctive [3.3.1] scaffold. This modification can be reversed under mild conditions in a controlled fashion, restoring the original protein motif. The effectiveness of this approach has been demonstrated in the modification and quantifiable regeneration of insulin protein.

Harnessing a bis-electrophilic boronic acid lynchpin for azaborolo thiazolidine (ABT) grafting in cyclic peptides

Chemical modifications of native peptides have significantly advanced modern drug discovery in recent decades. On this front, the installation of multitasking molecular grafts onto macrocyclic peptides offers numerous opportunities in biomedical applications. Here, we showcase a new class of borono-cyclic peptides featuring an azaborolo thiazolidine (ABT) graft, which can be readily assembled utilizing a bis-electrophilic boronic acid lynchpin while harnessing the inherent reactivity difference (>103 M-1 s-1) between the N-terminal cysteine and backbone cysteine for rapid and highly regioselective macrocyclization (∼1 h) under physiological conditions. The ABT-crosslinked peptides are fairly stable in endogenous environments, but can provide the linear diazaborine peptides via treatment with α-nucleophiles. This efficient peptide crosslinking protocol was further extended for regioselective bicyclizations and engineering of α-helical structures. Finally, ABT-grafted peptides were exploited in biorthogonal conjugation, leading to highly effective intracellular delivery of an apoptotic peptide (KLA) in cancer cells. The mechanism of action by which ABT-grafted KLA peptide induces apoptosis was also explored.

Dynamic Covalent Bond-Based Polymer Chains Operating Reversibly with Temperature Changes

Dynamic bonds can facilitate reversible formation and dissociation of connections in response to external stimuli, endowing materials with shape memory and self-healing capabilities. Temperature is an external stimulus that can be easily controlled through heat. Dynamic covalent bonds in response to temperature can reversibly connect, exchange, and convert chains in the polymer. In this review, we introduce dynamic covalent bonds that operate without catalysts in various temperature ranges. The basic bonding mechanism and the kinetics are examined to understand dynamic covalent chemistry reversibly performed by equilibrium control. Furthermore, a recent synthesis method that implements dynamic covalent coupling based on various polymers is introduced. Dynamic covalent bonds that operate depending on temperature can be applied and expand the use of polymers, providing predictions for the development of future smart materials.

Evaluation of digoxin-boronate ester formation through in-capillary derivatisation-large volume sample stacking-capillary zone electrophoresis

Determination of digoxin through in-capillary derivatisation based on the formation of o-tolyl- and 2-naphthyl-anionic boronate esters in combination with large volume sample stacking-capillary electrophoresis is proposed. The derivatisation reaction was performed at basic pH values to obtain compounds with a charge and chromophore group during the stacking process. After stacking, the species were separated and detected at 225 nm using p-nitrophenol as an internal standard. Stacking and derivatisation parameters such as pre-concentration time, preconcentration voltage and injection time (relation between the analyte and the derivatisation agent) were evaluated using a Box-Behnken design. Under optimal conditions, the proposed method exhibits a linear range of 1.08-50.00 μM with a limit of detection of 0.36 μM; additionally, adequate repeatability and reproducibility was obtained (%RSD ≤ 5.0%). The methodology was validated by comparing it to an HPLC-UV established methodology and was successfully applied for the determination of digoxin in pharmaceutical tablets and blood serum samples, showing a positive performance for these matrices.

Emerging and Re-emerging Warheads for Targeted Covalent Inhibitors: An Update

Covalent inhibitors and other types of covalent modalities have seen a revival in the past two decades, with a variety of new targeted covalent drugs having been approved in recent years. A key feature of such molecules is an intrinsically reactive group, typically a weak electrophile, which enables the irreversible or reversible formation of a covalent bond with a specific amino acid of the target protein. This reactive group, often called the "warhead", is a critical determinant of the ligand's activity, selectivity, and general biological properties. In 2019, we summarized emerging and re-emerging warhead chemistries to target cysteine and other amino acids (Gehringer, M.; Laufer, S. A. J. Med. Chem. 2019, 62, 5673-5724; DOI: 10.1021/acs.jmedchem.8b01153). Since then, the field has rapidly evolved. Here we discuss the progress on covalent warheads made since our last Perspective and their application in medicinal chemistry and chemical biology.

CdSe/ZnS Quantum Rods (QRs) and Phenyl Boronic Acid BODIPY as Efficient Förster Resonance Energy Transfer (FRET) Donor-Acceptor Pair

The reversibility of the covalent interaction between boronic acids and 1,2- or 1,3-diols has put the spotlight on this reaction for its potential in the development of sensors and for the fishing of bioactive glycoconjugates. In this work, we describe the investigation of this reaction for the reversible functionalization of the surface of CdSe/ZnS Quantum Rods (QRs). With this in mind, we have designed a turn-off Förster resonance energy transfer (FRET) system that ensures monitoring the extent of the reaction between the phenyl boronic residue at the meso position of a BODIPY probe and the solvent-exposed 1,2-diols on QRs' surface. The reversibility of the corresponding boronate ester under oxidant conditions has also been assessed, thus envisioning the potential sensing ability of this system.

Boron catalysis in a designer enzyme

Enzymes play an increasingly important role in improving the benignity and efficiency of chemical production, yet the diversity of their applications lags heavily behind chemical catalysts as a result of the relatively narrow range of reaction mechanisms of enzymes. The creation of enzymes containing non-biological functionalities facilitates reaction mechanisms outside nature's canon and paves the way towards fully programmable biocatalysis1-3. Here we present a completely genetically encoded boronic-acid-containing designer enzyme with organocatalytic reactivity not achievable with natural or engineered biocatalysts4,5. This boron enzyme catalyses the kinetic resolution of hydroxyketones by oxime formation, in which crucial interactions with the protein scaffold assist in the catalysis. A directed evolution campaign led to a variant with natural-enzyme-like enantioselectivities for several different substrates. The unique activation mode of the boron enzyme was confirmed using X-ray crystallography, high-resolution mass spectrometry (HRMS) and 11B NMR spectroscopy. Our study demonstrates that genetic-code expansion can be used to create evolvable enantioselective enzymes that rely on xenobiotic catalytic moieties such as boronic acids and access reaction mechanisms not reachable through catalytic promiscuity of natural or engineered enzymes.

The Rise of Boron-Containing Compounds: Advancements in Synthesis, Medicinal Chemistry, and Emerging Pharmacology

Boron-containing compounds (BCC) have emerged as important pharmacophores. To date, five BCC drugs (including boronic acids and boroles) have been approved by the FDA for the treatment of cancer, infections, and atopic dermatitis, while some natural BCC are included in dietary supplements. Boron's Lewis acidity facilitates a mechanism of action via formation of reversible covalent bonds within the active site of target proteins. Boron has also been employed in the development of fluorophores, such as BODIPY for imaging, and in carboranes that are potential neutron capture therapy agents as well as novel agents in diagnostics and therapy. The utility of natural and synthetic BCC has become multifaceted, and the breadth of their applications continues to expand. This review covers the many uses and targets of boron in medicinal chemistry.

Capturing Sialyl-glycan on Live Cancer Cells by Tailored Boronopeptide

Boronic acid-containing molecules are substantially popularized in chemical biology and medicinal chemistry due to the broad spectrum of covalent conjugations as well as interaction modules offered by the versatile boron atom. Apparently, the WGA peptide (wheat germ agglutinin, 62-73), which shows a considerably low binding affinity to sialic acid, turned into a selective and >5 folds potent binder with the aid of a suitable boronic acid probe installed chemoselectively. In silico studies prompted us to install BA probes on the cysteine residue, supposedly located in close proximity to the bound sialic acid. In vitro studies revealed that the tailored boronopeptides show enhanced binding ability due to the synergistic recognition governed by selective non-covalent interactions and cis-diol boronic acid conjugation. The intense binding is observed even in 10 % serum, thus enabling profiling of sialyl-glycan on cancer cells, as compared with the widely used lectin, Sambucus nigra. The synergistic binding mode between the best boronopeptide (P3) binder and sialic acid was analyzed via 1 H and 11 B NMR.

Modular Synthesis of Complex Benzoxaboraheterocycles through Chelation-Assisted Rh-Catalyzed [2 + 2 + 2] Cycloaddition

Benzoxaboraheterocycles (BOBs) are moieties of increasing interest in the pharmaceutical industry; however, the synthesis of these compounds is often difficult or impractical due to the sensitivity of the boron moiety, the requirement for metalation-borylation protocols, and lengthy syntheses. We report a straightforward, modular approach that enables access to complex examples of the BOB framework through a Rh-catalyzed [2 + 2 + 2] cycloaddition using MIDA-protected alkyne boronic acids. The key to the development of this methodology was overcoming the steric barrier to catalysis by leveraging chelation assistance. We show the utility of the method through synthesis of a broad range of BOB scaffolds, mechanistic information on the chelation effect, intramolecular alcohol-assisted BMIDA hydrolysis, and linear/cyclic BOB limits as well as comparative binding affinities of the product BOB frameworks for ribose-derived biomolecules.

Enantiomer Recognition Based on Chirality Transfer from Chiral Amines to Ternary Dynamic Covalent Systems

Enantiomer recognition is usually required in organic synthesis and materials and life sciences. This paper describes an enantiomer recognition method based on ternary dynamic covalent systems constructed via the complexation of chiral amines with a chiral boronate derived from 1,4-phenylenediboric acid and an L-DOPA-modified naphthalenediimide. The ternary systems aggregate into chiral assemblies driven by π-π interactions, and the chirality is transferred from the chiral amines to assemblies with high stereospecificity. Consequently, the enantiomer composition of chiral amines and the absolute configuration of the major enantiomer can be determined according to the sign of the Cotton effect of the ternary system by using circular dichroism (CD) spectroscopy. This method offers the advantage of using the long wavelength CD signals of the boronate at around 520 nm, thereby avoiding interference with those of the carbon skeleton. This ternary system provides a novel approach to the design of enantiomer recognition systems.

Stimuli-Responsive Boronate Formation to Control Nucleic Acid-Based Functional Architectures

Boronate esters, formed by the reaction of an oligonucleotide bearing a 5'-boronic acid moiety with the 3'-terminal cis-diol of another oligonucleotide, support the assembly of functional nucleic acid architectures. Reversible formation of boronate esters occurs in templated fashion and has been shown to restore the activity of split DNA and RNA enzymes as well as a split fluorescent light-up aptamer. Apart from their suitability for the design and application of split nucleic acid enzymes and aptamers in the field of biosensing, boronate esters may have played an important role in early life as surrogates of the natural phosphodiester bond. Their formation is reversible and thus fulfills an important requirement for biological self-assembly. Here we discuss the general concept of stimuli-dependent boronate formation and its application in biomolecules with implications for future research.

The Nitrile Bis-Thiol Bioconjugation Reaction

Electron-poor aryl nitriles are promising reagents for bioconjugation due to their high electrophilicity and selectivity for reaction with thiols, albeit generally in a reversible manner. A transient species has previously been observed in such reactions, involving the addition of two thiols to the nitrile functional group, forming a tetrahedral amino dithioacetal (ADTA). In this work, the reaction of heteroaryl nitriles with bis-thiols is explored in an attempt to generate stable ADTAs, which could facilitate new bioconjugation protocols. By use of a 1,2-dithiol, or the incorporation of an electrophilic trap into the aryl nitrile design, the formation of stable products is achieved. The resultant "nitrile bis-thiol" (NBT) reaction is then explored in the context of protein modification, specifically to carry out antibody conjugation. By addition of these nitriles to the reduced disulfide bond of an antibody fragment, it is shown that, depending on the reagent design, cysteine-to-lysine transfer or disulfide bridged NBT products can be generated. Both represent site-selective conjugates and are shown to be stable when challenged with glutathione under physiological conditions and upon incubation in serum. Furthermore, the NBT reaction is tested in the more challenging context of a full antibody, and all four disulfide bonds are effectively modified by these new one-carbon bridging reagents. Overall, this reaction of heteroaryl-nitriles with bis-thiols is shown to be highly efficient and versatile, of tunable reversibility, and offers enticing prospects as a new addition to the toolbox of biocompatible "click"-type reactions.

The linkage-type and the exchange molecule affect the protein-labeling efficiency of iminoboronate probes

Reversible bioorthogonal conjugation reactions have been exploited in the chemoproteomic field to prepare protein labeling reagents and to visualize labeled proteins. We recently demonstrated that reversible iminoboronates can be used to prepare probes from fragment libraries and that the linkage subsequently can be used to detect the labeled proteins. In this study, we determined the effect of the stability of the iminoboronate linkage on the efficiency of the labeling protocol. Our study reveals that the linkage should be stable enough to allow for efficient targeting, but should be labile enough to detect the labeled protein. Acyl hydrazides were identified as the most suitable handles for the probe synthesis step. Anthranilic hydrazides and N-hydroxy semicarbazides were found to be the most efficient read-out molecules. With these novel exchange molecules, native probe-labeled proteins could be visualized under physiological conditions.

BINOL as a Chiral Solvating Agent for Sulfiniminoboronic Acids

1H NMR spectroscopic studies using BINOL as a chiral solvating agent (CSA) for a scalemic sulfiniminoboronic acid (SIBA) have revealed concentration- and enantiopurity-dependent variations in the chemical shifts of diagnostic imine protons used to determine enantiopurity levels. 11B/15N NMR spectroscopic studies and X-ray structural investigations revealed that unlike other iminoboronate species, BINOL-SIBA assemblies do not contain N-B coordination bonds, with 1H NMR NOESY experiments indicating that intermolecular H-bonding networks between BINOL and the SIBA analyte are responsible for these variations. These effects can lead to diastereomeric signal overlap at certain er values that could potentially lead to enantiopurity/configuration misassignments. Consequently, it is recommended that hydrogen-bonding-CSA-based 1H NMR protocols should be repeated using both CSA enantiomers to ensure that any concentration- and/or er-dependent variations in diagnostic chemical shifts are accounted for when determining the enantiopurity of a scalemic analyte.

Dynamic covalent nano-networks comprising antibiotics and polyphenols orchestrate bacterial drug resistance reversal and inflammation alleviation

New antimicrobial strategies are urgently needed to meet the challenges posed by the emergence of drug-resistant bacteria and bacterial biofilms. This work reports the facile synthesis of antimicrobial dynamic covalent nano-networks (aDCNs) composing antibiotics bearing multiple primary amines, polyphenols, and a cross-linker acylphenylboronic acid. Mechanistically, the iminoboronate bond drives the formation of aDCNs, facilitates their stability, and renders them highly responsive to stimuli, such as low pH and high H₂O₂ levels. Besides, the representative A1B1C1 networks, composed of polymyxin B1(A1), 2-formylphenylboronic acid (B1), and quercetin (C1), inhibit biofilm formation of drug-resistant Escherichia coli, eliminate the mature biofilms, alleviate macrophage inflammation, and minimize the side effects of free polymyxins. Excellent bacterial eradication and inflammation amelioration efficiency of A1B1C1 networks are also observed in a peritoneal infection model. The facile synthesis, excellent antimicrobial performance, and biocompatibility of these aDCNs potentiate them as a much-needed alternative in current antimicrobial pipelines.

Sulfonyl Diazaborine 'Click' Chemistry Enables Rapid and Efficient Bioorthogonal Labeling

Finding an ideal bioorthogonal reaction that responds to a wide range of biological queries and applications is of great interest in biomedical applications. Rapid diazaborine (DAB) formation in water by the reactions of ortho-carbonyl phenylboronic acid with α-nucleophiles is an attractive conjugation module. Nevertheless, these conjugation reactions demand to satisfy stringent criteria for bioorthogonal applications. Here we show that widely used sulfonyl hydrazide (SHz) offers a stable DAB conjugate by combining with ortho-carbonyl phenylboronic acid at physiological pH, competent for an optimal biorthogonal reaction. Remarkably, the reaction conversion is quantitative and rapid (k2 >103  M-1  s-1 ) at low micromolar concentrations, and it preserves comparable efficacy in a complex biological milieu. DFT calculations support that SHz facilitates DAB formation via the most stable hydrazone intermediate and the lowest energy transition state compared to other biocompatible α-nucleophiles. This conjugation is extremely efficient on living cell surfaces, enabling compelling pretargeted imaging and peptide delivery. We anticipate this work will permit addressing a wide range of cell biology queries and drug discovery platforms exploiting commercially available sulfonyl hydrazide fluorophores and derivatives.

Ring Opening Copolymerization of Boron-Containing Anhydride with Epoxides as a Controlled Platform to Functional Polyesters

Boron-functionalized polymers are used in opto-electronics, biology, and medicine. Methods to produce boron-functionalized and degradable polyesters remain exceedingly rare but relevant where (bio)dissipation is required, for example, in self-assembled nanostructures, dynamic polymer networks, and bio-imaging. Here, a boronic ester-phthalic anhydride and various epoxides (cyclohexene oxide, vinyl-cyclohexene oxide, propene oxide, allyl glycidyl ether) undergo controlled ring-opening copolymerization (ROCOP), catalyzed by organometallic complexes [Zn(II)Mg(II) or Al(III)K(I)] or a phosphazene organobase. The polymerizations are well controlled allowing for the modulation of the polyester structures (e.g., by epoxide selection, AB, or ABA blocks), molar masses (9.4 < M_n < 40 kg/mol), and uptake of boron functionalities (esters, acids, "ates", boroxines, and fluorescent groups) in the polymer. The boronic ester-functionalized polymers are amorphous, with high glass transition temperatures (81 < T_g < 224 °C) and good thermal stability (285 < T_d < 322 °C). The boronic ester-polyesters are deprotected to yield boronic acid- and borate-polyesters; the ionic polymers are water soluble and degradable under alkaline conditions. Using a hydrophilic macro-initiator in alternating epoxide/anhydride ROCOP, and lactone ring opening polymerization, produces amphiphilic AB and ABC copolyesters. Alternatively, the boron-functionalities are subjected to Pd(II)-catalyzed cross-couplings to install fluorescent groups (BODIPY). The utility of this new monomer as a platform to construct specialized polyesters materials is exemplified here in the synthesis of fluorescent spherical nanoparticles that self-assemble in water (D_h = 40 nm). The selective copolymerization, variable structural composition, and adjustable boron loading represent a versatile technology for future explorations of degradable, well-defined, and functional polymers.

Dynamic Multicomponent Reactions-Directed Self-Assembled G-quadruplex Inherent Antibacterial Hydrogel

Nowadays, inherent antibacterial hydrogels have gained significant attention due to their utilization against infectious bacteria. Herein, we focus on the development of an injectable, self-healable, dynamic, and G-quadruplex hydrogel with inherent antibacterial activity. The dynamic self-assembled hydrogel is constructed upon multicomponent reactions (MCR) among guanosine, 2-formylphenylboronic acid, and amino acid/peptides in the presence of potassium ions. The role of amino acid/peptides in the formation of the G-quadruplex hydrogel is studied in detail. The G-quadruplex structure is formed via the π-π stacking of G-quartets. The formation of G-quadruplex is investigated by thioflavin T binding assay, CD spectroscopy, and PXRD. The formation of the dynamic imino-boronate bond in the hydrogels is well characterized by temperature-dependent ¹¹B NMR (VT-NMR) and FT-IR spectroscopy. Furthermore, HR-TEM images and rheological experiments reveal the fibrillar networks and viscoelastic property of the hydrogels. The presence of the dynamic imino-boronate ester bonds makes the hydrogel injectable and self-healable in nature. These dynamic G-quadruplex hydrogels show potential antibacterial activity against a series of Gram-positive and Gram-negative bacteria. The hydrogels have been used for the entrapment and sustained release of an anticancer drug doxorubicin over 48 h at different pHs (4.8, 7.4, and 8.5) and temperature without the influence of any external stimuli. Such injectable and self-healable hydrogels could be used in various applications in the field of biomedical science.

Alkynone β-trifluoroborates: A new class of amine-specific biocompatible click reagents

Amine-targeting reactions that work under biocompatible conditions or in water are green processes that are extremely useful for the synthesis of functional materials and biotherapeutics. Unfortunately, despite the usefulness of this reaction, there are very few good amine-specific click methods reported thus far. Here, we report an amine-specific click reagent using alkynone β-trifluoroborates as the electrophiles. These boron-containing alkynyl reagents exhibit extremely high chemoselectivity toward amines even in the presence of thiols. The resulting oxaboracycle products are bench-stable, displaying the reactivities of both organoborates and enaminones. Intrinsic advantages of this methodology include benign reaction conditions, operational simplicity, remarkable product stability, and excellent chemoselectivity, which satisfy the criteria of click chemistry and demonstrate the high potential in bioconjugation. Hence, this water-based chemical approach is also applicable to the modification of native amino acids, peptides, and proteins. Ultimately, the essential role of water during the reaction was elucidated.

Cysteine-Selective Installation of Functionally Diverse Boronic Acid Probes on Peptides

The current methods for direct late-stage and residue-selective installation of a versatile boronic acid (BA) repertoire on peptides are inadequate for a wide range of applications. Here, we show the suitability and efficiency of thiol-ene radical click chemistry to install functionally versatile BA derivatives on numerous bioactive, native peptides. Our work highlights that the methodology is operationally simple and adaptable for applications with BA-modified peptides, such as cyclization, conjugation, and functional group alteration.

A simple method for fabricating drugs containing a cis-o-diol structure into guanosine-based supramolecular hydrogels for drug delivery

Supramolecular hydrogels are attractive biomaterials for local drug delivery owing to their excellent self-healing, injectable, biodegradable, and biocompatible properties. However, traditional drug-loading approaches based on non-covalent encapsulation and covalent bonding have shown problems such as rapid or difficult drug release, complex reaction processes, low reaction efficiency, and decreased drug activity. Therefore, there is a need to find a simple and efficient method to load drugs into hydrogels, which possess stable drug release ability without impairing drug efficacy. In this study, we introduce dynamic borate ester bonds via a simple one-pot method to load cis-o-diol-containing drugs into guanosine (G)-based supramolecular hydrogels. The experimental results confirm that the dynamic covalent borate ester bonds are formed based on the cis-o-diol groups of the drug and the G in these hydrogels. Meanwhile, the as-prepared G-based hydrogels not only possess self-healing properties and injectability but also have satisfactory biodegradability and biocompatibility. Additionally, the drug can be released from the G-based hydrogel according to the pH-responsive cleavage of the borate ester bonds without affecting drug activity. Overall, these results indicate that the simple one-pot method of utilizing the dynamic borate bond can provide a valuable reference for the design of hydrogel dosage forms.

Sticking the Landing: Enhancing Liposomal Cell Delivery using Reversible Covalent Chemistry and Caged Targeting Groups

Liposomes are highly effective nanocarriers for encapsulating and delivering a wide range of therapeutic cargo. While advancements in liposome design have improved several pharmacological characteristics, an important area that would benefit from further progress involves cellular targeting and entry. In this concept article, we will focus on recent progress utilizing strategies including reversible covalent bonding and caging groups to activate liposomal cell entry. These approaches take advantage of advancements that have been made in complementary fields including molecular sensing and chemical biology and direct this technology toward controlling liposome cell delivery properties. The decoration of liposomes with groups including boronic acids and cyclic disulfides is presented as a means for driving delivery through reaction with functional groups on cell surfaces. Additionally, caging groups can be exploited to activate cell delivery only upon encountering a target stimulus. These approaches provide promising new avenues for controlling cell delivery in the development of next-generation liposomal therapeutic nanocarriers.

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.

Insight into ortho-boronoaldehyde conjugation via a FRET-based reporter assay

Ortho-boronoaldehydes react with amine-based nucleophiles with dramatically increased rates and product stabilities, relative to unfunctionalised benzaldehydes, leading to exciting applications across biological and material chemistry. We have developed a novel Förster resonance energy transfer (FRET)-based assay to provide key new insights into the reactivity of these boronoaldehydes, allowing us to track conjugation with unprecedented sensitivity and accuracy under standardised conditions. Our results highlight the key role played by reaction pH, buffer additives, and boronoaldehyde structure in controlling conjugation speed and stability, providing design criteria for further innovations and applications in the field.

Direct discrimination of cell surface glycosylation signatures using a single pH-responsive boronic acid-functionalized polymer

Cell surface glycans serve fundamental roles in many biological processes, including cell-cell interaction, pathogen infection, and cancer metastasis. Cancer cell surface have alternative glycosylation to healthy cells, making these changes useful hallmarks of cancer. However, the diversity of glycan structures makes glycosylation profiling very challenging, with glycan 'fingerprints' providing an important tool for assessing cell state. In this work, we utilized the pH-responsive differential binding of boronic acid (BA) moieties with cell surface glycans to generate a high-content six-channel BA-based sensor array that uses a single polymer to distinguish mammalian cell types. This sensing platform provided efficient discrimination of cancer cells and readily discriminated between Chinese hamster ovary (CHO) glycomutants, providing evidence that discrimination is glycan-driven. The BA-functionalized polymer sensor array is readily scalable, providing access to new diagnostic and therapeutic strategies for cell surface glycosylation-associated diseases.

Synthesis of 5'-Thymidine-Conjugated Formylphenylboronic Acids as Potential Lysine Targeting Iminoboronate Reversible Covalent Enzyme Probes

The design of reversible-covalent molecules to selectively target the ε-amino functionality of lysine residues in enzymes or proteins is a highly desirable goal. Herein, we describe synthetic methodology used to prepare a series of 5'-thymidine-linked formylphenylboronic acids as probes to interrogate sugar nucleotide processing enzymes that recognize thymidine. The first synthetic strategy mitigated the need for protecting group manipulations of thymidine by capitalizing upon the straightforward preparation, isolation, and reactivity of 5'-azidothymidine. An alkyne cycloaddition partner was installed through either a propargyl or ethynyl phenyl ketone derived boronic acid. The second strategy directly linked formylphenylboronic acids to 5-thymidine through an ether linkage installed using Mitsunobu conditions with 3'-O,3-dibenzoylthymidine. Iminoboronate formation was observed with a selected probe.

M, Zarrintaj P, Formela K, Saeb MR, Bencherif SA. Clickable Polysaccharides for Biomedical Applications: A Comprehensive Review

Recent advances in materials science and engineering highlight the importance of designing sophisticated biomaterials with well-defined architectures and tunable properties for emerging biomedical applications. Click chemistry, a powerful method allowing specific and controllable bioorthogonal reactions, has revolutionized our ability to make complex molecular structures with a high level of specificity, selectivity, and yield under mild conditions. These features combined with minimal byproduct formation have enabled the design of a wide range of macromolecular architectures from quick and versatile click reactions. Furthermore, copper-free click chemistry has resulted in a change of paradigm, allowing researchers to perform highly selective chemical reactions in biological environments to further understand the structure and function of cells. In living systems, introducing clickable groups into biomolecules such as polysaccharides (PSA) has been explored as a general approach to conduct medicinal chemistry and potentially help solve healthcare needs. De novo biosynthetic pathways for chemical synthesis have also been exploited and optimized to perform PSA-based bioconjugation inside living cells without interfering with their native processes or functions. This strategy obviates the need for laborious and costly chemical reactions which normally require extensive and time-consuming purification steps. Using these approaches, various PSA-based macromolecules have been manufactured as building blocks for the design of novel biomaterials. Clickable PSA provides a powerful and versatile toolbox for biomaterials scientists and will increasingly play a crucial role in the biomedical field. Specifically, bioclick reactions with PSA have been leveraged for the design of advanced drug delivery systems and minimally invasive injectable hydrogels. In this review article, we have outlined the key aspects and breadth of PSA-derived bioclick reactions as a powerful and versatile toolbox to design advanced polymeric biomaterials for biomedical applications such as molecular imaging, drug delivery, and tissue engineering. Additionally, we have also discussed the past achievements, present developments, and recent trends of clickable PSA-based biomaterials such as 3D printing, as well as their challenges, clinical translatability, and future perspectives.

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.

Access to Unexplored 3D Chemical Space: cis ‐Selective Arene Hydrogenation for the Synthesis of Saturated Cyclic Boronic Acids

A new class of saturated boron‐incorporated cyclic molecules has been synthesized employing an arene‐hydrogenation methodology. cis‐Selective hydrogenation of easily accessible, and biologically important molecules comprising benzoxaborole, benzoxaborinin, and benzoxaboripin derivatives is reported. Among the various catalysts tested, rhodium cyclic(alkyl)(amino)carbene [Rh‐CAAC] (1) pre‐catalyst revealed the best hydrogenation activity confirming turnover number up to 1400 with good to high diastereoselectivity. A broad range of functional groups was tolerated including sensitive substituents such as −F, −CF₃, and −silyl groups. The utility of the synthesized products was demonstrated by the recognition of diols and sugars under physiological conditions. These motifs can have a substantial importance in medicinal chemistry as they possess a three‐dimensional structure, are highly stable, soluble in water, form hydrogen bonds, and interact with diols and sugars.

Selective Synthesis of Ketones and Chiral Allylic Alcohols from the Addition of Arylboronic Acids to α,β-Unsaturated Aldehydes Mediated by a Transition Metal/Monophosphorus Ligand System

Here, we demonstrated a transition metal-mediated/monophosphorus ligand system for the selective synthesis of ketones or chiral allylic alcohols in high yields/enantiomeric excess from the 1,2-addition of arylboronic acids to α,β-unsaturated aldehydes. Notably, isomerization of the chiral allylic alcohols to ketones was suppressed by the Ru-catalyzed/monophosphorus ligand system. The asymmetric catalytic system provides an alternative and efficient method of preparing chiral allylic alcohols.

High-fidelity carbon dots polarity probes: revealing the heterogeneity of lipids in oncology

Polarity is an integral microenvironment parameter in biological systems closely associated with a multitude of cellular processes. Abnormal polarity variations accompany the initiation and development of pathophysiological processes. Thus, monitoring the abnormal polarity is of scientific and practical importance. Current state-of-the-art monitoring techniques are primarily based on fluorescence imaging which relies on a single emission intensity and may cause inaccurate detection due to heterogeneous accumulation of the probes. Herein, we report carbon dots (CDs) with ultra-sensitive responses to polarity. The CDs exhibit two linear relationships: one between fluorescence intensity and polarity and the other between polarity and the maximum emission wavelength. The emission spectrum is an intrinsic property of the probes, independent of the excitation intensity or probe concentration. These features enable two-color imaging/quantitation of polarity changes in lipid droplets (LDs) and in the cytoplasm via in situ emission spectroscopy. The probes reveal the polarity heterogeneity in LDs which can be applied to make a distinction between cancer and normal cells, and reveal the polarity homogeneity in cytoplasm.

Unraveling the Silent Hydrolysis of Cyclic B-X/C═C Isosteres: The Striking Impact of a Single Heteroatom on the Aromatic, Acidic, and Dynamic Properties of Hemiboronic Phenanthroids

Although heterocyclic hemiboronic acids are represented in several recently approved drugs, many questions remain unanswered regarding the physical properties and reactivity of these boranol (BOH)-containing compounds in aqueous media. Over the past 60 years, studies on the acidic and aromatic character of 10-hydroxy-10,9-boroxarophenanthrene and its boraza analog have been conflicting. In contradiction with the Lewis acidic behavior of arylboronic acids in aqueous conditions, it has been proposed that the central boroheterocyclic ring of these borophenanthroids confers sufficient aromatic character to compel the boranol unit to behave as a Brønsted acid and favor the boron oxy conjugate base, thereby avoiding the disruption of cyclic resonance that would otherwise occur with a tetravalent boronate anion. These questions are addressed with a combination of physical and spectroscopic characterizations, X-ray crystallographic analysis, and computational studies. Although both oxa and aza derivatives are conclusively shown to behave as Lewis acids in aqueous solutions, according to pK_a measurements and MO and NICS calculations, only the boraza derivatives possess an appreciable aromatic character within the boroheterocyclic ring. For the first time, the possibility of dynamic chemical exchange via a reversible hydrolysis of the endocyclic B-heteroatom bond was examined using VT and EXSY NMR with suitable probe compounds. Whereas the boraza analog is static at neutral pH, its oxa analog undergoes a rapid hydrolytic ring opening-closing equilibrium with the transient boronic acid. Altogether, this study will guide the methodical application of these heterocycles as reaction catalysts, in bioconjugation, and as new-drug chemotypes and bioisosteres of pharmaceutically important classes of heterocycles.

Applications of the Reversible Boronic Acids/Boronate Switch to Nucleic Acids

Over the last decades, boron and nucleic acids chemistries have gained a lot of attention for biological, medicinal and analytical applications. Our laboratory has a long-standing interest in both chemistries and owing to the ability of boronic acids to react with cis-diol function in aqueous media we developed over the years a variety of applications ranging from molecular recognition and sensing to the development of reversible dynamic systems in which the natural phosphodiester linkage was replaced by a boronate. In this account, we summarize research results from our group from our preliminary studies on molecular recognition of ribonucleosides to the dynamic assembly of functional DNAzymes. In particular, the various parameters influencing the dynamic nature of these reversible covalent bonds able to respond to external stimuli are discussed. Finally, current challenges and opportunities for boron-based nucleic acids are also addressed.

Investigating a Boronate-Affinity-Guided Acylation Reaction for Labelling Native Antibodies

The excellent molecular recognition capabilities of monoclonal antibodies (mAbs) have opened up exciting opportunities for biotherapeutic discovery. Taking advantage of the full potential of this tool necessitates affinity ligands capable of conjugating directly with small molecules to a defined degree of biorthogonality, especially when modifying natural Abs. Herein, a bioorthogonal boronate-affinity-based Ab ligand featuring a 4-(dimethylamino)pyridine and an S-aryl thioester to label full-length Abs is reported. The photoactivatable linker in the acyl donor facilitated purification of azide-labelled Ab (N₃ -Ab) was quantitatively cleaved upon brief exposure to UV light while retaining the original Ab activity. Click reactions enabled the precise addition of biotin, a fluorophore, and a pharmacological agent to the purified N₃ -Abs. The resulting immunoconjugate showed selectivity against targeted cells. Bioorthogonal traceless design and reagentless purification allow this strategy to be a powerful tool to engineer native antibodies amenable to therapeutic intervention.

Programmable Transient Supramolecular Chiral G-quadruplex Hydrogels via a Chemically Fueled Non-Equilibrium Self-assembly Strategy

The temporal and spatial control of natural systems has aroused great interest in the creation of synthetic mimics. Operating with boronic ester-based dynamic covalent chemistry and coupling it with an internal pH feedback system, herein, we developed a new chemically fueled reaction network to design non-equilibrium supramolecular chiral G-quadruplex hydrogels with programmable lifetime from minutes, to hours, to days, as well as high transparency and conductivity, excellent injectability and rapid self-healability. The cycle system can be controlled via in-situ kinetically-controlled formation and dissociation of dynamic boronic ester bonds between cis-diols of guanosine (G) and 5-fluorobenzoxaborole (B) under chemical fuels (KOH and 1,3-propanesultone), leading to the formation of a precipitate-solution-gel-precipitate cycle under non-equilibrium conditions. A combined experimental-computational approach revealed that the underlying mechanism of the non-equilibrium self-assembly involves aggregation and disaggregation of right-handed helical G-quadruplex superstructure. With consecutive cycles of fuel addition, the non-equilibrium system can be easily refueled at least 6 cycles without obvious loss in the rheological moduli of the transient hydrogels. The proposed dynamic boronic ester-based non-equilibrium self-assembly strategy offers a new option to design next-generation adaptive and interactive smart materials.

The modern role of boron as a 'magic element' in biomedical science: chemistry perspective

Boron was misconstrued as a toxic element for animals, which retarded the growth of boron-containing drug discovery in the last century. Nevertheless, modern applications of boronic acid derivatives are attractive in biomedical applications after the declaration that boron is a 'probable essential element' for humans by the WHO. Additionally, the approval of five boronic acid-containing drugs by the FDA has vastly impacted the use of boron in medicinal chemistry, chemical biology, drug delivery, biomaterial exploration, pharmacological improvements, and nutrition. This review article focuses on the chemistries attributed to boronic acids at physiological pH, enticing chemists to multidisciplinary applications. Prospective uses of boronic acid in pharma and chemical biology, along with prospects and challenges, are also part of the deliberation. Understanding these fundamental chemistries and interactions of boronic acid in biological systems will enable solving future challenges in drug discovery and executing space-age applications.

1,2-Dihydro-1,3,2-diazaborinine tautomer as an electron-pair donor in hydrogen-bonded complexes

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to investigate 1,2-dihydro-1,3,2-diazaborinine:HX complexes for HX = H+, HF, HCl, H2O, HCN, NH3, HCP, and HCCH. Most complexes are stabilized by linear, traditional hydrogen bonds except for those with H2O and NH3, which have bridging structures and nonlinear hydrogen bonds. H-atom transfer from N to B can occur in complexes with HF and HCl, with formation of a traditional F–H···N bond and a proton-shared Cl···H···N bond. The binding energies of the uncharged complexes range from 25 to 88 kJ mol−1. Spin-spin coupling constants have been used to characterize these hydrogen-bonded complexes.

Protein Modification at Tyrosine with Iminoxyl Radicals

Post-translational modifications (PTMs) of proteins are a biological mechanism for reversibly controlling protein function. Synthetic protein modifications (SPMs) at specific canonical amino acids can mimic PTMs. However, reversible SPMs at hydrophobic amino acid residues in proteins are especially limited. Here, we report a tyrosine (Tyr)-selective SPM utilizing persistent iminoxyl radicals, which are readily generated from sterically hindered oximes via single-electron oxidation. The reactivity of iminoxyl radicals with Tyr was dependent on the steric and electronic demands of oximes; isopropyl methyl piperidinium oxime 1f formed stable adducts, whereas the reaction of tert-butyl methyl piperidinium oxime 1o was reversible. The difference in reversibility between 1f and 1o, differentiated only by one methyl group, is due to the stability of iminoxyl radicals, which is partly dictated by the bond dissociation energy of oxime O-H groups. The Tyr-selective modifications with 1f and 1o proceeded under physiologically relevant, mild conditions. Specifically, the stable Tyr-modification with 1f introduced functional small molecules, including an azobenzene photoswitch, to proteins. Moreover, masking critical Tyr residues by SPM with 1o, and subsequent deconjugation triggered by the treatment with a thiol, enabled on-demand control of protein functions. We applied this reversible Tyr modification with 1o to alter an enzymatic activity and the binding affinity of a monoclonal antibody with an antigen upon modification/deconjugation. The on-demand ON/OFF switch of protein functions through Tyr-selective and reversible covalent-bond formation will provide unique opportunities in biological research and therapeutics.