Reactivity of Boronic Acids toward Catechols in Aqueous Solution

Spatiotemporal Imaging of Catechol Aldehydes in Neural Tissue

Catechol aldehydes (CAs), particularly 3,4-dihydroxyphenylacetaldehyde (DOPAL) and 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL), are potently cytotoxic and have been implicated in pathogenesis of neurodegenerative disorders. Understanding the dynamics of CAs in the brain is crucial for elucidating neurodegenerative pathways. Herein, we present an innovative fluorescent sensor system designed for the selective imaging of CAs within cells and neural tissues. This system employs a dual-reaction trigger, utilizing o-phenylenediamine's selectivity for aldehydes and phenylboronic acid for catechols, generating a specific Förster Resonance Energy Transfer (FRET) signal for CAs. Importantly, we have integrated fluorescence lifetime imaging microscopy (FLIM) with FRET (FLIM-FRET) to enhance detection accuracy while mitigating issues like spectral crosstalk and photobleaching. This dual-reaction FLIM-FRET system allows for the precise visualization of endogenous CAs in the substantia nigra and locus coeruleus of mice, the primary sites of CA production. Notably, this method represents a significant advancement in our ability to study these critical brain regions, as it uniquely enables the tracking of CAs spread across different parts of the brain, addressing a critical gap in the field, as no existing methods allow for such detailed localization of CAs across different brain regions. By enabling precise visualization of CAs within neural tissues, our method enhances understanding of their roles in disease progression.

Purification-Free Bortezomib-Drug Conjugates Optimize Drug Economy and Cancer Therapeutic Synergy

As a promising candidate in overcoming resistance, providing synergy, and developing treatments, conjugated combination drugs mostly prevail over drug cocktails in establishing prodrugs and precisely codelivering multiple drugs for combination chemotherapies. However, current drug-drug conjugation methods (e.g., esterification, amidation, etherification, etc.) do not allow quantitative drug conversion and require necessary purification of crude products, resulting in a limited economy of initial drugs. Meanwhile, practical stimulus concentration in vivo usually fails to efficiently activate parent drug release from drug conjugates in target sites, which diminishes their efficacy. Herein, we report a click conjugation strategy based on boronic acid-cis diol complexation, realizing a fast (<30 min), quantitative, and purification-free conjugation of bortezomib (BTZ) and azacytidine (AZA) or capecitabine or doxifluridine. Notably, the BTZ-AZA conjugate spontaneously self-assembles into nanomedicine and exhibits enhanced synergistic efficacy. Furthermore, BTZ and AZA could be conjugated into a polyprodrug with controlled size and composition, and different organelle uptakes augment the synergy of BTZ-AZA conjugate by approximately 1000-fold versus free BTZ toward A549 adenocarcinoma cells (IC50: 0.55 nM versus 536.7 nM). This click strategy would expand the vision for developing smart combination drugs.

Catechol-based chemistry for hypoglycemia-responsive delivery of zinc-glucagon via hydrogel-based microneedle patch technology

Hypoglycemia is a serious and potentially life-threatening condition for people with insulin-dependent diabetes. To provide a safeguard against hypoglycemia, we introduce a "smart" microneedle (MN) patch that senses glucose levels and delivers a blood glucose-raising agent (Zinc-Glucagon (Z-GCN)) in response to hypoglycemia. Herein, we describe the use of catechol and boronic acid chemistry to design a self-crosslinkable hydrogel-based MN that stimulates the release of Z-GCN during hypoglycemia. In this design, the catechol groups bind to Z-GCN through metal-ligand complexation. At hyperglycemia, boronic acids react with glucose to generate cyclic boronate esters. As the glucose concentration decreases, the boronic acid groups dissociate and are favored over Z-GCN in binding with catechol, which promotes the release of Z-GCN. We fully characterize the fabricated MN in vitro. Moreover, we further evaluate the MN and demonstrate the in vivo glucose-responsive delivery of Z-GCN from the patch. We also show its effectiveness in preventing hypoglycemia for up to 6 h in type 1 diabetic male rats against two consecutive insulin overdose challenges. Since many proteins/peptides have a high binding affinity to metal ions, the introduced mechanism driven by the competitive binding of catechol-metal ions has great implications in drug delivery applications of various protein/peptide-based therapeutics.

pH- and Saccharide-Responsive Cyclometalated Iridium(III) Complexes with Boronic Acid Moieties Displaying a Wide Range of Phosphorescence Colors

Single compounds displaying a wide range of luminescent colors are attractive optical materials for sensor applications. In this study, we present the beneficial combination of a cyclometalated iridium(III) complex scaffold and boronic acid units for designing stimulus-responsive luminescent materials with various emission colors. Five iridium(III) complexes bearing a diboronic acid ligand (bpyB2) were synthesized: Ir(C^N)bpyB2 (C^N=2-phenylpyridine (1), 2-(2,4-difluorophenyl)pyridine (2), 2-(4-methoxyphenyl)pyridine (3), benzo[h]quinoline (4), 1-phenylisoquinoline (5)). The luminescence color of Complexes 1-4 changed in response to the solution pH or saccharide concentration. Complex 1 exhibited a color change from orange to green-blue due to structural alteration of the boronic acid moiety from trigonal to tetrahedral. Furthermore, the luminescence color of Complex 1 changed reversibly due to repetitive changes in the solution pH between 5 and 10, enabling tuning of the luminescence color and pH tracking. Furthermore, the color range was tuned by selecting an appropriate C^N ligand. Time-dependent density functional theory investigations revealed that the dramatic and reversible color changes could be ascribed to a switch in the electronic distribution in the lowest excited state from bpyB2 to C^N. The stimulus-responsive iridium(III) complexes provide a prospective scaffold for future applications in color-tunable optical devices and chemosensing systems.

Epoxy-based vitrimeric semi-interpenetrating network/MXene nanocomposites for hydrogen gas barrier applications

Herein, we report MXene-filled epoxy-based vitrimeric nanocomposites featuring a semi-interpenetrating network (S-IPN) to develop a hydrogen gas (H2) barrier coating with self-healing characteristics for compressed H2 storage applications. The reversible epoxy network was formed by synthesizing linear epoxy chains with pendent bis-hydroxyl groups using amino diol, which were then crosslinked with 1,4-benzenediboronic acid to generate dynamic boronic ester linkages. To achieve the S-IPN-type molecular arrangement, the epoxy chains were in situ crosslinked in the presence of poly(ethylene-co-vinyl alcohol) (EVOH), giving rise to a self-healing network (EEP) with a healing efficiency of 87%. Into the S-IPN vitrimer (EEP), a 2D platelet-type nanofiller MXene was incorporated to introduce a tortuous path for H2 gas diffusion along with improved mechanical properties. The nanocomposite coating was applied to nylon 6 liner material, which is conventionally used in all-composite H2 storage vessels. The application of a 2 wt% MXene/EEP nanocomposite coating showed a permeability coefficient of 0.062 cm3 mm m-2 d-1 atm-1 exhibiting ∼96% reduction in gas permeability compared to uncoated nylon 6. The same nanocomposite exhibited a healing efficiency of 79%. Increasing the MXene loading to 10 wt% further reduced the permeability coefficient to 0.002 cm3 mm m-2 d-1 atm-1; however, the healing efficiency decreased due to restricted chain mobility. In essence, the current work highlights the potential of vitrimeric S-IPN nanocomposite coatings for H2 gas-barrier applications, enhancing safety and performance.

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.

A 11B-NMR Method for the In Situ Monitoring of the Formation of Dynamic Covalent Boronate Esters in Dendrimers

The in situ monitoring of dynamic covalent macromolecular boronate esters represents a difficult task. In this report, we present an in situ method using fluoride coordination and 11B NMR spectroscopy to determine the amount of boronate esters in a mixture of boronic acids and cis-diols. With fluoride coordination, the boronic acid and boronate esters afforded trifluoroborate and fluoroboronate esters, giving identical resonances at 3 and 9 ppm in the 11B NMR spectra. The same titration did not alter the resonance of amine-coordinated boronate esters, which gave resonances of 14 ppm in the 11B NMR spectra. Therefore, boronic acids, boronate esters, and amine-coordinated boronate esters gave three identical resonances, and the ratio of each component was obtained by deconvolution for a further equilibrium analysis. This method monitored the conversion among three species in various conditions, including separation. Accordingly, boronate esters were more stable after precipitation than chromatography, in which 29% and 20% of boronate esters were lost after purification. This method was applied to study the reaction between the boronic acid-decorated defect lysine dendron (16) and dopamine. No boronic acid signal was observed after adding 1 equivalent of dopamine; no boronic acid signal was observed in the NMR spectrum. According to the spectrum, the product contains 65% boronate ester and 35% N-B-coordinated derivatives. This method helps identify the presence of the three intermediates and provides more insights into this reaction.

Optical sensing of L-dihydroxy-phenylalanine in water by a high-affinity molecular receptor involving cooperative binding of a metal coordination bond and boronate-diol

Selective recognition and sensing of catecholamine-based neurotransmitters by fluorescent synthetic receptors capable of operating in pure water is a central topic of modern supramolecular chemistry that impacts biological and analytical chemistry. Despite advances achieved in the recognition of some neurotransmitters such as dopamine, little effort has been invested in the optical recognition of other neurotransmitters of paramount importance in biochemistry and medicinal chemistry such as the drug L-dihydroxy-phenylalanine (levodopa). Herein, a cationic Cu(II)-terpyridine complex bearing an intramolecular fluorescent quinolinium ring covalently linked to phenylboronic acid (CuL1) was synthesized, structurally described by single-crystal X-ray diffraction and studied in-depth as a fluorescent receptor for neurotransmitters in water. The complex CuL1 was designed to act as a receptor for levodopa through two Lewis acids of different natures (Cu(II) and B atoms) as cooperative binding points. The receptor CuL1 was found to have a strongly acidified -B(OH)2 group (pKa = 6.2) and exceptionally high affinity for levodopa (K = 4.8 × 106 M-1) with selectivity over other related neurotransmitters such as dopamine, epinephrine, norepinephrine and nucleosides in the micromolar concentration range at physiological pH. Such levodopa affinity/selectivity for a boronic acid-based receptor in water is still rare. On the basis of spectroscopic tools (11B NMR, UV-vis, EPR, and fluorescence), high-resolution ESI-MS, crystal structure, and DFT calculations, the interaction mode of CuL1 with levodopa is proposed in a 1 : 1 model using two-point recognition involving a boronate-catechol esterification and a coordination bond Cu(II)-carboxylate. Furthermore, a visual sensing ensemble was constructed using CuL1 and the commercial fluorescent dye eosin Y. Levodopa is efficiently detected by the displacement of the eosin Y bound to the Cu(II)-receptor, monitoring its green emission. The use of Cu(II)-boronate complexes for fast and selective neurotransmitter sensing was unexplored until now.

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.

Analysis of tri-benzeneboronic esters of monosaccharides formed in aqueous solution by MALDI-TOF MS and DFT calculations

The affinity interactions between boronic acids and sugars have been successfully exploited in many fields, such as the sensing of saccharides, selective enrichment of glycoconjugates, and drug delivery. However, despite multiple techniques having been adopted to investigate the reaction of boronate affinity, the pathway of boronate esters formation under aqueous conditions remains controversial. We report a MALDI-MS approach to investigate the interactions between phenylboronic acid and monosaccharides in neutral aqueous solution by using polylevodopa as an innovative substrate instead of conventional matrix. A series of unusual tri-benzeneboronic esters were then revealed. The mass spectrometry data indicate that they bear a dibenzenepyroboronate cyclic ester moiety with seven-membered ring or eight-membered ring. With the aid of theoretical computations, their most likely geometrical structures are elucidated, and these tri-benzeneboronic esters are proposed to be formed via a boroxine binding monosaccharide pathway. This work provides more insight into the mechanism of boronate affinity interaction between boronic acid and sugars and proves the developed MALDI-MS approach is promising for studying interactions between small molecules.

Mechanistic Study of the Copper(II)-Mediated Site-Selective O-Arylation of Glycosides with Arylboronic Acids

Glycosides having multiple free OH groups have been shown to undergo site-selective O-arylations in the presence of arylboronic acids and copper(II) acetate. Herein, a mechanistic analysis of these Chan-Evans-Lam-type couplings is presented based on reaction kinetics, mass spectrometric analysis of reaction mixtures, and substituent effect studies. The results establish that the formation of a substrate-derived boronic ester accelerates the rate-determining transmetalation step. Intramolecular transfer of the aryl group from the boronic ester is ruled out in favor of a pathway in which the key pre-transmetalation assembly is generated from a boronic ester, a copper complex, and a second equivalent of arylboronic acid.

Single-Molecule Interconversion between Chiral Configurations of Boronate Esters Observed in a Nanoreactor

Isomers of some chemical compounds may be dynamically interconvertible. Due to a lack of sensing methods with a sufficient resolution, however, direct monitoring of such processes can be difficult. Engineered Mycobacterium smegmatis porin A (MspA) nanopores can be applied as nanoreactors so that chemical reactions can be directly monitored. Here, an MspA modified with a phenylboronic acid (PBA) adapter was prepared and was used to observe dynamic interconversion between chiral configurations of boronate esters, which appears as telegraphic switching on top of nanopore events. The mechanism of this behavior was further confirmed by trials with different halogenated catechols, dopamine, adenosine, 1,2-propanediol, and (2R,3R)-2,3-butanediol, and its generality has been demonstrated. These results suggest that an engineered MspA possesses an exceptional resolution in its monitoring of chemical reaction processes and may inspire the future design of nanopore small-molecule sensors.

Structure-Reactivity-Property Relationships in Covalent Adaptable Networks

Polymer networks built out of dynamic covalent bonds offer the potential to translate the control and tunability of chemical reactions to macroscopic physical properties. Under conditions at which these reactions occur, the topology of covalent adaptable networks (CANs) can rearrange, meaning that they can flow, self-heal, be remolded, and respond to stimuli. Materials with these properties are necessary to fields ranging from sustainability to tissue engineering; thus the conditions and time scale of network rearrangement must be compatible with the intended use. The mechanical properties of CANs are based on the thermodynamics and kinetics of their constituent bonds. Therefore, strategies are needed that connect the molecular and macroscopic worlds. In this Perspective, we analyze structure-reactivity-property relationships for several classes of CANs, illustrating both general design principles and the predictive potential of linear free energy relationships (LFERs) applied to CANs. We discuss opportunities in the field to develop quantitative structure-reactivity-property relationships and open challenges.

pH Responsive Antibacterial Hydrogel Utilizing Catechol-Boronate Complexation Chemistry

Bacteria such as Methicillin-resistant Staphylococcus aureus (MRSA) causes acidic microenvironment during infection. A biomaterial that exhibits tunable antimicrobial property in a pH dependent manner is potentially attractive. Herein, we presented a novel antibacterial hydrogel consisting of pH responsive and reversible catechol-boronate linkage formed between intrinsically bactericidal chlorinated catechol (catechol-Cl) and phenylboronic acid. Fourier transformed infrared spectroscopy (FTIR), oscillatory rheometry, and Johnson Kendall Roberts (JKR) contact mechanics testing confirmed the formation and dissociation of the complex in a pH dependent manner. When the hydrogel was treated with an acidic buffer (pH 3), the hydrogel exhibited excellent antimicrobial property against multiple strains of Gram-positive and negative bacteria including MRSA (up to 4 log₁₀ reduction from 10⁸ colony forming units/mL). At an acidic pH, catechol-Cl was unbound from the phenylboronic acid and available for killing bacteria. Conversely, when the hydrogel was treated with a basic buffer (pH 8.5), the hydrogel lost its antimicrobial property but also became non-cytotoxic. At a basic pH, the formation of catechol-boronate complex effectively reduce the exposure of the cytotoxic catechol-Cl to the surrounding. When further incubating the hydrogel in an acidic pH, the reversible complex dissociated to re-expose catechol-Cl and the hydrogel recovered its antibacterial property. Overall, the combination of catechol-Cl and phenylboronic acid provide a new strategy for designing hydrogels with pH responsive antibacterial activity and reduced cytotoxicity.

Remote-Controlled Exchange Rates by Photoswitchable Internal Catalysis of Dynamic Covalent Bonds

The transesterification rate of boronate esters with diols is tunable over 14 orders of magnitude. Rate acceleration is achieved by internal base catalysis, which lowers the barrier for proton transfer. Here we report a photoswitchable internal catalyst that tunes the rate of boronic ester/diol exchange over 4 orders of magnitude. We employed an acylhydrazone molecular photoswitch, which forms a thermally stable but photoreversible intramolecular H-bond, to gate the activity of the internal base catalyst in 8-quinoline boronic ester. The photoswitch is bidirectional and can be cycled repeatedly. The intramolecular H-bond is found to be essential to the design of this photoswitchable internal catalyst, as protonating the quinoline with external sources of acid has little effect on the exchange rate.

Modulating Boronic Ester Stability in Block Copolymer Micelles via the Neighbor Effect of Copolymerized Tertiary Amines for Controlled Release of Polyphenolic Drugs

The traceless and pH-sensitive properties of boronic esters are attractive for the synthesis of polymer-drug conjugates, but current platforms suffer from both low stability under physiologically relevant conditions and synthetically demanding optimization to tune drug release profiles. We hypothesized that the high catechol affinity and stability of Wulff-type boronic acids could be mimicked by copolymerization of phenyl boronic acid with a tertiary amine and subsequent micellization. This strategy yielded a versatile platform for the preparation of reversible polymer-drug conjugates, which more than doubled the oxidative stability of encapsulated polyphenolic drug cargo at physiologically relevant pH and enabled simple and incremental tuning of drug release kinetics. Moreover, we validated, with ¹⁹F NMR, that these copolymers exhibit uniquely high catechol affinity that could not be replicated by combinations of similarly functionalized small molecules. Overall, this report demonstrates that copolymerization of boronic acid and tertiary amine monomers is a powerful and modular approach to improving boronic ester chemistry for drug delivery applications.

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.

Catechol Moiety Integrated Tri-Aryl Type AIEgen for Visual and Quantitative Boronic Acid Detection

Novel functional AIEgen based on three compact bound aryl skeletons is designed and synthesized. This tri-aryl type luminogen (TA-Catechol) embedded with catechol moiety responds rapidly to series of boronic acids. Real-time visual and quantitative dual-mode detection method is established for the first time with modest precision and low detection limit (8.0 μM). Detailed mechanistic discussion identifies tetra-coordinated boronic species as the key intermediate within sensing procedure. Wide range of organic boronic acids compatible with this strategy is displayed which is promising in high throughput screening technology. Furthermore, solid-state sensing capability of TA-Catechol is also demonstrated.

Protodeboronation of (Hetero)Arylboronic Esters: Direct versus Prehydrolytic Pathways and Self-/Auto-Catalysis

The kinetics and mechanism of the base-catalyzed hydrolysis (ArB(OR)₂ → ArB(OH)₂) and protodeboronation (ArB(OR)₂ → ArH) of a series of boronic esters, encompassing eight different polyols and 10 polyfluoroaryl and heteroaryl moieties, have been investigated by in situ and stopped-flow NMR spectroscopy (¹⁹F, ¹H, and ¹¹B), pH-rate dependence, isotope entrainment, ²H KIEs, and KS-DFT computations. The study reveals the phenomenological stability of boronic esters under basic aqueous-organic conditions to be highly nuanced. In contrast to common assumption, esterification does not necessarily impart greater stability compared to the corresponding boronic acid. Moreover, hydrolysis of the ester to the boronic acid can be a dominant component of the overall protodeboronation process, augmented by self-, auto-, and oxidative (phenolic) catalysis when the pH is close to the pK_a of the boronic acid/ester.

Boric Acid-Fueled ATP Synthesis by Fo F1 ATP Synthase Reconstituted in a Supramolecular Architecture

Significant strides toward producing biochemical fuels have been achieved by mimicking natural oxidative and photosynthetic phosphorylation. Here, different from these strategies, we explore boric acid as a fuel for tuneable synthesis of energy-storing molecules in a cell-like supramolecular architecture. Specifically, a proton locked in boric acid is released in a modulated fashion by the choice of polyols. As a consequence, controlled proton gradients across the lipid membrane are established to drive ATP synthase embedded in the biomimetic architecture, which facilitates tuneable ATP production. This strategy paves a unique route to achieve highly efficient bioenergy conversion, holding broad applications in synthesis and devices that require biochemical fuels.