Reversible Covalent Bond Formation Research Articles

New Advances in Covalent Network Polymers via Dynamic Covalent Chemistry.

Covalent network polymers, as materials composed of atoms interconnected by covalent bonds in a continuous network, are known for their thermal and chemical stability. Over the past two decades, these materials have undergone significant transformations, gaining properties such as malleability, environmental responsiveness, recyclability, crystallinity, and customizable porosity, enabled by the development and integration of dynamic covalent chemistry (DCvC). In this review, we explore the innovative realm of covalent network polymers by focusing on the recent advances achieved through the application of DCvC. We start by examining the history and fundamental principles of DCvC, detailing its inception and core concepts and noting its key role in reversible covalent bond formation. Then the reprocessability of covalent network polymers enabled by DCvC is thoroughly discussed, starting from the significant milestones that marked the evolution of these polymers and progressing to their current trends and applications. The influence of DCvC on the crystallinity of covalent network polymers is then reviewed, covering their bond diversity, synthesis techniques, and functionalities. In the concluding section, we address the current challenges faced in the field of covalent network polymers and speculates on potential future directions.

Dynamic covalent interface engineering for 2D MXene membrane with interchangeable surface groups and tunable layer spacing for water treatment

Two-dimensional (2D) MXene-based materials are promising for water purification due to their nanometer interlayers, however, achieving adaptable and engineerable nanochannel surface properties in the 2D MXene membrane remains challenging. We propose a pioneering approach, called dynamic covalent interface engineering (DCIE), to address this issue. A poly (4-vinylphenylboric acid)-grafted MXene membrane was prepared, binding interchangeable diols via dynamic boronic ester chemistry. Alternative diols enhanced interlayer distancing to 1.49–1.60 nm. The interlayer-anchored diols transition from anionic to cationic and from hydrophilic to hydrophobic states through reversible covalent bond formation and cleavage, with no residue effect of previous diols. As a result, the membrane with swiftly tailored nanochannel properties exhibits exceptional switchable permeability and selectivity (98 % for Congo Red, 97 % for Gentian Violet) for various water treatment scenarios. This innovative method presents a promising strategy for fabricating customizable and switchable 2D MXene-based membranes with enhanced molecular transport and sieving effects, thus advancing water purification applications.

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.

Reversible bond formation via sulfur free reversible addition fragmentation in photo-3D printing

An addition-fragmentation monomer (AFM) has been used as a crosslinker in photopolymerization-based 3D printer resins in order to try and reduce shrinkage due to polymerisation. This results in reversible network formation via photo crosslinking with reversible covalent bond formation to effect mechanical performance. The 3D printing conditions were optimized with regards to print quality, print speed, etc. The AFM was synthesized via catalytic chain transfer polymerization (CCTP) and characterized using NMR and size exclusion chromatography (SEC). The incorporation of the AFM as a crosslinker allows for rapid reversible covalent bond formation during network formation in 3D photopolymerization printing resulting in a reduction in both polymerisation shrinkage and stress higher molecular weight crosslinkers lead to less shrinkage and reversible bond formation to less stress build up. Curing was monitored via photo-rheology and Fourier Transform Infrared Spectroscopy (FT-IR). At lower contents of AFM, the mechanical properties (strength and Young’s modulus) are improved without compromising material properties, printing conditions and curing time. At higher AFM content, the kinetic analysis of the photopolymerization reaction shows a reduced final conversion of the vinylic bonds along with a delay of the gel point. The thermal and mechanical properties were evaluated with incorporation of different concentrations of AFM added to the resin formulations.

Shape‐ and Size‐Tunable Synthesis of Covalent Organic Cages through Rh‐Catalyzed Regioselective [2+2+2] Cycloaddition

AbstractCovalent organic cages have potential applications in molecular inclusion/recognition and porous organic crystals. Bridging arene units with sp3 atoms enables facile construction of rigid isolated internal vacancies, and various prismatic arene cages have been synthesized by kinetically controlled covalent bond formation. However, the synthesis of a tetrahedral one, which requires twice as much bond formation as prismatic ones, has been limited to a thermodynamically controlled dynamic SNAr reaction, and this reversible covalent bond formation made the resulting cage product chemically unstable. Here we report the Rh‐catalyzed high‐yielding and highly 1,3,5‐selective room temperature [2+2+2] cycloaddition of push‐pull alkynes and its application to the synthesis of chemically stable aryl ether cages of various shapes and sizes, including prismatic and tetrahedral forms. These aryl ether cages are highly crystalline and intertwine with each other to form regular packing structures. Some aryl ether cages encapsulated isolated water molecules in their hydrophobic cavity by hydrogen bonding with the multiple ester moieties.

Shape- and Size-Tunable Synthesis of Covalent Organic Cages through Rh-Catalyzed Regioselective [2+2+2] Cycloaddition.

Covalent organic cages have potential applications in molecular inclusion/recognition and porous organic crystals. Bridging arene units with sp3 atoms enables facile construction of rigid isolated internal vacancies, and various prismatic arene cages have been synthesized by kinetically controlled covalent bond formation. However, the synthesis of a tetrahedral one, which requires twice as much bond formation as prismatic ones, has been limited to a thermodynamically controlled dynamic SN Ar reaction, and this reversible covalent bond formation made the resulting cage product chemically unstable. Here we report the Rh-catalyzed high-yielding and highly 1,3,5-selective room temperature [2+2+2] cycloaddition of push-pull alkynes and its application to the synthesis of chemically stable aryl ether cages of various shapes and sizes, including prismatic and tetrahedral forms. These aryl ether cages are highly crystalline and intertwine with each other to form regular packing structures. Some aryl ether cages encapsulated isolated water molecules in their hydrophobic cavity by hydrogen bonding with the multiple ester moieties.

Deciphering dynamic combinatorial libraries of glycoclusters with miniaturized weak affinity chromatography coupled with mass spectrometry (nano-FAC-MS)

We report an original methodology based on affinity chromatography coupled with mass spectrometry to decipher the complexity of dynamic combinatorial libraries (DCLs) of glycoclusters. Such libraries are intended to boost the design of potential therapeutic anti-infectious agents targeting Pseudomonas aeruginosa, which is responsible for numerous diseases, mostly found in hospitals as major a cause of nosocomial infections. Dynamic combinatorial chemistry provides a rapid access to an equilibrating mixture of glycocluster candidates through the formation of reversible covalent bonds under thermodynamic control. Identifying each molecule in the complex mixture overcomes challenges due to the dynamic process. Selection of glycoclusters candidates was first realized on a model lectin (Concanavalin A, ConA). Home-made affinity nanocolumns, containing covalently immobilized ConA and have volumes in the microliter range, were used to separate DCLs of glycoclusters with respect to their specific lectin binding properties under buffered aqueous conditions. Miniaturization facilitates the inline coupling with MS detection in such purely aqueous and buffered conditions and reduces target protein consumption. Monolithic lectin-affinity columns prepared by immobilization of ConA were first characterized using a known ligand. The amount of active binding immobilized lectin is 61 ± 5 pmol on 8.5-cm length column. We demonstrated the ability of our approach to evaluate individual dissociation constants of species directly in the complex mixture. The concept was then successfully applied to the screening of DCLs of more complex glycoclusters to identify (by mass spectrometry) and rank the ligands (by relative breakthrough curve delay) according to their affinity for the immobilized lectin in a single experiment.

Lysine-Targeted Reversible Covalent Ligand Discovery for Proteins via Phage Display.

Binding via reversible covalent bond formation presents a novel and powerful mechanism to enhance the potency of synthetic inhibitors for therapeutically important proteins. Work on this front has yielded the anticancer drug bortezomib as well as the antisickling drug voxelotor. However, the rational design of reversible covalent inhibitors remains difficult even when noncovalent inhibitors are available as a scaffold. Herein, we report chemically modified phage libraries, both linear and cyclic, that incorporate 2-acetylphenylboronic acid (APBA) as a warhead to bind lysines via reversible iminoboronate formation. To demonstrate their utility, these APBA-presenting phage libraries were screened against sortase A of Staphylococcus aureus, as well as the spike protein of SARS-CoV-2. For both protein targets, peptide ligands were readily identified with single-digit micromolar potency and excellent specificity, enabling live-cell sortase inhibition and highly sensitive spike protein detection, respectively. Furthermore, our structure-activity studies unambiguously demonstrate the benefit of the APBA warhead for protein binding. Overall, this contribution shows for the first time that reversible covalent inhibitors can be developed via phage display for a protein of interest. The phage display platform should be widely applicable to proteins including those involved in protein-protein interactions.

Alginate-based aerogels as wound dressings for efficient bacterial capture and enhanced antibacterial photodynamic therapy

The development of novel wound dressings, such as aerogels, with rapid hemostasis and bactericidal capacities for pre-hospital care is necessary. To prevent the occurrence of bacterial resistance, antibacterial photodynamic therapy (aPDT) with broad-spectrum antibacterial ability and negligible bacterial resistance has been intensively studied. However, photosensitizers often suffer from poor water solubility, short singlet oxygen (1O2) half-life and restricted 1O2 diffusion distance. Herein, sodium alginate was covalently modified by photosensitizers and phenylboronic acid, and cross-linked by Ca(II) ions to generate SA@TPAPP@PBA aerogel after lyophilization as an antibacterial photodynamic wound dressing. Afterwards, its photodynamic and bacterial capture activities were intensively evaluated. Furthermore, its hemostasis and bactericidal efficiency against Staphylococcus aureus were assessed via in vitro and in vivo assays. First, chemical immobilization of photosensitizers led to an enhancement of its solubility. Moreover, it showed an excellent hemostasis capacity. Due to the formation of reversible covalent bonds between phenylboronic acid and diol groups on bacterial cell surface, the aerogel could capture S. aureus tightly and dramatically enhance aPDT. To sum up, the prepared aerogel illustrated excellent hemostasis capacity and antibacterial ability against S. aureus. Therefore, they have great potential to be utilized as wound dressing in clinical trials.

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.

Oligoboroxine-based architectures

Boroxines are well-known six-membered ring compounds and their fundamental properties such as aromaticity, Lewis acidity, photophysical property, interactions with anions, thermal stability, and reactivity have been investigated. Since the 2000s, boroxine formation has been paid much attention as a reversible covalent bond formation. Boronic acids (RB–(OH)2) reversibly trimerize to afford the tripodal structure of boroxines. The equilibrium between boronic acids and boroxines can be controlled simply by the addition or removal of water. The addition of N-donor Lewis bases also promotes the reversible formation of boroxine⋅N-donor Lewis base adducts. To date, by using this unique reversible trimerization, various self-assembled boroxine-based covalent architectures and polymeric materials have been reported. Boroxine-based architectures are categorized into three types as follows. (a) Tripodal structures by monoboroxine formation, (b) network structures by polyboroxine formation including covalent organic frameworks (= COFs) and dynamic polymer materials, and (c) discrete structures by oligoboroxine formation. Compared to mono or polyboroxine-based architectures (a) or (b), oligoboroxine-based discrete architectures (c) containing a precise number of boroxines are an emerging research subject and have the potential to create a novel type of discrete covalent architectures. Thus, in this review, we summarize recent progress including our works in this field.

Reversible Reaction‐Based Fluorescent Probes for Dynamic Sensing and Bioimaging

Diverse chemical species such as ions and molecules exist within living cells and organisms undergoing dynamic changes in their local environment by a web of continuously interacting reactions. Reaction‐based fluorescent probes with a highly sought reversible feature can provide a real‐time monitor of the concentration dynamics (increases and decreases) of such chemical species, thus ideally suited to understand the physiological function, and pathogenic mechanisms of corresponding bio‐species in the regulation of cellular function and disease progression. This review summarizes the current methods for constructing reversible reaction‐based fluorescent probes. The sensing mechanisms and biological applications of these probes are also discussed. The representative examples reported recently are categorized according to the type of reversible chemical reactions utilized: nucleophilic additions (Michael additions, chromophore reactions), nucleophilic addition‐condensation reactions, and redox reactions. Finally, we present the potential challenges and suggestions for developing probes based on dynamic and reversible covalent bond formation reactions.

Engineered Dynamic Boronate Ester-Mediated Self-Healable Biocompatible G-Quadruplex Hydrogels for Sustained Release of Vitamins.

Injectable, self-healable, and biocompatible dynamic hydrogels prepared from the molecular self-assembly and reversible covalent bond formation of low-molecular-weight hydrogelators are increasing in the field of drug delivery. Herein, we report the formation of G-quadruplex hydrogels via the multicomponent self-assembly and reversible bond formation between guanosine (G) and 1-naphthaleneboronic acid in the presence of the monovalent cation K+. The cation-templated stacking interaction of G4 quartets and the formation of dynamic cyclic boronate esters are responsible for the construction of dynamic G-quadruplex assembly. The in situ-synthesized dynamic cyclic boronate esters are well characterized by 11B nuclear magnetic resonance and Fourier transform infrared spectroscopy methods. The formation and morphology of the G-quadruplex hydrogel are well supported by several spectroscopic and microscopic techniques. The injectability and self-healing ability of the G-quadruplex hydrogel are also investigated. The in vivo cytotoxicity of the G-quadruplex hydrogel is extensively evaluated over different cell lines (HeLa, MCF-7, and HEK293) to observe the biostability and broad-spectrum biocompatibility of the hydrogel. Further, this injectable, biocompatible G-quadruplex hydrogel has been used for encapsulation and sustained release of two important vitamins (B2 and B12) over 40 h at physiological pH (7.46) and temperature (37 °C) without the influence of any external stimuli.

Light-Stabilized Dynamic Materials.

The light-responsive adaptation of polymer materials typically requires different wavelengths or additional heat to induce reversible covalent bond formation and dissociation. Here, we bypass the use of invasive triggers by introducing light-stabilized dynamic materials that can undergo a repeatable change in topology from a covalently cross-linked material into a liquid polymer formulation by switching one visible light source on-and-off without the need for any additional triggers. Specifically, we exploit the photo-Diels-Alder reaction of triazolinediones with naphthalenes as a dynamic covalent cross-linking platform that enables green light-induced network formation, while the cross-linked material collapses through spontaneous cycloreversion upon standing in the dark at ambient temperature. Importantly, the covalent cross-links remain stabilized for as long as visible light is present, thereby retaining the material's structural integrity. This enables their potential use in an array of light-directed applications whereby network properties such as stiffness can be tuned by the mildest trigger of all: darkness.

Simultaneous Determination of Aminoglycoside Residues in Livestock and Fishery Products by Phenylboronic Acid Solid-Phase Extraction and Liquid Chromatography-Tandem Mass Spectrometry.

A rapid and simple method has been developed for simultaneous determinations of the concentrations of nine aminoglycosides (AGs) in livestock and fishery products using phenylboronic acid (PBA) solid-phase extraction (SPE) clean-up. Unlike the widely employed SPE approaches, based on cation-exchange, PBA SPE relies on the reversible formation of covalent bonds with the analytes. The advantage of using PBA lies in the fact that this compound strongly and stably retains analytes, and the pH of the loading solution can be easily adjusted using a high-concentration buffer. The clean-up conditions, such as the pH of the loading solution and the acetonitrile concentration in the elution and wash solvents, were optimized. The degree of recovery measured for nine AGs in six samples (bovine muscle, bovine liver, milk, chicken egg, fish and shrimp) were in the 73 - 98% range, and the values for the relative standard deviation were 9.3% or less.

Molecular Biodynamers: Dynamic Covalent Analogues of Biopolymers.

ConspectusConstitutional dynamic chemistry (CDC) features the use of reversible linkages at both molecular and supramolecular levels, including reversible covalent bonds (dynamic covalent chemistry, DCC) and noncovalent interactions (dynamic noncovalent chemistry, DNCC). Due to its inherent reversibility and stimuli-responsiveness, CDC has been widely utilized as a powerful tool for the screening of bioactive compounds, the exploitation of receptors or substrates driven by molecular recognition, and the fabrication of constitutionally dynamic materials. Implementation of CDC in biopolymer science leads to the generation of constitutionally dynamic analogues of biopolymers, biodynamers, at the molecular level (molecular biodynamers) through DCC or at the supramolecular level (supramolecular biodynamers) via DNCC. Therefore, biodynamers are prepared by reversible covalent polymerization or noncovalent polyassociation of biorelevant monomers.In particular, molecular biodynamers, biodynamers of the covalent type whose monomeric units are connected by reversible covalent bonds, are generated by reversible polymerization of bio-based monomers and can be seen as a combination of biopolymers with DCC. Owing to the reversible covalent bonds used in DCC, molecular biodynamers can undergo continuous and spontaneous constitutional modifications via incorporation/decorporation and exchange of biorelevant monomers in response to internal or external stimuli. As a result, they behave as adaptive materials with novel properties, such as self-healing, stimuli-responsiveness, and tunable mechanical and optical character. More specifically, molecular biodynamers combine the biorelevant characters (e.g., biocompatibility, biodegradability, biofunctionality) of bioactive monomers with the dynamic features of reversible covalent bonds (e.g., changeable, tunable, controllable, self-healing, and stimuli-responsive capacities), to realize synergistic properties in one system. In addition, molecular biodynamers are commonly produced in aqueous media under mild or even physiological conditions to suit their biorelated applications.In contrast to static biopolymers emphasizing structural stability and unity by using irreversible covalent bonds, molecular biodynamers are seeking relative structural adaptability and diversity through the formation of reversible covalent bonds. Based on these considerations, molecular biodynamers are capable of reorganizing their monomers, generating, identifying, and amplifying the fittest structures in response to environmental factors. Hence, molecular biodynamers have received considerable research attention over the past decades. Accordingly, the construction of molecular biodynamers through equilibrium polymerization of nucleobase-, carbohydrate- or amino-acid-based monomers can lead to the fabrication of dynamic analogues of nucleic acids (DyNAs), polysaccharides (glycodynamers), or proteins (dynamic proteoids), respectively. In this Account, we summarize recent advances in developing different types of molecular biodynamers as structural or functional biomimetics of biopolymers, including DyNAs, glycodynamers, and dynamic proteoids. We introduce how chemists utilize various reversible reactions to generate molecular biodynamers with specific sequences and well-ordered structures in aqueous medium. We also discuss and list their potential applications in various research fields, such as drug delivery, drug discovery, gene sensing, cancer diagnosis, and treatment.

Amine-selective affinity resins based on pH-sensitive reversible formation of covalent bonds.

A new class of affinity resins using reversible covalent bonds is introduced for the separation of amine-containing molecules. pH-sensitive reversible formation of amic acid bonds between amines and carboxylate dimethyl maleic anhydride-decorated wrinkled silica nanoparticle resins was used to selectively retain and release amine-containing molecules, by controlling the pH.

ChemInform Abstract: Calcium‐Catalyzed Dynamic Multicomponent Reaction.

AbstractAn air and moisture insensitive calcium‐based catalytic system is developed for the reversible formation of covalent bonds allowing for the diastereoselective amplification of a pharmacologically relevant bicyclic amine scaffold.

BODIPY based self-healing fluorescent gel formation via acylhydrazone linkage

Polymeric BODIPY derivatives with reactive functional groups undergo reversible formation of covalent bonds leading to self-healing properties.

Disulfide-Based Diblock Copolymer Worm Gels: A Wholly-Synthetic Thermoreversible 3D Matrix for Sheet-Based Cultures.

It is well-known that 3D in vitro cell cultures provide a much better model than 2D cell cultures for understanding the in vivo microenvironment of cells. However, significant technical challenges in handling and analyzing 3D cell cultures remain, which currently limits their widespread application. Herein, we demonstrate the application of wholly synthetic thermoresponsive block copolymer worms in sheet-based 3D cell culture. These worms form a soft, free-standing gel reversibly at 20-37 °C, which can be rapidly converted into a free-flowing dispersion of spheres on cooling to 5 °C. Functionalization of the worms with disulfide groups was found to be essential for ensuring sufficient mechanical stability of these hydrogels to enable long-term cell culture. These disulfide groups are conveniently introduced via statistical copolymerization of a disulfide-based dimethacrylate under conditions that favor intramolecular cyclization and subsequent thiol/disulfide exchange leads to the formation of reversible covalent bonds between adjacent worms within the gel. This new approach enables cells to be embedded within micrometer-thick slabs of gel with good viability, permits cell culture for at least 12 days, and facilitates recovery of viable cells from the gel simply by incubating the culture in buffer at 4 °C (thus, avoiding the enzymatic degradation required for cell harvesting when using commercial protein-based gels, such as Matrigel).