Chemistry of periodate-mediated cross-linking of 3,4-dihydroxylphenylalanine-containing molecules to proteins

Tyrosinase-Modified UHMW SELP Polymers as Wet and Underwater Adhesives to Achieve Multi-interface Adhesion

The presence of a hydration layer in humid and underwater environments challenges adhesive-substrate interactions and prevents effective bonding, which has become a significant obstacle to the development of adhesives in the industrial and biomedical fields. In this study, ultrahigh-molecular-weight (UHMW) silk-elastin-like proteins (SELP) with 3,4-dihydroxyphenylalanine (DOPA) converted from tyrosine residues by tyrosinase exhibited excellent adhesive properties on different interfaces, such as glass, aluminum, wood, polypropylene sheets, and pigskin, under both dry and wet conditions. Additionally, by incorporating trace amounts of cross-linking agents like Fe3+, NaIO4, and tris(hydroxymethyl) phosphine (THP), the mussel-inspired adhesives maintained a stable and excellent adhesion, broadening the conditions of application. Notably, the UHMW SELP adhesive exhibited remarkable underwater adhesion properties with a shear strength of 0.83 ± 0.17 MPa on glass. It also demonstrated good adhesion to biological tissues including the kidney, liver, heart, and lungs. In vitro cytocompatibility testing using L929 cells showed minimal toxicity, highlighting its potential application in the biomedical field. The sustainable, cytocompatible, cost-effective, and highly efficient adhesive provides valuable insights for the design and development of a new protein-based underwater adhesive for medical application.

Systemic Convergent Multitarget Interactions of Plant Polyphenols Revealed by Affinity-Based Protein Profiling of Bone Cells Using C-Glucosidic Vescal(ag)in-Bearing Chemoproteomic Probes

The ellagitannins vescalagin and vescalin, known as actin-dependent inhibitors of osteoclastic bone resorption, were mounted onto chemical probes to explore their interactions with bone cell proteins by means of affinity-based chemoproteomics and bioinformatics. The chemical reactivity of the pyrogallol units of these polyphenols toward oxidation into electrophilic ortho-quinones was exploited using NaIO4 to promote the covalent capture of target proteins, notably those expressed at lower abundance and those interacting with polyphenols at low-to-moderate levels of affinity. Different assays revealed the multitarget nature of both ellagitannins, with 100-370 statistically significant proteins captured by their corresponding probes. A much higher number of proteins were captured from osteoclasts than from osteoblasts. Bioinformatic analyses unveiled a preference for the capture of proteins having phosphorylated ligands and GTPase regulators and enabled the identification of 33 potential target proteins with systemic relevance to osteoclast differentiation and activity, as well as to the regulation of actin dynamics.

Synthesis of Flavonol-Bearing Probes for Chemoproteomic and Bioinformatic Analyses of Asteraceae Petals in Search of Novel Flavonoid Enzymes

This study aimed at searching for the enzymes that are responsible for the higher hydroxylation of flavonols serving as UV-honey guides for pollinating insects on the petals of Asteraceae flowers. To achieve this aim, an affinity-based chemical proteomic approach was developed by relying on the use of quercetin-bearing biotinylated probes, which were thus designed and synthesized to selectively and covalently capture relevant flavonoid enzymes. Proteomic and bioinformatic analyses of proteins captured from petal microsomes of two Asteraceae species (Rudbeckia hirta and Tagetes erecta) revealed the presence of two flavonol 6-hydroxylases and several additional not fully characterized proteins as candidates for the identification of novel flavonol 8-hydroxylases, as well as relevant flavonol methyl- and glycosyltransferases. Generally speaking, this substrate-based proteome profiling methodology constitutes a powerful tool for the search for unknown (flavonoid) enzymes in plant protein extracts.

Flexible dual-functionalized hyaluronic acid hydrogel adhesives formed in situ for rapid hemostasis

Hydrogel adhesives integrating both rapid and strong adhesion to blooding tissues and biocompatibility are highly desired for fast hemostasis. Herein, a flexible hyaluronic acid hydrogel adhesive is fabricated via photocrosslinking of the solution originating from dopamine-conjugated maleic hyaluronic acid (DMHA) in situ. The introduction of acrylate groups with high substitutions into the hydrogel matrix endows the adhesive with rapid gelation and strong tissue adhesion properties through photopolymerization. Moreover, the high substitution of catechol groups with unoxidized state can not only induce red blood cell aggregation and platelets adhesion but also adhere to wound tissue to further enhance hemostasis. Based on its bio-adhesion and procoagulant activity, the DMHA hydrogel formed in situ reveals superior hemostatic performance in the rat liver injury model and noncompressible hemorrhage model, and rabbit femoral artery puncture model, compared to commercial products (gauze, absorbable gelatin sponge) and oxidized DMHA (SMHA) hydrogel. Besides, the hydrogel exhibited good adaptability, biodegradability, and superior cytocompatibility as well as negligible inflammation. This hydrogel adhesive is a promising biological adhesive for hemorrhage control.

Preparation and Application of an Inexpensive α-Formylglycine Building Block Compatible with Fmoc Solid-Phase Peptide Synthesis

α-Formylglycine (fGly) is a rare residue located in the active site of sulfatases and serves as a precursor to pharmaceutically relevant motifs. The installation of fGly motifs into peptides is currently challenging due to degradation under the acidic and nucleophile-rich conditions accompanying resin cleavage during solid-phase peptide synthesis. We report the synthesis of acid- and nucleophile-tolerant α-formylglycine building blocks from vitamin C and use them to prepare callyaerin A, a macrocyclic peptide containing an fGly-derived motif.

Bioinspired Injectable Self-Healing Hydrogel Sealant with Fault-Tolerant and Repeated Thermo-Responsive Adhesion for Sutureless Post-Wound-Closure and Wound Healing

Hydrogels with multifunctionalities, including sufficient bonding strength, injectability and self-healing capacity, responsive-adhesive ability, fault-tolerant and repeated tissue adhesion, are urgently demanded for invasive wound closure and wound healing. Motivated by the adhesive mechanism of mussel and brown algae, bioinspired dynamic bonds cross-linked multifunctional hydrogel adhesive is designed based on sodium alginate (SA), gelatin (GT) and protocatechualdehyde, with ferric ions added, for sutureless post-wound-closure. The dynamic hydrogel cross-linked through Schiff base bond, catechol-Fe coordinate bond and the strong interaction between GT with temperature-dependent phase transition and SA, endows the resulting hydrogel with sufficient mechanical and adhesive strength for efficient wound closure, injectability and self-healing capacity, and repeated closure of reopened wounds. Moreover, the temperature-dependent adhesive properties endowed mispositioning hydrogel to be removed/repositioned, which is conducive for the fault-tolerant adhesion of the hydrogel adhesives during surgery. Besides, the hydrogels present good biocompatibility, near-infrared-assisted photothermal antibacterial activity, antioxidation and repeated thermo-responsive reversible adhesion and good hemostatic effect. The in vivo incision closure evaluation demonstrated their capability to promote the post-wound-closure and wound healing of the incisions, indicating that the developed reversible adhesive hydrogel dressing could serve as versatile tissue sealant.

Substrate Specificity of the Flavoenzyme BhaC1 That Converts a C-Terminal Trp to a Hydroxyquinone

The preparation of protein-protein, protein-peptide, and protein-small molecule conjugates is important for a variety of applications, such as vaccine production, immunotherapies, preparation of antibody-drug conjugates, and targeted delivery of therapeutics. To achieve site-selective conjugation, selective chemical or enzymatic functionalization of proteins is required. We have recently reported biosynthetic pathways in which small, catalytic scaffold peptides are utilized for the generation of amino acid-derived natural products called pearlins. In these systems, peptide amino-acyl tRNA ligases (PEARLs) append amino acids to the C-terminus of a scaffold peptide, and tailoring enzymes encoded in the biosynthetic gene clusters modify the PEARL-appended amino acid to generate a variety of natural products. Herein, we investigate the substrate selectivity of one such tailoring enzyme, BhaC₁, that participates in pyrroloiminoquinone biosynthesis. BhaC₁ converts the indole of a C-terminal tryptophan into an o-hydroxy-p-quinone, a promising moiety for site-selective bioconjugation. Our studies demonstrate that BhaC₁ requires a 20-amino acid peptide for substrate recognition. When this peptide was appended at the C-terminus of proteins, the C-terminal Trp was modified by BhaC₁. The enzyme is sufficiently selective that only small changes to the sequence of the peptide are tolerated. An AlphaFold model for substrate recognition explains the selectivity of the enzyme, which may be used to install a reactive handle onto the C-terminus of proteins.

Peptidomimetic Wet-Adhesive PEGtides with Synergistic and Multimodal Hydrogen Bonding

The remarkable underwater adhesion of mussel foot proteins has long been an inspiration in the design of peptidomimetic materials. Although the synergistic wet adhesion of catechol and lysine has been recently highlighted, the critical role of the polymeric backbone has remained largely underexplored. Here, we present a peptidomimetic approach using poly(ethylene glycol) (PEG) as a platform to evaluate the synergistic compositional relation between the key amino acid residues (i.e., DOPA and lysine), as well as the role of the polyether backbone in interfacial adhesive interactions. A series of PEG-based peptides (PEGtides) were synthesized using functional epoxide monomers corresponding to catechol and lysine via anionic ring-opening polymerization. Using a surface force apparatus, highly synergistic surface interactions among these PEGtides with respect to the relative compositional ratio were revealed. Furthermore, the critical role of the catechol-amine synergy and diverse hydrogen bonding within the PEGtides in the superior adhesive interactions was verified by molecular dynamics simulations. Our study sheds light on the design of peptidomimetic polymers with reduced complexity within the framework of a polyether backbone.

Tannic acid: a crosslinker leading to versatile functional polymeric networks: a review

With the thriving of mussel-inspired polyphenol chemistry as well as the demand for low-cost analogues to polydopamine in adhesive design, tannic acid has gradually become a research focus because of its wide availability, health benefits and special chemical properties. As a natural building block, tannic acid could be used as a crosslinker either supramolecularly or chemically, ensuring versatile functional polymeric networks for various applications. Up to now, a systematic summary on tannic-acid-based networks has still been waiting for an update and outlook. In this review, the common features of tannic acid are summarized in detail, followed by the introduction of covalent and non-covalent crosslinking methods leading to various tannic-acid-based materials. Moreover, recent progress in the application of tannic acid composites is also summarized, including bone regeneration, skin adhesives, wound dressings, drug loading and photothermal conversion. Above all, we also provide further prospects concerning tannic-acid-crosslinked materials.

A novel tyrosine hyperoxidation enables selective peptide cleavage

A novel tyrosine hyperoxidation enabling selective peptide cleavage is reported. The scission of the N-terminal amide bond of tyrosine was achieved with Dess–Martin periodinane under mild conditions, generating a C-terminal peptide fragment bearing the unprecedented hyperoxidized tyrosine motif, 4,5,6,7-tetraoxo-1H-indole-2-carboxamide, along with an intact N-terminal peptide fragment. This reaction proceeds with high site-selectivity for tyrosine and exhibits broad substrate scope for various peptides, including those containing post-translational modifications. More importantly, this oxidative cleavage was successfully applied to enable sequencing of three naturally occurring cyclic peptides, including one depsipeptide and one lipopeptide. The linearized peptides generated from the cleavage reaction significantly simplify cyclic peptide sequencing by MS/MS, thus providing a robust tool to facilitate rapid sequence determination of diverse cyclic peptides containing tyrosine. Furthermore, the highly electrophilic nature of the hyperoxidized tyrosine unit disclosed in this work renders it an important electrophilic target for the selective bioconjugation or synthetic manipulation of peptides containing this unit.

A Tyr-selective peptide cleavage was reported using Dess–Martin periodinane. The cleavage generates an unprecedented hyperoxidized tyrosine motif in the C-terminal fragment and showed excellent site-specificity and broad scope for various peptides.

Freeze-Thawing-Induced Macroporous Catechol Hydrogels with Shape Recovery and Sponge-like Properties

Catechol-containing hydrogels have been exploited in biomedical fields due to their adhesive and cohesive properties, hemostatic abilities, and biocompatibility. Catechol moieties can be oxidized to o-catecholquinone, a chemically active intermediate, in the presence of oxygen to act as an electrophile to form catechol-catechol or catechol-amine/thiol adducts. To date, catechol cross-linking chemistry to fabricate hydrogels has been mostly performed at room temperature. Herein, we report large increases in catechol cross-linking reaction kinetics by the freeze-thawing process. The formation of ice crystals during freezing steps spatially condenses catechol-containing polymers into nearly frozen (yet unfrozen) regions, resulting in decreases in the polymeric chain distances. This environment allows great increases in catechol cross-linking kinetics, a phenomenon that can also occur during thawing steps. The increased cross-linking rate and spatial condensation in the cryogels provide unique wall and pore structures, which result in elastic, spongelike hydrogels. The moduli of the cryogels prepared by glycol-chitosan-catechol (g-chitosan-c) were improved by 3-6-fold compared to room temperature-cured conventional hydrogels, and the degree of improvement increased depending on the freezing time and the number of freeze-thawing cycles. Unlike typical cell encapsulations before cross-linking, which have often been a source of cytotoxicity, the macroporosity of cryogels allows nontoxic cell seeding with ease. This research offers a new way to utilize catechol cross-linking chemistry by freeze-thawing processes to simultaneously regulate mechanical strength and porous structures in catechol-containing hydrogels.

Tannic Acid Radicals in the Presence of Alkali Metal Salts and Their Impact on the Formation of Silicate-Phenolic Networks

Polyphenolic molecules have become attractive building blocks for bioinspired materials due to their adhesive characteristics, capacity to complex ions, redox chemistry, and biocompatibility. For the formation of tannic acid (TA) surface modifications based on silicate-phenolic networks, a high ionic strength is required. In this study, we investigated the effects of NaCl, KCl, and LiCl on the formation of TA coatings and compared it to the coating formation of pyrogallol (PG) using a quartz-crystal microbalance. We found that the substitution of NaCl with KCl inhibited the TA coating formation through the high affinity of K+ to phenolic groups resulting in complexation of TA. Assessment of the radical formation of TA by electron paramagnetic resonance spectroscopy showed that LiCl resulted in hydrolysis of TA forming gallic acid radicals. Further, we found evidence for interactions of LiCl with the Siaq crosslinker. In contrast, the coating formation of PG was only little affected by the substitution of NaCl with LiCl or KCl. Our results demonstrate the interaction potential between alkali metal salts and phenolic compounds and highlight their importance in the continuous deposition of silicate-phenolic networks. These findings can be taken as guidance for future biomedical applications of silicate-phenolic networks involving monovalent ions.

Tyrosine bioconjugation - an emergent alternative

A review of the heretofore less explored approach of tyrosine bioconjugation, which is rapidly becoming a constructive alternative/complement to the more well-established strategies, is provided.

Protein bioconjugation is an increasingly important field of research, with wide-ranging applications in areas such as therapeutics and biomaterials. Traditional cysteine and lysine bioconjugation strategies are widely used and have been extensively researched, but in some cases they are not appropriate and alternatives are needed or they are not compatible with one another to enable the formation of dually (and distinctly) modified dual-conjugates (an increasingly desired class of bioconjugates). Here we review the heretofore less explored approach of tyrosine bioconjugation, which is rapidly becoming a constructive alternative/complement to the more well-established strategies. Herein we present an overview of the field, and then focus on promising recent methods that can achieve high conversion and chemoselectivity. This suggests that not only can tyrosine bioconjugation be used in conjunction with cysteine and lysine modification to obtain proteins with multiple different modifications, it is also becoming a stand-alone alternative to these more traditional methods.

An alternative approach to the synthesis of the three fragments of anachelin H

The synthesis of the fully protected peptide, polyketide and alkaloid fragments of anachelin H is presented. The peptide fragment was prepared using a liquid phase peptide synthesis; the polyketide fragment was synthetized using a cross metathesis and an intramolecular oxa-Michael reaction as the key steps to introduce the desired stereochemistry; finally, the alkaloid fragment was obtained by an oxidative cyclization of a catechol derivative using potassium ferricyanide. The synthesis of all fragments was based on the use of natural amino acids as sources of asymmetry. The independent synthesis of the three fragments should allow more efficient biological studies on the fragments instead of the whole natural product. Experiments to illustrate the coupling of fragments and the effectiveness of the convergent strategy are also described.

Tyrosinase-Mediated Oxidative Coupling of Tyrosine Tags on Peptides and Proteins

Oxidative coupling (OC) through o-quinone intermediates has been established as an efficient and site-selective way to modify protein N-termini and the unnatural amino acid p-aminophenylalanine (paF). Recently, we reported that the tyrosinase-mediated oxidation of phenol-tagged cargo molecules is a particularly convenient method of generating o-quinones in situ. The coupling partners can be easily prepared and stored, the reaction takes place under mild conditions (phosphate buffer, pH 6.5, 4 to 23 °C), and dissolved oxygen is the only oxidant required. Here, we show an important extension of this chemistry for the activation of tyrosine residues that project into solution from the N or C-termini of peptide and protein substrates. Generating the o-quinone electrophiles from tyrosine allows greater flexibility in choosing the nucleophilic coupling partner and expands the scope of the reaction to include C-terminal positions. We also introduce a new bacterial tyrosinase enzyme that shows improved activation for some tyrosine substrates. The efficacy of several secondary amines and aniline derivatives was evaluated in the coupling reactions, providing important information for coupling partner design. This strategy was used to modify the C-termini of an antibody scFv construct and of Protein L, a human IgG kappa light chain binding protein. The use of the modified proteins as immunolabeling agents was also demonstrated.

Polydopamine as a stable and functional nanomaterial

The mussel inspired chemistry of dopamine leading to versatile coatings on the surface of all kinds of materials in a one pot process was considered as the unique aspect of catecholamine for a long time. Only recently, research has been undertaken to valorize the simultaneous oxidation and colloid formation in dopamine solutions in the presence of an oxidant. This mini review summarizes the synthesis methods allowing to get controlled nanomaterials, either nanoparticles, hollow capsules or nanotubes and even chiral nanomaterials from dopamine solutions. Finally the applications of those nanomaterials will be described.

Photocaged Quinone Methide Crosslinkers for Light-Controlled Chemical Crosslinking of Protein-Protein and Protein-DNA Complexes

Small-molecule crosslinkers are invaluable for probing biomolecular interactions and for crosslinking mass spectrometry. Existing chemical crosslinkers target only a small selection of amino acids, while conventional photo-crosslinkers target almost all residues non-specifically, complicating data analysis. Herein, we report photocaged quinone methide (PQM)-based crosslinkers that target nine nucleophilic residues through Michael addition, including Gln, Arg, and Asn, which are inaccessible to existing chemical crosslinkers. PQM crosslinkers were used in vitro, in Escherichia coli, and in mammalian cells to crosslink dimeric proteins and endogenous membrane receptors. The heterobifunctional crosslinker NHQM could crosslink proteins to DNA, for which few crosslinkers exist. The photoactivatable reactivity of these crosslinkers and their ability to target multiple amino acids will enhance the use of chemical crosslinking for studies of protein-protein and protein-DNA networks and for structural biology.

The molecular mechanisms underlying mussel adhesion

Marine mussels are able to firmly affix on various wet surfaces by the overproduction of special mussel foot proteins (mfps). Abundant fundamental studies have been conducted to understand the molecular basis of mussel adhesion, where the catecholic amino acid, l-3,4-dihydroxyphenylalanine (DOPA) has been found to play the major role. These studies continue to inspire the engineering of novel adhesives and coatings with improved underwater performances. Despite the fact that the recent advances of adhesives and coatings inspired by mussel adhesive proteins have been intensively reviewed in literature, the fundamental biochemical and biophysical studies on the origin of the strong and versatile wet adhesion have not been fully covered. In this review, we show how the force measurements at the molecular level by surface force apparatus (SFA) and single molecule atomic force microscopy (AFM) can be used to reveal the direct link between DOPA and the wet adhesion strength of mussel proteins. We highlight a few important technical details that are critical to the successful experimental design. We also summarize many new insights going beyond DOPA adhesion, such as the surface environment and protein sequence dependent synergistic and cooperative binding. We also provide a perspective on a few uncharted but outstanding questions for future studies. A comprehensive understanding on mussel adhesion will be beneficial to the design of novel synthetic wet adhesives for various biomedical applications.

Coordination of Covalent Cross-Linked Gelatin Hydrogels via Oxidized Tannic Acid and Ferric Ions with Strong Mechanical Properties

The design of gelatin-based hydrogels with high mechanical strength, high gelation temperature, and a rapid self-healing property still presents a challenge to researchers. In the present study, single cross-linked gelatin-oxidized tannic acid (SC-GT/OTA) hydrogels were fabricated through covalent cross-linking between gelatin and tannic acid (TA) oxidized by using sodium periodate (NaIO₄). Double cross-linked gelatin-OTA-FeCl₃·6H₂O (DC-GT/OTA/Fe^III) hydrogels were also created using metal coordination bonds formed between the catechol groups present in OTA and Fe^III in ferric chloride. As a result, the maximum gelling temperature of the SC-GT/OTA hydrogel (37 °C) was considerably higher than that of the pure gelatin hydrogel (15.4 °C). Moreover, the maximum values of compressive stress of SC-GT/OTA hydrogels increased significantly by almost seven times the original value as the molar ratio of NaIO₄ to TA increased from 3:1 to 5:1. When the molar ratio of NaIO₄ to TA was maintained at the constant of 4:1, the storage modulus values of DC-GT/OTA/Fe^III hydrogels with the Fe^III-to-TA molar ratio of 1.5:1 were three to 4 orders of magnitude higher than those of SC-GT/OTA hydrogels in the whole angular frequency range. The double cross-linked gelatin hydrogels developed in this research can be used widely in agriculture and material science fields.

Fingerprinting of Proteins that Mediate Quagga Mussel Adhesion using a De Novo Assembled Foot Transcriptome

The European freshwater mollusk Dreissena bugensis (quagga mussel), an invasive species to North America, adheres to surfaces underwater via the byssus: a non-living protein ‘anchor’. In spite of its importance as a biofouling species, the sequence of the majority of byssal proteins responsible for adhesion are not known, and little genomic data is available. To determine protein sequence information, we utilized next-generation RNA sequencing and de novo assembly to construct a cDNA library of the quagga mussel foot transcriptome, which contains over 200,000 transcripts. Quagga mussel byssal proteins were extracted from freshly induced secretions and analyzed using LC-MS/MS; peptide spectra were matched to the transcriptome to fingerprint the entire protein primary sequences. We present the full sequences of fourteen novel quagga mussel byssal proteins, named Dreissena bugensis foot proteins 4 to 17 (Dbfp4–Dbfp17), and new sequence data for two previously observed byssal proteins Dbfp1 and Dbfp2. Theoretical masses of the newly discovered proteins range from 4.3 kDa to 21.6 kDa. These protein sequences are unique but contain features similar to glue proteins from other species, including a high degree of polymorphism, proteins with repeated peptide motifs, disordered protein structure, and block structures.

A facile method for high level dual expression of recombinant and congener protein in a single expression system

Protein engineering is an emerging field for developing novel therapeutic proteins and commercial enzymes, along with a major impact on the global market. In recent decades, advanced methods employing protein modification through expansion of the genetic code have led to the development of proteins with new biochemical and physical properties. These techniques have produced engineered proteins with improved attribute comprising substrate relaxation, protein drug conjugation and high stability under extreme conditions of high temperatures, pH and organic solvents. Furthermore, residue specific incorporation is the simplest method for the global incorporation of non-canonical amino acid (NCAA) for protein modification; however it has the major drawbacks of high production cost and manpower requirement. In the present study, we developed a method for the incorporation of single NCAA in two different proteins by using Escherichia coli (E. coli) expression system. For that, the dual protein expressing Escherichia coli JW2581 strain was constructed by transforming pQE80L and pD881-PpiBT vectors with different promoters, selectable markers and AnnexinV, GFPHS gene. To modify the protein, the 3,4 dihydroxy phenyl alanine (DOPA) was globally incorporated into the GFPHS and Annexin V protein using dual protein expression system. The incorporation efficiency during the dual protein expression was achieved through optimized concentrations of amino acids, carbohydrate and inducers in minimal medium. This method for the incorporation of single NCAA into two different proteins using a single expression host system saves the production cost, manpower and time substantially.

Polydopamine Nanostructures as Biomaterials for Medical Applications

Polydopamine is a versatile and organic material that can be deposited as a conformal film with nanometer thickness on virtually any substrate. Much of the initial foundational work regarding polydopamine synthesis and processing was reported during the 2000s. Latter years have witnessed increasing interest and widespread adoption of polydopamine as a material for many applications including medicine. Conformal polydopamine coatings confer unique chemical and physical properties to many substrate materials including metals, ceramics, polymers, and beyond. Polydopamine-modified surfaces permit facile bioconjugation of many biomedical materials for potential use as bioadhesives, contrast agents, drug delivery systems, and protein-adsorption resistant interfaces. Polydopamine-based materials and interfaces may improve the performance of biomedical devices used in neurotechnology, diagnostics, and cardiovascular applications. This highlight article reviews recent advances in polydopamine processing capabilities. The use of polydopamine as a material in various biomedical applications is also discussed. Finally, challenges and opportunites in translating polydopamine for future biomedical technologies are summarized.

Bioinspired Underwater Adhesives by Using the Supramolecular Toolbox

Nature has developed protein-based adhesives whose underwater performance has attracted much research attention over the last few decades. The adhesive proteins are rich in catechols combined with amphiphilic and ionic features. This combination of features constitutes a supramolecular toolbox, to provide stimuli-responsive processing of the adhesive, to secure strong adhesion to a variety of surfaces, and to control the cohesive properties of the material. Here, the versatile interactions used in adhesives secreted by sandcastle worms and mussels are explored. These biological principles are then put in a broader perspective, and synthetic adhesive systems that are based on different types of supramolecular interactions are summarized. The variety and combinations of interactions that can be used in the design of new adhesive systems are highlighted.

Polydopamine Surface Chemistry: A Decade of Discovery

Polydopamine is one of the simplest and most versatile approaches to functionalizing material surfaces, having been inspired by the adhesive nature of catechols and amines in mussel adhesive proteins. Since its first report in 2007, a decade of studies on polydopamine molecular structure, deposition conditions, and physicochemical properties have ensued. During this time, potential uses of polydopamine coatings have expanded in many unforeseen directions, seemingly only limited by the creativity of researchers seeking simple solutions to manipulating surface chemistry. In this review, we describe the current state of the art in polydopamine coating methods, describe efforts underway to uncover and tailor the complex structure and chemical properties of polydopamine, and identify emerging trends and needs in polydopamine research, including the use of dopamine analogs, nitrogen-free polyphenolic precursors, and improvement of coating mechanical properties.

Robust Coatings via Catechol-Amine Codeposition: Mechanism, Kinetics, and Application

Bioinspired polyphenol/polyamine codeposition has been demonstrated by the competence for surface modification; however, the reaction processes including mechanism and kinetics remain superficially understood. In this work, the catechol (CA)-amine reaction has been thoroughly investigated by using CA and two amines m-phenylenediamine and piperazine. We verify that both primary and secondary amines are prone to link with CA through Michael addition to form polyphenol/polyamine oligomers under aerobic and mild-alkaline conditions. Molecular simulations indicate that the Michael addition products are dominant for both aromatic and aliphatic amines with CA, which supports the durable chem- and phystability of the codeposited coatings. The aggregation kinetics of polyphenol/polyamine is provided for the first time, and the formed aggregates show high-adhesive properties, which can be deposited as the skin layers for high-performance nanofiltration membranes.

Perspectives of ruthenium(ii) polyazaaromatic photo-oxidizing complexes photoreactive towards tryptophan-containing peptides and derivatives

This review focuses on recent advances in the search for Ru^II polyazaaromatic complexes as molecular photoreagents for tryptophan-containing peptides and proteins, in view of future biomedical applications.

ortho-Quinones and Analogues Thereof: Highly Reactive Intermediates for Fast and Selective Biofunctionalization

Fast, selective and facile functionalization of biologically relevant molecules is a pursuit of ever‐growing importance. A promising approach in this regard employs the high reactivity of quinone and quinone analogues for fast conjugation chemistry by nucleophilic addition or cycloadditions. Combined with in situ generation of these compounds, selective conjugation on proteins and surfaces can be uniquely induced in a time and spatially resolved manner: generation of a quinone can often be achieved by simple addition of an enzyme or stoichiometric amounts of chemoselective oxidant, or by exposure to light. In this Minireview, we discuss the generation and subsequent functionalization of quinones, iminoquinones, and quinone methides. We also discuss practical applications regarding these conjugation strategies.

Covalent Capture of Aligned Self-Assembling Nanofibers

A great deal of effort has been invested in the design and characterization of systems which spontaneously assemble into nanofibers. These systems are interesting for their fundamental supramolecular chemistry and have also been shown to be promising materials, particularly for biomedical applications. Multidomain peptides are one such assembler, and in previous work we have demonstrated the reversibility of their assembly under mild and easily controlled conditions, along with their utility for time-controlled drug delivery, protein delivery, cell encapsulation, and cell delivery applications. Additionally, their highly compliant criteria for sequence selection allows them to be modified to incorporate protease susceptibility and biological-recognition motifs for cell adhesion and angiogenesis. However, control of their assembly has been limited to the formation of disorganized nanofibers. In this work, we expand our ability to manipulate multidomain-peptide assembly into parallel-aligned fiber bundles. Albeit this alignment is achieved by the shearing forces of syringe delivery, it is also dependent on the amino acid sequence of the multidomain peptide. The incorporation of the amino acid DOPA (3,4-dihydroxyphenylalanine) allows the self-assembled nanofibers to form an anisotropic hydrogel string under modest shear stress. The hydrogel string shows remarkable birefringence, and highly aligned nanofibers are visible in scanning electronic microscopy. Furthermore, the covalent linkage induced by DOPA oxidation allows covalent capture of the aligned nanofiber bundles, enhancing their birefringence and structural integrity.

ortho-Methoxyphenols as Convenient Oxidative Bioconjugation Reagents with Application to Site-Selective Heterobifunctional Cross-Linkers

The synthesis of complex protein-based bioconjugates has been facilitated greatly by recent developments in chemoselective methods for biomolecular modification. The oxidative coupling of o-aminophenols or catechols with aniline functional groups is chemoselective, mild, and rapid; however, the oxidatively sensitive nature of the electron-rich aromatics and the paucity of commercial sources pose some obstacles to the general use of these reactive strategies. Herein, we identify o-methoxyphenols as air-stable, commercially available derivatives that undergo efficient oxidative couplings with anilines in the presence of periodate as oxidant. Mechanistic considerations informed the development of a preoxidation protocol that can greatly reduce the amount of periodate necessary for effective coupling. The stability and versatility of these reagents was demonstrated through the synthesis of complex protein-protein bioconjugates using a site-selective heterobifunctional cross-linker comprising both o-methoxyphenol and 2-pyridinecarboxaldehyde moieties. This compound was used to link epidermal growth factor to genome-free MS2 viral capsids, affording nanoscale delivery vectors that can target a variety of cancer cell types.

Microwave-Accelerated Rapid, Chemical Oxidant-Free, Material-Independent Surface Chemistry of Poly(dopamine)

A simple strategy for the rapid preparation of multifunctional polydopamine (pDA) coatings is demonstrated. Microwave irradiation of the coating solution enables the formation of a ≈18 nm thick, genuine pDA coating in 15 min, which is ≈18 times faster than conventional coating. The acceleration effect results from the radical generation and temperature increase, which facilitate thermally accelerated radical polymerization of dopamine.

Thiazolidine-Masked α-Oxo Aldehyde Functionality for Peptide and Protein Modification

α-Oxo aldehyde-based bioconjugation chemistry has been widely explored in peptide and protein modifications for various applications in biomedical research during the past decades. The generation of α-oxo aldehyde via sodium periodate oxidation is usually limited to the N-terminus of a target protein. Internal-site functionalization of proteins with the α-oxo aldehyde handle has not been achieved yet. Herein we report a novel method for site-specific peptide and protein modification using synthetically or genetically incorporated thiazolidine-protected α-oxo aldehyde. Efficient unmasking of the aldehyde was achieved by silver ion-mediated hydrolysis of thiazolidine under mild conditions for the first time. A model peptide and a recombinant protein were used to demonstrate the utility of this new method, which were site-specifically modified by oxime ligation with an oxyamine-functionalized peptide labeling reagent. Therefore, our current method has enriched the α-oxo aldehyde synthetic tool box in peptide and protein bioconjugation chemistry and holds great potential to be explored in novel applications in the future.

Reaction Pathways in Catechol/Primary Amine Mixtures: A Window on Crosslinking Chemistry

Catechol chemistry is used as a crosslinking tool abundantly in both natural organisms (e.g. mussels, sandcastle worms) and synthetic systems to achieve the desired mechanical properties. Despite this abundance and success, the crosslinking chemistry is still poorly understood. In this study, to simplify the system, yet to capture the essential chemistry, model compounds 4-methyl catechol and propylamine are used. The reaction of 4-methyl catechol (2 mM) with propylamine (6 mM) is carried out in the presence of NaIO4 (2 mM) in 10 mM Na2CO3 aqueous solution. A variety of spectroscopic/spectrometric and chromatographic methods such as 1H NMR, LC-MS, and UV-VIS are used to track the reaction and identify the products/intermediates. It is found that the crosslinking chemistry of a catechol and an amine is both fast and complicated. Within five minutes, more than 60 products are formed. These products encompass 19 different masses ranging from molecular weight of 179 to 704. By combining time-dependent data, it is inferred that the dominant reaction pathways: the majority is formed via aryloxyl-phenol coupling and Michael-type addition, whereas a small fraction of products is formed via Schiff base reactions.

Chemical Tools To Monitor and Manipulate Adaptive Immune Responses

Methods to monitor and manipulate the immune system are of enormous clinical interest. For example, the development of vaccines represents one of the earliest and greatest accomplishments of the biomedical research enterprise. More recently, drugs capable of "reawakening" the immune system to cancer have generated enormous excitement. But, much remains to be done. All drugs available today that manipulate the immune system cannot distinguish between "good" and "bad" immune responses and thus drive general and systemic immune suppression or activation. Indeed, with the notable exception of vaccines, our ability to monitor and manipulate antigen-specific immune responses is in its infancy. Achieving this finer level of control would be highly desirable. For example, it might allow the pharmacological editing of pathogenic immune responses without restricting the ability of the immune system to defend against infection. On the diagnostic side, a method to comprehensively monitor the circulating, antigen-specific antibody population could provide a treasure trove of clinically useful biomarkers, since many diseases expose the immune system to characteristic molecules that are deemed foreign and elicit the production of antibodies against them. This Perspective will discuss the state-of-the-art of this area with a focus on what we consider seminal opportunities for the chemistry community to contribute to this important field.

Bio-inspired adhesive catechol-conjugated chitosan for biomedical applications: A mini review

The development of adhesive materials, such as cyanoacrylate derivatives, fibrin glues, and gelatin-based adhesives, has been an emerging topic in biomaterial science because of the many uses of these materials, including in wound healing patches, tissue sealants, and hemostatic materials. However, most bio-adhesives exhibit poor adhesion to tissue and related surfaces due to the presence of body fluid. For a decade, studies have aimed at addressing this issue by developing wet-resistant adhesives. Mussels demonstrate robust wet-resistant adhesion despite the ceaseless waves at seashores, and mussel adhesive proteins play a key role in this adhesion. Adhesive proteins located at the distal end (i.e., those that directly contact surfaces) are composed of nearly 60% of amino acids called 3,4-dihydroxy-l-phenylalanine (DOPA), lysine, and histidine, which contain side chains of catechol, primary amines, and secondary amines, respectively. Inspired by the abundant catecholamine in mussel adhesive proteins, researchers have developed various types of polymeric mimics, such as polyethylenimine-catechol, chitosan-catechol, and other related catecholic polymers. Among them, chitosan-catechol is a promising adhesive polymer for biomedical applications. The conjugation of catechol onto chitosan dramatically increases its solubility from zero to nearly 60mg/mL (i.e., 6% w/v) in pH 7 aqueous solutions. The enhanced solubility maximizes the ability of catecholamine to behave similar to mussel adhesive proteins. Chitosan-catechol is biocompatible and exhibits excellent hemostatic ability and tissue adhesion, and thus, chitosan-catechol will be widely used in a variety of medical settings in the future. This review focuses on the various aspects of chitosan-catechol, including its (1) preparation methods, (2) physicochemical properties, and (3) current applications.

High-performance mussel-inspired adhesives of reduced complexity

Despite the recent progress in and demand for wet adhesives, practical underwater adhesion remains limited or non-existent for diverse applications. Translation of mussel-inspired wet adhesion typically entails catechol functionalization of polymers and/or polyelectrolytes, and solution processing of many complex components and steps that require optimization and stabilization. Here we reduced the complexity of a wet adhesive primer to synthetic low-molecular-weight catecholic zwitterionic surfactants that show very strong adhesion (∼50 mJ m⁻²) and retain the ability to coacervate. This catecholic zwitterion adheres to diverse surfaces and self-assembles into a molecularly smooth, thin (<4 nm) and strong glue layer. The catecholic zwitterion holds particular promise as an adhesive for nanofabrication. This study significantly simplifies bio-inspired themes for wet adhesion by combining catechol with hydrophobic and electrostatic functional groups in a small molecule.

Mussels use strong filaments to adhere to rocks, preventing them from being swept away in strong currents. Here, the authors borrow and simplify chemistries from the mussel foot to create a one component adhesive system which holds potential for employment in nanofabrication protocols.

Peptide Length and Dopa Determine Iron-Mediated Cohesion of Mussel Foot Proteins

Mussel adhesion to mineral surfaces is widely attributed to 3,4-dihydroxyphenylalanine (Dopa) functionalities in the mussel foot proteins (mfps). Several mfps, however, show a broad range (30-100%) of Tyrosine (Tyr) to Dopa conversion suggesting that Dopa is not the only desirable outcome for adhesion. Here, we used a partial recombinant construct of mussel foot protein-1 (rmfp-1) and short decapeptide dimers with and without Dopa and assessed both their cohesive and adhesive properties on mica using a surface forces apparatus (SFA). Our results demonstrate that at low pH, both the unmodified and Dopa-containing rmfp-1s show similar energies for adhesion to mica and self-self interaction. Cohesion between two Dopa-containing rmfp-1 surfaces can be doubled by Fe3+ chelation, but remains unchanged with unmodified rmfp-1. At the same low pH, the Dopa modified short decapeptide dimer did not show any change in cohesive interactions even with Fe3+. Our results suggest that the most probable intermolecular interactions are those arising from electrostatic (i.e., cation-π) and hydrophobic interactions. We also show that Dopa in a peptide sequence does not by itself mediate Fe3+ bridging interactions between peptide films: peptide length is a crucial enabling factor.

Hydrolytically Stable Site-Specific Conjugation at the N-Terminus of an Engineered Antibody

Antibody-drug conjugates (ADCs) have emerged as an important class of therapeutics for cancer treatment that combine the target specificity of antibodies with the killing activity of anticancer chemotherapeutics. Early conjugation technologies relied upon random conjugation to either lysine or cysteine residues, resulting in heterogeneous ADCs. Recent technology advancements have resulted in the preparation of homogeneous ADCs through the site-specific conjugation at engineered cysteines, glycosylated amino acids, and bioorthogonal unnatural amino acids. Here we describe for the first time the conjugation of an anti-mitotic drug to an antibody following the mild and selective oxidation of a serine residue engineered at the N-terminus of the light chain. Using an alkoxyamine-derivatized monomethyl auristatine E payload, we have prepared a hydrolytically stable ADC that retains binding to its antigen and displays potent in vitro cytotoxicity and in vivo tumor growth inhibition.

Development of oxidative coupling strategies for site-selective protein modification

As the need to prepare ever more complex but well-defined materials has increased, a similar need for reliable synthetic strategies to access them has arisen. Accordingly, recent years have seen a steep increase in the development of reactions that can proceed under mild conditions, in aqueous environments, and with low concentrations of reactants. To enable the preparation of well-defined biomolecular materials with novel functional properties, our laboratory has a continuing interest in developing new bioconjugation reactions. A particular area of focus has been the development of oxidative reactions to perform rapid site- and chemoselective couplings of electron rich aromatic species with both unnatural and canonical amino acid residues. This Account details the evolution of oxidative coupling reactions in our laboratory, from initial concepts to highly efficient reactions, focusing on the practical aspects of performing and developing reactions of this type. We begin by discussing our rationale for choosing an oxidative coupling approach to bioconjugation, highlighting many of the benefits that such strategies provide. In addition, we discuss the general workflow we have adopted to discover protein modification reactions directly in aqueous media with biologically relevant substrates. We then review our early explorations of periodate-mediated oxidative couplings between primary anilines and p-phenylenediamine substrates, highlighting the most important lessons that were garnered from these studies. Key mechanistic insights allowed us to develop second-generation reactions between anilines and anisidine derivatives. In addition, we summarize the methods we have used for the introduction of aniline groups onto protein substrates for modification. The development of an efficient and chemoselective coupling of anisidine derivatives with tyrosine residues in the presence of ceric ammonium nitrate is next described. Here, our logic and workflow are used to highlight the challenges and opportunities associated with the optimization of site-selective chemistries that target native amino acids. We close by discussing the most recent reports from our laboratory that have capitalized on the unique reactivity of o-iminoquinone derivatives. We discuss the various oxidants and conditions that can be used to generate these reactive intermediates from appropriate precursors, as well as the product distributions that result. We also describe our work to determine the nature of iminoquinone reactivity with proteins and peptides bearing free N-terminal amino groups. Through this discussion, we hope to facilitate the use of oxidative approaches to protein bioconjugation, as well as inspire the discovery of new reactions for the site-selective modification of biomolecular targets.

Microphase Behavior and Enhanced Wet-Cohesion of Synthetic Copolyampholytes Inspired by a Mussel Foot Protein

Numerous attempts have been made to translate mussel adhesion to diverse synthetic platforms. However, the translation remains largely limited to the Dopa (3,4-dihydroxyphenylalanine) or catechol functionality, which continues to raise concerns about Dopa's inherent susceptibility to oxidation. Mussels have evolved adaptations to stabilize Dopa against oxidation. For example, in mussel foot protein 3 slow (mfp-3s, one of two electrophoretically distinct interfacial adhesive proteins in mussel plaques), the high proportion of hydrophobic amino acid residues in the flanking sequence around Dopa increases Dopa's oxidation potential. In this study, copolyampholytes, which combine the catechol functionality with amphiphilic and ionic features of mfp-3s, were synthesized and formulated as coacervates for adhesive deposition on surfaces. The ratio of hydrophilic/hydrophobic as well as cationic/anionic units was varied in order to enhance coacervate formation and wet adhesion properties. Aqueous solutions of two of the four mfp-3s-inspired copolymers showed coacervate-like spherical microdroplets (ϕ ≈ 1-5 μm at pH ∼4 (salt concentration ∼15 mM). The mfp-3s-mimetic copolymer was stable to oxidation, formed coacervates that spread evenly over mica, and strongly bonded to mica surfaces (pull-off strength: ∼17.0 mJ/m(2)). Increasing pH to 7 after coacervate deposition at pH 4 doubled the bonding strength to ∼32.9 mJ/m(2) without oxidative cross-linking and is about 9 times higher than native mfp-3s cohesion. This study expands the scope of translating mussel adhesion from simple Dopa-functionalization to mimicking the context of the local environment around Dopa.