Single-step azide introduction in proteins via an aqueous diazo transfer

Radical-Mediated Covalent Azidylation of Hydrophobic Microdomains in Water-Soluble Proteins

Hydrophobic microdomains, also known as hydrophobic patches, are essential for many important biological functions of water-soluble proteins. These include ligand or substrate binding, protein-protein interactions, proper folding after translation, and aggregation during denaturation. Unlike transmembrane domains, which are easily recognized from stretches of contiguous hydrophobic sidechains in amino acids via primary protein sequence, these three-dimensional hydrophobic patches cannot be easily predicted. The lack of experimental strategies for directly determining their locations hinders further understanding of their structure and function. Here, we posit that the small triatomic anion N3- (azide) is attracted to these patches and, in the presence of an oxidant, forms a radical that covalently modifies C-H bonds of nearby amino acids. Using two model proteins (BSA and lysozyme) and a cell-free lysate from the model higher plant Arabidopsis thaliana, we find that radical-mediated covalent azidylation occurs within buried catalytic active sites and ligand binding sites and exhibits similar behavior to established hydrophobic probes. The results herein suggest a model in which the azido radical is acting as an "affinity reagent" for nonaqueous three-dimensional protein microenvironments and is consistent with both the nonlocalized electron density of the azide moiety and the known high reactivity of azido radicals widely used in organic chemistry syntheses. We propose that the azido radical is a facile means of identifying hydrophobic microenvironments in soluble proteins and, in addition, provides a simple new method for attaching chemical handles to proteins without the need for genetic manipulation or specialized reagents.

Highly selective preparation of N-terminus Horseradish peroxidase-DNA conjugate with fully retained enzymatic activity: HRP-DNA structure - activity relation

Within the last decade, the field of bio-nanoengineering has achieved significant advances allowing us to generate, e.g., nanoscaled molecular machineries with arbitrary shapes. To unleash the full potential of novel methods such as DNA origami technology, it is important to functionalise complex molecules and nanostructures precisely. Thus, considerable attention has been given to site-selective modifications of proteins allowing further incorporation of various functionalities. Here, we describe a method for the covalent attachment of oligonucleotides to the glycosylated horseradish peroxidase protein (HRP) with high N-terminus selectivity and significant yield while conserving the enzymatic activity. This two-step process includes a pH-controlled metal-free diazotransfer reaction using imidazole-1-sulfonyl azide hydrogen sulfate, which at pH 8.5 results in an N-terminal azide-functionalized protein, followed by the Cu-free click SPAAC reaction to dibenzocyclooctyne- (DBCO) modified oligonucleotides. The reaction conditions were optimised to achieve maximum yield and the best performance. The resulting protein-oligonucleotide conjugates (HRP-DNA) were characterised by electrophoresis and mass spectrometry (MS). Native-PAGE experiments demonstrated different migration patterns for HRP-DNA and the azido-modified protein allowing zymogram experiments. Structure-activity relationships of novel HRP-DNA conjugates were assessed using molecular dynamics simulations, characterising the molecular interactions that define the structural and dynamical properties of the obtained protein-oligonucleotide conjugates (POC).

Chemoselective restoration of para-azido-phenylalanine at multiple sites in proteins

The site-specific incorporation of nonstandard amino acids (nsAAs) during translation has expanded the chemistry and function of proteins. The nsAA para-azido-phenylalanine (pAzF) encodes a biorthogonal chemical moiety that facilitates "click" reactions to attach diverse chemical groups for protein functionalization. However, the azide moiety is unstable in physiological conditions and is reduced to para-amino-phenylalanine (pAF). Azide reduction decreases the yield of pAzF residues in proteins to 50%-60% per azide and limits protein functionalization by click reactions. Here, we describe the use of a pH-tunable diazotransfer reaction that converts pAF to pAzF at >95% efficiency in proteins. The method selectively restores pAzF at multiple sites per protein without introducing off-target modifications. This work addresses a key limitation in the production of pAzF-containing proteins by restoring azides for multi-site functionalization with diverse chemical moieties, setting the stage for the production of genetically encoded biomaterials with broad applications in biotherapeutics, materials science, and biotechnology.

Improved Diazo-Transfer Reaction for DNA-Encoded Chemistry and Its Potential Application for Macrocyclic DEL-Libraries

DNA-encoded libraries (DEL) are increasingly being used to identify new starting points for medicinal chemistry in drug discovery. Herein, we discuss the development of methods that allow the conversion of both primary amines and anilines, attached to DNA, to their corresponding azides in excellent yields. The scope of these diazo-transfer reactions was investigated, and a proof-of-concept has been devised to allow for the synthesis of macrocycles on DNA.

Guiding Oligodendrocyte Precursor Cell Maturation With Urokinase Plasminogen Activator-Degradable Elastin-like Protein Hydrogels

Demyelinating injuries and diseases, like multiple sclerosis, affect millions of people worldwide. Oligodendrocyte precursor cells (OPCs) have the potential to repair demyelinated tissues because they can both self-renew and differentiate into oligodendrocytes (OLs), the myelin producing cells of the central nervous system (CNS). Cell-matrix interactions impact OPC differentiation into OLs, but the process is not fully understood. Biomaterial hydrogel systems help to elucidate cell-matrix interactions because they can mimic specific properties of native CNS tissues in an in vitro setting. We investigated whether OPC maturation into OLs is influenced by interacting with a urokinase plasminogen activator (uPA) degradable extracellular matrix (ECM). uPA is a proteolytic enzyme that is transiently upregulated in the developing rat brain, with peak uPA expression correlating with an increase in myelin production in vivo. OPC-like cells isolated through the Mosaic Analysis with Double Marker technique (MADM OPCs) produced low-molecular-weight uPA in culture. MADM OPCs were encapsulated into two otherwise similar elastin-like protein (ELP) hydrogel systems: one that was uPA degradable and one that was nondegradable. Encapsulated MADM OPCs had similar viability, proliferation, and metabolic activity in uPA degradable and nondegradable ELP hydrogels. Expression of OPC maturation-associated genes, however, indicated that uPA degradable ELP hydrogels promoted MADM OPC maturation although not sufficiently for these cells to differentiate into OLs.

Azido-functionalized gelatin via direct conversion of lysine amino groups by diazo transfer as a building block for biofunctional hydrogels

Gelatin is one of the most prominent biopolymers in biomedical material research and development. It is frequently used in hybrid hydrogels, which combine the advantageous properties of bio-based and synthetic polymers. To prevent the biological component from leaching out of the hydrogel, the biomolecules can be equipped with azides. Those groups can be used to immobilize gelatin covalently in hydrogels by the highly selective and specific azide-alkyne cycloaddition. In this contribution, we functionalized gelatin with azides at its lysine residues by diazo transfer, which offers the great advantage of only minimal side-chain extension. Approximately 84-90% of the amino groups are modified as shown by ¹ H-NMR spectroscopy, 2,4,6-trinitrobenzenesulfonic acid assay as well as Fourier-transform infrared spectroscopy, rheology, and the determination of the isoelectric point. Furthermore, the azido-functional gelatin is incorporated into hydrogels based on poly(ethylene glycol) diacrylate (PEG-DA) at different concentrations (0.6, 3.0, and 5.5%). All hydrogels were classified as noncyctotoxic with significantly enhanced cell adhesion of human fibroblasts on their surfaces compared to pure PEG-DA hydrogels. Thus, the new gelatin derivative is found to be a very promising building block for tailoring the bioactivity of materials.

Using a Membrane-Penetrating-Peptide to Anchor Ligands in the Liposome Membrane Facilitates Targeted Drug Delivery

Antimicrobial peptides (AMPs) are typical cell penetrating peptides (CPPs) that intercalate into biomembranes and exhibit broad activities. We designed a triple fusion protein consisting of an AMP, Ib-AMP4 at the N-terminus, a fluorescent GFP probe in the center, and the tumor-targeting peptide P1c at the other terminus. After purification from E. coli, the interaction between the Ib-AMP4-GFP-P1c fusion protein (IGP) and the lipid membrane was characterized. Experiments using isothermal titration calorimetry (ITC) and quartz crystal microbalance with dissipation (QCM-D) demonstrated that IGP proteins spontaneously bound the lipid bilayer with a maximal molar ratio of 1:52 (protein:lipid). Furthermore, transmission electron microscopy (TEM) confirmed that the IGP protein was present in the liposome membrane. After decoration with IGP proteins, the DOPC:DOPG liposomes were applied to cancer cells. Microscopy and flow cytometry reveal that the decorated liposomes selectively bound integrin αvβ3-positive A549 cells. In addition, compared with the common chemical conjugation method, the reported method seemed to be superior in certain aspects, such as simple sample preparation and cost-effectiveness. Next, the IGP protein was applied to decorate red blood cell (RBC) liposomes for targeted delivery in both in vitro and in vivo applications. The IGP-decorated RBC liposomes preferentially targeted integrin αvβ3 expressing A549 cancer cells. The in vivo imaging showed that IGP-decorated RBC liposomes were concentrated in tumor tissue and were primarily metabolized by the liver and kidney.

Polyester-based ink platform with tunable bioactivity for 3D printing of tissue engineering scaffolds

In this work, we synthesized a novel polymeric biomaterial platform with tunable functionalizability for extrusion-based 3D printing. Biodegradable polymers were synthesized using 4-hydroxyphenethyl 2-(4-hydroxyphenyl)acetate (HTy), which is derived from Tyrosol and 2-(4-hydroxyphenyl)acetic acid. p-Phenylenediacetic acid (PDA) was introduced to enhance crystallinity. To enable functionalizability without deteriorating printability, glutamic acid derivatives were introduced into the polymer design, forming copolymers including poly(HTy-co-45%PDA-co-5%Gluhexenamide ester) (HP5GH), poly(HTy-co-45%PDA-co-5%Glupentynamide ester) (HP5GP), and poly(HTy-co-45%PDA-co-5%BocGlu ester) (HP5BG). The resulting polymers have: two melting temperatures (125-131 °C and 141-147 °C), Young's moduli of 1.9-2.4 GPa, and print temperatures of 170-190 °C. The molecular weight (Mw) loss due to hydrolytic degradation was gradual with ∼30% Mw retained after 25 weeks for HP5BG, whereas it was much faster for HP5GP and HP5GH with only 18% Mw retained after 8 weeks. HP5GH and HP5GP were successfully functionalized in solution (bulk) or on the surface using click-based chemistry. Finally, the utilization of this novel platform was demonstrated by studying osteogenic differentiation of human mesenchymal stem cells (hMSCs) using 3D printed scaffolds from HP5GP. Scaffolds were functionalized with azide-Heparin (az-Heparin) to bind and deliver bone morphogenetic protein 2 (BMP-2). This sample group significantly enhanced osteogenic differentiation of hMSCs as compared to unfunctionalized scaffolds incubated directly with az-Heparin or BMP-2 prior to cell culture. This novel polymer platform with tunable functionalizability could be utilized for additive manufacturing of biodegradable devices and scaffolds with tailored mechanical and bioactive properties for a wide range of medical applications including bone fixation devices and scaffolds for bone regeneration.

Bile Acid Binding Protein Functionalization Leads to a Fully Synthetic Rhodopsin Mimic

Rhodopsins are photoreceptive proteins using light to drive a plethora of biological functions such as vision, proton and ion pumping, cation and anion channeling, and gene and enzyme regulation. Here we combine organic synthesis, NMR structural studies, and photochemical characterization to show that it is possible to prepare a fully synthetic mimic of rhodopsin photoreceptors. More specifically, we conjugate a bile acid binding protein with a synthetic mimic of the rhodopsin protonated Schiff base chromophore to achieve a covalent complex featuring an unnatural protein host, photoswitch, and photoswitch-protein linkage with a reverse orientation. We show that, in spite of its molecular-level diversity, light irradiation of the prepared mimic fuels a photochromic cycle driven by sequential photochemical and thermal Z/E isomerizations reminiscent of the photocycles of microbial rhodopsins.

Site-Selective Synthesis of Insulin Azides and Bioconjugates

A synthetic method to access novel azido-insulin analogs directly from recombinant human insulin (RHI) was developed via diazo-transfer chemistry using imidazole-1-sulfonyl azide. Systematic optimization of reaction conditions led to site-selective azidation of amino acids B1-phenylalanine and B29-lysine present in RHI. Subsequently, the azido-insulin analogs were used in azide-alkyne [3 + 2] cycloaddition reactions to synthesize a diverse array of triazole-based RHI bioconjugates that were found to be potent human insulin receptor binders. The utility of this method was further demonstrated by the concise and controlled synthesis of a heterotrisubstituted insulin conjugate.

Bacillus subtilis MraY in detergent-free system of nanodiscs wrapped by styrene-maleic acid copolymers

As an integral membrane protein, purification and characterization of phospho-N- acetylmuramyl- pentapeptide translocase MraY have proven difficult. Low yield and concerns of retaining stability and activity after detergent solubilization have hampered the structure-function analysis. The recently developed detergent-free styrene-maleic acid (SMA) co-polymer system offers an alternative approach that may overcome these disadvantages. In this study, we used the detergent free system to purify MraY from Bacillus subtilis. This allowed efficient extraction of MraY that was heterologously produced in Escherichia coli membranes into SMA-wrapped nanodiscs. The purified MraY embedded in these nanodiscs (SMA-MraY) was comparable to the micellar MraY extracted with a conventional detergent (DDM) with regard to the yield and the purity of the recombinant protein but required significantly less time. The predominantly alpha-helical secondary structure of the protein in SMA-wrapped nanodiscs was also more stable against heat denaturation compared to the micellar protein. Thus, this detergent-free system is amenable to extract MraY efficiently and effectively while maintaining the biophysical properties of the protein. However, the apparent activity of the SMA-MraY was reduced compared to that of the detergent-solubilized protein. The present data indicates that this is caused by a lower accessibility of the enzyme in SMA-wrapped nanodiscs towards its polyisoprenoid substrate.

Synthesis of Modular Brush Polymer-Protein Hybrids Using Diazotransfer and Copper Click Chemistry

Proteoglycans are important brush-like biomacromolecules, which serve a variety of functions in the human body. While protein-bottlebrush hybrids are promising proteoglycan mimics, many challenges still exist to robustly produce such polymers. In this paper, we report the modular synthesis of protein-brush hybrids containing elastin-like polypeptides (ELP) as model proteins by copper-catalyzed azide-alkyne cycloaddition. We exploit the recently discovered imidazole-1-sulfonyl azide (ISA) in a diazotransfer reaction to introduce an N-terminal azide onto an ELP. Next, we use a click reaction to couple the azido-ELP to an alkyne-terminated amine-rich polymer followed by a second diazotransfer step to produce an azide-rich backbone that serves as a scaffold. Finally, we used a second click reaction to graft alkyne-terminated poly(oligoethylene glycol methacrylate) (POEGMA) bristles to the azide-rich backbone to produce the final protein-bottlebrush hybrid. We demonstrate the effectiveness of this synthetic path at each step through careful characterization with ¹H NMR, FTIR, GPC, and diagnostic test reactions on SDS-PAGE. Final reaction products could be consistently obtained for a variety of different molecular weight backbones with final total grafting efficiencies around 70%. The high-yielding reactions employed in this highly modular approach allow for the synthesis of protein-bottlebrush hybrids with different proteins and brush polymers. Additionally, the mild reaction conditions used have the potential to avoid damage to proteins during synthesis.

Site-Selective Functionalization of Flagellin by Steric Self-Protection: A Strategy To Facilitate Flagellin as a Self-Adjuvanting Carrier in Conjugate Vaccine

Flagellin (FliC) can act as a carrier protein in the preparation of conjugate vaccines to elicit a T-cell-dependent immune response and as an intrinsic adjuvant to activate the toll-like receptor 5 (TLR5) to enhance vaccine potency. To enable the use of FliC as a self-adjuvanting carrier, an effective method for site-selective modification (SSM) of pertinent amino-acid residues in the D2 and D3 domains of FliC is explored without excessive modification of the D0 and D1 domains, which are responsible for activating and binding with TLR5. In highly concentrated Na₂ SO₄ solution, FliC monomers form flagellar filaments, in which the D0 and D1 domains are situated inside the tubular structure. Thus, the lysine residues (K219, K224, K324, and K331) in the D2 and D3 domains of flagellin are selectively modified by a diazo-transfer reaction with imidazole-1-sulfonyl azide. The sites with azido modification are confirmed by MALDI-TOF-MS, ESI-TOF-MS, and LC-MS/MS analyses along with label-free quantitation. The azido-modified filament dissolves to give FliC monomers, which can conjugate with alkyne-hinged saccharides by the click reaction. Transmission electron microscopy imaging, dynamic light scattering measurements, and the secreted embryonic alkaline phosphatase reporter assay indicate that the modified FliC monomers retain the ability either to bind with TLR5 or to reassemble into filaments. Overall, this study establishes a feasible method for the SSM of FliC by steric self-protection of the D0 and D1 domains.

Modular, Bioorthogonal Strategy for the Controlled Loading of Cargo into a Protein Nanocage

Virus capsids, i.e., viruses devoid of their genetic material, are suitable nanocarriers for biomedical applications such as drug delivery and diagnostic imaging. For this purpose, the reliable encapsulation of cargo in such a protein nanocage is crucial, which can be accomplished by the covalent attachment of the compounds of interest to the protein domains positioned at the interior of the cage. This approach is particularly valid for the capsid proteins of the cowpea chlorotic mottle virus (CCMV), which have their N-termini located at the inside of the capsid structure. Here, we examined several site-selective modification methods for covalent attachment and encapsulation of cargo at the N-terminus of the CCMV protein. Initially, we explored approaches to introduce an N-terminal azide functionality, which would allow the subsequent bioorthogonal modification with a strained alkyne to attach the desired cargo. As these methods showed compatibility issues with the CCMV capsid proteins, a strategy based on 2-pyridinecarboxaldehydes for site-specific N-terminal protein modification was employed. This method allowed the successful modification of the proteins, and was applied for the introduction of a bioorthogonal vinylboronic acid moiety. In a subsequent reaction, the proteins could be modified further with a fluorophore using the tetrazine ligation. The application of capsid assembly conditions on the functionalized proteins led to successful particle formation, showing the potential of this covalent encapsulation strategy.

Targeted Diazotransfer Reagents Enable Selective Modification of Proteins with Azides

In chemical biology, azides are used to chemically manipulate target structures in a bioorthogonal manner for a plethora of applications ranging from target identification to the synthesis of homogeneously modified protein conjugates. While a variety of methods have been established to introduce the azido group into recombinant proteins, a method that directly converts specific amino groups in endogenous proteins is lacking. Here, we report the first biotin-tethered diazotransfer reagent DtBio and demonstrate that it selectively modifies the model proteins streptavidin and avidin and the membrane protein BioY on cell surface. The reagent converts amines in the proximity of the binding pocket to azides and leaves the remaining amino groups in streptavidin untouched. Reagents of this novel class will find use in target identification as well as the selective functionalization and bioorthogonal protection of proteins.

Synthetic Aspects of Peptide– and Protein–Polymer Conjugates in the Post-click Era

Biomacromolecules offer complex and precise functions embedded in their monomer sequence such as enzymatic activity or specific interactions towards other molecules. Their informational content and capability to organize in higher ordered structures is superior to those of synthetic molecules. In comparison, synthetic polymers are easy to access even at large production scales and they are chemically more diverse. Solubilization, shielding against enzymatic degradation to more advanced functions like switchability or protein mimicry, etc., are accessible through the world of polymer chemistry. Bio-inspired hybrid materials consisting of peptides or proteins and synthetic polymers thereby combine the properties of both molecules to give rise to a new class of materials with unique characteristics and performance. To obtain well-defined bioconjugate materials, high yielding and site-specific as well as biorthogonal ligation techniques are mandatory. Since the first attempts of protein PEGylation in the 1970s and the concept of “click” chemistry arising in 2001, continuous progress in the field of peptide– and protein–polymer conjugate preparation has been gained. Herein, we provide an overview on ligation techniques to prepare functional bioconjugates published in the last decade, also referred to as “post-click” methods. Furthermore, chemoenzymatic approaches and biotransformation reactions used in peptide or protein modification, as well as highly site-specific and efficient reactions originated in synthetic macromolecular chemistry, which could potentially be adapted for bioconjugation, are presented. Finally, future perspectives for the preparation and application of bioconjugates at the interface between biology and synthetic materials are given.

Bio-Orthogonally Crosslinked, Engineered Protein Hydrogels with Tunable Mechanics and Biochemistry for Cell Encapsulation

Covalently-crosslinked hydrogels are commonly used as 3D matrices for cell culture and transplantation. However, the crosslinking chemistries used to prepare these gels generally cross-react with functional groups present on the cell surface, potentially leading to cytotoxicity and other undesired effects. Bio-orthogonal chemistries have been developed that do not react with biologically relevant functional groups, thereby preventing these undesirable side reactions. However, previously developed biomaterials using these chemistries still possess less than ideal properties for cell encapsulation, such as slow gelation kinetics and limited tuning of matrix mechanics and biochemistry. Here, engineered elastin-like proteins (ELPs) are developed that cross-link via strain-promoted azide-alkyne cycloaddition (SPAAC) or Staudinger ligation. The SPAAC-crosslinked materials form gels within seconds and complete gelation within minutes. These hydrogels support the encapsulation and phenotypic maintenance of human mesenchymal stem cells, human umbilical vein endothelial cells, and murine neural progenitor cells. SPAAC-ELP gels exhibit independent tuning of stiffness and cell adhesion, with significantly improved cell viability and spreading observed in materials containing a fibronectin-derived arginine-glycine-aspartic acid (RGD) domain. The crosslinking chemistry used permits further material functionalization, even in the presence of cells and serum. These hydrogels are anticipated to be useful in a wide range of applications, including therapeutic cell delivery and bioprinting.

Encapsulated Hydrogels by E-beam Lithography and Their Use in Enzyme Cascade Reactions

Electron beam (e-beam) lithography was employed to prepare one protein immobilized hydrogel encapsulated inside another by first fabricating protein-reactive hydrogels of orthogonal reactivity and subsequently conjugating the biomolecules. Exposure of thin films of eight arm star poly(ethylene glycol) (PEG) functionalized with biotin (Biotin-PEG), alkyne (Alkyne-PEG) or aminooxy (AO-PEG) end-groups to e-beam radiation resulted in cross-linked hydrogels with the respective functionality. It was determined via confocal microscopy that a nominal size exclusion effect exists for streptavidin immobilized on Biotin-PEG hydrogels of feature sizes ranging from 5 to 40 μm. AO-PEG was subsequently patterned as an encapsulated core inside a contiguous outer shell of Biotin-PEG. Similarly, Alkyne-PEG was patterned as a core inside an AO-PEG shell. The hydrogel reactive end-groups were conjugated to dyes or proteins of complementary reactivity, and the three-dimensional (3-D) spatial orientation was determined for both configurations using confocal microscopy. The enzyme glucose oxidase (GOX) was immobilized in the core of the encapsulated Alkyne-PEG core/ AO-PEG shell architecture, and horseradish peroxidase (HRP) was conjugated to the shell periphery. Bioactivity for the HRP-GOX enzyme pair was observed in this encapsulated configuration by demonstrating that the enzyme pair was capable of enzyme cascade reactions.

CuAAC: An Efficient Click Chemistry Reaction on Solid Phase

Click chemistry is an approach that uses efficient and reliable reactions, such as Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC), to bind two molecular building blocks. CuAAC has broad applications in medicinal chemistry and other fields of chemistry. This review describes the general features and applications of CuAAC in solid-phase synthesis (CuAAC-SP), highlighting the suitability of this kind of reaction for peptides, nucleotides, small molecules, supramolecular structures, and polymers, among others. This versatile reaction is expected to become pivotal for meeting future challenges in solid-phase chemistry.

Azide-rich peptides via an on-resin diazotransfer reaction

Azide-containing amino acids are valuable building blocks in peptide chemistry, because azides are robust partners in several bioorthogonal reactions. Replacing polar amino acids with apolar, azide-containing amino acids in solid-phase peptide synthesis can be tricky, especially when multiple azide residues are to be introduced in the amino acid sequence. We present a strategy for effectively incorporating multiple azide-containing residues site-specifically.

Liposome functionalization with copper-free "click chemistry"

The modification of liposomal surfaces is of interest for many different applications and a variety of chemistries are available that makes this possible. A major disadvantage of commonly used coupling chemistries (e.g. maleimide-thiol coupling) is the limited control over the site of conjugation in cases where multiple reactive functionalities are present, leading to heterogeneous products and in some cases dysfunctional conjugates. Bioorthogonal coupling approaches such as the well-established copper-catalyzed azide-alkyne cycloaddition (CuAAC) "click" reaction are attractive alternatives as the reaction kinetics are favorable and azide-containing reagents are widely available. In the work described here, we prepared lipids containing a reactive cyclooctyne group and, after incorporation into liposomes, demonstrated successful conjugation of both a small molecule dye (5'-TAMRA-azide) as well as a larger azide-containing model protein based upon a designed ankyrin repeat protein (azido-DARPin). By applying the strain-promoted azido-alkyne cycloaddition (SPAAC) the use of Cu(I) as a catalyst is avoided, an important advantage considering the known deleterious effects associated with copper in cell and protein studies. We demonstrate complete control over the number of ligands coupled per liposome when using a small molecule azide with conjugation occurring at a reasonable reaction rate. By comparison, the conjugation of a larger azide-modified protein occurs more slowly, however the number of protein ligands coupled was found to be sufficient for liposome targeting to cells. Importantly, these results provide a strong proof of concept for the site-specific conjugation of protein ligands to liposomal surfaces via SPAAC. Unlike conventional approaches, this strategy provides for the homogeneous coupling of proteins bearing a single site-specific azide modification and eliminates the chance of forming dysfunctional ligands on the liposome. Furthermore, the absence of copper in the reaction process should also make this approach much more compatible with cell-based and in vivo applications.

Synthesis and Protein Incorporation of Azido-Modified Unnatural Amino Acids

Two new azidophenylalanine residues (3 and 4) have been synthesized and, in combination with 4-azido-L-phenylalanine (1) and 4-azidomethyl-L-phenylalanine (2), form a series of unnatural amino acids (UAAs) containing the azide vibrational reporter at varying distances from the aromatic ring of phenylalanine. These UAAs were designed to probe protein hydration with high spatial resolution by utilizing the large extinction coefficient and environmental sensitivity of the azide asymmetric stretch vibration. The sensitivity of the azide reporters was investigated in solvents that mimic distinct local protein environments. Three of the four azido-modified phenylalanine residues were successfully genetically incorporated into a surface site in superfolder green fluorescent protein (sfGFP) utilizing an engineered, orthogonal aminoacyl-tRNA synthetase in response to an amber codon with high efficiency and fidelity. SDS-PAGE and ESI-Q-TOF mass analysis verified the site-specific incorporation of these UAAs. The observed azide asymmetric stretch in the linear IR spectra of these UAAs incorporated into sfGFP indicated that the azide groups were hydrated in the protein.

An immunogen synthesis strategy for the development of specific anti-deoxynivalenol monoclonal antibodies

An immunogen synthesis strategy was designed to develop anti-deoxynivalenol (DON) monoclonal antibodies with low cross-reactivity against structurally similar trichothecenes. A total of eight different DON immunogens were synthesised, differing in the type and position of the linker on the DON molecule. After immunisation, antisera from mice immunised with different DON immunogens were checked for the presence of relevant antibodies. Then, both homologous and heterologous enzyme-linked immunosorbent assays (ELISAs) were performed for hybridoma screening. Finally, three monoclonal antibodies against DON and its analogues were generated. In addition, monoclonal antibody 13H1 could recognise DON and its analogues in the order of HT-2 toxin > 15-acetyldeoxynivalenol (15-ADON) > DON, with IC₅₀ ranging from 1.14 to 2.13 µg ml⁻¹. Another monoclonal antibody 10H10 manifested relatively close sensitivities to DON, 3-acetyldeoxynivalenol (3-ADON) and 15-ADON, with IC₅₀ values of 22, 15 and 34 ng ml⁻¹, respectively. Using an indirect ELISA format decreases the 10H10 sensitivity to 15-ADON with 92%. A third monoclonal antibody 2A9 showed to be very specific and sensitive to 3-ADON, with IC₅₀ of 0.38 ng ml⁻¹. Using both 2A9 and 10H10 monoclonal antibodies allows determining sole DON contamination.

Protein-DNA chimeras for nano assembly

In synthetic biology, "understanding by building" requires exquisite control of the molecular constituents and their spatial organization. Site-specific coupling of DNA to proteins allows arrangement of different protein functionalities with emergent properties by self-assembly on origami-like DNA scaffolds or by direct assembly via Single-Molecule Cut & Paste (SMC&P). Here, we employed the ybbR-tag/Sfp system to covalently attach Coenzyme A-modified DNA to GFP and, as a proof of principle, arranged the chimera in different patterns by SMC&P. Fluorescence recordings of individual molecules proved that the proteins remained folded and fully functional throughout the assembly process. The high coupling efficiency and specificity as well as the negligible size (11 amino acids) of the ybbR-tag represent a mild, yet versatile, general and robust way of adding a freely programmable and highly selective attachment site to virtually any protein of interest.

Finding the right (bioorthogonal) chemistry

Bioorthogonal chemistries can be used to tag diverse classes of biomolecules in cells and other complex environments. With over 20 unique transformations now available, though, selecting an appropriate reaction for a given experiment is challenging. In this article, we compare and contrast the most common classes of bioorthogonal chemistries and provide a framework for matching the reactions with downstream applications. We also discuss ongoing efforts to identify novel biocompatible reactions and methods to control their reactivity. The continued expansion of the bioorthogonal toolkit will provide new insights into biomolecule networks and functions and thus refine our understanding of living systems.

Chemoselective immobilization of proteins by microcontact printing and bio-orthogonal click reactions

Herein, a combination of microcontact printing of functionalized alkanethiols and site-specific modification of proteins is utilized to chemoselectively immobilize proteins onto gold surfaces, either by oxime- or copper-catalyzed alkyne-azide click chemistry. Two molecules capable of click reactions were synthesized, an aminooxy-functionalized alkanethiol and an azide-functionalized alkanethiol, and self-assembled monolayer (SAM) formation on gold was confirmed by IR spectroscopy. The alkanethiols were then individually patterned onto gold surfaces by microcontact printing. Site-specifically modified proteins-horse heart myoglobin (HHMb) containing an N-terminal α-oxoamide and a red fluorescent protein (mCherry-CVIA) with a C-terminal alkyne-were immobilized by incubation onto respective stamped functionalized alkanethiol patterns. Pattern formation was confirmed by fluorescence microscopy.

Bioorthogonal chemistry: strategies and recent developments

The use of covalent chemistry to track biomolecules in their native environment-a focus of bioorthogonal chemistry-has received considerable interest recently among chemical biologists and organic chemists alike. To facilitate wider adoption of bioorthogonal chemistry in biomedical research, a central effort in the last few years has been focused on the optimization of a few known bioorthogonal reactions, particularly with respect to reaction kinetics improvement, novel genetic encoding systems, and fluorogenic reactions for bioimaging. During these optimizations, three strategies have emerged, including the use of ring strain for substrate activation in the cycloaddition reactions, the discovery of new ligands and privileged substrates for accelerated metal-catalysed reactions, and the design of substrates with pre-fluorophore structures for rapid "turn-on" fluorescence after selective bioorthogonal reactions. In addition, new bioorthogonal reactions based on either modified or completely unprecedented reactant pairs have been reported. Finally, increasing attention has been directed toward the development of mutually exclusive bioorthogonal reactions and their applications in multiple labeling of a biomolecule in cell culture. In this feature article, we wish to present the recent progress in bioorthogonal reactions through the selected examples that highlight the above-mentioned strategies. Considering increasing sophistication in bioorthogonal chemistry development, we strive to project several exciting opportunities where bioorthogonal chemistry can make a unique contribution to biology in the near future.

Soluble, Clickable Polypeptides from Azide-Containing N-Carboxyanhydride Monomers

We report a method for the synthesis of soluble, well-defined, azide-functionalized polypeptides via living polymerization of new azide-containing amino acid N-carboxyanhydride (NCA) monomers. Homo and diblock azidopolypeptides were prepared with controlled segment lengths using (PMe₃)₄Co initiator and were subsequently modified by reaction with functional alkyne reagents. The azide groups were found to be quantitatively converted to the corresponding triazole derivatives, and the functionalized polymers were obtained in high yield. This methodology provides a facile and straightforward method for preparation of functional and side-chain reactive, high molecular weight polypeptides.

Traceless Staudinger acetylation of azides in aqueous buffers

In this paper, we demonstrate the applicability of water-soluble p-dimethylaminoethyl substituted phosphinomethanethiol in acetyl transfer reactions by the traceless Staudinger ligation with unprotected ε-azido lysine containing peptides in aqueous buffer systems. Additionally, we present an improved synthesis pathway for the water-soluble phosphinothiol linkers requiring less steps in a comparable overall yield in comparison to previously published protocols.

Dendronized albumin core-shell transporters with high drug loading capacity

We describe the synthesis of a core-shell biohybrid consisting of a human serum albumin (HSA) core that serves as a reservoir for lipophilic molecules and a cationized shell region consisting of ethynyl-G2.0-PAMAM or ethynyl-G3.0-PAMAM dendrons. The binding capacity of lipophilic guests was quantified applying electron paramagnetic resonance (EPR) spectroscopy, and five to six out of seven pockets were still available compared with HSA. The attachment of ethynyl-G2.0-PAMAM dendrons to HSA yielded a nontoxic core-shell macromolecule that was clearly uptaken by A549 human epithelial cells due to the presence of the dendritic PAMAM shell. Significantly higher loading of doxorubicin was observed for dendronized G2-DHSA compared with the native protein due to the availability of binding pockets of the HSA core, and interaction with the dendritic shell. Dendronized G2-DHSA-doxorubicin displayed significant cytotoxicity resulting from high drug loading and high stability under different conditions, thus demonstrating its great potential as a transporter for drug molecules.

Immobilization of enzymes on ethynyl-modified electrodes via click chemistry

This paper describes a novel, simple, and versatile protocol for covalent immobilization of enzyme on electrode. The immobilization method is based on the combination of diazonium salt electrografting and click chemistry. The ethynyl-terminated monolayers are obtained by diazonium salt electrografting, then, in the presence of copper (I) catalyst, the ethynyl modified surfaces reacts efficiently and rapidly with enzyme bearing an azide function (azido-enzyme), thus forming a covalent 1,2,3-triazole linkage by means of click chemistry. The ethynyl-terminated film preserves the activity of the immobilized enzyme. The click chemistry along with binary film of diazonium salts offers a variety of good characteristics including high sensitivity, good repeatability and reusability, rapid response and long term stability of the system. Thus, because of the chemoselective reactivity and quantitative yield of the click reaction, an ethynyl-terminated monolayer can be treated as a general platform for obtaining reliable coverage of a wide range of azido-terminated species of interest for various sensing applications.