Genetically Encoding Fluorosulfate-l-tyrosine To React with Lysine, Histidine, and Tyrosine via SuFEx in Proteins in Vivo

Late-Stage 18F/19F Isotopic Exchange for the Synthesis of 18F-Labeled Sulfamoyl Fluorides

Synthesis of sulfamoyl [¹⁸F]fluorides has been a challenging topic owing to the inefficient nucleophilic radiofluorination of sulfamoyl derivatives. Herein, we report an ¹⁸F/¹⁹F isotopic exchange approach to synthesize various sulfamoyl [¹⁸F]fluorides, otherwise inaccessible via direct synthesis from amines, with high radiochemical yields up to 97% (30 examples). This late-stage labeling protocol offers an efficient route to yield functionalized molecules by diversifying the chemical library possessing sulfamoyl functionalities through nucleophilic ¹⁸F incorporation within nitrogen-containing sulfur(VI) frameworks.

A Broad-Spectrum Catalytic Amidation of Sulfonyl Fluorides and Fluorosulfates*

A broad-spectrum, catalytic method has been developed for the synthesis of sulfonamides and sulfamates. With the activation by the combination of a catalytic amount of 1-hydroxybenzotriazole (HOBt) and silicon additives, amidations of sulfonyl fluorides and fluorosulfates proceeded smoothly and excellent yields were generally obtained (87-99 %). Noticeably, this protocol is particularly efficient for sterically hindered substrates. Catalyst loading is generally low and only 0.02 mol % of catalyst is required for the multidecagram-scale synthesis of an amantadine derivative. In addition, the potential of this method in medicinal chemistry has been demonstrated by the synthesis of the marketed drug Fedratinib via a key intermediate sulfonyl fluoride 13. Since a large number of amines are commercially available, this route provides a facile entry to access Fedratinib analogues for biological screening.

A synthetic 5,3-cross-link in the cell wall of rod-shaped Gram-positive bacteria

Gram-positive bacteria assemble a multilayered cell wall that provides tensile strength to the cell. The cell wall is composed of glycan strands cross-linked by nonribosomally synthesized peptide stems. Herein, we modify the peptide stems of the Gram-positive bacterium Bacillus subtilis with noncanonical electrophilic d-amino acids, which when in proximity to adjacent stem peptides form novel covalent 5,3-cross-links. Approximately 20% of canonical cell-wall cross-links can be replaced with synthetic cross-links. While a low level of synthetic cross-link formation does not affect B. subtilis growth and phenotype, at higher levels cell growth is perturbed and bacteria elongate. A comparison of the accumulation of synthetic cross-links over time in Gram-negative and Gram-positive bacteria highlights key differences between them. The ability to perturb cell-wall architecture with synthetic building blocks provides a novel approach to studying the adaptability, elasticity, and porosity of bacterial cell walls.

Sulfur [18F]Fluoride Exchange Click Chemistry Enabled Ultrafast Late-Stage Radiosynthesis

The lack of efficient [¹⁸F]fluorination processes and target-specific organofluorine chemotypes remains the major challenge of fluorine-18 positron emission tomography (PET). We report here an ultrafast isotopic exchange method for the radiosynthesis of novel PET agent aryl [¹⁸F]fluorosulfate enabled by the emerging sulfur fluoride exchange (SuFEx) click chemistry. The method has been applied to the fully automated ¹⁸F-radiolabeling of 25 structurally and functionally diverse aryl fluorosulfates with excellent radiochemical yield (83-100%, median 98%) and high molar activity (280 GBq μmol^-1) at room temperature in 30 s. The purification of radiotracers requires no time-consuming HPLC but rather a simple cartridge filtration. We further demonstrate the imaging application of a rationally designed poly(ADP-ribose) polymerase 1 (PARP1)-targeting aryl [¹⁸F]fluorosulfate by probing subcutaneous tumors in vivo.

Radical Fluorosulfonylation: Accessing Alkenyl Sulfonyl Fluorides from Alkenes

Sulfonyl fluorides have widespread applications in many fields. In particular, their unique biological activity has drawn considerable research interest in the context of chemical biology and drug discovery in the past years. Therefore, new and efficient methods for the synthesis of sulfonyl fluorides are highly in demand. In contrast to extensive studies on FSO₂ ⁺ -type reagents, a radical fluorosulfonylation reaction with a fluorosulfonyl radical (FSO₂ ^. ) remains elusive so far, probably owing to its instability and difficulty in generation. Herein, the development of the first radical fluorosulfonylation of alkenes based on FSO₂ radicals generated under photoredox conditions is reported. This radical approach provides a new and general access to alkenyl sulfonyl fluorides, including structures that would otherwise be challenging to synthesize with previously established cross-coupling methods. Moreover, extension to the late-stage fluorosulfonylation of natural products is also demonstrated.

A novel three-component reaction for constructing indolizine-containing aliphatic sulfonyl fluorides

A copper-catalyzed three-component reaction for transforming quinolines, isoquinolines and pyridines to a class of indolizine-containing alkyl sulfonyl fluorides was developed.

Complementary Base Lowers the Barrier in SuFEx Click Chemistry for Primary Amine Nucleophiles

The sulfur(VI) fluoride exchange (SuFEx) reaction is an emerging scheme for connecting molecular building blocks. Due to its broad functional group tolerance and rather stable resulting linkage, it is seeing rapid adoption in various fields of chemistry. Still, to date the reaction mechanism is poorly understood, which hampers further development. Here, we show that the mechanism of the SuFEx reaction for the prototypical example of methanesulfonyl fluoride reacting with methylamine can be understood as an SN2-type reaction. By analyzing the reaction path with the help of density functional theory in vacuo and under consideration of solvent and co-reactant influence, we identify the often used complementary base as a crucial ingredient to lower the reaction barrier significantly by increasing the nucleophilicity of the primary amine. With the help of energy decomposition analysis at the transition state structures, we quantify the underlying stereoelectronic effects and propose new avenues for experimental exploration of the potential of SuFEx chemistry.

Suzuki Cross-Coupling Reaction with Genetically Encoded Fluorosulfates for Fluorogenic Protein Labeling

A palladium-catalyzed cross-coupling reaction with aryl halide functionalities has recently emerged as a valuable tool for protein modification. Herein, a new fluorogenic modification methodology for proteins, with genetically encoded fluorosulfate-l-tyrosine, which exhibits high efficiency and biocompatibility in bacterial cells as well as in aqueous medium, is described. Furthermore, the cross-coupling of 4-cyanophenylboronic acid on green fluorescent protein was shown to possess a unique fluorogenic property, which could open up the possibility of a responsive "off/on" switch with great potential to enable spectroscopic imaging of proteins with minimal background noise. Taken together, a convenient and efficient catalytic system has been developed that may provide broad utilities in protein visualization and live-cell imaging.

From Differential Stains to Next Generation Physiology: Chemical Probes to Visualize Bacterial Cell Structure and Physiology

Chemical probes have been instrumental in microbiology since its birth as a discipline in the 19th century when chemical dyes were used to visualize structural features of bacterial cells for the first time. In this review article we will illustrate the evolving design of chemical probes in modern chemical biology and their diverse applications in bacterial imaging and phenotypic analysis. We will introduce and discuss a variety of different probe types including fluorogenic substrates and activity-based probes that visualize metabolic and specific enzyme activities, metabolic labeling strategies to visualize structural features of bacterial cells, antibiotic-based probes as well as fluorescent conjugates to probe biomolecular uptake pathways.

Genetically Encoded Quinone Methides Enabling Rapid, Site-Specific, and Photocontrolled Protein Modification with Amine Reagents

Site-specific modification of proteins with functional molecules provides powerful tools for researching and engineering proteins. Here we report a new chemical conjugation method which photocages highly reactive but chemically selective moieties, enabling the use of protein-inert amines for selective protein modification. New amino acids FnbY and FmnbY, bearing photocaged quinone methides (QMs), were genetically incorporated into proteins. Upon light activation, they generated highly reactive QM, which rapidly reacted with amine derivatives. This method features a rare combination of desired properties including fast kinetics, small and stable linkage, compatibility with low temperature, photocontrollability, and widely available reagents. Moreover, labeling via FnbY occurs on the β-carbon, affording the shortest linkage to protein backbone which is essential for advanced studies involving orientation and distance. We installed various functionalities onto proteins and attached a spin label as close as possible to the protein backbone, achieving high resolution in double electron-electron paramagnetic resonance distance measurements.

Increasing the chemical space of proteins in living cells via genetic code expansion

In recent years it has become possible to genetically encode an expanded set of designer amino acids with tailored chemical and physical properties (dubbed unnatural amino acids, UAAs) into proteins in living cells by expanding the genetic code. Together with developments in chemistries that are amenable to and selective within physiological settings, these strategies have started to have a big impact on biological studies, as they enable exciting in cellulo applications. Here we highlight recent advances to covalently stabilize transient protein-protein interactions and capture enzyme substrate-complexes in living cells using proximity-triggered and residue-selective photo-induced crosslinking approaches. Furthermore, we describe recent efforts in controlling enzyme activity with photocaged UAAs and in extending their application to a variety of enzymatic scaffolds. In addition, we discuss the site-specific incorporation of UAAs mimicking post-translational modifications (PTMs) and approaches to generate natively-linked ubiquitin-protein conjugates to probe the role of PTMs in modulating complex cellular networks.

Sulfuryl Fluoride Mediated Synthesis of Amides and Amidines from Ketoximes via Beckmann Rearrangement

A metal-free and redox-neutral method for Beckmann rearrangement employing inexpensive and readily available SO₂ F₂ gas is described. The reported transformation proceeds at ambient temperature and is compatible with a wide range of sterically and electronically diverse aromatic, heteroaromatic, aliphatic and lignin-like oximes providing amides in good to excellent yields. The reaction proceeds through the formation of an imidoyl fluoride intermediate that can also be used for the synthesis of amidines.

Genetically encoding thyronine for fluorescent detection of peroxynitrite

Peroxynitrite is a highly reactive oxidant effecting cell signaling and cell death. Here we report a fluorescent protein probe to selectively detect peroxynitrite. A novel unnatural amino acid, thyronine (Thy), was genetically encoded in E. coli and mammalian cells by evolving an orthogonal tRNA^Pyl/ThyRS pair. Incorporation of Thy into the chromophore of sfGFP or cpsGFP afforded a virtually non-fluorescent reporter. Upon treatment with peroxynitrite, Thy was converted into tyrosine via O-dearylation, regenerating GFP fluorescence in a time- and concentration-dependent manner. Genetically encoded thyronine may also be valuable for other redox applications.

But-3-ene-1,3-disulfonyl difluoride (BDF): a highly selective SuFEx clickable hub for the quick assembly of sultam-containing aliphatic sulfonyl fluorides

A versatile selectively addressable SuFEx click chemistry hub, but-3-ene-1,3-disulfonyl difluoride (BDF) was designed and synthesized through head-to-tail dimerization of ethenesulfonyl fluoride (ESF). BDF possesses three active sites to selectively participate in the construction of 4-membered, 5-membered and 6-membered cyclic sulfonamide (sultams) bearing aliphatic sulfonyl fluoride moieties for further manipulations in a fast, efficient and practical manner.

Developing bioorthogonal probes to span a spectrum of reactivities

Bioorthogonal chemistries enable researchers to interrogate biomolecules in living systems. These reactions are highly selective and biocompatible and can be performed in many complex environments. However, like any organic transformation, there is no perfect bioorthogonal reaction. Choosing the "best fit" for a desired application is critical. Correspondingly, there must be a variety of chemistries-spanning a spectrum of rates and other features-to choose from. Over the past few years, significant strides have been made towards not only expanding the number of bioorthogonal chemistries, but also fine-tuning existing reactions for particular applications. In this Review, we highlight recent advances in bioorthogonal reaction development, focusing on how physical organic chemistry principles have guided probe design. The continued expansion of this toolset will provide more precisely tuned reagents for manipulating bonds in distinct environments.

Synthesis of 18F-Labeled Aryl Fluorosulfates via Nucleophilic Radiofluorination

Sulfuryl fluoride gas is a key reagent for SO₂F transfer. However, conventional SO₂F transfer reactions have limited ¹⁸F-radiochemistry translation, due to the inaccessibility of gaseous [¹⁸F]SO₂F₂. Herein, we report the first SO₂F₂-free synthesis of aryl [¹⁸F]fluorosulfates from both phenolic and isolated aryl imidazylate precursors with cyclotron-produced ¹⁸F^-. The radiochemical yields ranged from moderate to good with excellent functional group tolerance. The reliability of our approach was validated by the automated radiosynthesis of 4-acetamidophenyl [¹⁸F]fluorosulfate.

Semisynthesis of a Bacterium with Non-canonical Cell-Wall Cross-Links

The cell wall is an elaborate framework of peptidoglycan that serves to protect the bacterium against osmotic challenge. This exoskeleton is composed of repeating saccharides covalently cross-linked by peptide stems. The general structure of the cell wall is widely conserved across diverse Gram-negative bacteria. To begin to explore the biological consequence of introducing non-canonical cross-links into the cell wall of Escherichia coli, we generated a bacterium where up to 31% of the cell-wall cross-links are formed by a non-enzymatic reaction between a sulfonyl fluoride and an amino group. Bacteria with these non-canonical cell-wall cross-links achieve a high optical density in culture, divide and elongate successfully, and display no loss of outer membrane integrity. This work represents a first step in the design of bacteria with non-canonical "synthetic" cell walls.

Sulfur(VI) Fluoride Exchange (SuFEx)-Enabled High-Throughput Medicinal Chemistry

Optimization of small-molecule probes or drugs is a synthetically lengthy, challenging, and resource-intensive process. Lack of automation and reliance on skilled medicinal chemists is cumbersome in both academic and industrial settings. Here, we demonstrate a high-throughput hit-to-lead process based on the biocompatible sulfur(VI) fluoride exchange (SuFEx) click chemistry. A high-throughput screening hit benzyl (cyanomethyl)carbamate (K_i = 8 μM) against a bacterial cysteine protease SpeB was modified with a SuFExable iminosulfur oxydifluoride [RN═S(O)F₂] motif, rapidly diversified into 460 analogs in overnight reactions, and the products were directly screened to yield drug-like inhibitors with 480-fold higher potency (K_i = 18 nM). We showed that the improved molecule is active in a bacteria-host coculture. Since this SuFEx linkage reaction succeeds on picomole scale for direct screening, we anticipate our methodology can accelerate the development of robust biological probes and drug candidates.

SO2F2-Mediated one-pot cascade process for transformation of aldehydes (RCHO) to cyanamides (RNHCN)

A simple, mild and practical cascade process for the direct conversion of aldehydes to cyanamides was developed featuring a wide substrate scope and great functional group tolerability. This method allows for transformations of readily available, inexpensive, and abundant aldehydes to highly valuable cyanamides in a pot, atom, and step-economical manner with a green nitrogen source. This protocol will serve as a robust tool for the installation of the cyanamide moiety in various complicated molecules.

Acid-brightening fluorescent protein (abFP) for imaging acidic vesicles and organelles

Acidic organelles and vesicles, such as endosomes, lysosomes, autophagosomes, trans-Golgi network, and synaptic vesicles, are known to play important roles in a broad range of cellular events. To facilitate studying these multifunctional systems, we describe here an acid-brightening fluorescent protein (abFP), which fluoresces strongly at acidic pH, but is almost nonfluorescent at or above physiological pH, making it well suited for imaging molecules residing in acidic microenvironment in live cells. Specifically, a quinoline-containing unnatural amino acid Qui is incorporated into the chromophore of EGFP via genetic code expansion to generate the abFP. When being exposed to acidic environment, protonation of Qui results in a cationic chromophore and fluorescence increase. Protocols are presented to express abFP in E. coli and mammalian cells, and to fluorescently image the endocytosis of δ opioid receptor-abFP fusion protein in mammalian cells. This strategy may be similarly applicable to other fluorescent proteins to enable acidic imaging.

The Application of Fluorine-Containing Reagents in Structural Proteomics

Structural proteomics refers to large-scale mapping of protein structures in order to understand the relationship between protein sequence, structure, and function. Chemical labeling, in combination with mass-spectrometry (MS) analysis, have emerged as powerful tools to enable a broad range of biological applications in structural proteomics. The key to success is a biocompatible reagent that modifies a protein without affecting its high-order structure. Fluorine, well-known to exert profound effects on the physical and chemical properties of reagents, should have an impact on structural proteomics. In this Minireview, we describe several fluorine-containing reagents that can be applied in structural proteomics. We organize their applications around four MS-based techniques: a) affinity labeling, b) activity-based protein profiling (ABPP), c) protein footprinting, and d) protein cross-linking. Our aim is to provide an overview of the research, development, and application of fluorine-containing reagents in protein structural studies.

Arenesulfonyl Fluoride Synthesis via Copper-Catalyzed Fluorosulfonylation of Arenediazonium Salts

We report herein a general and practical copper-catalyzed fluorosulfonylation reaction of a wide range of abundant arenediazonium salts to smoothly prepare various arenesulfonyl fluorides using the 1,4-diazabicyclo[2.2.2]octane-bis(sulfur dioxide) adduct as a convenient sulfonyl source in combination with KHF₂ as an ideal fluorine source and without the need for additional oxidants. Interestingly, the electronic character of the arene ring in the starting arenediazonium salts has a significant impact on the reaction mechanistic pathway.

Structure-based design and analysis of SuFEx chemical probes

The discerning reactivity of sulfur(vi)-fluoride exchange (SuFEx) chemistry has enabled the context-specific labeling of protein binding sites by chemical probes that incorporate these versatile warheads. Emerging information from protein-probe structures and proteomic mapping experiments is helping advance our understanding of the protein microenvironment that dictates the reactivity of targetable amino acid residues. This review explores these new findings that should influence the future rational design of SuFEx probes for a multitude of applications in chemical biology and drug discovery.

Rh-Catalyzed Highly Enantioselective Synthesis of Aliphatic Sulfonyl Fluorides

Rh-catalyzed, highly enantioselective (up to 99.8% ee) synthesis of aliphatic sulfonyl fluorides was accomplished. This protocol provides a portal to a class of novel 2-aryl substituted chiral sulfonyl fluorides, which are otherwise extremely difficult to access. This asymmetric synthesis has the feature of mild conditions, excellent functional group compatibility, and wide substrate scope (51 examples) generating a wide array of structurally unique chiral β-arylated sulfonyl fluorides for sulfur(VI) fluoride exchange (SuFEx) click reaction and drug discovery.

A Genetically Encoded, Phage-Displayed Cyclic-Peptide Library

Superior to linear peptides in biological activities, cyclic peptides are considered to have great potential as therapeutic agents. To identify cyclic-peptide ligands for therapeutic targets, phage-displayed peptide libraries in which cyclization is achieved by the covalent conjugation of cysteines have been widely used. To resolve drawbacks related to cysteine conjugation, we have invented a phage-display technique in which its displayed peptides are cyclized through a proximity-driven Michael addition reaction between a cysteine and an amber-codon-encoded Nϵ -acryloyl-lysine (AcrK). Using a randomized 6-mer library in which peptides were cyclized at two ends through a cysteine-AcrK linker, we demonstrated the successful selection of potent ligands for TEV protease and HDAC8. All selected cyclic peptide ligands showed 4- to 6-fold stronger affinity to their protein targets than their linear counterparts. We believe this approach will find broad applications in drug discovery.

A cascade process for directly converting nitriles (RCN) to cyanamides (RNHCN) via SO2F2-activated Tiemann rearrangement

A simple, mild and practical process for the direct conversion of nitriles to cyanamides was newly discovered and exhibited a wide substrate scope as well as great functional group-tolerability (36 examples). In this efficient strategy, the in situ generated amidoximes obtained from the reaction of nitriles with hydroxylamine subsequently underwent Tiemann rearrangement, producing the corresponding cyanamides with great isolated yields under SO2F2. Additionally, the control experiments reportedly shed light on the tentative mechanism involved in the formation and elimination of the key intermediate: a sulfonyl ester.

SuFEx-enabled, agnostic discovery of covalent inhibitors of human neutrophil elastase

Sulfur fluoride exchange (SuFEx) has emerged as the new generation of click chemistry. We report here a SuFEx-enabled, agnostic approach for the discovery and optimization of covalent inhibitors of human neutrophil elastase (hNE). Evaluation of our ever-growing collection of SuFExable compounds toward various biological assays unexpectedly revealed a selective and covalent hNE inhibitor: benzene-1,2-disulfonyl fluoride. Synthetic derivatization of the initial hit led to a more potent agent, 2-(fluorosulfonyl)phenyl fluorosulfate with IC₅₀ 0.24 μM and greater than 833-fold selectivity over the homologous neutrophil serine protease, cathepsin G. The optimized, yet simple benzenoid probe only modified active hNE and not its denatured form.

A rapid access to aliphatic sulfonyl fluorides

The past few years have witnessed a fast-growing research interest on the study of sulfonyl fluorides as reactive probes in chemical biology and molecular pharmacology, which raises an urgent need for the development of effective synthetic methods to expand the toolkit. Herein, we present the invention of a facile and general approach for the synthesis of aliphatic sulfonyl fluorides via visible-light-mediated decarboxylative fluorosulfonylethylation. The method is based on abundant carboxylic acid feed stock, applicable to various alkyl carboxylic acids including primary, secondary, and tertiary acids, and is also suitable for the modification of natural products like amino acids, peptides, as well as drugs, forging a rapid, metal-free approach to build sulfonyl fluoride compound libraries of considerable structural diversity. Further diversification of the SO₂F-containing products is also demonstrated, which allows for access to a range of pharmaceutically important motifs such as sultam, sulfonate, and sulfonamide.

Sulfonyl fluorides are important probes in chemical biology and molecular pharmacology. Here, the authors report a mild visible light-mediated decarboxylative fluorosulfonylethylation for the synthesis of aliphatic sulfonyl fluorides from a wide range of carboxylic acids, including natural products and drug derivatives.

Using genetically incorporated unnatural amino acids to control protein functions in mammalian cells

Genetic code expansion allows unnatural (non-canonical) amino acid incorporation into proteins of interest by repurposing the cellular translation machinery. The development of this technique has enabled site-specific incorporation of many structurally and chemically diverse amino acids, facilitating a plethora of applications, including protein imaging, engineering, mechanistic and structural investigations, and functional regulation. Particularly, genetic code expansion provides great tools to study mammalian proteins, of which dysregulations often have important implications in health. In recent years, a series of methods has been developed to modulate protein function through genetically incorporated unnatural amino acids. In this review, we will first discuss the basic concept of genetic code expansion and give an up-to-date list of amino acids that can be incorporated into proteins in mammalian cells. We then focus on the use of unnatural amino acids to activate, inhibit, or reversibly modulate protein function by translational, optical or chemical control. The features of each approach will also be highlighted.

Genetically Encoding Photocaged Quinone Methide to Multitarget Protein Residues Covalently in Vivo

Genetically introducing covalent bonds into proteins in vivo with residue specificity is affording innovative ways for protein research and engineering, yet latent bioreactive unnatural amino acids (Uaas) genetically encoded to date react with one to few natural residues only, limiting the variety of proteins and the scope of applications amenable to this technology. Here we report the genetic encoding of (2 R)-2-amino-3-fluoro-3-(4-((2-nitrobenzyl)oxy) phenyl) propanoic acid (FnbY) in Escherichia coli and mammalian cells. Upon photoactivation, FnbY generated a reactive quinone methide (QM), which selectively reacted with nine natural amino acid residues placed in proximity in proteins directly in live cells. In addition to Cys, Lys, His, and Tyr, photoactivated FnbY also reacted with Trp, Met, Arg, Asn, and Gln, which are inaccessible with existing latent bioreactive Uaas. FnbY thus dramatically expanded the number of residues for covalent targeting in vivo. QM has longer half-life than the intermediates of conventional photo-cross-linking Uaas, and FnbY exhibited cross-linking efficiency higher than p-azido-phenylalanine. The photoactivatable and multitargeting reactivity of FnbY with selectivity toward nucleophilic residues will be valuable for addressing diverse proteins and broadening the scope of applications through exploiting covalent bonding in vivo for chemical biology, biotherapeutics, and protein engineering.

A Diene-Containing Noncanonical Amino Acid Enables Dual Functionality in Proteins: Rapid Diels-Alder Reaction with Maleimide or Proximity-Based Dimerization

Here, we describe a diene-containing noncanonical amino acid (ncAA) capable of undergoing fast and selective normal electron-demand Diels-Alder (DA) reactions following its incorporation into antibodies. A cyclopentadiene derivative of lysine (CpHK) served as the reactive handle for DA transformations and the substrate for genetic incorporation. CpHK incorporated into antibodies with high efficiency and was available for maleimide conjugation or self-reaction depending on position in the amino acid sequence. CpHK at position K274 reacted with the maleimide drug-linker AZ1508 at a rate of ≈79 m^-1  s^-1 to produce functional antibody-drug conjugates (ADCs) in a one-step process. Incorporation of CpHK at position S239 resulted in dimerization, which covalently linked antibody heavy chains together. The diene ncAA described here is capable of producing therapeutic protein conjugates with clinically validated and widely available maleimide compounds, while also enabling proximity-based stapling through a DA dimerization reaction.