Oligonucleotide Bioconjugation with Bifunctional Palladium Reagents

A Versatile Method for Site-Specific Chemical Installation of Aromatic Posttranslational Modification Analogs into Proteins

Posttranslational modifications (PTMs) of proteins play central roles in regulating the protein structure, interactome, and functions. A notable modification site is the aromatic side chain of Tyr, which undergoes modifications such as phosphorylation and nitration. Despite the biological and physiological importance of Tyr-PTMs, our current understanding of the mechanisms by which these modifications contribute to human health and disease remains incomplete. This knowledge gap arises from the absence of natural amino acids that can mimic these PTMs and the lack of synthetic tools for the site-specific introduction of aromatic PTMs into proteins. Herein, we describe a facile method for the site-specific chemical installation of aromatic PTMs into proteins through palladium-mediated S-C(sp2) bond formation under ambient conditions. We demonstrate the incorporation of novel PTMs such as Tyr-nitration and phosphorylation analogs to synthetic and recombinantly expressed Cys-containing peptides and proteins within minutes and in good yields. To demonstrate the versatility of our approach, we employed it to prepare 10 site-specifically modified proteins, including nitrated and phosphorylated analogs of Myc and Max proteins. Furthermore, we prepared a focused library of site-specifically nitrated and phosphorylated α-synuclein (α-Syn) protein, which enabled, for the first time, deciphering the role of these competing modifications in regulating α-Syn conformation aggregation in vitro. Our strategy offers advantages over synthetic or semisynthetic approaches, as it enables rapid and selective transfer of rarely explored aromatic PTMs into recombinant proteins, thus facilitating the generation of novel libraries of homogeneous posttranslationally modified proteins for biomarker discovery, mechanistic studies, and drug discovery.

ABNOH-Linked Nucleotides and DNA for Bioconjugation and Cross-linking with Tryptophan-Containing Peptides and Proteins

Reactive N-hydroxy-9-azabicyclo[3.3.1]nonane (ABNOH) linked 2'-deoxyuridine 5'-O-mono- and triphosphates were synthesized through a CuAAC reaction of ABNOH-PEG4-N3 with 5-ethynyl-dUMP or -dUTP. The modified triphosphate was used as substrate for enzymatic synthesis of modified DNA probes with KOD XL DNA polymerase. The keto-ABNO radical reacted with tryptophan (Trp) and Trp-containing peptides to form a stable tricyclic fused hexahydropyrrolo-indole conjugates. Similarly modified ABNOH-linked nucleotides reacted with Trp-containing peptides to form a stable conjugate in the presence but surprisingly even in the absence of NaNO2 (presumably through activation by O2). The reactive ABNOH-modified DNA probe was used for bioconjugations and crosslinking with Trp-containing peptides or proteins.

Electrochemical Bioconjugation of Tryptophan Residues: A Strategy for Peptide Modification

Interest in electrocatalytic bioconjugation reactions has surged, particularly for modifying tryptophan and tyrosine residues in proteins. We used a cost-effective graphite felt electrode and low-current methodology to achieve selective bioconjugation of tryptophan with thiophenols, yielding up to 92%. This method exclusively labeled tryptophan residues and incorporated fluorinated tryptophan for NMR analysis. Eight polypeptides, including lanreotide and leuprorelin, were effectively coupled, demonstrating the method's versatility and potential for novel diagnostic and therapeutic agents.

Ultrafast Au(III)-Mediated Arylation of Cysteine

Through mechanistic work and rational design, we have developed the fastest organometallic abiotic Cys bioconjugation. As a result, the developed organometallic Au(III) bioconjugation reagents enable selective labeling of Cys moieties down to picomolar concentrations and allow for the rapid construction of complex heterostructures from peptides, proteins, and oligonucleotides. This work showcases how organometallic chemistry can be interfaced with biomolecules and lead to a range of reactivities that are largely unmatched by classical organic chemistry tools.

An organometallic swap strategy for bottlebrush polymer-protein conjugate synthesis

Polymer-protein bioconjugation offers a powerful strategy to alter the physical properties of proteins, and various synthetic polymer compositions and architectures have been investigated for this purpose. Nevertheless, conjugation of molecular bottlebrush polymers (BPs) to proteins remains an unsolved challenge due to the large size of BPs and a general lack of methods to transform the chain ends of BPs into functional groups suitable for bioconjugation. Here, we present a strategy to address this challenge in the context of BPs prepared by "graft-through" ring-opening metathesis polymerization (ROMP), one of the most powerful methods for BP synthesis. Quenching ROMP of PEGylated norbornene macromonomers with an activated enyne terminator facilitates the transformation of the BP Ru alkylidene chain ends into Pd oxidative addition complexes (OACs) for facile bioconjugation. This strategy is shown to be effective for the synthesis of two BP-protein conjugates (albumin and ERG), setting the stage for a new class of BP-protein conjugates for future therapeutic and imaging applications.

Chemical Engineering of Artificial Transcription Factors by Orthogonal Palladium(II)-Mediated S-Arylation Reactions

Site-selective functionalization strategies are in high demand to prepare well-defined homogeneous proteins for basic research and biomedical applications. In this regard, cysteine-based reactions have enabled a broad set of transformations to produce modified proteins for various applications. However, these approaches were mainly employed to modify a single reactive site with a specific transformation. Achieving site selectivity or multiple transformations, essential for preparing complex biomolecules, remains challenging. Herein we demonstrate the power of combining palladium(II)-mediated C-S bond formation and C-S bond cleavage reactions to selectively edit desired cysteine sites in complex and uniquely modified proteins. We developed an orthogonal palladium(II) strategy for rapid and effective diversification of multiple cysteine sites (3-6 residues) with various transformations. Importantly, we employed our approach to prepare 10 complex analogues, including modified, stapled, and multimeric proteins on a milligram scale. Furthermore, we also synthesized a focused library of stabilized artificial transcription factors that displayed enhanced stability and potent DNA binding activity. Our approach enables rapid and effective protein editing and opens new avenues to engineer new biomolecules for fundamental research and therapeutic applications.

Nickel-Catalyzed Antibody Bioconjugation

New biocompatible methods for post-translational protein modification are challenging to develop but crucial to create improved chemical probes and optimize next-generation biologic therapies such as antibody-drug conjugates (ADCs). Herein, we describe the bottom-up construction of an aqueous nickel-catalyzed cross-coupling for the chemospecific arylation of cysteine residues on peptides and proteins and its use for the preparation of ADCs. A variety of arene linkages are exemplified, enabling the incorporation of small molecules, probes, and cytotoxic payloads. The utility of this new bioconjugation platform in a drug discovery setting is highlighted by the construction of novel ADCs with target-mediated in vitro cytotoxic activity.

Recent Advances in Bioorthogonal Ligation and Bioconjugation

The desire to create biomolecules modified with functionalities that go beyond nature's toolbox has resulted in the development of biocompatible and selective methodologies and reagents, each with different scope and limitations. In this overview, we highlight recent advances in the field of bioconjugation from 2016 to 2023. First, (metal-mediated) protein functionalization by exploiting the specific reactivity of amino acids will be discussed, followed by novel bioorthogonal reagents for bioconjugation of modified biomolecules.

The Dawn of a New Era: Targeting the "Undruggables" with Antibody-Based Therapeutics

The high selectivity and affinity of antibodies toward their antigens have made them a highly valuable tool in disease therapy, diagnosis, and basic research. A plethora of chemical and genetic approaches have been devised to make antibodies accessible to more "undruggable" targets and equipped with new functions of illustrating or regulating biological processes more precisely. In this Review, in addition to introducing how naked antibodies and various antibody conjugates (such as antibody-drug conjugates, antibody-oligonucleotide conjugates, antibody-enzyme conjugates, etc.) work in therapeutic applications, special attention has been paid to how chemistry tools have helped to optimize the therapeutic outcome (i.e., with enhanced efficacy and reduced side effects) or facilitate the multifunctionalization of antibodies, with a focus on emerging fields such as targeted protein degradation, real-time live-cell imaging, catalytic labeling or decaging with spatiotemporal control as well as the engagement of antibodies inside cells. With advances in modern chemistry and biotechnology, well-designed antibodies and their derivatives via size miniaturization or multifunctionalization together with efficient delivery systems have emerged, which have gradually improved our understanding of important biological processes and paved the way to pursue novel targets for potential treatments of various diseases.

Covalent Modification of Nucleobases using Water-Soluble Palladium Catalysts

Nucleosides represent one of the key building blocks of biochemistry. There is significant interest in the synthesis of nucleoside-derived materials for applications as probes, biochemical models, and pharmaceuticals. Palladium-catalyzed cross-coupling reactions are effective methods for making covalent modification of carbon and nitrogen sites on nucleobases under mild conditions. Water-soluble catalysts derived from palladium and hydrophilic ligands, such as tris(3-sulfonatophenyl)phosphine trisodium (TPPTS), are efficient catalysts for a range of coupling reactions of unprotected halonucleosides. Over the past two decades, these methods have been extended to direct functionalization of halonucleotides, as well as RNA and DNA oligonucleotides (ONs) containing halogenated bases. These methods can be run under biocompatible conditions, including examples of Suzuki coupling of modified DNA in whole cells and tissue samples. In this account, development of this methodology by our group and others is highlighted along with the extension of these catalyst systems to modification of nucleotides and ONs.

Metal-Free and Open-Air Arylation Reactions of Diaryliodonium Salts for DNA-Encoded Library Synthesis

A successful DNA-encoded library (DEL) will consist of diverse skeletons and cover chemical space as comprehensive as possible to fully realize its potential in drug discovery and chemical biology. However, the lack of versatile on-DNA arylation methods for phenols that are less nucleophilic and reactive poses a great hurdle for DEL to include diaryl ether, a privileged chemotype in pharmaceuticals and natural products. This work describes the use of "substrate activation" approach to address the arylation of DNA-conjugated phenols. Diaryliodonium salt, a highly electrophilic and reactive arylation reagent, is employed as Ar⁺ sources to ensure highly selective on-DNA arylation of phenols and oximes with both high yields and DNA fidelity. Notably, the new on-DNA arylation reaction can be applied to the late-stage modification of peptides containing tyrosine side-chain and to synthesize DNA-tagged analogues of existing drug molecules such as sorafenib, a known pan-kinase inhibitor. The new on-DNA diaryliodonium salts chemistry affords a greater flexibility in DEL design and synthesis.

Posttranslational Chemical Mutagenesis Methods to Insert Posttranslational Modifications into Recombinant Proteins

Posttranslational modifications (PTMs) dramatically expand the functional diversity of the proteome. The precise addition and removal of PTMs appears to modulate protein structure and function and control key regulatory processes in living systems. Deciphering how particular PTMs affect protein activity is a current frontier in biology and medicine. The large number of PTMs which can appear in several distinct positions, states, and combinations makes preparing such complex analogs using conventional biological and chemical tools challenging. Strategies to access homogeneous and precisely modified proteins with desired PTMs at selected sites and in feasible quantities are critical to interpreting their molecular code. Here, we summarize recent advances in posttranslational chemical mutagenesis and late-stage functionalization chemistry to transfer novel PTM mimicry into recombinant proteins with emphasis on novel transformations.

Palladium Mediated Synthesis of Protein-Polyarene Conjugates

Catalyst transfer polymerization (CTP) is widely applied to the synthesis of well-defined π-conjugated polymers. Unlike other polymerization reactions that can be performed in water (e.g., controlled radical polymerizations and ring-opening polymerizations), CTP has yet to be adapted for the modification of biopolymers. Here, we report the use of protein-palladium oxidative addition complexes (OACs) that enable catalyst transfer polymerization to furnish protein-polyarene conjugates. These polymerizations occur with electron-deficient monomers in aqueous buffers open to air at mild (≤37 °C) temperatures with full conversion of the protein OAC and an average polymer length of nine repeating units. Proteins with polyarene chains terminated with palladium OACs can be readily isolated. Direct evidence of protein-polyarene OAC formation was obtained using mass spectrometry, and all protein-polyarene chain ends were uniformly functionalized via C-S arylation to terminate the polymerization with a small molecule thiol or a cysteine-containing protein.

Pd(II)-Mediated C-H Activation for Cysteine Bioconjugation

Selective bioconjugation remains a significant challenge for the synthetic chemist due to the stringent reaction conditions required by biomolecules coupled with their high degree of functionality. The current trailblazer of transition-metal mediated bioconjugation chemistry involves the use of Pd(II) complexes prepared via an oxidative addition process. Herein, the preparation of Pd(II) complexes for cysteine bioconjugation via a facile C-H activation process is reported. These complexes show bioconjugation efficiency competitive with what is seen in the current literature, with a user-friendly synthesis, common Pd(II) sources, and a more cost-effective ligand. Furthermore, these complexes need not be isolated, and still achieve high conversion efficiency and selectivity of a model peptide. These complexes also demonstrate the ability to selectively arylate a single surface cysteine residue on a model protein substrate, further demonstrating their utility.

Convertible and Constrained Nucleotides: The 2'-Deoxyribose 5'-C-Functionalization Approach, a French Touch

Many strategies have been developed to modulate the biological or biotechnical properties of oligonucleotides by introducing new chemical functionalities or by enhancing their affinity and specificity while restricting their conformational space. Among them, we review our approach consisting of modifications of the 5'-C-position of the nucleoside sugar. This allows the introduction of an additional chemical handle at any position on the nucleotide chain without disturbing the Watson-Crick base-pairing. We show that 5'-C bromo or propargyl convertible nucleotides (CvN) are accessible in pure diastereoisomeric form, either for nucleophilic displacement or for CuAAC conjugation. Alternatively, the 5'-carbon can be connected in a stereo-controlled manner to the phosphate moiety of the nucleotide chain to generate conformationally constrained nucleotides (CNA). These allow the precise control of the sugar/phosphate backbone torsional angles. The consequent modulation of the nucleic acid shape induces outstanding stabilization properties of duplex or hairpin structures in accordance with the preorganization concept. Some biological applications of these distorted oligonucleotides are also described. Effectively, the convertible and the constrained approaches have been merged to create constrained and convertible nucleotides (C2NA) providing unique tools to functionalize and stabilize nucleic acids.

1,3-Diketone-Modified Nucleotides and DNA for Cross-Linking with Arginine-Containing Peptides and Proteins

Linear or branched 1,3-diketone-linked thymidine 5'-O-mono- and triphosphate were synthesized through CuAAC click reaction of diketone-alkynes with 5-azidomethyl-dUMP or -dUTP. The triphosphates were good substrates for KOD XL DNA polymerase in primer extension synthesis of modified DNA. The nucleotide bearing linear 3,5-dioxohexyl group (HDO) efficiently reacted with arginine-containing peptides to form stable pyrimidine-linked conjugates, whereas the branched 2-acetyl-3-oxo-butyl (PDO) group was not reactive. Reaction with Lys or a terminal amino group formed enamine adducts that were prone to hydrolysis. This reactive HDO modification in DNA was used for bioconjugations and cross-linking with Arg-containing peptides or proteins (e.g. histones).

Engineering Bioactive Dimeric Transcription Factor Analogs via Palladium Rebound Reagents

Transcription factors (TF), such as Myc, are proteins implicated in disease pathogenesis, with dysregulation of Myc expression in 50% of all human cancers. Still, targeting Myc remains a challenge due to the lack of small molecule binding pockets in the tertiary structure. Here, we report synthetic covalently linked TF mimetics that inhibit oncogenic Myc-driven transcription by antagonistic binding of the target DNA-binding site. We combined automated flow peptide chemistry with palladium(II) oxidative addition complexes (OACs) to engineer covalent protein dimers derived from the DNA-binding domains of Myc, Max, and Omomyc TF analogs. Palladium-mediated cross-coupling of synthesized protein monomers resulted in milligram quantities of seven different covalent homo- and heterodimers. The covalent helical dimers were found to bind DNA and exhibited improved thermal stability. Cell-based studies revealed the Max-Max covalent dimer is cell-penetrating and interfered with Myc-dependent gene transcription resulting in reduced cancer cell proliferation (EC₅₀ of 6 μM in HeLa). RNA sequencing and gene analysis of extracted RNA from treated cancer cells confirmed that the covalent Max-Max homodimer interferes with Myc-dependent transcription. Flow chemistry, combined with palladium(II) OACs, has enabled a practical strategy to generate new bioactive compounds to inhibit tumor cell proliferation.