Palladium-Mediated Arylation of Lysine in Unprotected Peptides

Selective Stepwise Arylation of Unprotected Peptides by PtIV Complexes

LPtIV F(Aryl) complexes bearing a bulky bidentate 2-[bis(adamant-1-yl)phosphino]phenoxide ligand (L) demonstrate excellent reactivity and selectivity in the arylation of X-H (X=S, N) bonds of amino acid residues in unprotected peptides under mild, including aqueous, conditions. Stepwise addition of these complexes allowed a convenient one-pot introduction of different aromatic groups in the X-H bonds of Cys and N terminus. PtIV reagents can also be used to further arylate N-H bonds in Lys and Trp providing access to peptides bearing multiple aromatic groups.

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.

Peptide stapling by late-stage Suzuki–Miyaura cross-coupling

The development of peptide stapling techniques to stabilise α-helical secondary structure motifs of peptides led to the design of modulators of protein–protein interactions, which had been considered undruggable for a long time. We disclose a novel approach towards peptide stapling utilising macrocyclisation by late-stage Suzuki–Miyaura cross-coupling of bromotryptophan-containing peptides of the catenin-binding domain of axin. Optimisation of the linker length in order to find a compromise between both sufficient linker rigidity and flexibility resulted in a peptide with an increased α-helicity and enhanced binding affinity to its native binding partner β-catenin. An increased proteolytic stability against proteinase K has been demonstrated.

Azaphilones as Activation-Free Primary-Amine-Specific Bioconjugation Reagents for Peptides, Proteins and Lipids

Residue-selective bioconjugation methods for biomolecules are highly sought to expand the scope of their biological and medical applications. Inspired by the mechanism of the generation of natural vinylogous γ-pyridones (vPDNs), we have developed a novel unique azaphilone-based, activation-free primary-amine-selective bioconjugation method for biomolecules. Our strategy allows facile functionalization of primary amine groups in peptides and proteins, including the clinically used therapeutic antibody trastuzumab, by generating a highly stable vPDN linkage. Excellent chemoselectivity toward primary amines also enables the azaphilone derivatives to specifically modify the lipid components of Gram-positive bacteria while bypassing Gram-negative bacteria and mammalian cells. The new method shows significant advantages including chemoselectivity, efficiency, flexibility and biocompatibility, and therefore provides a valuable addition to the current toolbox for biomolecule conjugation.

Macrocyclization strategies for cyclic peptides and peptidomimetics

Peptides are a growing therapeutic class due to their unique spatial characteristics that can target traditionally "undruggable" protein-protein interactions and surfaces. Despite their advantages, peptides must overcome several key shortcomings to be considered as drug leads, including their high conformational flexibility and susceptibility to proteolytic cleavage. As a general approach for overcoming these challenges, macrocyclization of a linear peptide can usually improve these characteristics. Their synthetic accessibility makes peptide macrocycles very attractive, though traditional synthetic methods for macrocyclization can be challenging for peptides, especially for head-to-tail cyclization. This review provides an updated summary of the available macrocyclization chemistries, such as traditional lactam formation, azide-alkyne cycloadditions, ring-closing metathesis as well as unconventional cyclization reactions, and it is structured according to the obtained functional groups. Keeping peptide chemistry and screening in mind, the focus is given to reactions applicable in solution, on solid supports, and compatible with contemporary screening methods.

Selective N-Arylation of p-Aminophenylalanine in Unprotected Peptides with Organometallic Palladium Reagents

The selective N-arylation of p-aminophenylalanine in polypeptides with pre-formed palladium oxidative addition complexes is described. The depressed pKa of the aniline NH₂ group enables chemoselective C-N bond formation on peptides containing multiple other aliphatic amino groups at lysines or the N-terminus via Curtin-Hammett control under mild conditions. Using palladium complexes derived from electron-poor aryl halides, p-aminophenylalanine is fully arylated in aqueous buffer in as little as one hour at micromolar concentrations. A complementary protocol using the non-nucleophilic, organic base 1,5-diazabicyclo(4.3.0)non-5-ene (DBN), expands the substrate scope to tolerate electron-rich functional groups provides up to 97 % conversion. These procedures enable the chemoselective conjugation of functionally diverse small molecule pharmaceuticals to p-aminophenylalanine containing derivatives of cell-penetrating peptides.

Gold-Based Pharmacophore Inhibits Intracellular MYC Protein

Direct targeting of intrinsically disordered proteins, including MYC, by small molecules for biomedical applications would resolve a longstanding issue in chemical biology and medicine. Thus, we developed gold-based small-molecule MYC reagents that engage MYC inside cells and modulate MYC transcriptional activity. Lead compounds comprise an affinity ligand and a gold(I) or gold(III) warhead capable of protein chemical modification. Cell-based MYC target engagement studies via CETSA and co-immunoprecipitation reveal specific interaction of compounds with MYC in cells. The lead gold(I) reagent, 1, demonstrates superior cell-killing potential (up to 35-fold) in a MYC-dependent manner when compared to 10058-F4 in cells including the TNBC, MDA-MB-231. Subsequently, 1 suppresses MYC transcription factor activity via functional colorimetric assays, and gene-profiling using whole-cell transcriptomics reveals significant modulation of MYC target genes by 1. These findings point to metal-mediated ligand affinity chemistry (MLAC) based on gold as a promising strategy to develop chemical probes and anticancer therapeutics targeting MYC.

One‐Step Azolation Strategy for Site‐ and Chemo‐Selective Labeling of Proteins with Mass‐Sensitive Probes

The chemical modification of proteins in a site-selective manner leads to many advances in various scientific fields. The major challenges with conventional N-terminal bioconjugation techniques are the lack of universal sequence compatibility and poor mass-detection sensitivity of the resulting bioconjugates. This approach efficiently analyzes proteolytic fragments and native proteins in a complex mixture. Multiple chemical steps are usually required for the site-selective synthesis of bioconjugates with enhanced mass-detection sensitivity. We present a single-step, versatile strategy for the selective modification of protein N-termini with mass boosters. The chemical tag enhances the peptide detection by multiple orders thus leading to the unambiguous analysis of the resulting bioconjugates. We demonstrate that tagging proteolytic fragments with mass sensitivity probes in a complex mixture improves the detection of resulting bioconjugates.

Transition-Metal-Mediated Modification of Biomolecules

The site-selective modification of biomolecules has grown spectacularly in recent years. The presence of a large number of functional groups in a biomolecule makes its chemo- and regioselective modification a challenging goal. In this context, transition-metal-mediated reactions are emerging as a powerful tool owing to their unique reactivity and good functional group compatibility, allowing highly efficient and selective bioconjugation reactions that operate under mild conditions. This Minireview focuses on the current state of organometallic chemistry for bioconjugation, highlighting the potential of transition metals for the development of chemoselective and site-specific methods for functionalization of peptides, proteins and nucleic acids. The importance of the selection of ligands attached to the transition metal for conferring the desired chemoselectivity will be highlighted.

Fluorescent Isoindole Crosslink (FlICk) Chemistry: A Rapid, User-friendly Stapling Reaction

The stabilization of peptide secondary structure via stapling is a ubiquitous goal for creating new probes, imaging agents, and drugs. Inspired by indole-derived crosslinks found in natural peptide toxins, we employed ortho-phthalaldehydes to create isoindole staples, thus transforming inactive linear and monocyclic precursors into bioactive monocyclic and bicyclic products. Mild, metal-free conditions give an array of macrocyclic α-melanocyte-stimulating hormone (α-MSH) derivatives, of which several isoindole-stapled α-MSH analogues (K_i ≈1 nm) are found to be as potent as α-MSH. Analogously, late-stage intra-annular isoindole stapling furnished a bicyclic peptide mimic of α-amanitin that is cytotoxic to CHO cells (IC₅₀ =70 μm). Given its user-friendliness, we have termed this approach FlICk (fluorescent isoindole crosslink) chemistry.

Late-Stage Functionalization of Peptides and Cyclopeptides Using Organozinc Reagents

We report a new late-stage functionalization of small peptides and cyclopeptides relying on the Negishi cross-coupling of readily prepared iodotyrosine- or iodophenylalanine-containing peptides with aryl-, heteroaryl-, and alkylzinc pivalates or halides. In silico and in vitro determinations of membrane permeability parameters of the modified cyclopeptides showed that in most cases, the solubility was improved by the introduction of polar pyridyl units while the cell-membrane permeability was maintained.

Palladium-Catalyzed Decarboxylative Alkynylation of α-Acyloxyketones by C(sp3 )-O Bond Cleavage

Palladium-catalyzed decarboxylative alkynylation of α-acyloxyketones triggered by C(sp³ )-O bond cleavage is disclosed. The decarboxylation strategy featuring a neutral reaction condition enabled an unprecedent catalytic alkynylation of a ketone enolate. The reaction was applied to a variety of substrates, giving desired products in good yields. We successfully obtained X-ray crystallography of a new palladium-enolate intermediate that was synthesized by a reaction of [Pd(cod)(CH₂ TMS)₂ ] with XPhos and α-acyloxyketone at room temperature, indicating facile C(sp³ )-O bond disconnection.

Arylation Chemistry for Bioconjugation

Bioconjugation chemistry has been used to prepare modified biomolecules with functions beyond what nature intended. Central to these techniques is the development of highly efficient and selective bioconjugation reactions that operate under mild, biomolecule compatible conditions. Methods that form a nucleophile-sp2 carbon bond show promise for creating bioconjugates with new modifications, sometimes resulting in molecules with unparalleled functions. Here we outline and review sulfur, nitrogen, selenium, oxygen, and carbon arylative bioconjugation strategies and their applications to modify peptides, proteins, sugars, and nucleic acids.

Late-Stage Diversification through Manganese-Catalyzed C-H Activation: Access to Acyclic, Hybrid, and Stapled Peptides

Bioorthogonal C-H allylation with ample scope was accomplished through a versatile manganese(I)-catalyzed C-H activation for the late-stage diversification of structurally complex peptides. The unique robustness of the manganese(I) catalysis manifold was reflected by full tolerance of sensitive functional groups, such as iodides, esters, amides, and OH-free hydroxy groups, thereby setting the stage for the racemization-free synthesis of C-H fused peptide hybrids featuring steroids, drug molecules, natural products, nucleobases, and saccharides.

Homogeneous Catalysis: A Powerful Technology for the Modification of Important Biomolecules

Homogeneous catalysis plays an important and ubiquitous role in the synthesis of simple and complex molecules, including drug compounds, natural products, and agrochemicals. In recent years, the wide-reaching importance of homogeneous catalysis has made it an indispensable tool for the modification of biomolecules, such as carbohydrates (sugars), amino acids, peptides, nucleosides, nucleotides, and steroids. Such a synthetic strategy offers several advantages, which have led to the development of new molecules of biological relevance at a rapid rate relative to the number of available synthetic methods. Given the powerful nature of homogeneous catalysis in effecting these synthetic transformations, this Focus Review has been compiled to highlight these important developments.

Palladium(II)-Mediated C-H Tritiation of Complex Pharmaceuticals

Tritium-labeled molecules are critical tools for elucidating the binding and metabolic properties of bioactive compounds, particularly during pharmaceutical discovery. Direct tritiation of inert C-H bonds with T₂ gas is an ideal approach for tritium labeling, but significant gaps remain for direct tritiation of structurally complex molecules with diverse functional groups. Here we report the first application of palladium(II) C-H activation chemistry for tritiation with T₂ gas. This practical transformation exhibits novel substrate scope and greater functional group tolerance compared to previous state of the art tritiation methods, and has been applied to directly tritiate 9 complex pharmaceuticals and an unprotected dipeptide. The isolated tritium-labeled products exhibit >15 Ci mmol^-1 specific activity, exceeding the typical requirements for application in studies of molecular interaction and metabolism.

Internal Peptide Late-Stage Diversification: Peptide-Isosteric Triazoles for Primary and Secondary C(sp3 )-H Activation

Secondary C(sp³ )-H arylations were accomplished by palladium catalysis with triazoles as peptide bond isosteres. The unique power of this approach is highlighted by the possibility of achieving secondary C(sp³ )-H functionalizations on terminal peptides as well as the unprecedented positional-selective C(sp³ )-H functionalization of internal peptide positions, setting the stage for modular peptide late-stage diversification.