Protein Substrates for Reaction Discovery: Site-Selective Modification with Boronic Acid Reagents

A single backbone amide modification method to achieve single site-specific chemical protein conjugation

Efficient and precise chemical protein modification methods are highly sought after in biotechnology. However, chemically distinguishing a single site within a large protein is challenging. This study introduces a Copper Assisted Sequence-specific Conjugation Tag (CAST) method, enabling rapid (second order rate 8.1 M-1s-1) and site-specific chemical modification of the protein backbone with pinpoint accuracy. The versatility of this method is demonstrated through the preparation of antibody-drug conjugates, showcasing high plasma stability and potent efficacy in both in vitro and in vivo settings. Thus, CAST emerges as an efficient and quantitative approach for attaching payloads to large, native proteins.

Copper assisted sequence-specific chemical protein conjugation at a single backbone amide

Direct, site-specific methods of protein functionalization are highly desirable for biotechnology. However, such methods are challenging due to the difficulty of chemically differentiating a single site within a large protein. Herein, we propose "metal binding targeting" strategy and develop a Copper Assisted Sequence-specific conjugation Tag (CAST) method to achieve rapid (second order rate 8.1 M-1 s-1), site-specific protein backbone chemical modification with pinpoint accuracy. We demonstrate the versatility of CAST conjugation by preparing various on-demand modified recombinant proteins, including a homogeneous antibody-drug conjugate with high plasma stability and potent efficacy in vitro and in vivo. Thus, CAST provides an efficient and quantitative method to site-specifically attach payloads on large, native proteins.

Cysteine-Selective Installation of Functionally Diverse Boronic Acid Probes on Peptides

The current methods for direct late-stage and residue-selective installation of a versatile boronic acid (BA) repertoire on peptides are inadequate for a wide range of applications. Here, we show the suitability and efficiency of thiol-ene radical click chemistry to install functionally versatile BA derivatives on numerous bioactive, native peptides. Our work highlights that the methodology is operationally simple and adaptable for applications with BA-modified peptides, such as cyclization, conjugation, and functional group alteration.

Selective Stepwise Arylation of Unprotected Peptides by PtIV Complexes

LPt^IVF(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. Pt^IV reagents can also be used to further arylate N−H bonds in Lys and Trp providing access to peptides bearing multiple aromatic groups.

Gold(I)-Mediated Rapid Cyclization of Propargylated Peptides via Imine Formation

In fundamental research and drug discovery, there is still a need for effective and straightforward chemical approaches for generating cyclic peptides. The divergent synthesis of cyclic peptides remains a challenge, in particular when cyclization is carried out in the presence of unprotected side chains and a nonpeptidic component within the cycle is needed. Herein, we describe a novel and efficient strategy based on Au(I)-mediated cyclization of unprotected peptides through rapid (30-60 min) amine addition on a propargyl group to generate an imine linkage. Mechanistic insights reveal that the reaction proceeds via regioselective Markovnikov's addition of the amine on the Au(I)-activated propargyl. This strategy was successfully applied to prepare efficiently (56-94%) over 35 diverse cyclic peptides having different sequences and lengths. We have also achieved stereoselective reduction of cyclic imines employing chiral ligands. The practicality of our method was extended for the synthesis of cyclic peptides that bind Lys48-linked di-ubiquitin chains with high affinity, leading to apoptosis of cancer cells.

Merging the Versatile Functionalities of Boronic Acid with Peptides

Peptides inherently feature the favorable properties of being easily synthesized, water-soluble, biocompatible, and typically non-toxic. Thus, boronic acid has been widely integrated with peptides with the goal of discovering peptide ligands with novel biological activities, and this effort has led to broad applications. Taking the integration between boronic acid and peptide as a starting point, we provide an overview of the latest research advances and highlight the versatile and robust functionalities of boronic acid. In this review, we summarize the diverse applications of peptide boronic acids in medicinal chemistry and chemical biology, including the identification of covalent reversible enzyme inhibitors, recognition, and detection of glycans on proteins or cancer cell surface, delivery of siRNAs, development of pH responsive devices, and recognition of RNA or bacterial surfaces. Additionally, we discuss boronic acid-mediated peptide cyclization and peptide modifications, as well as the facile chemical synthesis of peptide boronic acids, which paved the way for developing a growing number of peptide boronic acids.

Organometallic AlaM Reagents for Umpolung Peptide Diversification

Selective modifications of peptides and proteins have emerged as a promising strategy to develop novel mechanistic probes and prepare compounds with translational potentials. Here, we report alanine carbastannatranes AlaSn as a universal synthon in various C-C and C-heteroatom bond-forming reactions. These reagents are compatible with peptide manipulation techniques and can undergo chemoselective conjugation in minutes when promoted by Pd(0). Despite their increased nucleophilicity and propensity to transfer the alkyl group, C(sp3)-C(sp2) coupling with AlaSn can be accomplished at room temperature under buffered conditions (pH 6.5-8.5). We also show that AlaSn can be easily transformed into several canonical L- and D-amino acids in arylation, acylation, and etherification reactions. Furthermore, AlaSn can partake in macrocyclizations exemplified by the synthesis of medium size cyclic peptides with various topologies. Taken together, metalated alanine AlaSn demonstrates unparalleled scope and represents a new type of umpolung reagents suitable for structure-activity relationship studies and peptide diversification.

Oxidative cross-coupling processes inspired by the Chan-Lam reaction

The Cu-catalyzed oxidative cross-coupling of N- and O-nucleophiles with aryl boronic acids (the Chan-Lam reaction) remains among the most useful approaches to prepare aniline and phenol derivatives. The combination of high chemoselectivity, mild reaction conditions, and the ability to use simple Cu-salts as catalysts makes this process a valuable alternative to aromatic substitutions and Pd-catalyzed reactions of aryl electrophiles (Buchwald-Hartwig coupling). Despite the widespread use of Chan-Lam reactions in synthesis, the analogous carbon-carbon bond forming variant of this process had not been developed prior to our work. This feature article describes our discovery and application of Cu-catalyzed oxidative coupling reactions of activated methylene derivatives or carboxylic acids with nucleophiles including aryl boronic esters and amines.

Site-selective modification of peptide backbones

Exciting developments in the site-selective modification of peptide backbones are allowing an outstanding fine-tuning of peptide conformation, folding ability, and physico-chemical and biological properties.

Boronic acids as building blocks for the construction of therapeutically useful bioconjugates

This review summarizes boronic acid's contribution to the development of bioconjugates with a particular focus on the molecular mechanisms underlying its role in the construction and function of the bioconjugate, namely as a bioconjugation warhead, as a payload and as part of a bioconjugate linker.