The bioconjugation of proteins—that is, the creation of a covalent link between a protein and any other molecule—has been studied for decades, partly because of the numerous applications of protein conjugates, but also due to the technical challenge it represents. Indeed, proteins possess inner physico-chemical properties—they are sensitive and polynucleophilic macromolecules—that make them complex substrates in conjugation reactions. This complexity arises from the mild conditions imposed by their sensitivity but also from selectivity issues, viz the precise control of the conjugation site on the protein. After decades of research, strategies and reagents have been developed to address two aspects of this selectivity: chemoselectivity—harnessing the reacting chemical functionality—and site-selectivity—controlling the reacting amino acid residue—most notably thanks to the participation of synthetic chemistry in this effort. This review offers an overview of these chemical bioconjugation strategies, insisting on those employing native proteins as substrates, and shows that the field is active and exciting, especially for synthetic chemists seeking new challenges.

Efficient methods to introduce bioorthogonal groups, such as terminal alkynes, into biomolecules are important tools for chemical biology. State-of-the-art approaches are based on the introduction of a linker between the targeted amino acid and the alkyne, and still present limitations of either reactivity, selectivity or adduct stability. Herein, we present an ethynylation method of cysteine residues based on the use of ethynylbenziodoxolone (EBX) reagents. In contrast to other approaches, the acetylene group is directly introduced onto the thiol group of cysteine and can be used in onepot in a copper-catalyzed alkyne-azide cycloaddition (CuAAC) for further functionalization. Labeling proceeded with reaction rates comparable or higher than the most often used iodoacetamide on peptides or maleimide on the antibody trastuzumab. Under optimized conditions, high cysteine selectivity was observed. The reagents were also used in living cells for cysteine proteomic profiling and displayed an improved coverage of the cysteinome compared to previously reported iodoacetamide or hypervalent iodine-reagent based probes. Fine-tuning of the EBX reagents allowed optimization of their reactivity and physical properties for the desired application. Scheme 1. Well-established alkyne-linker approach for cysteine functionalization.

A natural material of extra-terrestrial origin yields a high-performance electrocatalyst for alkaline water oxidation.

Site-selective modificationo fp roteins hasb een the object of intense studies over the past decades, especially in the therapeutic field. Prominent resultsh ave been obtained with recombinant proteins,f or which site-specific conjugation is made possible by the incorporation of particular aminoa cid residues or peptides equences. In parallel, methods for the site-selective and site-specific conjugation of native andn atural proteins are starting to thrive, allowing the controlled functionalization of various types of amino acid residues.P ursuingt he efforts in this field, we planned to develop an ew typeo fs ite-selective method, aiming at the simultaneous conjugation of two amino acid residues. We reasonedt hat this should give higher chances of developing as ite-selective strategy compared to the great majority of existing methods that solely target as ingle residue. We opted for the Ugi four-centre three-component reaction to implement this idea, with the aim of conjugating the sidechain amine and carboxylate groups of two neighbouring lysine and aspartate/glutamate. Herein,w es how that this strategyc an give access to valuable antibody conjugates bearings everald ifferent payloads;f urthermore, the approach limits the potential conjugation sites to only six on the model antibody trastuzumab.