Development of Photocatalysts towards the Visible-Light driven C-Terminal Bioconjugation of Peptides and Proteins

If small molecules have led to tremendous progress in curing diseases, new therapeutic agents such as peptides or antibody-drugs conjugates have emerged as a highly promising next generation of pharmaceuticals. As for small molecules, their derivatization is often the key to custom activity or physicochemical properties. Peptide and protein modification methods are thus in important. This is however particularly challenging as it requires physiological conditions, as well as excellent chemoselectivities. Mild activation techniques are thus under special attention.

For that purpose, photoredox catalysis stands as a mild strategy to generate radicals selectively. Visible-light is thus attractive for biomolecule functionalization and impressive reports have been disclosed in the last years, even if the field is still at an early stage. Another mild strategy towards protein functionalization is the use of hypervalent iodine reagents as an electrophilic alkyne source. This has been successful for biomolecule bioconjugation on several residues. Gratifyingly, those two concepts could be combined and a decarboxylative alkynylation of amino acids using photoredox catalysis and hypervalent iodine reagents was previously released.

The goal of my PhD was to contribute to the field of biomolecules site-selective chemical modification by providing a novel labeling of carboxylic acids. To this end, it was envisioned to extend the existing photoredox-catalyzed decarboxylative alkynylation, from simple carboxylic acids to peptides, and later proteins. Among the diverse possible positions for protein bioconjugation, the C-terminal extremity is extremely attractive as it allows for a single-site selectivity. Available methodologies remain however scarce and are not general.

Our investigations started with the development of novel fine-tuned organic dyes. A rational design allowed us to report organophotocatalysts filling existing gaps in redox properties. The following extensions of this concept and the use of this library to unlock novel reactivities demonstrated the impact of this contribution.

Those organophotocatalysts were precious in the success of our peptide decarboxylative alkynylation. This reaction was found to proceed with an excellent regio and chemo-selectivity towards the C-terminal position. Native peptides up to hexamers were suitable substrates and the potential was confirmed by the introduction of bioorthogonal functional groups.

Unfortunately, larger native peptides could not be modified under those conditions, leading us to design a novel strategy. We envisioned that a covalent linkage of an organophotocatalyst on proteins would allow an alkynylation at the C-terminus through a proximity-induced photoredox-catalyzed decarboxylation. For this unprecedented concept, 4CzIPN derivatives were thus designed to be introduced on a single-site on peptides. Ongoing work aims to assess the success of this approach.

Finally, another direction explored during this thesis was a decarboxylative oxidation of peptides towards the formation of N,O-acetals. They were employed as a platform for derivatization. Through Friedel-crafts arylations, phenols and indoles could be successfully introduced. This strategy was further extended towards the cross-linking of peptides, providing a novel scaffold of unnatural tetramers of potential interest for drug discovery.