Palladium chemistry in peptide and protein synthesis and modification

Peptides and proteins that bear site-selective modifications are of great interest for various biochemical studies. Total chemical and semi-synthesis of proteins based on chemoselective ligation of unprotected peptides enable the preparation of modified peptides/proteins while overcoming the limitations of molecular biology approaches. The thiolate functionality of the Cys residue coupled with the wise selection of orthogonal protecting groups (PGs) has been employed to control the directionality of synthesis and/or the modification sites. Nevertheless, the deprotection conditions of the Cys PGs reduce the efficiency and applicability of the syntheses. Recently, we reported an unprecedented palladium chemoselectivity for on-demand deprotection of the Cys PGs; thiazolidine, acetamidomethyl (Acm) and tert-butyl under aqueous conditions. These novel tools were applied for the synthesis of complex peptides and proteins such as the activity-based probe of ubiquitinated histone H2A, which was cross-linked with Calypso/ASX heterodimer deubiquitinase to form a stable covalent nucleosome-enzyme complex. These findings also prompted us to exploit the use of palladium chemistry for the preparation of peptides and proteins bearing disulfide bridges. We developed a strategy for the direct disulfide bond formation directly from Cys(Acm) by applying palladium and diethyldithiocarbamate. The utility of this chemistry that was performed in a one-pot and highly efficient manner was exemplified in the synthesis of oxytocin peptide and in the first total chemical synthesis of Thioredoxin-1 protein. These discoveries open exciting avenues for the efficient preparation of unique and complex peptides and proteins.