[Metal-Assisted N-Terminal Modification of Polypeptides via Amino Acid-Metal Ion Interactions]

Selective chemical modification of proteins under mild aqueous conditions is an important technique in chemical biology and biopharmaceutical research. Among the various targets for protein modification, the N-terminus is particularly attractive because it is uniquely present in each polypeptide and is often exposed on the protein surface. However, because the N-terminal amino group exhibits reactivity similar to that of lysine side chains, selective N-terminal modification based solely on the intrinsic amino-group reactivity remains a challenge. Our approach utilizes metal coordination involving the N-terminus and neighboring functional groups to selectively activate N-terminal intermediates. Using this approach, we developed several N-terminal modification reactions, including nickel(II)-mediated C-N coupling of pyroglutamic acid-histidine (pGlu-His) sequences, copper(II)-mediated aldol reactions, sequential multifunctionalization via nickel(II)-mediated aldol and Pictet-Spengler reactions, and one-step dual-functionalization via copper(II)-mediated [3+2] cycloaddition. Unlike conventional bioconjugation methods that rely on side-chain-specific reactions, our approach is based on metal-complex formation involving the peptide backbone, allowing broad applicability of our approach to diverse peptides and proteins without depending on specific side-chain functionalities. Furthermore, these reactions have been successfully applied to protein assembly and antibody-drug conjugate (ADC) construction. We believe that these methods will serve as useful tools in a wide range of studies in protein chemistry and chemical biology.