Inert Complexes Unlock Ligand-Accelerated Transition-Metal Catalysis on Proteins

Reactions that excel in small-molecule settings typically require metal loadings far exceeding the number of protein reaction sites (often ≥10-fold) once transplanted into proteinaceous media-conditions that are not truly "catalytic." Here, we show that biologically inert metal-ligand complexes based on bathocuproine disulfonic acid disodium salt (BCS) overcome this barrier and enable ligand-accelerated catalysis (LAC) on proteins under substoichiometric conditions. For example, Ni-BCS effects complete deprotection of green fluorescent protein bearing N^ε-propargyloxycarbonyl-L-lysine (GFP-ProcLys) at 5 mol% catalyst with an observed turnover number (TON) ≈ 20, surpassing all previously reported metal-catalyzed depropargylation reactions. Mechanistic studies indicate that an in situ Ni-H intermediate mediates multiple transformations on proteins, including reductive deuteration of terminal alkenes/alkynes and efficient decaging across diverse amino acid side chains. Likewise, Cu-BCS enables copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) on proteins at 10 mol% with low residual copper and no protein oxidation, in sharp contrast to the benchmark Cu-BTTAA (tris((1-tert-butyl-1H-1,2,3-triazol-4-yl)methyl)amine) system. These outcomes stem from a screening strategy that prioritized metal-ligand stability, eliminating metal complexes susceptible to protein sequestration and selecting strongly coordinating, physiologically inert pairs. The resulting rational ligand-design framework for protein-level transition-metal catalysis expands the frontier of protein chemistry and paves the way to translate advanced small-molecule LAC strategies onto protein substrates for posttranslational mutagenesis.