Proteome-Wide Target Identification Using Reactive Metallo-Scaffolds (r-mS): A Platform for Metallodrug Discovery.

Metal complexes offer unique opportunities as scaffolds in chemical biology and drug discovery, with tunable geometries, modular coordination environments, and structural features not readily accessible with organic molecules. Here, we introduce reactive metallo-scaffolds (r-mS) as a class of metal complexes designed to map ligandable cysteines across the mammalian proteome. These covalent warhead-bearing metal complexes use the metal centre and ligand architecture to modulate cysteine engagement, with cysteine labelling occurring through the electrophilic chloroacetamide warhead. Using chemoproteomics, we profiled a focused r-mS series in HEK293T lysate, identifying novel cysteine ligandability and demonstrating how metal identity, arene substituents and overall molecular topography govern cysteine reactivity and proteome-wide targeting. Among the series screened, r-mS-2 emerged as the most productive scaffold, which engaged cysteine 119 within the functionally relevant SAM-binding domain of PRMT1. This interaction was validated by intact protein LC-MS and was determined to functionally inhibit the activity of PRMT1. Structural modelling and docking provided insights into the molecular basis of binding, which implied π-stacking and electrostatic complementarity in driving covalent engagement. Together, these results position reactive metallo-scaffolds (r-mS) as a versatile platform for proteome-wide covalent ligand discovery and the rational development of next-generation metallodrugs.