Nucleophilic covalent ligand discovery for the cysteine redoxome

Covalent drugs have revolutionized modern therapeutics, yet their design has focused almost exclusively on electrophilic chemistries that target reduced cysteine thiols. In contrast, many disease-relevant proteins exist in oxidized states that render these residues unreactive to standard covalent ligands. Here, we introduce a complementary strategy that exploits the reversed polarity of oxidized cysteines (sulfenic acids) to enable covalent ligation by nucleophilic small molecules. Using quantitative chemoproteomics, we globally mapped more than 500 ligandable sulfenic acids across diverse functional protein classes—most inaccessible to electrophilic fragments. Nucleophilic ligands were further shown to inhibit enzyme activity and disrupt oncogenic protein–protein interactions in cells. This study expands the accessible chemical space for covalent ligand discovery, revealing that the human “sulfenome” represents a vast, previously untapped landscape for chemical probe and drug development that is inherently sensitive to redox state and thus potentially targetable in oxidative disease contexts.