Covalent Targeting of Histidine Residues: A Ligand-First Approach.

netic properties compared to reversible ligands. Nevertheless, most ligands to date are designed to target a cysteine (Cys) residue, which is not a very common amino acid and only rarely occurs in protein target binding sites, thereby limiting the applicability of this covalent targeting approach. Recent work from our laboratory and others suggests that, after Cys, histidine (His) residues can be particularly suitable for covalent substitution with proper electrophiles. Using a ligand-first, structure-based approach, we assessed the possibility of using different electrophiles including acrylamides, chloroacetamides, or aryl fluorosulfates to target His residues covalently. Targeting His224 of hMcl-1 with model peptides, we demonstrate that both aryl fluorosulfates and chloroacetamides can be used to target His residues efficiently. Our studies also report strategies and biophysical approaches useful for the design and characterization of such His-covalent agents.