Covalent chemical probes – chemical probes that form a covalent bond with their target biomacromolecule(s) represent powerful tools that can be used for biology discovery, target validation (or off-target identification), and as starting points for drug discovery programmes.

Drugs with a covalent mode of action have been known and used for over a century, although historically, pursuit of such bioactive compounds has been avoided due to concerns over lack of selectivity. Over the last three decades, however, the use of ligands bearing reactive handles has seen increased interest spawning efforts to rationally design covalent drugs and new approaches to study protein function such as activity-based protein-profiling. Where covalent bioactive ligands are concerned, increased selectivity and duration of action represent advantages. In the context of new methods to study biology, chemical methods uniquely offer the ability to develop new reactive warheads, optimize bioorthogonal chemistries, and label a protein-of-interest with a (multi)functional group of choice, including natural or non-natural functionalities harbouring a recognition, reporting or affinity handle. Collectively, this offers new opportunities to modulate, track and isolate proteins of interest in/from the complex cellular milieu. In turn, this has created demand for new regioselective chemistries and analytical methods to assess biomacromolecule labelling in the complex environment of the cell.