Activity‐Based Proteome Profiling Probes Based on Woodward's Reagent K with Distinct Target Selectivity

Woodward's reagent K (WRK) is a reactive heterocyclic compound that has been employed in protein chemistry to covalently and unspecifically label proteins at nucleophilic amino acids, notably at histidine and cysteine. We have developed a panel of WRK‐derived activity‐based probes and show that surprisingly and unexpectedly, these probes are fairly selective for a few proteins in the human proteome. The WRK‐derived probes show unique reactivity towards the catalytic N‐terminal proline in the macrophage migration inhibitory factor (MIF) and can be used to label and, if equipped with a fluorophore, to image MIF activities in living cells.