Covalent, irreversible small-molecule inhibitors that modify the sulfenyl form (i.e., sulfenic acid, RSOH and sulfenamide, RSNR′2) of therapeutically important proteins (particularly kinases and phosphatases) are disclosed, where the compositions include a compound having a substituted aryl or heterocyclic core structure that promotes binding interactions with a specific protein, and a nucleophilic reaction center (carbon, nitrogen, sulfur, or phosphorous) that is capable of forming a covalent bond with a sulfenic acid- or sulfenamide-modified cysteine residue in the protein. Methods for synthesizing these compounds are also disclosed, as well as methods of using them for determining the bioactivity of a chemical composition comprising an active compound toward a specific protein and for determining the potency of an inhibitor against a specific protein.
