NEM is a Michael acceptor in the Michael reaction, which means that it adds nucleophiles such as thiols.  The resulting thioether features a strong C-S bond and the reaction is virtually irreversible.  Reaction with thiols occur in the pH range 6.5–7.5, NEM may react with amines or undergo hydrolysis at a more alkaline pH.  NEM has been widely used to probe the functional role of thiol groups in enzymology. NEM is an irreversible inhibitor of all cysteine peptidases, with alkylation occurring at the active site thiol group (see schematic).