A thiol-labelling reagent and reactivity probe containing electrophilic mercury and a chromophoric leaving group

2-(2′-Pyridylmercapto)mercuri-4-nitrophenol was synthesized and evaluated as a thiol-labelling reagent containing a chromophoric leaving group and as a reactivity probe by studies on its reactions with 2-mercaptoethanol and with papain (EC 3.4.22.2).