On-line electrochemical tagging of cysteines in proteins during nanospray

An electrochemical modification of free cysteine residues is studied and characterized by means of quinone addition. Taking advantage of the electrolytic nature of electrospray interfaces (ESI), an electrochemical tagging is performed prior to mass spectrometry (MS) analyses. The tagging has been studied by MS and different mechanisms, involving electrochemical and/or chemical steps, could be characterized. It is demonstrated that the present nanospray is a very efficient tool to obtain cysteine modification. Using the high voltage electrode of the nanospray interface to perform protein specific tagging is a novel method that can be associated to analytical or preparative techniques, such as digestion of proteins or capillary electrophoresis, for post-column modifications.