Differential Protein Labeling based on Electrochemically Generated Reactive Intermediates Lars Büter, Helene Faber, Tina Wigger and Uwe Karst Institute of Inorganic and Analytical Chemistry, University of Münster, Corrensstr. 28/30, 48149 Münster, lars.bueter@uni-muenster.de Electrochemical generation of quinoid intermediates via dehydrogenation leads to a Micheal-type addition of these electrophilic structures to nucleophilic compounds. Therefore, modification of peptides and proteins that contain the amino acid cysteine can be easily achieved due to the nucleophilic nature of their free thiol groups. In this poster, we present an approach for the electrochemical generation of reactive benzoquinone intermediates of phenol in its native and isotope labeled form (13C6-phenol). Subsequent reaction with peptides and proteins results in a defined mass shift between the differentially labeled biomolecules. The hyphenation of an electrochemical cell to electrospray ionization mass spectrometry enables the immediate detection of generated peptide adducts. By applying a potential ramp, the optimum potential for the generation of reactive intermediates can be determined. An extension of the instrumental set-up by means of liquid chromatography allows the online generation, separation and identification of modified proteins. Furthermore, after tryptic digestion, the binding position in the proteins can be determined and a mass shift can be observed, which can be traced back to the heavier, labeled species phenol. Hence, the identification of these tryptic peptides is simplified, thus allowing the identification of proteins in a complex mixture.