The role of copper in cysteine oxidation: study of intra- and inter-molecular reactions in mass spectrometry

Cysteine-containing peptide oxidation was studied both by using an inert platinum electrode and a sacrificial electrode (copper or zinc) generating metallic ions in electrospray ionization mass spectrometry (ESI-MS). Using peptides containing one, two and three cysteines, we have compared the different chemical and electrochemical oxidation pathways of cysteine (RS IIH) to cystine (RS IS IR) and to sulfenic, sulfinic and sulfonic acid (RS0OH, RSIIO2H and RSIVO3H, respectively). In the absence of copper ions, intra-molecular reactions were the most abundant, whereas inter-molecular reactions were found to be enhanced by the presence of copper ions. These cations favor the formation of 2 : 1 (peptide : copper) complexes compared to 1 : 1 complexes, thus enhancing the formation of inter-molecular bridges. This study highlights the importance of the position of cysteine inside a peptide during disulfide bridge formation.