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. Oxidation of cysteine-containing peptides was studied in the absence or presence of copper ions electrogenerated from a sacrificial electrode in on-line mass spectrometry. This highlights differences in intra- and inter-molecular reactions.