Study of the oxidation dynamics of ethyl cysteinate dimer in solution by ultra-performance liquid chromatography with...

Diaminodithiol (N2S2)-type compounds readily oxidize to produce disulfides. We found that some ligands failed to produce a prospective protonated molecular ion peak but gave a peak of [M-2+H]+, whereas others produced both [M+H]+ and [M-2+H]+ peaks in electrospray ionization mass spectra. In this study, an important N2S2 ligand, the ethyl cysteinate dimer (ECD), was investigated with high-resolution accurate mass measurements and tandem mass spectrometric analysis. The elemental compositions of ECD and its oxidized product were analyzed. The oxidation of ECD was confirmed. An ultra-performance liquid chromatography/tandem mass spectrometry (UPLC/MS/MS) method in multiple reaction monitoring mode was developed, and ECD and its oxidized product were quantitated in solution. The dynamic oxidation process of ECD in solution was studied in detail. The full time course of the decrease in ECD and the increase in its oxide was observed; the oxidation procedure followed first-order kinetics, and the half-life time of ECD was 51 min.