Reduction of copper(II) complexes by electron capture in an electrospray ionization source

The relative proportion of 1:1 Cu(I)– and Cu(II)–peptide complexes PeptCu(I)⁺ and [Pept − H+Cu(II)]⁺ yielded by electrospray ionization of copper sulfate and GlyHisLys solutions in water/methanol was examined under different source conditions. Two factors leading to an increase in Cu(I) complex ratio were found. (1) Increase of nozzle–skimmer voltages caused collision-induced dissociation of Cu(II) complexes, and most probably favor ligand-to-metal electron transfers that result in the decoordination of oxydated ligands to form PeptCu⁺. (2) Independent of these “innersphere” processes that involve only electron exchange inside the coordination sphere around the metal cation, an increase in source voltages with a concomitant increase of current and, supposedly, electron counterflow between the counterelectrode and the capillary caused an increase in PeptCu⁺ relative proportion. The hypothesis that an “outersphere” electron capture might happen in these conditions was verified by using discharge supressing SF₆ gas as nebulizing gas. The electronegative gas reduced the current brought on by high voltages and inhibited the PeptCu⁺ increase phenomenon.