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 Pep tCu(I)(+) and [Pept H-+Cu(II)](+) yielded by electrospray ionization o f copper sulfate and GlyHisLys solutions in water/methanol was examine d under different source conditions. Two factors leading to an increas e in Cu(I) complex ratio were found. (1) Increase of nozzle-skimmer vo ltages 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) Independ ent of these ''innersphere'' processes that involve only electron exch ange inside the coordination sphere around the metal cation, an increa se in source voltages with a concomitant increase of current and, supp osedly, electron counterflow between the counterelectrode and the capi llary caused an increase in PeptCu(+) relative proportion. The hypothe sis that an ''outersphere'' electron capture might happen in these con ditions was verified by using discharge supressing SF6 gas as nebulizi ng gas. The electronegative gas reduced the current brought on by high voltages and inhibited the PeptCu(+) increase phenomenon. (J Am Soc M ass Spectrom 1998, 9, 1217-1221) (C) 1998 American Society for Mass Sp ectrometry.