Anionic copper complex fragmentations from enkephalins under low-energy collision-induced dissociation in an ion trap mass spectrometer

Peptide metallation with Cu2+ was explored in the negative ESI mode using an ion trap mass spectrometer. Under these conditions, the [(M - 3H) + Cu-II]- species formed were investigated under low-energy collision-induced dissociation conditions. MS' experiments indicate a very different behavior of Cull metallated complexes compared with [M - H](-) species. Cull induces an easy loss Of CO2 and specific side-chain cleavages (by radical losses) at the C-terminal residue, as observed previously by prompt 'in source' dissociation experiments. The loss Of CO2 yields an unstable carbylide that leads to further dissociations involving the migration of a proton or a hydrogen radical (through the reduction of Cu-II). Multistage MS3 experiments were carried out to rationalize this behavior. Fragmentation pathways are proposed in order to explain the product ions observed. The side-chain radical loss at the C-terminus was demonstrated to be a consecutive process. Finally, evidence is provided that the specific side-chain cleavages can be used for the differentiation of Leu/Ile and GIn/Lys residues when they are located at the C-terminus. The existence of a zwitterionic form in the case of the anionic YGGFK-Cu-II complex is proposed. Copyright (C) 2004 John Wiley Sons, Ltd.