Abstract
Peptide metallation with Cu^2+^ 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. MS2 experiments indicate a very different behavior of Cu^II^ metallated complexes compared with [M − H]^−^ species. Cu^II^ induces an easy loss of CO~2~ 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 CO~2~ 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 Gln/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 © 2004 John Wiley & Sons, Ltd.