Electron transfer dissociation (ETD) has been used for peptide sequencing. Since ETD preferentially produces the c'/z center dot fragment pair, peptide sequencing is generally performed by interpretation of mass differences between series of consecutive c' and z center dot ions. However, the presence of free cysteine residues in a precursor promotes peptide bond cleavage, hindering interpretation of the ETD spectrum. In the present study, the divalent group XII metals, such as Zn2+, Cd2+ and Hg2+, were used as charge carriers to produce metal-peptide complexes. The thiol group is deprotonated by complexation with the group XII metal. The formation of b and y' ions was successfully suppressed by using the zinc-peptide complex as a precursor, indicating Zn2+-aided ETD to be a useful method for sequencing of cysteine-containing peptides. By contrast, ETD of Cd2+ and Hg(2+)peptide complexes mainly led to SH2 loss and radical cation formation, respectively. These processes were mediated by recombination energy between the metal cation and an electron. The presence of monovalent cadmium and neutral mercury in ETD products was confirmed by MS3 analysis with collision-induced dissociation.