Peptides adducted with different divalent Group IIB metal ions (Zn 2+ , Cd 2+ , and Hg 2+ ) were found to give very different ECD mass spectra. ECD of Zn 2+ adducted peptides gave series of c-/z-type fragment ions with and without metal ions. ECD of Cd 2+ and Hg 2+ adducted model peptides gave mostly a-type fragment ions with M +• and fragment ions corresponding to losses of neutral side chain from M +• . No detectable a-ions could be observed in ECD spectra of Zn 2+ adducted peptides. We rationalized the present findings by invoking both proton-electron recombination and metal-ion reduction processes. . The relative population of these precursor ions depends largely on the acidity of the metal-ion peptide complexes. Peptides adducted with divalent metal-ions of small ionic radii (i.e., Zn 2+ ) would form predominantly species (b) and (c); whereas peptides adducted with metal ions of larger ionic radii (i.e., Hg 2+ ) would adopt predominantly species (a). Species (b) and (c) are believed to be essential for proton-electron recombination process to give c-/z-type fragments via the labile ketylamino radical intermediates. Species (c) is particularly important for the formation of non-metalated c-/z-type fragments. Without any mobile protons, species (a) are believed to undergo metal ion reduction and subsequently induce spontaneous electron transfer from the peptide moiety to the charge-reduced metal ions. Depending on the exothermicity of the electron transfer reaction, the peptide radical cations might be formed with substantial internal energy and might undergo further dissociation to give structural related fragment ions.

yielded abundant metalated a-/y-type fragment ions; whereas ECD of Cu 2+ adducted peptides generated predominantly metalated b-/y-type fragment ions. From the present experimental results, it was postulated that electronic configuration of metal ions is an important factor in determining the ECD behavior of the metalated peptides. Due presumably to the stability of the electronic configuration, metal ions with fully-filled (i.e., Zn 2+ ) and half filled (i.e., Mn 2+ ) d-orbitals might not capture the incoming electron. Dissociation of the metal ions adducted peptides would proceed through the usual ECD channel(s) via "hot-hydrogen" or "superbase" intermediates, to form series of c-/z• -fragments. For other transition metal ions studied, reduction of the metal ions might occur preferentially. The energy liberated by the metal ion reduction would provide enough internal energy to generate the "slow-heating" type of fragment ions, i.e., metalated a-/yfragments and metalated b-/y-fragments.

Our results indicate that survival of the intact reduced peptide ion after electron capture depends also on the length of the peptide. The amide hydrogen was critical in forming the resonance structure that suppressed the ECD fragmentation.