Formation of Peptide Radical Cations (M^+·) in Electron Capture Dissociation of Peptides Adducted with Group IIB Metal Ions

Abstract

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.

Transition Metal Ions: Charge Carriers that Mediate the Electron Capture Dissociation Pathways of Peptides

“…Such nonmetalated fragment ions generated by ECD of metalated peptides have been reported previously [24,26,29]. The formation of such protonated fragment ions indicates the existence of zwitterionic structures in the precursor ions.…”

“…Only some platinum ions adducted a-, b-, and y-ions were occasionally observed. More recently, our group observed peptide radical cation (M +• ) and fragment ions corresponding to neutral side chain losses from M +• in the ECD spectra of Cd 2+ and Hg 2+ adducted peptides [29]. The experimental results were rationalized by a two-step process involving an initial charge reduction the metal ions by the incoming electron and then an electron transfer from the peptide moiety to the charge reduced metal ions.…”

“…Sequence ions are labeled according to our previously proposed nomenclature, in which " cÀand " zÀdenote the corresponding N-and C-terminal fragments originated from the homolytic cleavage of the N-C α linkage, respectively. Any surplus of proton/hydrogen atom or metal ion/atom is indicated in the label [24,29]. As summarized in the bond cleavage inserts, many sequence specific fragment ions were generated in the ECD spectra.…”

“…We have previously demonstrated that the precursor ion heterogeneity was one of the important factors the affects the ECD fragmentation of peptides adducted with alkaline earth metal ions and group IIB metal ions [24,29]. To explore the possible correlation between the precursor ion heterogeneity and the fragmentation behavior, the deprotonation reaction mediated by first row divalent transition metal ions were investigated computationally by using a truncated peptide model.…”

“…Many previous studies have investigated the ECD of biomolecules and model peptides adducted with metal ions [23][24][25][26][27][28][29][30][31][32][33]. The key questions in ECD of metal ion peptide complexes are the electron neutralization site(s) and the determining factor(s) for various fragmentations.…”