The effect of charge state and the localization of charge on the collision-induced dissociation of peptide ions.

The effect that charge state has on the collision-induced dissociation (CID) of peptide ions is examined in detail for several representative peptides under high-energy collision conditions. The CID spectra of singly and doubly charged precursor ions (generated by fast-atom bombardment and electrospray ionization, respectively) are compared for several peptides with similar primary structure. It is shown that for peptides that contain highly basic amino acids, the dissociation of doubly charged ions is strongly influenced by the position of these residues within the peptide and the general observations reported concerning the dissociation of singly charged ions can be extended to precursors with higher charge states. Based on the dissociation behavior of the doubly charged ions of these peptides, it is demonstrated that two charges can reside in close proximity in the precursor ions, overcoming possible repulsion effects, when favored by a high concentration of basic sites. In addition)' this work illustrates that in the case of doubly charged ions..the charge state of some fragment ions can be determined directly from the mass-to-charge ratio assignments of the CID spectrum.