Charge promotion of low-energy fragmentations of peptide ions.

We have examined the hypothesis that structural features which predispose to localization of charge at a strongly favored site are not conducive to the low-energy fragmentation of peptide ions via a multiplicity of pathways. Consistent with this proposal, it is demonstrated that the formation of N- or C-terminal pre-charged derivatives is detrimental to the formation of sequence-specific product ions following low-energy collisional activation. Protonation of pre-charged derivatives (yielding doubly charged ions) restores favorable fragmentation properties; the effect is attributed to the fragmentation-directing properties of the proton which may occupy one of several sites. Similarly, a doubly protonated peptide which incorporates a C-terminal arginine residue as a single strongly favored site of protonation exhibits favored low-energy fragmentations attributable to location of the second proton at one of several sites remote from the C-terminus.