ROLE OF THE SITE OF PROTONATION IN THE LOW-ENERGY DECOMPOSITIONS OF GAS-PHASE PEPTIDE IONS

The dissociation of singly or multiply protonated peptide ions by usin g low-energy collisional activation (CA) is highly dependent on the si tes of protonation. The presence of strongly basic amino acid residues in the peptide primary structure dictates the sites of protonation, w hich generates a precursor ion population that is largely homogeneous with respect to charge sites. Attempts to dissociate this type of prec ursor ion population by low-energy CA result in poor fragmentation via few pathways. The work described here represents a systematic investi gation of the effects of charge heterogeneity in the precursor ion pop ulation of a series of model peptides in low-energy CA experiments. In corporation of acidic residues in the peptide RLCIFSC*FR (where C* in dicates a cysteic acid residue), for example, balances the charge on t he basic arginine residues, which enables the ionizing protons to resi de on a number of less basic sites along the peptide backbone. This re sults in a precursor ion population that is heterogeneous with respect to charge site. Low-energy CA of these ions results in diverse and ef ficient fragmentation. Molecular modeling has been utilized to demonst rate that energetically preferred conformations incorporate an intraio nic interaction between arginine and cysteic acid residues.