Simplifying fragmentation patterns of multiply charged peptides by N-terminal derivatization and electron transfer collision activated dissociation.

N-terminal peptide derivatization strategies used in conjunction with tandem mass spectrometry to yield simplified fragmentation patterns have shown limited success for the de novo sequencing of multiply charged peptides, including those predominantly formed in LC-ESI-MS experiments. Significant proton mobilization occurs for multiply charged peptides upon collisional activation, resulting in the formation of both N-terminal and C-terminal product ions rather than an exclusive series of C-terminal ions preferred for de novo sequencing algorithms. To circumvent this problem, multiply charged, N-terminally derivatized peptides were subjected to electron transfer reactions with fluoranthene anions to produce singly charged, radical species. Upon subsequent "soft" collision induced dissociation (CID), highly abundant z-type ions were formed nearly exclusively, which yielded simplified fragmentation patterns amenable to de novo sequencing methods. Furthermore, the derivatized peptides retained labile phosphoric acid moieties, and the enhanced set of z ions were also observed for peptides not possessing basic C-terminal residues, a type of peptide that poses more challenges to traditional simplification methods based on collision activated dissociation. This improved LC-MS(n) strategy was demonstrated for a variety of multiply charged model peptides and a tryptic digest of myoglobin.