Abstract
As part of De Novo sequencing issue, new approaches have to be found to sequence small natural peptides (<15-20 residues) which often present amino acid compositions inducing merely singly charged species that are thus quite difficult to thoroughly fragment under low energy activation conditions in MS/MS experiments. Cationization by alkali metals, like Na+, followed by collision-induced dissociations (CID) or post-source metastable decay (PSD) of such cationized molecular ions were found to improve sequence coverage of native peptides through the formation of [bn-1+Na+OH]+ ions issued from C-terminal residue exclusion. Concerned by the identification of peptides with a C-terminal amide, the fragmentation pattern of their sodiated molecular ions was investigated. In contrast to the peptides featuring unmodified C-termini, the C-terminal loss was not occurring, the amide function triggering different fragmentations pathways. Whereas several residue such as aspartic acid (D), glutamic acid (E), and arginine (R) influenced dissociation of fixed charge sodiated ions similarly to protonated peptides, more surprisingly, serine (S), threonine (T) and tyrosine (Y), which exhibit a hydroxyl function on their side chains, showed a very peculiar behavior that could help De Novo peptide sequencing.