Peptide de novo sequencing facilitated by a dual-labeling strategy.

A novel peptide derivatization strategy based on guanidination and amidination is presented. Mass-coded labels help distinguish N- and C-terminal fragment ions produced by collision-induced dissociation and are of general utility since peptide N-termini are coded. The amidine labels also promote specific fragmentation pathways that elucidate N-terminal residues and provide valuable internal calibrants. This strategy is demonstrated with the tryptic peptides of several model proteins, including two that are phosphorylated. Additionally, interpreted peptide sequences are matched against a database of over 80,000 proteins to assess the selectivity of this sequencing approach.