"Dissociation of Multiply-charged Negative Ions for Hirudin (54-65), Fibrinopeptide B, and Insulin A (Oxidized)", N.P. Ewing and C.J. Cassady, J. Am. Soc. Mass Spectrom. 12, 105-116 (2001).

Collision-induced dissociation (CID) was performed on multiply-deprotonated ions from three commercial peptides: hirudin (54-65), fibrinopeptide B, and oxidized insulin chain A. Ions were produced by electrospray ionization in a Fourier transform ion cyclotron resonance mass spectrometer. Each of these peptides contains multiple acidic residues, which make them difficult to ionize in the positive mode. However, the peptides deprotonate readily making negative ion studies a viable alternative. The CID spectra indicated that the likely deprotonation sites are acidic residues (aspartic, glutamic and cysteic acids) and the C-terminus. The spectra are rife with c-, y- and internal ions, although some a-, b-, x- and z-ions form. A majority of the fragment ions were formed from cleavage adjacent to acidic residues, both N- and C-terminal to the acidic site. In addition, neutral loss (e.g., NH₃, CH₃, H₂O and CO₂) was prevalent from both the parent ions and from fragment ions. These neutral eliminations were often indicative of specific amino acid residues. The fragmentation patterns from several charge states of the parent ions, when combined, provide significant primary sequence information.