"Anion and Cation Post-Source Decay Time-of-Flight Mass Spectrometry of Small Peptides: Substance P, Angiotensin II, and Renin Substrate," J. Jai-nhuknan and C.J. Cassady, Rapid Commun. Mass Spectrom. 10, 1678-1682 (1996).

Substance P, angiotensin II, and renin substrate tetradecapeptide were used to compare post-source decay (PSD) fragmentation of positive and negative peptide ions in a reflectron time-of-flight mass spectrometer. The peptides readily produced both [M+H]⁺ and [M-H]^- by matrix-assisted laser desorption/ionization (MALDI). Dissociation of the protonated molecular ions yielded primarily an+ and bn+ fragment ions and low mass ions characteristic of individual amino acid residues. In contrast, the distinguishing feature of the anion spectra is a complete, or almost complete, series of yn- ions. In addition, b_n^-, c_n^-, x_n^-, and [y_n-18]^- fragment ions are found in the anion spectra. Anion formation from substance P is particularly interesting because this peptide contains no acidic carboxyl groups to act as a charge site. In general, peptide cations produced predominantly fragment ions from the N-terminus, while anion dissociation gave primarily C-terminal fragment ions. The results suggest that cation and anion PSD yield complementary information on peptide primary structures.