Anion and Cation Post-source Decay Time-of-flight Mass Spectrometry of Small Peptides: Substance P, Angiotensin II, and Renin Substrate

Abstract

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]− ions by matrix-assisted laser desorption/ionization (MALDI). Dissociation of the protonated molecules yielded primarily a+n and b+n 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 y+n ions. In addition, b−n, c−n, x−n, and [yn18]− 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.