ANION AND CATION POST-SOURCE DECAY TIME-OF-FLIGHT MASS-SPECTROMETRY OF SMALL PEPTIDES - SUBSTANCE-P, ANGIOTENSIN-II, AND RENIN SUBSTRATE

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)(+) fragmen t ions and low-mass ions characteristic of individual amino acid resid ues. 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 [y(n)- 18](-) fragment ions are found in the anion spectra. Anion formation from substance P is particularly in teresting, because this peptide contains no acidic carboxyl groups to act as a charge site, In general, peptide rations produced predominant ly fragment ions from the N-terminus, while anion dissociation gave pr imarily C-terminal fragment ions. The results suggest that cation acid anion PSD yield complementary information on peptide primary structur es.