TANDEM MASS-SPECTROMETRY OF PEPTIDES - MECHANISTIC ASPECTS AND STRUCTURAL INFORMATION-BASED ON NEUTRAL LOSSES .2. TRI-PEPTIDES AND LARGER PEPTIDES

The neutral products arising during the collisionally activated dissociation of protonated oligopeptides (MH+) am post-ionized by collision and detected in neutral fragment-reionization (+N(f)R+) mass spectra. For the tripeptides Ala-Gly-Gly, Gly-Ala-Gly and Gly-Gly-Ala, the amino acid and dipeptide losses from the C-terminus and the diketopiperazine losses from the N-terminus allow for differentiation. These neutral fragments are identified in the corresponding +N(f)R+ spectra by comparison to reference collision-induced dissociative ionization (CIDI) mass spectra of individual amino acids, dipeptides and diketopiperazines. Peptides with distinct C-termini but otherwise identical sequences are found to yield +N(f)R+ products that are characteristic of the respective C-terminal amino acid. This is demonstrated for several peptide pairs, including leucine- and methioine-enkephalin. In general, +N(f)R+ spectra are dominated by the heavier neutral losses; further, +N(f)R+ and CIDI cause extensive dissociation, indicating that the collisional ionization process imparts high average internal energies.