Tandem mass spectrometry of peptides: Mechanistic aspects and structural information based on neutral losses. II—Tri- and larger peptides

The neutral products arising during the collisionally activated dissociation of protonated oligopeptides (MH⁺) are post-ionized by collision and detected in neutral fragment-reionization (⁺N_fR⁺) mass spectra. For the isomeric 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_fR⁺ 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_fR⁺ products that are characteristic of the respective C-terminal amino acid. This is demonstrated for several peptide pairs, including leucine- and methionine-enkephalin. In general, ⁺N_fR⁺ spectra are dominated by the heavier neutral losses; further, ⁺N_fR⁺ and CIDI cause extensive dissociation, indicating that the collisional ionization process imparts high average internal energies.