Tandem mass spectrometry of peptides: Sequence information based on neutral losses. I—isomeric dipeptides

The neutral losses arising on the collisionally activated dissociation (CAD) of protonated dipeptides (MH+) are reionized by collision and detected in neutral fragment‐reionization (NfR) mass spectra. For the dipeptides investigated, which encompass Ala–Gly/Gly–Ala, Asp–Gly/Gly–Asp, Phe–Gly/Gly–Phe and Trp–Ala/Ala–Trp, the NfR spectra of each pair of isomers are distinct and permit the identification of the correct C‐terminal amino acid. The precise structure of this amino acid is elucidated by comparison of the NfR spectra with reference collision‐induced dissociative ionization (CIDI) mass spectra of authentic Gly, Ala, Asp, Phe and Trp, acquired from beams of these amino acids formed by dissociation of their proton‐bound dimers.