TANDEM MASS-SPECTROMETRY OF PEPTIDES - SEQUENCE INFORMATION-BASED ON NEUTRAL LOSSES .1. 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 (N(f)R) 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 N(f)R 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 N(f)R 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.