Identification of a new fragment ion type in the collision-induced dissociation spectra of peptides: formation of a 2-16 ions

Abstract

The identification of an ion observed in the high-energy collision-induced dissociation spectra of several model peptides is reported. The ion, observed at m/z 99 for the peptide pentaalanine (Ala 5) and designated a 2-16, is shown to have an elemental formula of C 5H 9NO by high-resolution peak matching. The precursor ion spectrum of the a 2-16 ion and product ion spectra of the a 2 and the a 2 + 1 ions for Ala 5 suggest that this ion is formed by the loss of 17 u (presumably NH 3) from the a 2 + 1 ion and, to a lesser extent, by loss of 28 u (presumably CO) from the b 2-16 ion. On the basis of the data presented and other experimental evidence, a structure and mechanism for the formation of the a 2-16 ion is proposed.