Identification of a new fragment ion type in the collision-induced dissociation spectra of peptides: Formation of a2-16 ions.

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 (Ala5) and designated a2-16, is shown to have an elemental formula of C5H9NO by high-resolution peak matching. The precursor ion spectrum of the a2-16 ion and product ion spectra of the a2 and the a2+ 1 ions for Ala5 suggest that this ion is formed by the loss of 17 u (presumably NH3) from the a2+1 ion and, to a lesser extent, by loss of 28 u (presumably CO) from the b2-16 ion. On the basis of the data presented and other experimental evidence, a structure and mechanism for the formation of the a2-16 ion is proposed.