Investigation of the neutral loss of a full amino acid mass during collision‐induced dissociation of the b 3 + ion derived from a model peptide containing a 4‐aminobutyric acid residue

In a previous study we found that a dominant fragmentation pathway observed for collision‐induced dissociation (CID) of b   3 +derived from peptides with sequence AXAG, where X is γ ‐aminobutyric acid ( γ Abu) or ε ‐aminocaproic acid (Cap), involved the loss of 89 mass units (u). A neutral loss of 89 u corresponded to the free acid mass of an alanine (A) residue. This specific pathway was studied in greater detail here using a series of A( γ Abu)AG peptides with strategic positioning of 15 N, 13 C and 2 H isotope labels. Based on the extensive labeling, several possible routes to the net elimination of 89 u are proposed. One is based on initial elimination of either aziridinone or imine and CO, followed by opening of an oxazolinone, tautomerization and elimination of H 2 O. Another involves formation of an aziridinone by cleavage of the N‐terminal amide bond, and transfer of O and H atoms to this fragment via an H‐bonded ion‐molecule complex to complete the loss of 89 u. Both types of pathway include the transfer/migration of H atoms from the α ‐carbon position of γ Abu or A residues. Copyright © 2007 John Wiley & Sons, Ltd.