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 gamma-aminobutyric acid (gammaAbu) or epsilon-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(gammaAbu)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 H2O. 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 alpha-carbon position of gammaAbu or A residues.