摘要
Collision-induced dissociation of protonated AGabaAIG (where Gaba is gamma-amino butyric acid, NH2-(CH2)3-COOH) leads to an unusually stable a3 ion. Tandem mass spectrometry and theory are used here to probe the enhanced stability of this fragment, whose counterpart is not usually observed in CID of protonated peptides containing only alpha amino acids. Experiments are carried out on the unlabelled and 15N-Ala labeled AGabaAIG (labeled separately at residue one or three) probing the b3, a3, a3-NH3 (a3*), and b2 fragments while theory is used to characterize the most stable b3, a3, and b2 structures and the formation and dissociation of the a3 ion. Our results indicate the AGabaA oxazolone b3 isomer undergoes head-to-tail macrocyclization and subsequent ring opening to form the GabaAA sequence isomer while this chemistry is energetically disfavored for the AAA sequence. The AGabaA a3 fragment also undergoes macrocyclization and rearrangement to form the rearranged imine-amide isomer while this reaction is energetically disfavored for the AAA sequence. The barriers to dissociation of the AGabaA a3 ion via the a3 鈫?#xA0;b2 and a3 鈫?#xA0;a3* channels are higher than the literature values reported for the AAA sequence. These two effects provide a clear explanation for the enhanced stability of the AGabaA a3 ion.