Collision-Induced Dissociation Threshold Energies of Protonated Glycine, Glycinamide, and Some Related Small Peptides and Peptide Amino Amides

Abstract

The energy thresholds for fragment ions from the collision-induced dissociation of the protonated amino acid glycine (Gly), glycinamides Gly-NH2 and Gly-NHCH3, the dipeptides Gly-Gly and Gly-Gly-NH2, and the peptides Gly-Gly-Gly and Gly-Gly-Gly-Gly were determined with a modified triple quadrupole mass spectrometer. The precursor ions were produced by electrospray. The threshold energies for the formation of the major fragment ions were determined by fitting the experimentally determined threshold curves to theoretical threshold equations following the procedure developed by Armentrout and co-workers. To obtain corrections for the kinetic shifts, several of the transition states were modeled by using ab initio or semiempirical calculations. The immonium ion, CH2NH2+, which is an a1 ion, was observed as a major fragment ion from precursor ions (H2NCH2CO-X)H+, where X = OH, NH2, NHCH3, and NH2COOH. The activation energies obtained from the thresholds were between 40 and 50 kcal/mol. These energies were in g...