Collision-induced dissociation threshold energies of protonated glycine, glycinamide, and some related small peptides and peptide amino amides

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 a(1) ion, was observed as a major fragment ion from precursor ions (H2NCH2CO-X)H+, where X = OH, NH2, NNCH3, and NH2COOH. The activation energies obtained from the thresholds were between 40 and 50 kcal/mol. These energies were in good agreement with theoretically evaluated activation energies for transition states based on a mechanism proposed by Harrison and co-workers. A second pathway leading to XH2+ (y(1)) ions was observed for X = NHCH3 and NHCH2COOH. The threshold based activation energy was somewhat lower than that for the a(1) ions. Agreement between the threshold based activation energy and the calculated activation energy for a transition state involving an aziridinone intermediate provides support for the mechanism proposed by Wesdemiotis and co-workers. The lowest energy fragmentation pathway for ions with the general structure H+-(Gly-Gly-X) was found to be the b(2) (acylium) ion. In the case where X = NH2, the activation energy was only 20.4 kcal/mol. This very low energy was found consistent with the b(2) ions being cyclic, protonated oxazalones, as proposed by Harrison and co-workers. The thresholds for the fragment ions from larger precursor ions were affected by very large kinetic shifts and supression by competitive decompositions. Accurate activation energies for these reactions could not be obtained. Nevertheless, the threshold curves provide a view of the evolution into the complex pathways occurring for the higher polypeptides.