Fragmentation mechanisms of product ions from protonated tripeptides

Dissociation chemistries of the primary fragment ions from the tripeptides, GAG and AGG, were examined both experimentally and theoretically, and compared with those from GGG [El Aribi, H.; Rodriquez, C. F.; Almeida, D. R. P.; Ling, Y.; Mak, W. W.-N.; Hopkinson, A. C.; Siu, K. W. M. J. Am. Chem. Soc. 2003, 125, 9229-9236], Findings in this study with GAG and AGG confirm and extend those in the earlier study on GGG. Fragmentation of the b to a ion from GAG and AGG is qualitatively similar to that from GGG; stabilization by the methyl group, however, results in generally lower energies for GAG. Fragmentation of the a ion from GAG produces both the a (protonated methanimine) and the internal iminium ion (protonated ethanimine). By contrast, fragmentation of the a ion from AGG produces only the a ion (protonated ethanimine). In GAG, formation of the internal iminium ion requires an intramolecular proton transfer in the proton-bridged complex after cleavage of CO, which is absent in AGG. The a ion from GAG is postulated to form via cleavage of the vibrationally excited proton-bridged complex prior to proton transfer, favored under higher collision-energy conditions. The critical transition state in the fragmentation of the b to a ion is best described as a complex between the a ion and a carbene. Although details differ, in both GAG and AGG, there is a proton transfer from the terminal amino group to the carbene carbon and a second proton transfer from the ring nitrogen back to the terminal amino group. Separation of the components then yields the a ion and a neutral oxazolone.