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Theoretical study of the main fragmentation pathways for protonated glycylglycine.

Quantum chemical and RRKM calculations were carried out on protonated glycylglycine in order to determine the atomic details of the main fragmentation pathways leading to formation of a1 and y1 ions. Two possible mechanisms were considered. The first path results in elimination of aziridinone as a neutral counterpart of the y1 ion formed. Our calculations show that this pathway has a relatively high threshold energy (48.6 kcal/mol) and the corresponding unimolecular rate constants are quite small even at large internal energy. An alternative pathway (a1-y1) proposed in the present paper seems, however, to be favored against the above 'aziridinone' one from the points of view of both energetics and kinetics. The 'a1-y1' pathway leads to simultaneous formation of a1 and y1 ions, the ratio of which depends on the energy distribution of the fragmenting species for a particular dipeptide. However, even if y1 ions are formed via the 'a1-y1' pathway, the corresponding neutrals eliminated do not have a strained cyclic aziridinone structure. Instead, in a two-step process, CO and NHCH2 are formed leading to neutral products energetically more favored than aziridinone. The available experimental data reevaluated in the present paper lend support to the 'a1-y1' pathway.