Computational study of peptide bond formation in the gas phase through ion-molecule reactions

A computational study of peptide bond formation from gas-phase ion-molecule reactions has been carried out. We have considered the reaction between protonated glycine and neutral glycine, as well as the reaction between two neutral glycine molecules for comparison purposes. Two different mechanisms, concerted and stepwise, were studied. Both mechanisms show significant energy barriers for the neutral reaction. The energy requirements for peptide bond formation are considerably reduced upon protonation of one of the glycine molecules. For the reaction between neutral glycine and N-protonated glycine the lowest energy barrier is observed for the concerted mechanism. For the reaction between neutral glycine and protonated glycine at carbonyl oxygen, the preferred mechanism is the stepwise one, with a relatively small energy barrier (23 kJ mol(-1) at 0 K) and leading to the lowest-lying protonated glycylglycine isomer. In the case that the reaction could be initiated by protonated glycine at hydroxyl oxygen the process would be barrier-free and clearly exothermic. In that case peptide bond formation could take place even under interstellar conditions if glycine is present in space.