The fragmentation pathways of protonated glycine: a computational study.

Numerous studies have demonstrated that protonated aliphatic amino acids, [H2NCHRCO2H + H]+, fragment in the gas phase to form iminium ions, H2N=CHR+. Unfortunately none of these studies have probed the structure of the neutral(s) lost as well as the mechanism of fragmentation. Three main mechanisms have been previously proposed: (1) loss of the combined elements of H2O and CO; (2) loss of dihydroxycarbene (HO)2C: and (3) loss of formic acid, HC(=O)OH. Herein, ab initio and density functional theory calculations have been used to calculate the key reactants, transition states, and products of these and several other competing reaction channels in the fragmentation of protonated glycine. The loss of the combined elements of H2O and CO is thermodynamically and kinetically favored over the alternative formic acid or (HO)2C fragmentation processes.