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 neu tral(s) lost as well as the mechanism of fragmentation. Three main mechanis ms have been previously proposed: (1) loss of the combined elements of H2O and CO; (2) loss of dihydroxycarbene (HO)(2)C: and (3) loss of formic acid, HC(=O)OH. Herein, ab initio and density functional theory calculations hav e been used to calculate the key reactants, transition stares, and products of these and several other competing reaction channels in the fragmentatio n 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)(2)C fragmentation processes. (J Am Soc Mass Spectrom 2000, 11, 687 -696) (C) 2000 American Society for Mass Spectrometry.