An experimental and theoretical investigation of gas-phase reactions of Ca²⁺ with glycine

The gas-phase reactions between Ca²⁺ and glycine ([Ca(gly)] ²⁺) have been investigated through the use of mass spectrometry techniques and B3-LYP/cc-pWCVTZ density functional theory computations. The major peaks observed in the electrospray MS/MS spectrum of [Ca(gly)] ²⁺ correspond to the formation of the [Ca,C,O₂,H] ⁺, NH₂CH₂⁺, CaOH⁺, and NH₂CH₂CO⁺ fragment ions, which are produced in Coulomb explosion processes. The computed potential energy surface (PES) shows that not only are these species the most stable product ions from a thermodynamic point of view, but they may be produced with barriers lower than for competing processes. Carbon monoxide is a secondary product, derived from the unimolecular de composition of some of the primary ions formed in the Coulomb explosions. In contrast to what is found for the reactions of Ca ²⁺ with urea ([Ca-(urea)]²⁺), minimal unimolecular losses of neutral fragments are observed for the gas-phase fragmentation processes of [Ca(gly)]²⁺, which is readily explained in terms of the topological differences between their respective PESs.