Fragmentation mechanisms of glycine-Cu⁺ in the gas phase. An experimental and theoretical study

Electrosprayed copper complexes of glycine (GlyCu^-) were formed from a glycine/CuX₂ (X = Cl, CH₃CO₂) mixture, in methanol. The collision-induced fragmentation of the most abundant isotopic form Gly⁶³Cu⁺ was studied as a function of collision energy. Four fragment ions are observed: ⁶³CuNHCH₂ ⁺, ⁶³Cu⁺, CH₂NH₂ ⁺, plus a minor loss of H₂O. The potential energy surface for formation of these fragment ions has been investigated at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* level. Several types of mechanisms were considered, involving either metal insertion into covalent bonds (C-C, C-O, and C-N) or dissociative attachment whereby the metal ion catalyzes the fragmentation by its distant electronic influence. Mechanisms starting with copper insertion into the C-C bond account for the most favorable pathways for the formations of CuNHCH₂⁺ and CH₂NH ₂⁺. Dissociative attachment cannot be excluded to participate to the formation of CuNHCH₂⁺ and is the only way to explain H₂O loss. Finally, calculations on the possible mechanisms for NH₃ loss (not observed experimentally) indicate that the observed ions are the result of a competition between several fragmentation modes with relatively similar energetic requirements.