Modelling peptide-metal dication interactions: formamide-Ca2+ reactions in the gas phase.

The collision induced dissociation of formamide-Ca(2+) complexes produced in the gas phase through nanoelectrospray ionization yields as main products ions CaOH, HCNH, Ca(NH(2)), HCO(+) and Ca(NH(3)) and possibly Ca(H(2)O) and C,O,Ca, the latter being rather minor. The mechanisms behind these fragmentation processes have been established by analyzing the topology of the potential energy surface by means of B3LYP calculations carried out with a core-correlated cc-pWCVTZ basis set. The Ca(2+) complexes formed by formamide itself and formimidic acid play a fundamental role. The former undergoes a charge separation reaction yielding Ca(NH(2)) + HCO(+), and the latter undergoes the most favorable Coulomb explosion yielding Ca-OH + HCNH and is the origin of a multistep mechanism which accounts for the observed loss of water and HCN. Conversely, the other isomer of formamide, amino(hydroxyl)carbene, does not play any significant role in the unimolecular reactivity of the doubly charged molecular cation.