Modeling the interactions between peptide functions and Cu(I):: Formamide-Cu+ reactions in the gas phase

The gas-phase reactions between Cu+ and formamide, as ihe most simple model of a peptide function, have been investigated through the use of mass spectrometry techniques. The primary products formed in the ion source correspond mainly to three types of complexes: (i) those formed by direct interaction of Cu+ with formamide: [formamide-Cu](+), [(formamide)(2)Cu](+) complexes, (ii) secondary products generated by association of these ions with ammonia: [formamide-Cu-NH3](+) complexes, (iii) secondary products formed by interactions of [Cu2H](+) clusters with residual HNCO coming from the formamide-Cu(+ )complexes elimination, namely [HNCO,Cu2H](+) species. The structures and bonding characteristics of these systems were studied by means of the B3LYP DFT approach. The [formamide-Cu](+) potential energy surfaces were studied at the B3LYP/6-311+G(2df,2p) level in order to explore the validity of formamide to model peptidic reactivity with respect to Cu+. This survey shows that the attachment of Cu+ takes place preferentially at the carbonyl group, while attachment at the amino leads to a local minimum which lies 21 kcal/mol higher in energy. The estimated formamide-Cu+ binding energy (56.2 kcal/mol) is equal to that previously reported for ammonia, although its intrinsic basicity with respect to H+ is 7 kcal/mol smaller. The MIKE spectra of the different primary ions formed in the reaction have also been analyzed. For the particular case of formamide, CAD spectra have been also performed in order to have a more complete description of its reactivity. Starting from the [formamide-Cu](+) complexes, several reaction channels leading to the loss of Cu+, H2O, NH3, HCO, and HCN/CNH have been considered.