No abstract

The gas-phase reactions between Cu+ and formamide, as the 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)2Cu]+ 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.

Mass spectrometry measurements reveal that the reactions between guanidine and Cu+ in the gas phase leads predominantly to the loss of ammonia. The corresponding mechanisms were investigated by means of DFT calculations at the B3LYP/6-311+G(2df,2p)//B3LYP/6-311G(d,p) level. The attachment of Cu+ takes place preferentially at the imino nitrogen, while attachment at the amino leads to a local minimum which lies almost 26 kcal/mol higher in energy. The estimated guanidine−Cu+ binding energy is 77.9 kcal/mol. Hence, guanidine is predicted to be a stronger base than ammonia when the reference acid is Cu+, although this basicity difference is about 11 kcal/mol smaller than that found when the reference acid is H+. Two possible reaction channels, with almost equal activation barriers, lead to the loss of NH3. One of them yields the H2N−CN−Cu+ complex while the other yields the HNCNHCu+ complex. The structures and bonding characteristics of both products are discussed. Their estimated heats of formation are 225 ± 2 and 237 ± 2 kcal/mol, respectively. The structures and the relative stabilities of the complexes which can be formed by the solvation of these two products by ammonia have been also investigated since they are essential to explain the unimolecular NH3-loss processes observed.

The gas-phase reactions between Cu + and urea have been investigated by means of mass spectrometry techniques. The primary products formed in the ion source correspond to [urea-Cu] + , [(urea) 2 -Cu] + , and [Cu + ,C,N 2 ,H 2 ] complexes. The MIKE spectrum of [urea-Cu] + complex shows several spontaneous losses, namely, NH 3 and HNCO. A very weak peak corresponding to the loss of H 2 O is also observed, as well as a minor fragmentation of the adduct ion to yield Cu + . The structures and bonding characteristics of the different complexes involved in the urea-Cu + potential energy surface (PES) were investigated using density functional theory (DFT) at the B3LYP level of theory and a valence triple-ζ. Attachment of Cu + takes place preferentially at the carbonyl oxygen atom, while attachment at the amino group is 12.4 kcal/mol less exothermic. Insertion of the metal cation into the C-N bonds of the neutral is predicted to be slightly exothermic, in contrast with what was found for formamide and guanidine. The estimated urea-Cu + binding energy (62.3 kcal/mol) is 6.0 kcal/mol greater than that of formamide. The exploration of the PES indicates that there are several reaction paths leading to the loss of ammonia yielding as product ions HNCOCu + complexes where the metal cation is attached either to the oxygen or the nitrogen of the HNCO species. Also several reaction paths can be envisaged for the loss of HNCO, in which bisligated [HNCO-Cu-NH 3 ] + and [OC(NH)-Cu-NH 3 ] + complexes play an important role.