Negative ion photoelectron spectroscopy of the copper-aspartic acid anion and its hydrated complexes

Negative ions of copper-aspartic acid Cu(Asp)− and its hydrated complexes have been produced in the gas phase and studied by anion photoelectron spectroscopy. The vertical detachment energies (VDE) of Cu(Asp)− and Cu(Asp)−(H2O)1,2 were determined to be 1.6, 1.95, and 2.20 eV, respectively. The spectral profiles of Cu(Asp)−(H2O)1 and Cu(Asp)−(H2O)2 closely resembled that of Cu(Asp)−, indicating that hydration had not changed the structure of Cu(Asp)− significantly. The successive shifts to higher electron binding energies by the spectra of the hydrated species provided measures of their stepwise solvation energies. Density functional calculations were performed on anionic Cu(Asp)− and on its corresponding neutral. The agreement between the calculated and measured VDE values implied that the structure of the Cu(Asp)− complex originated with a zwitterionic form of aspartic acid in which a copper atom had inserted into the N–H bond.