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)(-)(H(2)O)(1,2) were determined to be 1.6, 1.95, and 2.20 eV, respectively. The spectral profiles of Cu(Asp)(-)(H(2)O)(1) and Cu(Asp)(-)(H(2)O)(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.