Metal-interacted histidine dimer: an ETS-NOCV and XANES study

Abstract

We have analyzed the metal coordination in a histidine dimer, hydrated with a water molecule, based on the extended transition state scheme with the theory of natural orbitals for chemical valence (ETS-NOCV). Metal interaction weakens and strengthens the CO and C–N bonds respectively, which indicates σ bond formation between the metal and the ligand. Frequency analysis reveals CO bond exhibits a red shift, which confirms that the π-back donation of electrons takes place through this bond. From the ETS-NOCV analysis, the electrostatic term is dominant over the orbital term. NOCV analysis indicates that the switching of bond strength between C–N bond and CO bond in neutral and zwitterionic systems respectively is due to the amount of back donation in TM²⁺–N and TM²⁺–O bonds (TM²⁺ = Cu, Zn, Ca, Mg). K-edge XANES spectra for the metal indicates that oxidation state of Zn and Ca increases while for Cu and Mg it decreases. Oxygen K-edge spectra indicate that metals like zinc and copper back-donate the electrons largely to ligands.