All metal coordination complexes were fully optimized using DFT methods as implemented in the Gaussion09 quantum chemistry package. The binding energies were estimated by employing ωB97xD///6-31G*(lanl2dz for metal) level of theory, followed by a higher level optimization using 6-311+G**(SDD for metal). Solvation effects were considered employing SMD, based on charge density of a solute molecule interacting with a continuum description of the solvent. The Gibbs free energy correction was obtained by frequency analysis. Gibbs free energies for histidine were calculated by computing the energy differences in the following equation: