Density functional study on UV/VIS spectra of copper-protein active sites: the effect of mutations

UV/VIS Electron excitation spectra have been computed for large, realistic model systems of the blue copper protein family. Fully quantum-chemical calculations at the density-functional theory level employing polarized triple-ζ basis sets have been performed on systems of over 120 atoms, without symmetry. Different mutants, with the ligating methionine of the wild type Cu center exchanged for histidine (M121H) and glutamine (M121Q), have been investigated in order to obtain insight about how the influence of the exact surrounding milieu of the Cu-atom affects the computed spectrum. With sufficiently large model sizes, inclusion of the environment by using continuum solvation models do not change the spectra significantly. More direct and rigorous treatments are needed to reliably assess the effect of the surrounding protein on the electronic structure of the active sites.