A QM/MM study on the spinach plastocyanin: Redox properties and absorption spectra

The absorption spectrum of spinach plastocyanin protein is studied at time dependent density functional theory level. The reversible copper I–copper II oxido-reduction is also taken into account and the role of the protein is considered. The effects of the macromolecule environment are taken into account by a hybrid quantum mechanics/molecular mechanics method. The electrostatic interactions between the active site and the rest of the molecule have been shown to modify greatly the properties of the system and non-equilibrium polarization response of the protein environment to the transition has been considered in our model by the inclusion of a dielectric constant in the molecular mechanical part. The geometrical effects have also been considered by comparing results in the protein with simple isolated model systems. Particular attention is devoted to the intense 600 nm absorption band, and both oxidized (copper II) and reduced (copper I) systems have been studied. The nature of the excited states has been analyzed by using natural transition orbitals formalism. The redox properties of the protein have been analyzed by comparison of oxidized and reduced system, and the easy electron transfer is explained by the fact that the coordination geometry of the copper ion is constrained by the protein structure.