Comparison of the inner-sphere reorganization energies of cytochromes, iron-sulfur clusters, and blue copper proteins

Inner-sphere reorganization energies have been calculated for a number of models of six-coordinate iron porphyrins (with varying axial ligands), using the density functional B3LYP method. If the axial ligands are uncharged, the reorganization energy is very low, 5-9 kJ/mol. If one of the axial ligands is charged, the reorganization energy is higher, 20-47 kJ/mol, but such sites are normally not used in electron carriers. The former reorganization energies are appreciably smaller than what was found for blue copper proteins (62-90 kJ/mol), the dimeric Cua site in cytochrome c oxidase and nitrous oxide reductase (43 kJ/mol), and six different types of iron-sulfur clusters with one, two, or four iron atoms (40-75 kJ/mol), even if these vacuum energies are typically halved inside the protein (as a result of hydrogen bonds and solvation effects). Therefore, the cytochromes seem to have the inherently lowest inner-sphere reorganization energy of the three commonly used electron carriers. All three types of sites have reduced the reorganization energy by using a delocalized charge and N- and S-donors (rather than O-donors) as metal ligands. Moreover, iron is a more appropriate metal for electron transfer than copper because Fe(II) and Fe(III) prefer the same coordination number and geometry and give bonds weaker than those of copper. The low-spin state of the cytochrome has a similar to 20 kJ/mol lower reorganization energy than that of the corresponding high-spin site. Moreover, ring strain in the porphyrin reduce the changes in the Fe-Np,, distances by 5 pm and therefore the reorganization energy by 8 kJ/mol.