Orbital interactions, electron delocalization and spin coupling in iron-sulfur clusters

Abstract

The interconnections among orbital interactions, electron delocalization and spin coupling in iron-sulfur clusters are reviewed, with special attention to the complex nature of spin and orbital states in 4Fe4S complexes. We summarize the uses of broken symmetry density functional calculations and spin projection methods for extracting Heisenberg spin coupling and electron delocalization parameters, as well as for understanding charge distributions and orbital aspects of electronic structure. The value of spin projection coefficients for sorting out spin coupling patterns in complex systems is also emphasized. Among the systems examined are oxidized, high-potential, iron-sulfur proteins, 4Fe ferredoxin proteins and related synthetic complexes. By analysis of experimental hyperfine parameters, a detailed model of spin coupling for the “double cubane” P cluster of nitrogenase has been proposed in recent work based on Mössbauer, electron paramagnetic resonance (EPR) and X-ray structural data; there is one pairwise valence delocalized and one trapped valence cubane in the P (oxidized) state. In the area of electron transfer energetics, we have found that Heisenberg spin coupling and electron delocalization both contribute substantially to the redox potentials of 4Fe4S complexes, and Heisenberg coupling contributes to the difference in redox potential between 1Fe and 2Fe2S complexes, based on recent density functional calculations for model systems in solvents.