DFT Studies of Cubane [4Fe-4S], [4Fe-4S]+, and [4Fe-4S]4+ Clusters

Iron sulfur clusters are metallic cofactors found in iron-sulfur proteins that have many important biological roles, including electron transfer. The versatile [4Fe-4S] cluster has a special electronic environment which leads to its unique reactivity. Using DFT calculations, the geometry of the [4Fe-4S] cluster at 0, +, and 4+ oxidation states was investigated at a variety of spin states. The B3LYP functional and 6-311+G(d,p) basis set were used to perform gas phase geometry optimization and vibrational frequency calculations on the clusters. The neutral [4Fe-4S] cluster's most stable spin state is the 17et (16 unpaired electrons), while the +1 cluster has a most stable spin of 18et, and the +4 cluster of 21et. Consistent with what was previously observed, the symmetry and the cubicity of the structures increase with increasing oxidation and spin state. Additionally, bond length is seen to consistently increase with spin and oxidation state. Interestingly, the HOMO/LUMO gap of the most stable forms of the 0, +1, and +4 clusters (1.87eV, 1.73eV, and 1.79eV, respectively) were considerably lower than those of the 2+ and 3+ oxidation states (4.25eV and 4.23eV). Our results support the hypothesis that the structure consists of J aggregates of FeS monomers.