DFT Studies of Cubane [4Fe-4S]2+/3+ Clusters: J-Aggregates of FeS Tetramers

Iron sulfur clusters are essential cofactors of numerous proteins that play many important biological roles due to their unique reactivity—a product of their geometry, oxidation, and spin states. The geometry of the iron sulfur clusters of [4Fe-4S]2+ and [4Fe-4S]3+ in various spin states was investigated using density functional theory (DFT). Geometry optimizations and vibrational frequency calculations were carried out on these clusters using the B3LYP functional and the 6-311+G(d,p) basis set. The most stable spin state for the [4Fe-4S]2+ cluster was found to be the 19-et, an antisymmetric structure. The most stable spin state for the [4Fe-4S]3+ cluster was found to be the 20-et, also an antisymmetric structure. Interestingly, these cubane clusters consist of J aggregates of four species of FeS and FeS+. The breakage of a monomer FeS from a [4Fe-4S] cluster under different environments may be responsible for the observations of noncubane [4Fe-4S] clusters. Furthermore, with increased spin state, the positive and negative charges on the iron and sulfur increased and decreased, respectively, and the structure of cubane [4Fe-4S]2+ cluster evolves from the structure with a Fe tetramer as core and S as ligands to J-aggregate of FeS and to a homogeneous [4Fe-4S]2+ cluster.