Electronic and Structural Properties of the Double Cubane Iron-Sulfur Cluster

• Main Authors: Nadia Elghobashi-Meinhardt, Daria Tombolelli, Maria Andrea Mroginski
• Format: Article
• Language: English
• Published: MDPI AG 2021-02-01
• Series: Catalysts
• Subjects: [Fe8−S9] iron-sulfur cluster; double-cubane cluster (DCC); broken symmetry (BS); density function theory (DFT); quantum mechanics (QM)/molecular mechanics (MM); geometry optimizations
• Online Access: https://www.mdpi.com/2073-4344/11/2/245

The double-cubane cluster (DCC) refers to an [Fe₈S₉] iron-sulfur complex that is otherwise only known to exist in nitrogenases. Containing a bridging µ₂-S ligand, the DCC in the DCC-containing protein (DCCP) is covalently linked to the protein scaffold via six coordinating cysteine residues. In this study, the nature of spin coupling and the effect of spin states on the cluster’s geometry are investigated computationally. Using density functional theory (DFT) and a broken symmetry (BS) approach to study the electronic ground state of the system, we computed the exchange interaction between the spin-coupled spins of the four FeFe dimers contained in the DCC. This treatment yields results that are in excellent agreement with both computed and experimentally determined exchange parameters for analogously coupled di-iron complexes. Hybrid quantum mechanical (QM)/molecular mechanical (MM) geometry optimizations show that cubane cluster A closest to charged amino acid side chains (Arg312, Glu140, Lys146) is less compact than cluster B, indicating that electrons of the same spin in a charged environment seek maximum separation. Overall, this study provides the community with a fundamental reference for subsequent studies of DCCP, as well as for investigations of other [Fe₈S₉]-containing enzymes.