Probing Ligand Effects on the Redox Energies of [4Fe-4S] Clusters Using Broken-Symmetry Density Functional Theory

A central issue in understanding redox properties of iron-sulfur proteins is determining the factors that tune the reduction potentials of the Fe-S Clusters. Recently, Solomon and coworkers have shown that the Fe-S bond covalency of protein analogs measured by %L, the percent ligand character of the Fe 3d orbitals, front ligand K-edge X-ray absorption spectroscopy (XAS) correlates with the electrochemical redox potentials. Also, Wang and coworkers have measured electron detachment energies for iron-sulfur clusters without environmental perturbations by gas-phase photoelectron spectroscopy (PES). Here the correlations of the ligand character with redox energy and %L character are examined in [Fe4S4L4](2-) Clusters with different ligands by broken symmetry density functional theory (BS-DFT) calculations using the B3LYP functional together with PES and XAS experimental results. These gas-phase Studies assess ligand effects independently of environmental perturbations and thus provide essential information for computational Studies of iron-sulfur proteins. The B3LYP oxidation energies agree well with PIES data, and the %L character obtained from natural bond orbital analysis correlates with XAS values, although it systematically underestimates them because of basis set effects. The results show that stronger electron-donating terminal ligands increase %L-t, the percent ligand character from terminal ligands, but decrease %S-b, the percent ligand character from the bridging sulfurs. Because the oxidized Orbital has significant Fe-L-t antibonding character, the oxidation energy correlates well with %L-t. However, because the reduced orbital has varying contributions of both Fe-L-t and Fe-S-b antibonding character, the reduction energy does not correlate with either %L-t or %S-b. Overall, BS-DFT calculations together with XAS and PIES experiments call unravel the complex underlying factors in the redox energy and chemical bonding of the [4Fe-4S] clusters in iron-sulfur proteins.