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

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• 作者：Shuqiang Niu ; Toshiko Ichiye
• 刊名：Journal of Physical Chemistry A
• 出版年：2009
• 出版时间：May 14, 2009
• 年：2009
• 卷：113
• 期：19
• 页码：5671-5676
• 全文大小：177K
• 年卷期：v.113,no.19(May 14, 2009)
• ISSN：1520-5215

文摘

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, from 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 PES 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 PES experiments can unravel the complex underlying factors in the redox energy and chemical bonding of the [4Fe−4S] clusters in iron−sulfur proteins.