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Density functional theory study of an all ferrous 4Fe-4S cluster.

ABSTRACT: The all-ferrous, carbene-capped Fe(4)S(4) cluster, synthesized by Deng and Holm (DH complex), has been studied with density functional theory (DFT). The geometry of the complex was optimized for several electronic configurations. The lowest energy was obtained for the broken-symmetry (BS) configuration derived from the ferromagnetic state by reversing the spin projection of one of the high spin (S(i) = 2) irons. The optimized geometry of the latter configuration contains one unique and three equivalent iron sites, which are both structurally and electronically clearly distinguishable. For example, a distinctive feature of the unique iron site is the diagonal Fe···S distance, which is 0.3 Å longer than for the equivalent irons. The calculated (57)Fe hyperfine parameters show the same 1:3 pattern as observed in the Mössbauer spectra and are in good agreement with experiment. BS analysis of the exchange interactions in the optimized geometry for the 1:3, M(S) = 4, BS configuration confirms the prediction of an earlier study that the unique site is coupled to the three equivalent ones by strong antiferromagnetic exchange (J > 0 in J ?(j<4)?(4)·?(j)) and that the latter are mutually coupled by ferromagnetic exchange (J' < 0 in J' ?(i

SUBMITTER: Chakrabarti M

PROVIDER: S-EPMC3107574 | biostudies-literature | 2011 May

REPOSITORIES: biostudies-literature

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Density functional theory study of an all ferrous 4Fe-4S cluster.

Chakrabarti Mrinmoy M Münck Eckard E Bominaar Emile L EL

Inorganic chemistry 20110408 10

The all-ferrous, carbene-capped Fe(4)S(4) cluster, synthesized by Deng and Holm (DH complex), has been studied with density functional theory (DFT). The geometry of the complex was optimized for several electronic configurations. The lowest energy was obtained for the broken-symmetry (BS) configuration derived from the ferromagnetic state by reversing the spin projection of one of the high spin (S(i) = 2) irons. The optimized geometry of the latter configuration contains one unique and three equ ...[more]

PMID: 21476577

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Project description: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 [Fe(4)S(4)L(4)](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.

Project description:The key enzyme of the fermentation of glutamate by Acidaminococcus fermentans, 2-hydroxyglutaryl-coenzyme A dehydratase, catalyzes the reversible syn-elimination of water from (R)-2-hydroxyglutaryl-coenzyme A, resulting in (E)-glutaconylcoenzyme A. The dehydratase system consists of two oxygen-sensitive protein components, the activator (HgdC) and the actual dehydratase (HgdAB). Previous biochemical and spectroscopic studies revealed that the reduced [4Fe-4S]+ cluster containing activator transfers one electron to the dehydratase driven by ATP hydrolysis, which activates the enzyme. With a tenfold excess of titanium(III) citrate at pH 8.0 the activator can be further reduced, yielding about 50% of a superreduced [4Fe-4S]0 cluster in the all-ferrous state. This is inferred from the appearance of a new Mössbauer spectrum with parameters delta = 0.65 mm/s and deltaE(Q) = 1.51-2.19 mm/s at 140 K, which are typical of Fe(II)S4 sites. Parallel-mode electron paramagnetic resonance (EPR) spectroscopy performed at temperatures between 3 and 20 K showed two sharp signals at g = 16 and 12, indicating an integer-spin system. The X-band EPR spectra and magnetic Mössbauer spectra could be consistently simulated by adopting a total spin S(t) = 4 for the all-ferrous cluster with weak zero-field splitting parameters D = -0.66 cm(-1) and E/D = 0.17. The superreduced cluster has apparent spectroscopic similarities with the corresponding [4Fe-4S]0 cluster described for the nitrogenase Fe-protein, but in detail their properties differ. While the all-ferrous Fe-protein is capable of transferring electrons to the MoFe-protein for dinitrogen reduction, a similar physiological role is elusive for the superreduced activator. This finding supports our model that only one-electron transfer steps are involved in dehydratase catalysis. Nevertheless we discuss a common basic mechanism of the two diverse systems, which are so far the only described examples of the all-ferrous [4Fe-4S]0 cluster found in biology.

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Density functional theory study of an all ferrous 4Fe-4S cluster.

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Density functional theory study of an all ferrous 4Fe-4S cluster.

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