Cleavage of [4Fe--4S]-Type Clusters: Breaking the Symmetry
The cleavage of [4Fe--4S]-type clusters is thought to be important in proteins such as Fe--S scaffold proteins and nitrogenase. However, most [4Fe--4S]2⁺ clusters in proteins have two antiferromagnetically coupled high-spin layers in which a minority spin is delocalized in each layer, thus forming a symmetric Fe2.5⁺+--Fe2.5⁺ pair, and how cleavage occurs between the irons is puzzling because of the shared electron. Previously, we proposed a novel mechanism for the fission of a [4Fe--4S] core into two [2Fe--2S] cores in which the minority spin localizes on one iron, thus breaking the symmetry and creating a transition state with two Fe3⁺--Fe2⁺ pairs. Cleavage first through the weak Fe2⁺--S bonds lowers the activation energy. Here, we propose a test of this mechanism: break the symmetry of the cluster by changing the ligands to promote spin localization, which should enhance reactivity. The cleavage reactions for the homoligand [Fe₄S₄L₄]2⁻ (L = SCH3, Cl, H) and heteroligand [Fe₄S₄ (SCH3)2L2]2⁻ (L = Cl, H) clusters in the gas phase were examined via broken-symmetry density functional theory calculations. In the heteroligand clusters, the minority spin localized on the iron coordinated by the weaker electron-donor ligand, and the reaction energy and activation barrier of the cleavage were lowered, which is in accord with our proposed mechanism and consistent with photoelectron spectroscopy and collision-induced dissociation experiments. These studies suggest that proteins requiring facile fission of their [4Fe--4S] cluster in their biological function might have spin-localized [4Fe--4S] clusters.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 969902
- Journal Information:
- Journal of Physical Chemistry A, 113(19):5710–5717, Vol. 113, Issue 19; ISSN 1089-5639
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
99 GENERAL AND MISCELLANEOUS//MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE
ACTIVATION ENERGY
BINDING ENERGY
BIOLOGICAL FUNCTIONS
CLEAVAGE
DISSOCIATION
ELECTRONS
FISSION
FUNCTIONALS
IRON
NITROGENASE
PHOTOELECTRON SPECTROSCOPY
PROTEINS
SPIN
SYMMETRY
VALENCE
Environmental Molecular Sciences Laboratory