title: Cleavage of [4Fe-4S]-type clusters: breaking the symmetry.
abstract: 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 Fe(2.5+)-Fe(2.5+) 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 Fe(3+)-Fe(2+) pairs. Cleavage first through the weak Fe(2+)-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 [Fe(4)S(4)L(4)](2-) (L = SCH(3), Cl, H) and heteroligand [Fe(4)S(4)(SCH(3))(2)L(2)](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.
authors.affiliation: Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227, USA.
authors.forename: Shuqiang
authors.initials: S
authors.lastname: Niu
authors.forename: Toshiko
authors.initials: T
authors.lastname: Ichiye
mesh_terms: D004583:Electrons; D007501:Iron / Q000737:chemistry*; D007506:Iron-Sulfur Proteins / Q000737:chemistry; D008024:Ligands; D011789:Quantum Theory; D013455:Sulfur / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D007506:Iron-Sulfur Proteins; D008024:Ligands; D013455:Sulfur; D007501:Iron