Niu Shuqiang, Ichiye Toshiko
Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227, USA.
J Phys Chem A. 2009 May 14;113(19):5710-7. doi: 10.1021/jp900402y.
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 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) (L = SCH(3), Cl, H) and heteroligand Fe(4)S(4)(SCH(3))(2)L(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.
[4Fe-4S]型簇的裂解在诸如铁硫支架蛋白和固氮酶等蛋白质中被认为是重要的。然而，蛋白质中的大多数4Fe-4S簇具有两个反铁磁耦合的高自旋层，其中少数自旋在每层中离域，从而形成对称的Fe(2.5+)-Fe(2.5+)对，由于电子共享，铁之间如何发生裂解令人困惑。此前，我们提出了一种将[4Fe-4S]核心裂变为两个[2Fe-2S]核心的新机制，其中少数自旋定位于一个铁上，从而打破对称性并产生具有两个Fe(3+)-Fe(2+)对的过渡态。首先通过弱的Fe(2+)-S键进行裂解降低了活化能。在此，我们提出对该机制的一个测试：通过改变配体来打破簇的对称性以促进自旋定域化，这应该会增强反应性。通过破缺对称性密度泛函理论计算研究了气相中同配体Fe(4)S(4)L(4)（L = SCH(3)、Cl、H）和异配体Fe(4)S(4)(SCH(3))(2)L(2)（L = Cl、H）簇的裂解反应。在异配体簇中，少数自旋定位于由较弱给电子配体配位的铁上，裂解的反应能和活化势垒降低，这与我们提出的机制一致，并且与光电子能谱和碰撞诱导解离实验相符。这些研究表明，在其生物学功能中需要其[4Fe-4S]簇易于裂变的蛋白质可能具有自旋定域化的[4Fe-4S]簇。
