Kästner Johannes, Hemmen Sascha, Blöchl Peter E
Institute for Theoretical Physics, Clausthal University of Technology, D-38678 Clausthal-Zellerfeld, Germany.
J Chem Phys. 2005 Aug 15;123(7):074306. doi: 10.1063/1.2008227.
The protonation of N2 bound to the active center of nitrogenase has been investigated using state-of-the-art density-functional theory calculations. Dinitrogen in the bridging mode is activated by forming two bonds to Fe sites, which results in a reduction of the energy for the first hydrogen transfer by 123 kJ/mol. The axial binding mode with open sulfur bridge is less reactive by 30 kJ/mol and the energetic ordering of the axial and bridged binding modes is reversed in favor of the bridging dinitrogen during the first protonation. Protonation of the central ligand is thermodynamically favorable but kinetically hindered. If the central ligand is protonated, the proton is transferred to dinitrogen following the second protonation. Protonation of dinitrogen at the Mo site does not lead to low-energy intermediates.
利用最先进的密度泛函理论计算方法，对与固氮酶活性中心结合的N₂的质子化过程进行了研究。桥连模式下的双氮通过与铁位点形成两个键而被激活，这使得第一次氢转移的能量降低了123 kJ/mol。具有开放硫桥的轴向结合模式的反应活性低30 kJ/mol，并且在第一次质子化过程中，轴向和桥连结合模式的能量顺序发生反转，有利于桥连双氮。中心配体的质子化在热力学上是有利的，但在动力学上受到阻碍。如果中心配体被质子化，质子会在第二次质子化后转移到双氮上。钼位点上双氮的质子化不会产生低能量中间体。
