Dioxygen binding to deoxyhemocyanin: electronic structure and mechanism of the spin-forbidden two-electron reduction of o(2).
Dioxygen binding to deoxyhemocyanin: electronic structure and mechanism of the spin-forbidden two-electron reduction of o(2).
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DOI：
10.1021/ja004166b
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发表时间：
2001-05
影响因子：
15
通讯作者：
M. Metz;E. Solomon
中科院分区：
文献类型：
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作者：
M. Metz;E. Solomon
采用光谱校正的密度泛函理论（DFT）研究了O（2）与血蓝蛋白（Hc）结合的反应坐标。计算了一个反应路径，其中O（2）接近双核铜位点，金属-配体重叠增加，这将配位模式从末端的eta（1）-eta（1）切换到mu-eta（1）：eta（2），然后切换到蝶形，最后切换到平面[Cu（2）（mu-eta（2）：eta（2）O（2））]结构。O（2）结合过程中的电子结构分析表明，同时发生两个电子转移（ET）。在O（2）结合的早期阶段，三重态和单重态之间的能量差通过电荷转移（CT）减小，这使未成对电子离域，从而降低了分离的铜中心的交换稳定性。铜离子上的电子自旋最初是铁磁耦合的，这是由于通过双氧桥连配体接近正交的磁轨道路径，并且Cu（2）O（2）核结构的变化开启了铜离子之间的超交换耦合。这有利于单重态超过三重态，从而实现系统间交叉。与单核模型复合物的比较表明，蛋白质基质中的两个铜（I）中心非常接近，由于还原双核位点的不稳定性，这在热力学和熵上有利于O（2）结合。这也允许通过改变脱氧Hc中的Cu-Cu距离来调节焓，这为Hc中的O（2）结合协同性提供了解释。这些结果进行了比较，我们早期的研究Hemerythrin（Hr）和一个共同的主题出现在O（2）结合的自旋禁忌是克服通过离域的未成对电子到金属中心和超交换耦合的金属中心通过配体桥。
Spectroscopically calibrated DFT is used to investigate the reaction coordinate of O(2) binding to Hemocyanin (Hc). A reaction path is calculated in which O(2) approaches the binuclear copper site with increasing metal-ligand overlap, which switches the coordination mode from end-on eta(1)-eta(1), to mu-eta(1):eta(2), then to butterfly, and finally to the planar [Cu(2)(mu-eta(2):eta(2)O(2))] structure. Analysis of the electronic structures during O(2) binding reveals that simultaneous two-electron transfer (ET) takes place. At early stages of O(2) binding the energy difference between the triplet and the singlet state is reduced by charge transfer (CT), which delocalizes the unpaired electrons and thus lowers the exchange stabilization onto the separated copper centers. The electron spins on the copper(II) ions are initially ferromagnetically coupled due to close to orthogonal magnetic orbital pathways through the dioxygen bridging ligand, and a change in the structure of the Cu(2)O(2) core turns on the superexchange coupling between the coppers. This favors the singlet state over the triplet state enabling intersystem crossing. Comparison with mononuclear model complexes indicates that the protein matrix holds the two copper(I) centers in close proximity, which enthalpically and entropically favors O(2) binding due to destabilization of the reduced binuclear site. This also allows regulation of the enthalpy by the change of the Cu--Cu distance in deoxyHc, which provides an explanation for the O(2) binding cooperativity in Hc. These results are compared to our earlier studies of Hemerythrin (Hr) and a common theme emerges where the spin forbiddeness of O(2) binding is overcome through delocalization of unpaired electrons onto the metal centers and the superexchange coupling of the metal centers via a ligand bridge.