Hansen D Flemming, Led Jens J
Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark.
J Am Chem Soc. 2004 Feb 4;126(4):1247-52. doi: 10.1021/ja0379464.
The biological function of metalloproteins stems from the electronic and geometric structures of their active sites. Thus, in blue copper proteins such as plastocyanins, an unusual electronic structure of the metal site is believed to contribute to the rapid, long-range electron-transfer reactivity that characterizes these proteins. To clarify this structure-function relationship, numerous quantum chemical calculations of the electronic structure of the blue copper proteins have been made. However, the obtained structures depend strongly on the applied model. Experimental approaches based on ENDOR spectroscopy and X-ray absorption have also been used to elucidate the electronic structure of the blue copper site. Still, the determination of the electronic structure relies on a calibration with quantum chemical calculations, performed on small model complexes. Here we present an approach that allows a direct experimental mapping of the electron spin delocalization in paramagnetic metalloproteins using oxidized plastocyanin from Anabaena variabilis as an example. The approach utilizes the longitudinal paramagnetic relaxation of protons close to the metal site and relies on the dependence of these relaxations on the spatial distribution of the unpaired electron of the metal ion. Surprisingly it is found that the unpaired electron of the copper ion in plastocyanin is less delocalized than predicted by most of the quantum chemical calculations.
金属蛋白的生物学功能源于其活性位点的电子结构和几何结构。因此，在诸如质体蓝素等蓝色铜蛋白中，金属位点异常的电子结构被认为有助于这些蛋白所特有的快速、长程电子转移反应活性。为了阐明这种结构-功能关系，人们对蓝色铜蛋白的电子结构进行了大量量子化学计算。然而，所得到的结构强烈依赖于所采用的模型。基于电子核双共振光谱和X射线吸收的实验方法也被用于阐明蓝色铜位点的电子结构。尽管如此，电子结构的确定仍依赖于对小分子模型配合物进行量子化学计算的校准。在此，我们以多变鱼腥藻的氧化质体蓝素为例，提出一种方法，可直接对顺磁性金属蛋白中的电子自旋离域进行实验测绘。该方法利用靠近金属位点的质子的纵向顺磁弛豫，并依赖于这些弛豫对金属离子未配对电子空间分布的依赖性。令人惊讶的是，发现质体蓝素中铜离子的未配对电子的离域程度比大多数量子化学计算所预测的要小。
