Observation and interpretation of temperature-dependent valence delocalization in the [2Fe-2S]+ cluster of a ferredoxin from Clostridium pasteurianum
Observation and interpretation of temperature-dependent valence delocalization in the [2Fe-2S]+ cluster of a ferredoxin from Clostridium pasteurianum
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DOI：
10.1021/ja983980k
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发表时间：
1999-04-21
影响因子：
15
通讯作者：
Münck, E
中科院分区：
文献类型：
--
作者：
Achim, C;Bominaar, EL;Münck, E
我们给出了野生型和C56S变异型的[2Fe-2S](+)团簇的穆斯堡尔谱和磁化强度的研究结果。在pH=11的Tris/Caps缓冲液中，C56S变体的样品在4.2K时含有价局域化的S=1/2(FD(1/2))和价离域化的S=9/2(FD(9/2))基态的[2Fe-2S](+)团簇的1：1混合物。用自旋哈密顿方法对8.0T以下的穆斯堡尔谱进行了分析，得到了FD(9/2)的精细结构和超精细参数：D-9/2=-1.5 cm(-1)，E/D=0.11，A=(-13，-13，-9.1)MHz，E-Q=1.83 mm/S，ETa=0，对FD(9/2)和FD(1/2)的超精细张量分量的比较分析表明，内禀A值不能直接从定域系统转移到离域系统。这一结果表明，价离域化伴随着电子结构的额外修改。高温穆斯堡尔谱研究表明，价离域团簇的比例增加，从4.2K时的约50%增加到200K时的约94%。磁化率研究排除了高温下产生的离域部分是FD(1/2)向Fd(9/2)自旋转化的结果，相反，离域部分的变化是由于Fd(1/2)的两个铁中心之间的分子内电子转移速率的快速增加。这种从局域到离域的转变以前没有在任何Fe-S团簇中观察到过。光谱特征和转变的温度范围(约100K)表明，Fd(1/2)铁位之间的电子转移速率分布，并指向不同构象的团簇与蛋白质相互作用产生的电子参数值的分散。用考虑了Heisenberg-Dirac-van Vleck交换、自旋相关的共振相互作用和振动俘获的量子力学方法计算了Fd(1/2)铁位间的电子转移速率。通过假设表征这些相互作用的参数的分布，我们已经能够模拟离域FD(1/2)分子分数的温度依赖关系。我们的研究表明，铁氧还蛋白在Fe-S团簇中的电子转移速率可能探索蛋白质部分的构象亚态。
We present the results of Mossbauer and magnetization studies of [2Fe-2S](+) clusters from the wild-type and the C56S variant 2Fe-ferredoxin from Clostridium pasteurianum. At pH = 11 in Tris/Caps buffer, samples of the C56S variant contain, at 4.2 K, a 1:1 mixture of [2Fe-2S](+) clusters with valence localized S = 1/2 (Fd(1/2)) and valence-delocalized S = 9/2 (Fd(9/2)) ground states. A spin Hamiltonian analysis of Mossbauer spectra recorded in applied fields up to 8.0 T provides the fine structure and hyperfine parameters for Fd(9/2): D-9/2 = -1.5 cm(-1), E/D = 0.11, A = (-13, -13, -9.1) MHz, Delta E-Q = 1.83 mm/s, eta = 0, and delta = 0.50 mm/s. A comparative analysis of the hyperfine tensor components for Fd(9/2) and Fd(1/2) shows that the intrinsic A-values cannot be directly transferred from localized to delocalized systems. This result indicates that valence delocalization is accompanied by additional modifications in the electronic structure. High temperature Mossbauer studies show an increase in the fraction of valence-delocalized clusters, from ca. 50% at 4.2 K to ca. 94% at 200 K. Magnetic susceptibility studies rule out that the delocalized fraction generated at high temperature results from a spin conversion of Fd(1/2) to Fd(9/2) Rather, the change in the delocalized fraction is due to a rapid increase in the intramolecular electron-transfer rate between the two iron sites of Fd(1/2). Such a localization-to-delocalization transition has not been observed previously for any Fe-S cluster. The spectral features and the temperature range of the transition (approximate to 100 K) suggest a distribution in the rate of electron transfer between the iron sites of Fd(1/2) and point toward a dispersion in the values for the electronic parameters arising from the interaction of the cluster with the protein in different conformations. The rate for electron transfer between the iron sites of Fd(1/2) was calculated using a quantum mechanical method which takes into account Heisenberg-Dirac-van Vleck exchange, spin-dependent resonance interaction, and vibronic trapping. By assuming a distribution in the parameters characterizing these interactions, we have been able to model the temperature dependence of the fraction of delocalized Fd(1/2) molecules. Our studies suggest that electron-transfer rates in Fe-S clusters from ferredoxins may probe the conformational substates of the protein moiety.