Xiao Yuming, Koutmos Markos, Case David A, Coucouvanis Dimitri, Wang Hongxin, Cramer Stephen P
Department of Applied Science, University of California, Davis, CA 95616, USA.
Dalton Trans. 2006 May 14(18):2192-201. doi: 10.1039/b513331a. Epub 2006 Feb 9.
We have used four vibrational spectroscopies--FT-IR, FT-Raman, resonance Raman, and 57Fe nuclear resonance vibrational spectroscopy (NRVS)--to study the normal modes of the Fe-S cluster in [(n-Bu)4N]2[Fe4S4(SPh)4]. This [Fe4S4(SR)4]2- complex serves as a model for the clusters in 4Fe ferredoxins and high-potential iron proteins (HiPIPs). The IR spectra exhibited differences above and below the 243 K phase transition. Significant shifts with 36S substitution into the bridging S positions were also observed. The NRVS results were in good agreement with the low temperature data from the conventional spectroscopies. The NRVS spectra were interpreted by normal mode analysis using optimized Urey-Bradley force fields (UBFF) as well as from DFT theory. For the UBFF calculations, the parameters were refined by comparing calculated and observed NRVS frequencies and intensities. The frequency shifts after 36S substitution were used as an additional constraint. A D 2d symmetry Fe4S4S'4 model could explain most of the observed frequencies, but a better match to the observed intensities was obtained when the ligand aromatic rings were included for a D 2d Fe4S4(SPh)4 model. The best results were obtained using the low temperature structure without symmetry constraints. In addition to stretching and bending vibrations, low frequency modes between approximately 50 and 100 cm(-1) were observed. These modes, which have not been seen before, are interpreted as twisting motions with opposing sides of the cube rotating in opposite directions. In contrast with a recent paper on a related Fe4S4 cluster, we find no need to assign a large fraction of the low frequency NRVS intensity to 'rotational lattice modes'. We also reassign the 430 cm(-1) band as primarily an elongation of the thiophenolate ring, with approximately 10% terminal Fe-S stretch character. This study illustrates the benefits of combining NRVS with conventional Raman and IR analysis for characterization of Fe-S centers. DFT theory is shown to provide remarkable agreement with the experimental NRVS data. These results provide a reference point for the analysis of more complex Fe-S clusters in proteins.
我们使用了四种振动光谱技术——傅里叶变换红外光谱（FT-IR）、傅里叶变换拉曼光谱（FT-Raman）、共振拉曼光谱和57Fe核共振振动光谱（NRVS）——来研究[(n-Bu)4N]2[Fe4S4(SPh)4]中Fe-S簇的正常模式。这种[Fe4S4(SR)4]2-配合物可作为4Fe铁氧化还原蛋白和高电位铁蛋白（HiPIPs）中簇的模型。红外光谱在243 K相变温度上下表现出差异。还观察到用36S取代桥连S位置时的显著位移。NRVS结果与传统光谱的低温数据吻合良好。NRVS光谱通过使用优化的尤里-布拉德利力场（UBFF）以及密度泛函理论（DFT）的正常模式分析进行解释。对于UBFF计算，通过比较计算和观察到的NRVS频率及强度来优化参数。36S取代后的频率位移用作额外的约束条件。D2d对称的Fe4S4S'4模型可以解释大部分观察到的频率，但当将配体芳香环包含在D2d Fe4S4(SPh)4模型中时，与观察到的强度匹配得更好。使用无对称约束的低温结构可获得最佳结果。除了伸缩和弯曲振动外，还观察到约50至100 cm(-1)之间的低频模式。这些以前未见的模式被解释为立方体相对两侧沿相反方向旋转的扭转运动。与最近一篇关于相关Fe4S4簇的论文不同，我们发现无需将大部分低频NRVS强度归为“旋转晶格模式”。我们还将430 cm(-1)波段重新指定为主要是硫酚盐环的伸长，约有10%的末端Fe-S伸缩特征。本研究说明了将NRVS与传统拉曼和红外分析相结合用于表征Fe-S中心的益处。DFT理论与实验NRVS数据显示出显著的一致性。这些结果为分析蛋白质中更复杂的Fe-S簇提供了参考点。
