Department of Chemistry, Stanford University, Stanford, California 94305, USA.
J Am Chem Soc. 2010 Sep 15;132(36):12639-47. doi: 10.1021/ja102807x.
S K-edge X-ray absorption spectroscopy (XAS) was performed on wild type Cp rubredoxin and its Cys --> Ser mutants in both solution and lyophilized forms. For wild type rubredoxin and for the mutants where an interior cysteine residue (C6 or C39) is substituted by serine, a normal solvent effect is observed, that is, the S covalency increases upon lyophilization. For the mutants where a solvent accessible surface cysteine residue is substituted by serine, the S covalency decreases upon lyophilization which is an inverse solvent effect. Density functional theory (DFT) calculations reproduce these experimental results and show that the normal solvent effect reflects the covalency decrease due to solvent H-bonding to the surface thiolates and that the inverse solvent effect results from the covalency compensation from the interior thiolates. With respect to the Cys --> Ser substitution, the S covalency decreases. Calculations indicate that the stronger bonding interaction of the alkoxide with the Fe relative to that of thiolate increases the energy of the Fe d orbitals and reduces their bonding interaction with the remaining cysteines. The solvent effects support a surface solvent tuning contribution to electron transfer, and the Cys --> Ser result provides an explanation for the change in properties of related iron-sulfur sites with this mutation.
S K 边 X 射线吸收光谱（XAS）在野生型 Cp 血蓝蛋白及其半胱氨酸到丝氨酸突变体的溶液和冻干形式下进行了研究。对于野生型血蓝蛋白和内部半胱氨酸残基（C6 或 C39）被丝氨酸取代的突变体，观察到正常的溶剂效应，即在冻干时 S 键合度增加。对于溶剂可及表面半胱氨酸残基被丝氨酸取代的突变体，冻干时 S 键合度降低，这是一种反溶剂效应。密度泛函理论（DFT）计算再现了这些实验结果，并表明正常的溶剂效应反映了由于与表面硫醇盐的溶剂氢键而导致的键合度降低，而反溶剂效应则来自于内部硫醇盐的键合度补偿。对于半胱氨酸到丝氨酸取代，S 键合度降低。计算表明，相对于硫醇盐，醇盐与 Fe 的键合相互作用更强，增加了 Fe d 轨道的能量，并降低了它们与其余半胱氨酸的键合相互作用。溶剂效应支持表面溶剂对电子转移的调谐贡献，而半胱氨酸到丝氨酸的结果为该突变导致相关铁硫位点性质变化提供了解释。
