Job R C, Bruice T C
Department of Chemistry, University of California, Santa Barbara, Calif. 93106.
Proc Natl Acad Sci U S A. 1975 Jul;72(7):2478-82. doi: 10.1073/pnas.72.7.2478.
The water-soluble tetranuclear iron-sulfur cluster ion Fe(4)S(4)(SCH(2)CH(2)CO(2) (-))(4) (6-) (II) has been prepared. The stability of II in water is sufficient to allow the spectrotitrimetric determination of the pK(a) of its Fe(4)S(4) core as 7.4. In our hands the one-electron reductions of compounds I [Fe(4)S(4)(SR)(4) (2-), R = alkyl or aryl,] are thermodynamically irreversible with associated E(1/2) values greater than those for one-electron reduction of ferredoxins. In contrast, the one-electron reduction of II is thermodynamically reversible and the associated potential (-0.58 V versus hydrogen electrode) approaches closely that of the ferredoxins. The kinetics for ligand exchange of II as a function of pH and thiol concentration are in accord with four reversible mercaptan/lyate species exchange reactions followed by product formation via specific acid and base catalysis. Preliminary experiments indicate the nucleophilic order towards II to be Cl(-) [unk] Br(-) < HO(-) < CN(-).
已制备出水溶性四核铁硫簇离子Fe(4)S(4)(SCH(2)CH(2)CO(2) (-))(4) (6 -) (II)。II在水中的稳定性足以通过分光滴定法测定其Fe(4)S(4)核心的pK(a)为7.4。在我们的实验中，化合物I [Fe(4)S(4)(SR)(4) (2 -)，R = 烷基或芳基]的单电子还原在热力学上是不可逆的，其相关的E(1/2)值大于铁氧化还原蛋白单电子还原的E(1/2)值。相比之下，II的单电子还原在热力学上是可逆的，其相关电位（相对于氢电极，为 - 0.58 V）与铁氧化还原蛋白的电位非常接近。II的配体交换动力学作为pH和硫醇浓度的函数，符合四个可逆的硫醇/羟离子物种交换反应，随后通过特定的酸和碱催化形成产物。初步实验表明，对II的亲核顺序为Cl(-) [unk] Br(-) < HO(-) < CN(-)。
