Iron-sulfur clusters II: Kinetics of ligand exchange studied on a water-soluble Fe 4 S 4 (SR) 4 n - cluster
- 1 July 1975
- journal article
- Published by Proceedings of the National Academy of Sciences in Proceedings of the National Academy of Sciences
- Vol. 72 (7), 2478-2482
- https://doi.org/10.1073/pnas.72.7.2478
Abstract
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(-).Keywords
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