Abstract
The water-soluble tetranuclear iron-sulfur cluster ion Fe4S4(SCH2CH2CO2-)46- (II) has been prepared. The stability of II in water is sufficient to allow the spectrotitrimetric determination of the pKa of its Fe4S4 core as 7.4. In our hands the one-electron reductions of compounds I [Fe4S4(SR)42-, R = alkyl or aryl,] are thermodynamically irreversible with associated E1/2 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: ferredoxin, nucleophilicity
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