Fe4S4 centers redox potential

Figure 2 Electron transfer series in native and synthetic model clusters of the Fe4S4 center in the Fe-protein. Redox potential versus NHE, Fe Mossbauer isomer shift (S at 4.2 K vs. iron), mean iron oxidation state, and spin state are indicated for each redox state. Syntheses and ligand-substitution reactions are indicated for synthetic clusters...

High-potential iron proteins (HiPIPs) comprise a subset of the 4Fe-4S cluster family of metalloproteins that are characterized by a positive reduction potential, F , in the range of +50 to +450 mV. This class is differentiated from the 4Fe-4S centers in low-potential ferredoxins, which show a negative E, tjrpically varying between -100 and -650 mV. The origin of these distinct redox properties has been rationalized in terms of the three-state hypothesis of Carter (1), summarized in Scheme 1, and can be attributed to the stability of the common [Fe4S4(SR)4] state. 

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