Electronic coupling, multinuclear iron

The multinuclear tetrahedral iron clusters have the potential for additional formal oxidation states. Because not all of these states have been found in proteins or model compounds, it is possible that some oxidation states may be unstable. For a given Fe S protein only one redox couple is used the other possible states appear to be excluded by restrictions of the protein structure. This selection rule is illustrated with two 4Fe 4S low-molecular-weight electron transfer proteins ferredoxin and high-potential iron protein (HiPIP). The 4Fe 4S clusters in both proteins were shown by X-ray crystallography to be virtually identical. However, the redox potential and oxidation states for the two proteins are vastly... 

Each major class of oxygenase and monooxygenase enzyme has a vast literature. A general pattern, however, is that most such enzymes have either Fe or Cu as active site metals, often in multinuclear cluster centers. For iron, the Fe(lI)/(III) couple is often accessed with Fe(IV) present in certain cases. For copper, the typical states are Cu(I)/(II) with Cu(lII) also present in some cases. Since O2 is a 4e oxidant, while the low-valent metals are 1 or 2e reductants, a cluster site can help attain compatibility of oxidant and reductant by allowing the site to take up more than one electron. 

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