Valence delocalization iron-sulfur proteins

A condition where metal ions within a coordination complex or cluster are present in more than one oxidation state. In such systems, there is often complete delocalization of the valence electrons over the entire complex or cluster, and this is thought to facilitate electron-transfer reactions. Mixed valency has been observed in iron-sulfur proteins. Other terms for this behavior include mixed oxidation state and nonintegral oxidation state. 

This cluster formally contains three iron(III) and one iron(E). It is present in a class of proteins called high potential iron-sulfur proteins (HiPIP). It has also been prepared through oxidation of [(RS)4Fe4S4]2 model compounds [57]. Both in the model compound at low temperatures and in proteins there is electron delocalization on one mixed valence pair [58-62]. Therefore, the polymetallic center is constituted by two iron ions at the oxidation state +2.5 and two iron ions at the oxidation state +3. Hamiltonian (6.20), or a more complicated one [40, 41,43], can be used to describe the electronic structure. Indeed, a delocalization operator is sometimes needed in the Hamiltonian [40,41,43]. Consistently with magnetic Mossbauer data the S M subspin involving the mixed valence pair is 9/2, whereas the S n subspin involving the iron(IH) ions is 4. Mossbauer and EPR data do not exclude % and 3, respectively, for the two pairs [57] in any case, the... 

Three stable oxidation states, 3+, 2+ and 1+, have been observed for the [4Fe-4S] cluster. The 3+ and 2+ states are stable In high potential iron-sulfur proteins, while the 2+ and 1+states are detected for bacterial ferredoxins and other 4Fe cluster-containing proteins (23). Both the 3+ and 1+ states are paramagnetic with S-1/2. The 2+ state is diamagnetic. In all three states, valence delocalized pairs are observed. 

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