Structure and magnetism of iron-sulfur clusters in proteins

Iron-sulfur clusters are mixed-valence systems exhibiting both localized and delocalized valence states. We discuss here spin-coupling models for two types of oxidized 3Fe-4S] clusters with localized Fe³⁺ valence states; a Heisenberg Hamiltonian with isotropic antiferromagnetic exchange fits the data well. Reduced 3Fe-4S] clusters, on the other hand, contain a trapped Fe³⁺ site and a delocalized Fe³⁺-Fe²⁺ pair. The pair has spin S₁₂=9/2 (formally ferromagnetic coupling) and is antiferromagnetically coupled to the Fe³⁺ S₃=5/2 spin to yield a system spin S=2. We discuss also recent results for 4Fe-4S] clusters such as 3Fe-4S]→4Fe-4S] conversions, incorporation of other metals into the iron-sulfur core, and the observation of novel spin states.