text: Iron–sulfur clusters occur in many biological systems, often as components of electron transfer proteins.  The ferredoxin proteins are the most common Fe–S clusters in nature.  They feature either 2Fe–2S or 4Fe–4S centers. They occur in all branches of life.
  Fe–S clusters can be classified according to their Fe:S stoichiometry [2Fe–2S], [4Fe–3S], [3Fe–4S], and [4Fe–4S]. The [4Fe–4S] clusters occur in two forms: normal ferredoxins and high potential iron proteins (HiPIP). Both adopt cuboidal structures, but they utilize different oxidation states.  They are found in all forms of life.
  The relevant redox couple in all Fe–S proteins is Fe(II)/Fe(III).
  Many clusters have been synthesized in the laboratory with the formula [FeS(SR)], which are known for many R substituents, and with many cations.  Variations have been prepared including the incomplete cubanes [FeS(SR)].