Figure 13 Proposed formation of a rapid intermediate in the oxidation of the hyper-reduced [3Fe-4S] cluster.

Questions have been raised about the possible wider occurrence of heterometal clusters such as the [Mo7Fe-8S] cofactor of nitrogenase [5, 6]; yet, whereas the [4Fe-4S] cluster is familiar to all biological chemists, no direct evidence exists to suggest that simple heterometal replacements might occur naturally. Two issues are (1) the spectroscopic characteristics that might be useful for detecting these species and (2) the chemical factors that influence and usually ensure the fidelity of assembly of all-Fe clusters. Under biological control (i.e., in the organism), cluster assembly in a protein must be linked to metal ion transport, availability, and homeostasis [82]. From the chemical viewpoint, however, it is important to determine what reactions are feasible, regardless of biological influences.