An Open-Cuboidal [Fe 3 S 4 ] Cluster Characterized in Both Biologically Relevant Redox States.

Synthetic analogues of the three common types of Fe-S clusters found in biology─diamond-core [Fe 2 S 2 ] clusters, open-cuboidal [Fe 3 S 4 ] clusters, and cuboidal [Fe 4 S 4 ] clusters─have been reported in each biologically relevant redox state with one exception: the open-cuboidal [Fe 3 S 4 ] + cluster. Here, we describe the synthesis and characterization of an open-cuboidal [Fe 3 S 4 ] cluster in both biologically relevant redox states: [Fe 3 S 4 ] + and [Fe 3 S 4 ] 0 . Like their biological counterparts, the oxidized cluster has a spin-canted, S = 1 / 2 ground state, and the reduced cluster has an S = 2 ground state. Structural analysis reveals that the [Fe 3 S 4 ] core undergoes substantial contraction upon oxidation, in contrast to the minimal structural changes observed for the only [Fe 3 S 4 ] protein for which high-resolution structures are available in both redox states ( Azotobacter vinelandii ferredoxin I; Av FdI). This difference between the synthetic models and Av FdI is discussed in the context of electron transfer by [Fe 3 S 4 ] proteins.