Electronic, Magnetic, and Redox Properties of [MFe3S4] Clusters (M = Cd, Cu, Cr) in Pyrococcus furiosus Ferredoxin

The ground- and excited-state properties of heterometallic [CuFe3S4]2+,+, [CdFe3S4]2+,+, and [CrFe3S4]2+,+ cubane clusters assembled in Pyrococcus furiosus ferredoxin have been investigated by the combination of EPR and variable-temperature/variable-field magnetic circular dichroism (MCD) studies. The results indicate Cd2+ incorporation into [Fe3S4]0,- cluster fragments to yield S = 2 [CdFe3S4]2+ and S = 5/2 [CdFe3S4]+ clusters and Cu+ incorporation into [Fe3S4]+,0 cluster fragments to yield S = 1/2 [CuFe3S4]2+ and S = 2 [CuFe3S4]+ clusters. This is the first report of the preparation of cubane type [CrFe3S4]2+,+ clusters, and the combination of EPR and MCD results indicates S = 0 and S = 3/2 ground states for the oxidized and reduced forms, respectively. Midpoint potentials for the [CdFe3S4]2+,+, [CrFe3S4]2+,+, and [CuFe3S4]2+,+ couples, E m = −470 ± 15, −440 ± 10, and +190 ± 10 mV (vs NHE), respectively, were determined by EPR-monitored redox titrations or direct electrochemistry at a glassy carbon electrode. The trends in redox potential, ground-state spin, and electron delocalization of [MFe3S4]2+,+ clusters in P. furiosus ferredoxin are discussed as a function of heterometal (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Tl).