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 combinati on of EPR and variable-temperature/variable-field magnetic circular di chroism (MCD) studies. The results indicate Cd2+ incorporation into [F e3S4](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 [CdFe3S4](+) and S = 2 [CuFe3S4](+) cluster s. 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, r espectively. 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-monitore d redox titrations or direct electrochemistry at a glassy carbon elect rode. The trends in redox potential, ground-state spin, and electron d elocalization of [MFe3S4](2+,+) clusters in P. furiosus ferredoxin are discussed as a fun ction of heterometal (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Tl).