Mößbauer Effect and Electron Spin Resonance of the (Iron) 4 ‐Sulfur Clusters of Ferredoxin from Clostriclium pasteuriunum

Ferredoxin from Clostridiurn pasteurianum which is characterized by two clusters containing four sulfur‐linked iron atoms each, was studied by MöBbaner and elect)ron spin resonance spectroscopy. The iron of oxidized ferredoxin is high‐spin Fe(III) ( S =5/2) coupled antiparallel magnetically as conaludcd from the magnitude of the quadrupole splitting and its slight temperature dependence. Thus the net spin of the cluster is 8=0. This explains why no magnetic hyperfine splitting was observed in the Mößbauer absorption spectrum of oxidized ferredoxin at 4.2 K. The observation of at least two quadrupole doublets with a ratio of intensities deviating from unity points to a slight inequivalence of the ligand field symmetry of the individual iron atoms. Reduction of oxidized ferredoxin leads to an uptake of one electron per cluster. The net electron spin of each cluster of reduced ferredoxin is thus S =1/2. Spin‐spin coupling of both the clusters prevents the appearance of a magnetic hyperfine splitting at low temperature. In an external magnetic field of 20 kG the coupling is removed and a hyperfine structure is observed. Under the conditions of X‐band electron spin resonance spectroscopy, i.e. in a magnetic field of 3.5 kG, the spin‐spin coupling is not totally removed. Thedore, an anisotropic 7‐line spectrum originating from a system with S = 1, is observed. Under chemical conditions producing partly reduced ferrcdoxin a 2‐line spectrum from the uncoupled S =1/2 systems is also found. It indicates axial symmetry of the innermolecular electric field. Möβhauer spectroscopy enabled the detection of various amounts of non‐cluster iron in samples of oxidized and reduced ferredoxin.