title: Spectroscopic studies of the seven-iron-containing ferredoxins from Azotobacter vinelandii and Thermus thermophilus.
abstract: The seven-iron-containing ferredoxins from Azotobacter vinelandii and Thermus thermophilus have been investigated by low-temperature magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectroscopies and room temperature ultraviolet-visible absorption spectroscopy. The results confirm the presence of one trinuclear and one tetranuclear iron-sulfur cluster in both ferredoxins and facilitate comparison of the electronic and magnetic properties of the oxidized and reduced [3Fe-xS] clusters. MCD magnetization data are consistent with an S = 2 ground state for both reduced [3Fe-xS] clusters, but indicate differences in the rhombicity of the zero-field splittings. The data permit rationalization of the absence of a delta M = 4 EPR transition for the reduced [3Fe-xS] cluster in A. vinelandii ferredoxin I. Spectroscopic studies of anaerobically isolated A. vinelandii ferredoxin I do not support the hypothesis that the [3Fe-xS] cluster arises as a result of aerial oxidative damage to a [4Fe-4S] cluster during isolation. The possibility that two distinct forms of [3Fe-xS] clusters can exist in A. vinelandii ferredoxin I was investigated by spectroscopic studies as a function of pH. The results reveal two distinct and interconvertible forms of the reduced [3Fe-xS] cluster, but do not permit rationalization of the inconsistencies in the structural data that have been reported for the oxidized clusters.
mesh_terms: D001395:Azotobacter / Q000032:analysis*; D002942:Circular Dichroism; D004578:Electron Spin Resonance Spectroscopy; D005288:Ferredoxins; D006863:Hydrogen-Ion Concentration; D007506:Iron-Sulfur Proteins; D010084:Oxidation-Reduction; D013053:Spectrophotometry; D013696:Temperature; D013824:Thermus / Q000032:analysis*
chemical_list: D005288:Ferredoxins; D007506:Iron-Sulfur Proteins