Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)




Iron-sulfur proteins play a vital role in metabolism; mediating such life-sustaining processes as aerobic and anaerobic respiration, nitrogen fixation, and photosynthesis. This work employs low temperature magnetic circular dichroism (MCD), electron paramagnetic resonance (EPR), and UV-visible spectroscopy to characterize the iron-sulfur clusters of the following bacterial proteins: Azotobacter vinelandii ferredoxinI, Thermus thermophilus ferredoxin, Escherichia coli nitrate reductase and the rubredoxin and ferredoxin from Clostridium pasteurianum. Novel 3Fe-xS clusters were identified in A. vinelandii FdI, T. thermophilus Fd, ferricyanide treated C. pasteurianum Fd, and E. coli nitrate reductase. The uniformity of the magnetic and electronic properties of these clusters in both the oxidized and the reduced states indicates a common iron-sulfur core structure for this type of cluster. E. coli nitrate reductase is the first example of an active enzyme which contains a 3Fe-xS cluster. This argues against the currently prevailing hypothesis that all such clusters are isolation artifacts. This work has also developed the potential of low temperature MCD for the detection and characterization of iron-sulfur clusters in multicluster enzymes. Studies on the well-characterized Clostridial proteins demonstrated that MCD magnetization curves provide a selective method of obtaining ground state g-values, spin states, and estimations of the polarizations of the electronic transitions for randomly oriented samples. Moreover, zero field splitting parameters were obtained by a detailed study of the temperature dependence of individual MCD transitions. This has led to a more detailed understanding of the complex Kramers' and non-Kramers' ground states exhibited by reduced 3Fe-xS clusters (S = 2) and oxidized and reduced rubredoxin (S = 5/2 and S = 2 respectively).