The Use of 13C Nuclear Magnetic Resonance of Aromatic Amino Acid Residues to Determine the Midpoint Oxidation-Reduction Potential of Each Iron-Sulfur Cluster of Clostridium acidi-urici and Clostridium pasteurianum Ferredoxins*

13C NMR of the aromatic residues of Clostridium acidi-urici [Phe*]ferredoxin (a chemically modified ferredoxin in which a phenylalanyl residue replaces a tyrosyl residue) and Clostridium pasteurianum ferredoxin permits one to distinguish and probe each iron-sulfur (Fe&*) cluster neighboring the aromatic residues within each protein. This is because the ring carbon resonance shifts of the phenylalanyl and tyrosyl residues can be distinguished. The 13C NMR results suggest that the midpoint oxidation-reduction potentials of the two Fe4S4* clusters in C. pasteurianum and C. acidi-urici ferredoxin differ by 10 f 5 mv and <lO mv, respectively. 13C! NMR of an equilibrium mixture of methyl viologen-reduced C. acidi-urici and C. pasteurianum ferredoxin shows that the protein midpoint oxidation-reduction potential of C. acidiurici ferredoxin is 47 f 10 mv lower than that of C. pasteurianum ferredoxin. We attribute the differences in cluster and protein midpoint oxidation-reduction potentials to differences in protein structure.