Double Integration and Titration of the Electron Paramagnetic Resonance Signal in the Molybdenum Iron Protein of Azotobacter Vinlandii
William B. Euler, Jens Martinsen, John W. MacDonald, Gerald D. Watt, Z.-C. Wang, Biochemistry, 1984, 23, 3021 – 3024
Abstract
The electron paramagnetic resonance (EPR) signal from the MoFe protein component of nitrogenase was doubly integrated over the temperature range 2.2–15 K. Comparison of the protein double integral with that from the spin standard copper ethylenediaminetetraacetate demonstrated that two S = 3/2 spin centers are responsible for the observed EPR signal. The zero-field splitting parameter was found to be 15 ± 1 cm–1 from variation of the double integral with temperature. The double integral varied with the Mo content of the protein, suggesting direct Mo involvement in the S = 3/2 spin centers. EPR titrations using methylene blue, thionine, or dichlorophenolindophenol as oxidants under a wide variety of solution conditions supported previous results of three "P clusters", two EPR centers, and a single center of a third but unknown type.