| Abstract: | The temperature dependence of EPR spectra of oxidized 4Fe-4S](-1,-2) ferredoxins (previously designated HiPIP) and a reduced 4Fe-4S](-2,-3) ferredoxin have been analyzed so as to determine the energy of a low-lying excited electronic state. The values obtained were: Center S-3 from beef heart, 44 cm-1; Center S-3 from mung bean, 53 cm-1; the 4Fe-4S](-1,-2) ferredoxin from Thermus thermophilus, 78 cm-1; Center N-2 of NADH ubiquinone reductase, 83 cm-1. Increasing axial distortion in the EPR spectra of the 4Fe-4S](-1,-2), ferrodoxins was associated with higher energy differences. Center N-2, a Fe-4S](-2,-3) iron-sulfur cluster does not fit this relationship. |
