High-multiplicity spin states of 2[4Fe-4Se]+ clostridial ferredoxins

The electron paramagnetic resonance (EPR) spectra of the reduced selenium-substituted 2-[4Fe-4Se]+ ferredoxins from three bacteria of the Clostridium genus display low-field signals at g = 5.17, g = 10.11, and g = 12.76. The positions, shapes, and temperature dependencies of these signals have allowed their assignments to the three excited states of an S = 7/2 spin multiplet, the fundamental state of which is observed as unusual features in low-temperature (T less than or equal to 20 K) Mössbauer spectra. The S = 7/2 spin state is present in 2[4Fe-4Se]+ clostridial ferredoxins together with the classical S = 1/2 state and with a S = 3/2 state, the fundamental doublet of which is observed as a broad signal in the g = 3-4 region. The relative intensities of the EPR signals corresponding to these spin states depend on the species of Clostridium that the ferredoxin is extracted from. In contrast with clostridial ferredoxins, the reduced selenium-substituted ferredoxin from Bacillus stearothermophilus, which differs significantly from the clostridial proteins by its primary structure and by its containing only one tetranuclear cluster, displays only the S = 1/2 state. Thus, the high-multiplicity spin states arise from a specific interaction between the clostridial ferredoxin polypeptide chain and the reduced [4Fe-4Se]+ clusters.