Proton n.m.r. of ferrodoxin from Clostridium pasteurianum

Proton magnetic resonance spectra at 500 MHz are reported for the oxidized and reduced forms of the 2[4Fe-4S]-ferredoxin from Clostridium pasteurianum. The reduced protein showed additional peaks in the 10–60 ppm region, which were previously unobserved, and there were significant differences between oxidized and reduced states in the whole region. The electron exchange rate in partially reduced ferredoxin is slow on the n.m.r. time scale when reduced with sodium dithionite, but fast when zinc reduced methyl viologen is used as reducing agent. We explain the difference between fast and slow exchange as being due to the different chemical properties of the two reducing agents.