The electronic structure of Fe4S4 clusters

We have investigated through NMR an extensive series of HiPIPs as well as some ferredoxins in both oxidation states. We have developed a theoretical model for the spin coupling within the cluster that allowed us to rationalize the hyperfine shifts of thecysteine l&and protons as due to the presence of two antierromagnetically coupled spin pairs. The NMR data can thus be interpreted in terms of each individual cysteine beingcoordinated to either an iron belonging to a pair of ferric ions or to a mixed-valence pair