An exchange coupling model for the Fe4S43+ polymetallic center present in high potential iron-sulfur proteins

We present here a theoretical structural model for the Fe4S43+ polymetallic center, present in proteins and synthetic compounds, which under magnetic exchange coupling and double exchange coupling provides the best agreement between calculated and experimental properties. To test our approach we have fit the hyperfine coupling constants of the unpaired electrons with the Fe-57 nuclei, the temperature dependence of the NMR contact shifts which are available for proteins, and the magnetic susceptibility data available for a synthetic compound. The model allows for either three or four different J values to start with, and a double exchange operator. Both the model and the fitting parameters are satisfactory from a chemical point of view.