AN EXCHANGE COUPLING MODEL FOR THE FE4S43-POTENTIAL IRON-SULFUR PROTEINS( POLYMETALLIC CENTER PRESENT IN HIGH)

We present here a theoretical structural model for the Fe4S43+ polymet allic 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 unpa ired electrons with the Fe-57 nuclei, the temperature dependence of th e NMR contact shifts which are available for proteins, and the magneti c susceptibility data available for a synthetic compound. The model al lows for either three or four different J values to start with, and a double exchange operator. Both the model and the fitting parameters ar e satisfactory from a chemical point of view.