A NEW APPROACH FOR THE STRUCTURAL STUDY OF METALLOPROTEINS - THE QUANTITATIVE-ANALYSIS OF INTERCENTER MAGNETIC-INTERACTIONS

The quantitative analysis of intercenter magnetic interactions, based on the simulation of EPR spectra recorded at different microwave frequ encies, is a powerful technique to determine the relative arrangement of paramagnetic centers in metalloproteins. Such simulations generally rely on a model Hamiltonian in which the interacting centers are appr oximated by point dipoles. This approximation is often sufficient when these centers are mononuclear metal complexes or organic radicals in which the spin density is not too delocalized and keeps a constant sig n. It is used in the present paper to study the magnetic interactions among several hemes and between a heme and a FMN radical in cytochrome s. In the case of metalloproteins containing polynuclear metal cluster s, the point dipole approximation is no longer valid and must be repla ced by a local spin model in which the magnetic interactions among all the paramagnetic sites of the system are explicitly considered. Numer ical simulations based an this model provide a description of the rela tive arrangement of the interacting centers at atomic resolution and c an be used to assign a valence state to the different metal ions of th e clusters. This is illustrated by recent studies carried out oil meta lloproteins containing [2Fe-2S](1+) and [4Fe-4S](1+) clusters.