BIOLOGICAL POLYNUCLEAR CLUSTERS COUPLED BY MAGNETIC-INTERACTIONS - FROM THE POINT DIPOLE APPROXIMATION TO A LOCAL SPIN MODEL

The structural organization of paramagnetic centers in biomolecules ca n be predicted on the basis of a quantitative study of their magnetic interactions. These studies are usually carried out within the framewo rk of the so-called point dipole approximation, in which the delocaliz ation of the magnetic moments over the centers is ignored. In this pap er, we examine how this delocalization can be taken into account in th e spin Hamiltonian describing the magnetic interactions between polynu clear paramagnetic centers. A local spin model is described and applie d first to a system made of a dinuclear center interacting with a mono nuclear center and then to a system comprising two dinuclear centers. In both cases, the EPR spectrum calculated from the local spin model i s compared to that given by the point dipole model for different geome trical configurations. The model is illustrated by a detailed quantita tive study of the magnetic interactions between the molybdenum center and one [2Fe-2S] center (center 1) in the enzyme xanthine oxidase. The se studies emphasize the effective character of some important structu ral parameters given by numerical simulations of EPR spectra based on the point dipole approximation.