P. Bertrand et al., BIOLOGICAL POLYNUCLEAR CLUSTERS COUPLED BY MAGNETIC-INTERACTIONS - FROM THE POINT DIPOLE APPROXIMATION TO A LOCAL SPIN MODEL, Journal of the American Chemical Society, 116(7), 1994, pp. 3078-3086
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.