P. Bertrand et al., A LOCAL SPIN MODEL TO DESCRIBE THE MAGNETIC-INTERACTIONS IN BIOLOGICAL MOLECULES CONTAINING [4FE-4S](-FE HYDROGENASES() CLUSTERS, APPLICATION TO NI), Journal of the American Chemical Society, 118(6), 1996, pp. 1426-1434
Structural information on the relative arrangement of paramagnetic cen
ters can be obtained through the quantitative study of their magnetic
interactions based on the numerical simulation of EPR spectra recorded
at different microwave frequencies. In a recent work, we have shown t
hat such studies must explicitly take into account the delocalization
of the magnetic moments over polynuclear clusters, and we have present
ed a local spin model suited for valence-localized clusters (Bertrand,
P.; More, C.; Guigliarelli, B.; Fournel, A.; Bennet, B,; Howes, B. J.
Am. Chern. Sec. 1994, 116, 3078-3086). In this paper, we examine the
more general situation in which the cluster contains partially or comp
letely delocalized mixed-valence pairs. This situation is encountered
in particular in the case of [4Fe-4S] clusters which are ubiquitous in
biological systems. A model is presented and developed for a system c
ontaining a [4Fe-4S](+) cluster magnetically coupled to a mononuclear
center. This model is then applied to the study of the magnetic intera
ctions between the nickel center and the proximal [4Fe-4S](+) cluster
in the active form of Desulfovibrio gigas hydrogenase. The arrangement
at atomic resolution deduced from this study is compared to that give
n by a recent X-ray crystal study carried out on the inactive form of
the same enzyme.