Hyperfine Couplings of Metal Nuclei

A question of major importance for the electronic structure of the active center is the spin density distribution over the heterobimetallic [NiFe] center. A direct approach is provided by measuring the spin density at the nickel and at flic iron in the paramagnetic states of the enzyme. This has become possible by labeling the enzyme with Ni (7= 3/2) and Fe (/= 1/2), respectively. Fe ENDOR experiments have shown that the Fe contains a negligible amount of electron spin density in both oxidized states [64]. Similar experiments on the Ni-C state also showed a very small Fe HFC ( 1 MHz) [64]. These experiments are in line with iron being in the low-spin Fe state (S = 0) in all redox states of the enzyme. The small amount of spin density at the iron is caused by spin polarization. 

Calculation of the Ni HFCs is difficult, in particular for the isotropic part [67]. The data reported recently using DFT [68] are, however, in satisfy- 

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