The Split Ni-C Signal

In general it is difficult to study the Ni-C state by EPR at low temperatures ( 40 K). The reason for this is that the proximal [4Fe4S] cluster is usually also 

The splitting of the EPR spectrum at low temperature can advantageously be used to measure the spin-spin interaction between the [NiFe] center and the reduced proximal [4Fe4S] cluster. This has been done by Guigliarelli et al. using multifrequency EPR [52] (see Fig. 8). It was found that the spin-spin interaction 

From such measurements information about the identity, coupling strength, distance, and even relative orientation of other paramagnetic centers in the enzyme can, in principle, be obtained. This is of utmost importance for understanding die electron transfer between the metal centers in the enzyme. In diis respect, more information is stiU required for understanding the exact electron transfer padiway in the hydrogenases. 

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