Hyperfme interaction tensor

Proton and Fe hyperfme interactions give insight into the spin density (sometimes denoted spin population) and its distribution, in the present context, over the iron ions and other nuelei of iron sulfur clusters. For protons, the dipolar part of the tensor is direetly eonneeted, apart from distanee to the spin center (iron) in the straeture, with the spin density at the center in the point-dipole approximation. This type of spin density is therefore not assoeiated with any iron orbital. The iso-tropie part that we have eonsidered only for the eysteine P-protons is connected with the 2 i-orbital spin density on the sulfur and the dihedral angle as typical P-protons [40]. The latter spin density is usually not more than about 3-5%. The Fe hyperfine interaetion is also associated with the iron spin density, and there have been several approaehes to its description. 

Examples are drawn from reduced [2Fe-2S] clusters in ferredoxins with all-cysteine coordination, Rieske-type centers, and oxidized [4Fe-4S] clusters in HiPIP proteins. Details regarding proton and Fe hyperfme interactions are provided. These coupled with the g-tensor orientation in the molecular frame provide detailed information regarding the site of reduction or oxidation within the cluster as well as valence delocalization of the iron ions. 

Nitroxide radicals are widely used as spin labels in biology, biochemistry and biophysics to gain information about the structure and the dynamics of biomolecules, membranes, and different nanostructures. Their widespread use is related to an unusual stability, which allows researchers to label specific sites and to detect the most informative EPR parameters (g and hyperfme tensors) that are very sensitive to interactions with the chemical surroundings. Figure 2.1 collects all the radicals used in the following to illustrate the different aspects mentioned in the preceding section. 

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