Isotopic spin, site-directed

Nevertheless, solid-state NMR can be usefiil for active site studies, which is nowadays predominantly employed for acid catalysis, e.g., with zeolites [32]. Chemical shifts obtained are related to the electronic state of the atom H-NMR data have been, for instance, used to determine acidity of Brpnsted sites in zeolites, NMR can discriminate in various environments in supported catalysts [33]. As long as the sites are sufficiently dilute, NMR intensity is directly related to concentration. Structural information is accessible by echo techniques which allow determining distances between spin sites. By analogy with IR methodology, adsorption sites have been investigated with probe molecules also in NMR studies. While work with N bases may require isotopic enrichment of the probe, there are opportunities to detect subtle structural features by cross-polarization and doubleresonance (e.g., " N/ A1) techniques [34]. 

So far, biological Mossbauer spectroscopy is only sensitive to iron (or to be more precise to the Fe isotope) which rules out the use of this technique to directly probe the Ni site. Unfortunately, the iron site seems to remain always in a low-spin state making its identification by Mossbauer very difficult since it will be probably unresolved from the [3Fe-4S] and [4Fe-4S] cluster components. Also, each iron atom in the protein contributes equally to the Mossbauer spectrum the Fe site of the Ni-Fe centre is only 8.3 per cent of the total absorption. 

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