Electron paramagnetic resonance assignment

The [NiFe] hydrogenase from D. gigas has been used as a prototype of the [NiFe] hydrogenases. The enzyme is a heterodimer (62 and 26 kDa subunits) and contains four redox active centers one nickel site, one [3Fe-4S], and two [4Fe-4S] clusters, as proven by electron paramagnetic resonance (EPR) and Mosshauer spectroscopic studies (174). The enzyme has been isolated with different isotopic enrichments [6 Ni (I = I), = Ni (I = 0), Fe (I = 0), and Fe (I = )] and studied after reaction with H and D. Isotopic substitutions are valuable tools for spectroscopic assignments and catalytic studies (165, 166, 175). 

Deep state experiments measure carrier capture or emission rates, processes that are not sensitive to the microscopic structure (such as chemical composition, symmetry, or spin) of the defect. Therefore, the various techniques for analysis of deep states can at best only show a correlation with a particular impurity when used in conjunction with doping experiments. A definitive, unambiguous assignment is impossible without the aid of other experiments, such as high-resolution absorption or luminescence spectroscopy, or electron paramagnetic resonance (EPR). Unfortunately, these techniques are usually inapplicable to most deep levels. However, when absorption or luminescence lines are detectable and sharp, the symmetry of a defect can be deduced from Zeeman or stress experiments (see, for example, Ozeki et al. 1979b). In certain cases the energy of a transition is sensitive to the isotopic mass of an impurity, and use of isotopically enriched dopants can yield a positive chemical identification of a level. 

The assumed carboxylato bridge in the assignment is consistent with earlier proposals that in the oxidized MnniMnm form the bridging is (p-O)tp-carboxy-lato)2 [72,73,93], Electron paramagnetic resonance spectroscopy shows that the... 

Conserved residues, particularly cysteine, histidine, and as-partate/glutamate, can signal likely metal ligands, which is an assignment that can be tested by mutagenesis. Detailed information on the coordination sphere can also be provided by spectroscopic techniques such as X-ray absorption spectroscopy, as weU as UV/visible spectroscopy, or electronic paramagnetic resonance spectroscopy for some metals. 

---