High-spin ferric forms of heme proteins

High-Spin Ferric Forms of Heme Proteins 

One obvious way to determine the zero-field splitting parameter, D, wifli cw-EPR spectroscopy, is to go to higher microwave frequencies, where die hv/D ratio becomes larger than 1. E. Reijerse, W. Hagen, and coworkers measured the cw-EPR spectra of aquometmyoglobin from 1 to 285 GHz [51]. As die microwave frequency approaches the zero-field splitting, the value of gr,eff reduces notably, giving direct information on the D value. From 130 GHz onwards, an anomalous increase of the linewidth is observed that relates directly to D strain effects. 

detailed information on the heme-pocket structure can only be obtained by performing additional ENDOR or pulsed EPR experiments. In a detailed X-band cw-ENDOR study of single crystals of aquometmyoglobin, C.P. Scholes et al. unraveled the hyperfine and nuclear-quadrupole tensors of the heme and histidine nitrogens [52]. From diis, the authors could derive a lot of information on the electronic and geometric structure of die heme pocket, and the study has become an important work of reference. 

The amount of information that one can derive using X-band cw-ENDOR decreases enormously when no single crystals of the protein are available for similar reasons as discussed in the previous section on low-spin ferric heme centers. Again the solution to overcome this problem lies in a multifrequency pulse-EPR/ENDOR approach. In their cw and pulse F and H ENDOR study of fluorometmyoglobin, B.M. Hoffman and coworkers demonstrated the advantage of combining X- and Q- 

A search of the literature reveals that the number of ESEEM studies performed on high-spin ferric heme proteins are very scarce, and the ESEEM spectra are only performed for observation positions corresponding to the gy efr = 2 position [55,56]. Two obvious reasons for this observation can be identified  

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See also in sourсe #XX -- [ , ]

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