ENDOR spectroscopy paramagnetic centers

In this review we have concentrated on explaining the basic mechanisms behind ENDOR and ESEEM spectroscopy. These two methods, along with field-swept EPR experiments, provide a means to obtain a detailed description of the EPR parameters of paramagnetic centers in single crystals, powders, and frozen solutions. To obtain the most accurate EPR parameters requires not one technique, but a combination, and preferably applied at several m.w. frequencies. Measurements at multi-frequencies allow possible ambiguities that arise from data measured at only one m.w. frequency to be resolved. 

Chapter 3 by Hiittermann and Kappl presents detailed strategies for the analysis of CW- and pulsed-ENDOR spectroscopy from Fe-S proteins. ENDOR is by now a well-known high-resolution technique ideal for resolving small interactions between unpaired electrons and metal nuclei or with nuclei in the coordination sphere that cannot be resolved using CW-EPR. It provides important information to characterize the functional, structural, and electronic properties of paramagnetic centers found in metalloproteins. It complements ESEEM and HYSCORE methods, which are more suited to determining hyperfine interactions from distant ligand nuclei. 

These techniques are applicable only to paramagnetic Mo(V) centers, but the EPR parameters are extremely sensitive to coordination changes at the molybdenum center 17, 64). The molybdenum and ligand hyperfine splittings can provide additional information about the coordination environment of the molybdenum(V) species and the chemical reactions at the molybdenum center. EPR spectra from xanthine oxidase were first reported in 1959 by Bray et al. (65), and Bray and co-workers have continued to develop the application of EPR spectroscopy to molybdenum enzymes 17, 64). In 1966 it was shown (66) that mixing [Mo04] with dithiols produced EPR signals with (g) and (A( Mo)) values similar to those of xanthine oxidase. Only recently, however, have the structures of such thiolate complexes been determined (see Section IV.B.2.b). 39) and P (67) ENDOR spec-... 

A variety of spectroscopic and physical techniques have been used to investigate the nature of these redox centers. EPR, Fe Mossbauer spectroscopy, and Mo and Fe X-ray absorption spectroscopy Mo, Fe, and H electron-nuclear double resonance (ENDOR) linear electric field effect and magnetic circular dichroism (MCD) have provided information about the environment of the Mo and Fe nuclei and their interaction with the unpaired spin of electrons in paramagnetic species of the MoFe proteins. 

---