Nuclear ENDOR

DeRose, V. J., Telser, J., Anderson, M. E., Lindahl, P. A., and Hoffman, B. M., 1998, A multi-nuclear ENDOR study of the C-cluster in CO dehydrogenase from Clostridium ther-moaceticum Evidence for HxO and histidine coordination to the [Ee4S4] center, J. Am. Chem. Soc. 120(34) 8767ii8776. 

B1.15.5.1 ELECTRON-NUCLEAR DOUBLE RESONANCE SPECTROSCOPY (ENDOR)... 

ENDOR transitions can be easily understood in temis of a simple system consisting of a single unpaired electron spin (S=2) coupled to a single nuclear spin (1=2). The interactions responsible for the various... 

Figure Bl.15.8. (A) Left side energy levels for an electron spin coupled to one nuclear spin in a magnetic field, S= I =, gj >0, a<0, and a l 2h)<(a. Right side schematic representation of the four energy levels with )= Mg= , Mj= ). +-)=1, ++)=2, -)=3 and -+)=4. The possible relaxation paths are characterized by the respective relaxation rates W. The energy levels are separated horizontally to distinguish between the two electron spin transitions. Bottom ENDOR spectra shown when a /(21j)< ca (B) and when co < a /(2fj) (C).

Since in general the nuclear g factors are different for ground and excited states of a Mdssbauer nucleus, the spin state must be quoted when giving numerical values for A in energy (which, however, is usually not necessary for NMR spectroscopy or other ground-state techniques). Thus, for a comparison of A values obtained from Mdssbauer and NMR or ENDOR spectra, usually the ground state is considered. 

This chapter concludes with a brief description of one advanced technique, Electron Nuclear Double Resonance (ENDOR), the capabilities for which, unlike pulsed methods, may be added as a relatively minor modification to commercial CW ESR spectrometers. 

In Chapter 2, ENDOR (electron-nuclear double resonance) was briefly described. To perform an ENDOR experiment it is necessary to apply both a radiofrequency and a microwave frequency, effectively performing simultaneous NMR and ESR, respectively, on the sample. The experiment is performed at a fixed magnetic field, with the ESR saturating frequency centered on a... 

As discussed in Chapter 6, in systems with more than one unpaired electron the ESR spectrum contains features that involve electron-electron coupling parameters analogous to the nuclear hyperfine parameters. In those types of samples the advantages of double resonance are carried out by employing the use of two different microwave frequencies, one fixed and saturating, and one variable frequency that searches for transitions. This technique is known as ELDOR (electron-electron double resonance).38,40,41,44 It has been used much less than ENDOR and usually requires custom-built equipment. 

M. Bennati, C.T. Farrar, J.A. Bryant, S.J. Inati, V. Weis, G.J. Gerfen, P. Riggs-Gelasco, J. Stubbe and R.G. Griffin, Pulsed electron-nuclear double resonance (ENDOR) at 140 GHz, J. Magn. Reson., 1999, 138, 232. 

The development of a wide range of special forms of EPR was initiated when the idea of double resonance (using simultaneous irradiation by two different sources) was cast in 1956 by G. Feher at Bell Telephone Labs in his seminal paper on ENDOR, electron nuclear double resonance (Feher 1956). BioEPR applications of ENDOR were later developed on flavoprotein radicals in a collaboration of A. Ehrenberg and L. E. G. Eriksson in Stockholm, Sweden, and J. S. Hyde at Varian in Palo Alto, California (Ehrenberg et al. 1968), and on metalloproteins in a joint effort of the groups of R. H. Sands in Ann Arbor, I. C. Gunsalus in Urbana, Illinois, and H. Beinert in Madison (Fritz et al. 1971). 

Double-resonance spectroscopy involves the use of two different sources of radiation. In the context of EPR, these usually are a microwave and a radiowave or (less common) a microwave and another microwave. The two combinations were originally called ENDOR (electron nuclear double resonance) and ELDOR (electron electron double resonance), but the development of many variations on this theme has led to a wide spectrum of derived techniques and associated acronyms, such as ESEEM (electron spin echo envelope modulation), which is a pulsed variant of ENDOR, or DEER (double electron electron spin resonance), which is a pulsed variant of ELDOR. The basic principle involves the saturation (partially or wholly) of an EPR absorption and the subsequent transfer of spin energy to a different absorption by means of the second radiation, leading to the detection of the difference signal. The requirement of saturability implies operation at close to liquid helium, or even lower, temperatures, which, combined with long experimentation times, produces a... 

Fritz, J., Anderson, R., Fee, J., Palmer, G, Sands, R.H., Tsibris, J.C.M., Gunsalus, I.C., Orme-Johnson, W.H., and Beinert, H. 1971. The iron electron-nuclear double resonance (ENDOR) of two-iron ferredoxins from spinach, parsley, pig adrenal cortex and Pseudomonas putida. Biochimica et Biophysica Acta 253 110-133. 

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