Magnetic resonance ENDOR

Some preliminary ENDOR investigations of the fascinating cooperative oxygenation effect in hemoglobin have been reported by Feher et al.243). Since magnetic resonance... 

The state of knowledge on iron-sulfur proteins which contain non-heme iron bonded to sulfur ligands (cysteinyl residues from the protein and inorganic sulfur) has been reviewed by several authors258"264. With magnetic resonance techniques it has been possible to obtain detailed information on the nature of the active site in many of these proteins. The contributions from ENDOR have recently been summarized by Sands265 so that we shall only give an outline of the crucial points. 

In this monograph it has been demonstrated that ENDOR is a very powerful tool to study transition metal ions in organic, inorganic and bioinorganic single crystals, polycrystalline samples and frozen solutions. Due to the high resolution of this double resonance technique, many problems in magnetic resonance, which cannot be solved with ordinary EPR, become accessible. 

Electron-nuclear double resonance (ENDOR) Nuclear magnetic resonance (NMR)... 

Electron-nuclear double resonance (ENDOR) spectroscopy A magnetic resonance spectroscopic technique for the determination of hyperfine interactions between electrons and nuclear spins. There are two principal techniques. In continuous-wave ENDOR the intensity of an electron paramagnetic resonance signal, partially saturated with microwave power, is measured as radio frequency is applied. In pulsed ENDOR the radio frequency is applied as pulses and the EPR signal is detected as a spin-echo. In each case an enhancement of the EPR signal is observed when the radiofrequency is in resonance with the coupled nuclei. 

Brecht, M., Stein, M., Trofanchuk, O., Lendzian, E, Bittl, R., Higuchi, Y. and Lubitz, W. (1998) Catalytic center of the [NiFe] hydrogenase A pulse ENDOR and ESEEM study. In D. Ziessow, W. Lubitz and E. Lendzian, (eds). Magnetic Resonance and Related Phenomena, TU Berlin, pp. 818-19. 

The new techniques of phosphorescence-microwave multiplet resonance spectroscopy with optical detection have been reviewed by El-Sayed and Kwiram Such exciting experiments as the optical detection on electron-nuclear double resonance (ENDOR) and of electron-electron double resonance (EEDOR) in zero magnetic field have been achieved, and it is certain that much detailed knowledge concerning the phosphorescent states will evolve from this field. 

Electron nuclear double resonance (ENDOR) and electron spin-echo envelope modulation (ESEEM) are two of a variety of pulsed EPR techniques that are used to study paramagnetic metal centers in metalloenzymes. The techniques are discussed in Chapter 4 of reference la and will not be discussed in any detail here. The techniques can define electron-nuclear hyperfine interactions too small to be resolved within the natural width of the EPR line. For instance, as a paramagnetic transition metal center in a metalloprotein interacts with magnetic nuclei such as H, H, P, or these... 

NMR and EPR techniques provide unique information on the microscopic properties of solids, such as symmetry of atomic sites, covalent character of bonds, strength of exchange interactions, and rates of atomic and molecular motion. The recent developments of nuclear double resonance, the Overhauser effect, and ENDOR will allow further elucidation of these properties. Since the catalytic characteristics of solids are presumably related to the detailed electronic and geometric structure of solids, a correlation between the results of magnetic resonance studies and cata lytic properties can occur. The limitation of NMR lies in the fact that only certain nuclei are suitable for study in polycrystalline or amorphous solids while EPR is limited in that only paramagnetic species may be observed. These limitations, however, are counter-balanced by the wealth of information that can be obtained when the techniques are applicable. 

Spectroscopic studies on the Fe-Mo protein by EPR and Mossbauer spectroscopy have shown six iron atoms each in a distinctive magnetic environment coupled to an overall S=3/2 spin system (6,7,8) and electron nuclear double resonance (ENDOR) studies suggest one molybdenum per spin system (8). The 5 Fe signals (five or six doublets) observed in the ENDOR spectra (8) indicate a rather asymmetric structure for the Fe/Mo/S aggregate in which the iron atoms roughly can be grouped into two sets of trios, each set having very similar hyperfme parameters. 

Electron spin resonance (ESR) measures the absorption spectra associated with the energy states produced from the ground state by interaction with the magnetic field. This review deals with the theory of these states, their description by a spin Hamiltonian and the transitions between these states induced by electromagnetic radiation. The dynamics of these transitions (spin-lattice relaxation times, etc.) are not considered. Also omitted are discussions of other methods of measuring spin Hamiltonian parameters such as nuclear magnetic resonance (NMR) and electron nuclear double resonance (ENDOR), although results obtained by these methods are included in Sec. VI. 

Magnetic resonance spectroscopy Nuclear magnetic resonance Electron paramagnetic resonance ENDOR... 

Endeward, B., Plato, M., Will, S., Vogel, E., Szyczewski, A. ,Moebius, K.. (1998) Liquid-phase EPR, ENDOR, and TRIPLE resonance studies on corrole and isocorrole cation radicals, Applied Magnetic Resonance 14, 69-80. 

Freed, J. H. and Mobeus, K. (1992) Magnetic resonance. High-field EPR-ENDOR. Israel J. Chem. 32, 475-481. 

Lubitz, W., Lendzian, F., Plato, M., Scheer, H., and Moebius, K. (1997) The bacteriochlorophyll a cation radical revisited. An ENDOR and triple resonance study, Applied Magnetic Resonance 13, 531-551. 

Makinen, M. W., Mustafi, D., and Kasa, S. (1998) ENDOR of spin labels for structure determination from small molecules to enzyme reaction intermediate, in Berliner, L. (eds.), Biological Magnetic Resonance 14, Spin Labeling Next Millennium, Plenum Press, N. Y. ppl81-249. 

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