Hyperfine-correlated ENDOR spectroscopy

The resolution of the basic ID Mims and Davies ENDOR sequenees can be improved by disentangling the spectrum into a second appropriately chosen dimension. One approach is to correlate the ENDOR frequencies with their corresponding hyperfme frequencies, so-called hyperfme-correlated ENDOR spectroscopy. We discuss two sequences that achieve this correlation 2D Mims ENDOR and HYEND (hyperfine correlated ENDOR). 

2D Mims ENDOR is restrieted to hyperfine eouplings smaller than the frequency range covered by the m.w. pulses, typieally 50 MHz, and can suffer from poor resolution along the hyperfine axis as the signal deeays with the phase memory time Tm (which is often of the order of only a few mieroseconds in transition metal complexes). 

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As schematically shown in Figure 6.18, an unpaired 7i-electron is associated with the soliton in trans-polyacetylene. In this case, ENDOR spectroscopy can directly measure the spin density distribution of the soliton by the study of hyperfine coupling [98], according to the discussion in the preceding section. In fact ENDOR observations of the spin density distribution close to those predicted theoretically in the case of finite electron correlation have been reported independently for stretch-oriented cA-rich samples prepared by the conventional Shirakawa method [102-105] and for stretch-oriented trans samples prepared by the Durham route [99,106,107]. 

There are also pulse EPR methods that probe the chemical or rather magnetic environment. These are pulse electron nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopy, which allow measuring hyperfine couplings from the unpaired electron spin to surrounding magnetically active nuclei ([20] in Fig. 3 this is a P nucleus). As these experiments are performed in frozen solution (e.g., in all examples of this chapter) or in solids, from the anisotropy and orientation dependence of the hyperfine coupling one can obtain valuable information on the structure up to 1 nm. 

IspG is a protein that carries out an essential reduction step in isoprenoid biosynthesis. Using electron-nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopies on labeled samples, Oldfield et al. have specifically assigned the hyperfine interactions in a reaction intermediate (144), which was created as a result of unusual 4Fe-4S cluster containing protein IspG catalysed reduction of 2-C-methyl-erythritol-cyc/o-2,4-diphosphate (143) to ( )-l-hydroxy-2-methyl-but-2-enyl-4-diphosphate (145) and then to dimethylallyl diphosphate (146) and isopentenyl diphosphate (147) (Scheme 39). " ... 

Advanced EPR techniques such as CW and pulsed ENDOR, electron spin-echo envelope modulation (ESEEM), and two-dimensional (2D)-hyperfine sublevel correlation spectroscopy (HYSCORE) have been successfully used to examine complexation and electron transfer between carotenoids and the surrounding media in which the carotenoid is located. 

Abstract Multi-resonance involves ENDOR, TRIPLE and ELDOR in continuous-wave (CW) and pulsed modes. ENDOR is mainly used to increase the spectral resolution of weak hyperfine couplings (hfc). TRIPLE provides a method to determine the signs of the hfc. The ELDOR method uses two microwave (MW) frequencies to obtain distances between specific spin-labeled sites in pulsed experiments, PELDOR or DEER. The electron-spin-echo (ESE) technique involves radiation with two or more MW pulses. The electron-spin-echo-envelope-modulation (ESEEM) method is particularly used to resolve weak anisotropic hfc in disordered solids. HYSCORE (Hyperfine Sublevel Correlation Spectroscopy) is the most common two-dimensional ESEEM method to measure weak hfc after Fourier transformation of the echo decay signal. The ESEEM and HYSCORE methods are not applicable to liquid samples, in which case the FID (free induction decay) method finds some use. Pulsed ESR is also used to measure magnetic relaxation in a more direct way than with CW ESR. 

Hyperfine splitting due to interaction with ligand nuclei with 7 > 0 reflects the extent of spin delocalization onto neighboring atoms and can be used to characterize the types and numbers of such nuclei. In cases where these couplings are too small to be resolved in the EPR spectra, electron nuclear double resonance (ENDOR) or electron spin echo envelope modulation (ESEEM) can be used to measure the couplings as discussed in Chapter 2.3. Modern calculational tools are approaching the capabilities required to calculate g and A values from electronic wave functions. However, much of the spectroscopy that has been performed to date has used empirical correlations to interpret g and A values. 

ENDOR techniques work rather poorly if the hyperfine interaction and the nuclear Zeeman interaction are of the same order of magnitude. In this situation, electron and nuclear spin states are mixed and formally forbidden transitions, in which both the electron and nuclear spin flip, become partially allowed. Oscillations with the frequency of nuclear transitions then show up in simple electron spin echo experiments. Although such electron spin echo envelope modulation (ESEEM) experiments are not strictly double-resonance techniques, they are treated in this chapter (Section 5) because of their close relation and complementarity to ENDOR. The ESEEM experiments allow for extensive manipulations of the nuclear spins and thus for a more detailed separation of interactions. From the multitude of such experiments, we select here combination-peak ESEEM and hyperfine sublevel correlation spectroscopy (HYSCORE), which can separate the anisotropic dipole-dipole part of the hyperfine coupling from the isotropic Fermi contact interaction. 

During the past two decades or so, CW-ENDOR has given way to Pulsed-ENDOR, and the growing availability of pulsed facilities has also opened up ESEEM (Electron Spin Echo Envelope Modulation) and HYSCORE (Hyperfine Sublevel Correlation Spectroscopy) methods. It is quite clear that there is no universal EPR experiment. 

The application of Mossbauer spectroscopy in diverse fields of qualitative and quantitative analysis is based on the ease with which hyperfine interactions can be observed. The information obtained from Mossbauer spectroscopy may be correlated with other methods by which HI can be examined such as NMR, EPR, ENDOR, PAC (perturbed angular correlations), nuclear orientation and neutron scattering. However, Mossbauer spectroscopy often proves to be experimentally simpler, more illustrative and an efficient method for studying applied problems. Mossbauer nuclei are ideal spies supplying information on both the microscopic and macroscopic properties of solids. 

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