Hyperfine sublevel correlation spectroscopy HYSCORE

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. 

With the commercial availability of pulsed EPR instrumentation, other pulsed methods such as electron-spin echo envelope modulation (ESEEM) or hyperfine sublevel correlation spectroscopy (HYSCORE), which are quite useful to study specific hyperfine and quadrupolar couplings, have also been applied to flavoproteins [59, 61, 62]. These studies have been reviewed recently, e.g., in [46]. 

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. 

Pulsed EPR Experiment Hyperfine Sublevel Correlated Spectroscopy (HYSCORE)... 

HexOMe, methyl glycoside Hex-onic, aldohexonic acid hmba, 2-hydroxy-2-methylbutanoate2 HYSCORE, hyperfine sublevel correlation spectroscopy mod. ampl., modulation amphtude polyGalA, galacturonan Qa, quinic acid XAFS, X-ray absorption fine structure spectroscopy... 

There are many other specialized methods electron-electron double resonance (ELDOR), TRIPLE, HYSCORE (hyperfine sublevel correlation spectroscopy, which is similar to 2D-EPR), electron spin-echo, and so on these methods are not discussed here. 

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. 

Overlap of lines can make analysis difficult when several nuclei contribute in the one-dimensional (ID) two- and three-pulse ESEEM spectra. Eollowing the development in NMR, methods to simplify the analysis involving two-dimensional (2D) techniques have therefore been designed. The Hyperfine Sublevel Correlation Spectroscopy, or HYSCORE method proposed in 1986 [14] is at present the most commonly used 2D ESEEM technique. The HYSCORE experiment has been applied successfully to study single crystals, but is more often applied to orienta-tionally disordered systems. It is a four-pulse experiment (Fig. 2.23(a)) with a k pulse inserted between the second and the third k/2 pulse of the three-pulse stimulated echo sequence. This causes a mixing of the signals due to the two nuclear transitions with m.s = Vi of an 5 = Vi species. For a particular nucleus two lines appear at (v , V ) and (V ", v ) in the 2D spectrum as shown most clearly in the contour map (d) of Fig. 2.23. The lines of a nucleus with a nuclear Zeeman frequency... 

More elaborate pulse sequences have been designed to selectively extract specific information about a spin system. A particularly useful experiment for structural studies of metal complexes is the two-dimensional four-pulse experiment called HYSCORE, hyperfine sublevel correlation spectroscopy. The resulting two-dimensional plot reveals the correlation between different modulation frequencies arising from the same nucleus, which greatly facilitates assignment of frequencies in complicated spin systems. ... 

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. 

Two-dimensional pulsed EPR experiments are also possible, such as HYSCORE (hyperfine sublevel correlation) spectroscopy. This uses a pulse sequence as in Figure 8B, separated by an inversion pulse into two variable time intervals. Fourier transformation in the two time-domains gives a 2D spectrum, in which the effects of multiple hyperfine couplings can be resolved. 

Hdfer P, Grupp A, Nebenfuhr H and Mehring M 1986 Hyperfine sublevel correlation (HYSCORE) spectroscopy a 2D ESR investigation of the squaric acid radical Chem. Phys. Lett. 132 279-82... 

Other pulse sequences are in use such as the three-pulse sequence (Figure 3.16) and hyperfine sublevel correlation (HYSCORE) spectroscopy, the latter being a two-dimensional technique.P ]... 

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. 

This report was followed by several studies that used pulsed EPR spectroscopy to determine the fete of chelated vanadyl complexes in the organs of chronically treated rats. Fukui et al. measured the 2- and 3-pulse ESEEM spectra of rat kidney, liver, and bone from animals treated with VOSO4 or VO(pic)2 [71]. Dikanov and coworkers used ESEEM and the two-dimensional ESEEM technique hyperfine sublevel correlation (HYSCORE) spectroscopy to examine in detail the coordina-... 

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). " ... 

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