Hyperfine sublevel correlation electron spin echo envelope modulation

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. 

DPPH = 2,2-diphenyl-1-picrylhydrazyl ENDOR= electron-nuclear double resonance EPR = electron paramagnetic resonance ESE = electron spin echoes ESEEM = electron spin echo envelope modulation EFT = fast fourier transformations FWHM = fidl width at half maximum HYSCORE = hyperfine sublevel correlation nqi = nuclear quadrupole interaction TauD = taurme/aKG dioxygenase TWTA = traveling wave tube amphfier ZFS = zero field sphtting. 

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]. 

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. 

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. 

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