Saturation transfer, electron spin resonance

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... 

As a result of relaxation, the nuclear Overhauser effect (NOE) is a phenomenon predicted by Albert Overhauser in 1953, which is the fractional change in intensity of one NMR resonance when another resonance is irradiated. It is the transfer of nuclear spin polarisation between nuclei by cross-relaxation and has become indispensable for the determination of the liquid structure of macromolecules, particularly biomolecules, since the first 2D methods were developed by K. Wiithrich, who was awarded the Nobel Prize in Chemistry in 2001 for his work [28]. It was first shown, theoretically, that saturating the electron magnetic resonance in a metal would cause the nuclear resonance intensity to increase by three orders of magnitude (Feiectron/ynuciei) Similar, albeit much less, enhancement was caused between two nuclei... 

The nature of lipid-protein interactions in photosynthetic membranes has been examined by means of electron spin resonance (ESR) and saturation transfer ESR (STESR) spectroscopy. Nitroxide-labelled acyl lipids were introduced into thylakoid membranes and sub-membrane preparations and the resultant ESR spectra were analysed under various conditions. 

A well-known and important phenomenon in the area of nuclear-spin resonance (NMR) in gases, liquids, or solid samples is dynamic nuclear-spin polarisation (DNP) (see e.g. [M6]). This term refers to deviations of the nuclear magnetisation from its thermal-equilibrium value, thus a deviation from the Boltzmann distribution of the populations of the nuclear Zeeman terms, which is produced by optical pumping (Kastler [31]), by the Overhauser effect [32], or by the effet solide or solid-state effect [33]. In all these cases, the primary effect is a disturbance of the Boltzmann distribution in the electronic-spin system. In the Overhauser effect and the effet solide, this disturbance is caused for example by saturation of an ESR transition. Owing to the hyperfine coupling, a nuclear polarisation then results from coupled nuclear-electronic spin relaxation processes, whereby the polarisation of the electronic spins is transferred to the nuclear spins. 

Federicamycin A (= NSC-305264) spontaneously forms an oxidised free radical with electron transfer to O2 (Hilton et al. 1986). The observed hyper-fine structure of this radical is consistent with one-electron oxidation of the quininoid group. After federicamycin A is exposed to O2, an electron paramagnetic resonance (EPR) signal is observed with axial symmetry with temperature and power saturation behaviour suggestive of 02 . Spin-trapping EPR studies demonstrated that the drug reduces O2 to 02 ". and H2O2 to HO. ... 

Another method of polarizing nuclear spins is dynamic nuclear polarization (DNP), whereby the comparably large electron spin polarization (see Fig. 1) is transferred to nuclear spins by saturating the electron resonance. DNP is almost as old as NMR spectroscopy, building on the aforementioned theoretical work by Overhauser [21] who predicted what is today known as the Overhauser effect (OE). DNP was soon after demonstrated experimentally by Carver and Slichter [22, 23]. The enhancement, s, that can be obtained by DNP is determined by the gamma ratio 7e/7n> which is 660 for protons and 2,625 for... 

As already proposed by Albert Overhauser [203] in 1953 and subsequently confirmed experimentally by Carver and Slichter [204, 205], electron spin polarization in solid metals can be transferred to nuclear spins if the unpaired conducting electrons are saturated by irradiation with the corresponding electron paramagnetic resonance (EPR) frequency. As a result, nuclear spin polarization may be enhanced by the factor yJyt,. Likewise, stable organic radicals can be used as polarizing agents. 

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