Hyperfine saturation

Although it is now established that CIDNP has a quite different origin from DNP, the two effects were initially thought to be related. Thus the Overhauser effect, in which saturation of unpaired electron spins leads to polarization of nuclei coupled to the electrons through the hyperfine couphng constant (ajj), can be observed in organic radicals, and CIDNP 3... 

As discussed in Chapter 6, in systems with more than one unpaired electron the ESR spectrum contains features that involve electron-electron coupling parameters analogous to the nuclear hyperfine parameters. In those types of samples the advantages of double resonance are carried out by employing the use of two different microwave frequencies, one fixed and saturating, and one variable frequency that searches for transitions. This technique is known as ELDOR (electron-electron double resonance).38,40,41,44 It has been used much less than ENDOR and usually requires custom-built equipment. 

At either frequency the sensitivity of the instrument is quite remarkable. The exact signal-to-noise ratio depends upon a number of factors including apparent line width (including g and hyperfine anisotropy), ease of saturation, the temperature, and the linear density of the sample in the quartz tube. For a relatively narrow line with peak-to-peak separation of two gauss it is possible to observe the spectrum for concentrations as low as 1014 spins per gram of sample. As the spectrum becomes more anisotropic, the sensitivity of course decreases. Lowering the temperature increases the sensitivity since the population difference An increases [(Eqs. (26) and (3°)]. 

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. 

Lepidocrocite is paramagnetic at room temperature. The Neel temperature of 77 K is much lower than that of the other iron oxides and is the result of the layer-like structure of this mineral. The sheets of Fe(0,0H)6 octahedra are linked by weak hydrogen bonds, hence magnetic interactions are relatively weak. The saturation hyperfine field is also lower than for any other iron oxide (Tab. 6.2). In the antiferromagnetic state, the spins are ordered parallel to the c-axis with spins in alternate layers having opposite signs. A decrease of T by 5 K was observed for Al-lepidocrocites with an Al/(Fe-i-Al) ratio of 0.1 (De Grave et al., 1995). 

The ENDOR technique refers to electron-nuclear double resonance. This consists of the effect on a partially saturated ESR line of simultaneously irradiating the sample with a radiofrequency to induce nuclear resonance transitions of hyperfine coupled nuclei. It may enable one to obtain information about signs of coupling constants. ELDOR is the technique corresponding to electron-electron double resonance. Such techniques, coupled with TRIPLE resonance, have been utilized and well described in a discussion of pyridine and 4,4-bipyridyl radical anion ESR spectra measured in sodium/liquid ammonia (80JMR<41)17). 

An interesting new experimental approach has been taken in order to separate overlapping EPR spectra as they appear e.g. in the multi Fe/S centre containing complex I. Inversion- and saturation-recovery measurements which allow to measure Ti relaxation times are used in a inversion-recovery filter which is subsequently applied to separate EPR signals on account of their Trdifferences. In addition, this filter can be used in conjunction with high-resolution hyperfine measurements e.g. by ESEEM and thus the separated centres can be characterized in depth.211... 

Fig. 7.3. NOE difference spectra obtained upon saturation of the hyperfine shifted signals corresponding to the P-CH2 protons of the Fe4S4-coordinated cysteines in oxidized HiPIP from C. vinosum [13]. Signals marked by x arise from saturation transfer to a small amount of reduced species [13].

Results similar to those shown in the slice of Fig. 8.22 can be obtained with the so-called NOE-NOESY sequence [36]. Here a hyperfine shifted signal, e.g. I2-CH3 of the above compound, is selectively saturated, and then the NOESY pulse sequence is applied. The NOESY difference spectrum obtained by subtracting a NOESY spectrum without presaturation of the I2-CH3 signal is shown in Fig. 8.23. Here, some more cross peaks are evident with respect to the 3D NOESY-NOESY experiment because secondary NOEs develop much more when the primary NOEs from the I2-CH3 signal evolve in a steady state experiment like the NOE-NOESY rather than in a transient-type experiment like the NOESY-NOESY. In Fig. 8.23, dipolar connectivity patterns are apparent among protons... 

Fig. 46. 250-MHz ferric hyperfine-shifted proton resonances of the abnormal p chains of 15% Hb M Milwaukee (p67Val — Glu, a2p2) as a function of oxygenation in 0.1 M Bis—Tris in D20 at pH 6.6 and 30°C. The proton chemical shifts are referenced to HDO. The range of the fractional saturation (y) was based on P50 = 5 torr or P5 = 10 torr. [Adapted from Fung et al. (1977)].

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