Hyperfine coupling modulation

The Orbach-type process as well as the collisional process (inducing either ZFS, g anisotropy or hyperfine coupling modulation) are mechanisms that can provide electron relaxation independently on reorientation. Electron relaxation is certainly not modulated by reorientational motions... 

Hyperfine coupling constants provide a direct experimental measure of the distribution of unpaired spin density in paramagnetic molecules and can serve as a critical benchmark for electronic wave functions [1,2], Conversely, given an accurate theoretical model, one can obtain considerable information on the equilibrium stmcture of a free radical from the computed hyperfine coupling constants and from their dependenee on temperature. In this scenario, proper account of vibrational modulation effects is not less important than the use of a high quality electronic wave function. 

ESR line widths are also sensitive to processes that modulate the g-value or hyperfine coupling constants or limit the lifetime of the electron spin state. The effects are closely analogous to those observed in NMR spectra of dynamical systems. However, since ESR line widths are typically on the order of 0.1-10 G... 

Since the phenoxyls possess an S = ground state, they have been carefully studied by electron paramagnetic spectroscopy (EPR) and related techniques such as electron nuclear double resonance (ENDOR), and electron spin-echo envelope modulation (ESEEM). These powerful and very sensitive techniques are ideally suited to study the occurrence of tyrosyl radicals in a protein matrix (1, 27-30). Careful analysis of the experimental data (hyperfine coupling constants) provides experimental spin densities at a high level of precision and, in addition, the positions of these tyrosyls relative to other neighboring groups in the protein matrix. 

The electron relaxes through modulation of the A and g anisotropy. Typical examples are copper(II), oxovanadium(IV) and silver(II) aqua ions. The electronic relaxation times are relatively long (10 -10 ° s at room temperature) and the hyperfine coupling with the metal nuclear spin is usually present. No field dependence of the electron relaxation time is usually evident up to 100 MHz. 

Fig. 3 Signal of metronidazole anion radicals generated by hydrogenosomes of T. vaginalis in the presence of 45 mM metronidazole, pyruvate, CoA, and ferredoxin (activity of PFOR). Recorded by EPR spectroscopy at 25 °C with 20 mW of microwave power, a frequency of 9.64 GHz, and a modulation amplitude of 0.19 mT. The hyperfine coupling constants were n0no2 = 1 565 mT, Hfl4 = 0.542 mT, and - 0.229 mT...

The three-pulse electron spin-echo envelope modulation (ESEEM) technique is particularly sensitive for detecting hyperfine couplings to nuclei with a weak nuclear moment, such as 14N. It has been used to probe the coordination state of nickel in two hydrogenases from M. tkermoautotrophicum, strain AH (56). One of these enzymes contains FAD and catalyzes the reduction of F420 (7,8-dimethyl-8-hydroxy-5-deazaflavin), while the other contains no FAD and has so far only been shown to reduce artificial redox agents such as methyl viologen. 

Previous lower-frequency electron spin echo envelope modulation (ESEEM) studies showed a histidine nitrogen interaction with the Mn cluster in the S2 state, but the amplitude and resolution of the spectra were relatively poor at these low frequencies. With the intermediate frequency instruments we are much closer to the exact cancellation limit, which optimizes ESEEM spectra for hyperfine-coupled nuclei such as 14N and 15N. We will report the results on 14N and 15N labeled PSII at these two frequencies, along with simulations constrained by both isotope datasets at both frequencies, with a focus on high-resolution spectral determination of the histidine ligation to the cluster in the S2 state. 

The contribution of the hyperfine interactions to the relaxation rates of the radical depends on whether the dominant contribution comes from the anisotropic (dipolar) or the isotropic (scalar) part of the hyperfine interaction. Usually, the anisotropic contribution predominates because this interaction can be readily modulated by the tumbling motion of the molecule. However, in radicals and radical anions such as the trifluoroaceto-phenone (115,116), the rotation of the CF3 group may modulate the isotropic part of the hyperfine interaction and the scalar relaxation W0 could dominate the dipolar transition W2. In such a case, the authors have pointed out that the sign of the resulting CIDNP will be independent of the sign of the isotropic hyperfine coupling constants. 

For spectra obtained at 110 and 119°C, additional linewidth to each transition equally is necessary to simulate the experimental data. As the temperature drops below 1(X)°C, incorporation of hyperfine modulation becomes necessary to simulate the data. Simply increasing the natural linewidth for all transitions without addition of hyperfine modulation does not lead to satisfactory fitting. As the temperature drops, the overall linewidth of all of the transitions increases, and also slight changes in the average hyperfine coupling constants from 90 to 30°C were observed. The simulated data in Fig. 14.11 gives values ofXc that closely fit a modified Arrhenius plot shown in Fig. 14.12. 

For a two-pulse (90° - t - 180°), or primary echo experiment, the integrated intensity of the spin echo, which occurs at time t after the 180° pulse, is measured as a fimction of increasing t from the probe s dead-time ( 100 ns) to a time where the echo amplitude has decayed to a few percent of its initial amplitude (2-8 ps for most powder samples). A two-pulse ESE decay envelope for the type-1 Cu(II) site of a multi-copper oxidase, Fet3p, is shown in Figure 1(a). The data show an overall decay characterized by a phase memory time, Tm or T, of < 1.0 ps. Superimposed on this decay are echo modulations that arise ft om hyperfine coupling to the N nuclei of two histidyl imidazole ligands and the protons of the snrronnding matrix. 

In addition, it is often possible to determine hyperfine couplings from modulations in the echo amplitude or by combining the microwave pulses with radiofrequency pulses, thus performing a pulsed ENDOR experiment. Recent applications of ESE to photosynthesis are discussed in Refs. 119-123 and in Ref. R14. 

Long range dipolar interactions between an unpaired electron and nuclear spins on adjacent atoms will not normally be resolved in conventional powder EPR spectra.The pulse technique of electron spin echo modulation (ESEM) is in favourable cases able to detect very weak hyperfine interactions not seen in CW EPR. The method measures modulation of the electron spin echo signal by dipolar hyperfine coupling in the time domain at fixed magnetic field. Until recently,... 

Another matching effect can occur when molecular motion, for example, rotation of a methyl group, simultaneously modulates the hyperfine coupling constants and the chemical reactivity, the result being a violation of magnetic equivalence of the nuclei of the group. This phenomenon, which... 

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