EPR line shape

Hagen, W.R. and Albracht, S.P.J. 1982. Analysis of strain-induced EPR-line shapes and anisotropic spin-lattice relaxation in a [2Fe-2S] ferredoxin. Biochimica et Biophysica Acta 702 61-71. 

The motional dynamics of O J adsorbed on Ti supported surfaces has been analyzed over the temperature range 4.2-400 K in a recent paper by Shiotani et al. (66). Of the several types of 02, a species noted as 02 (III), and characterized by gxx = 2.0025, gyy = 2.0092, g12 = 2.0271 at 4.2 K, exhibited highly anisotropic motion. While gxx and gzz varied with increasing temperature and were accompanied by drastic line shape changes, gyy was found to remain constant. This observation indicates that the molecular motion of this 02 can be described by rotation about the y axis perpendicular to the internuclear axis of 02 and perpendicular to the surface with the notation given in Fig. 4. The EPR line shapes were simulated for different possible models and it was found that a weak jump rotational diffusion gave a best fit of the observed spectra below 57.4 K, whereas some of the models could fit the data above this temperature. The rotational correlation time was found to range from 10 5 sec (below 14.5 K) to 10 9 sec (263 K), while the... 

Analytical expressions were derived for the CW EPR line-shape for spin-polarized radical pairs in the limit where the combined dipolar and exchange interaction is weak relative to the energy differences between the resonances of the two spins.19 The equations were applied to the case of charge-separated sites in Ti02 nanoparticles. This approach simplifies the analysis of the distributions of interspin distances. 

In eqn. (2), AH°m is the EPR line width in the limit of such dilution that one can neglect the dipole-dipole interaction and A is a coefficient depending on the EPR line shape and on the character of the spatial distribution of the spin-labelled molecules. In the case of random spatial distribution according to theoretical calculations A = 35 GM 1 for the Gaussian EPR line shape and A = 56GM-1 for the Lorentzian EPR line shape. 

Computations of the EPR line shape made in ref. 71 with the help of eqn. (18) for the model distribution functions f(R) that are frequently used in radiation chemistry, have shown that the shape of the wings of the EPR lines is far more sensitive to changes in the distribution functions of radical pairs over the distances than to changes in such a conventionally used parameter as the line width between the points of the maximum slope, A//p. Thus, to estimate the distances of tunneling and their variations in the course of a reaction it is necessary to analyze the shape of the wings of the EPR lines. 

Figure 29.21 The upper portion shows the EPR lines recorded prior to exposing polyacetylene film to gaseous AsFs and the transformation of a symmetric EPR line shape to a dysonian line for conductive material. The ratio of the low-field to high-field amplitudes is shown in the inset as a function of the composition of the doped film. The lower portion of the curve shows a single line fit to the theoretical dysonian line shape (circles). 

EPR spectroscopy has been used to study rotation on vanadyl (V02+) analogues of Gd(III) chelates [73-76]. Vanadyl EPR line shapes are sensitive to rotation, they even allow for the distinction of isotropic and anisotropic motions. One concern about this technique is that substituting the trivalent Gd ion with the divalent vanadyl may modify the rotational dynamics of the complex. 

The main advantage of a multi-frequency study is that it provides information on the frequency dispersion of magnetic resonance parameters. This approach (dispersion), for example, is the power in NMRD studies. Several laboratories pioneered in the application of multi-frequency EPR as a route to a more accurate evaluation of key spectroscopic parameters (g, A, Q, D, E), as well as a more sensitive methodology for studying dynamical processes, where an interplay between the frequency dependence of the spin process and the frequency dependence of the EPR observation often can provide exceptionally detailed information [64,65]. In order to take advantage of the method, the frequency dependence of spin systems must be understood. This has led to the development of several theoretical approaches for better analysis of multi-frequency data, and especially in BPCA research, for the analysis of the frequency dependence of geffective, Tle, T2e, and the overall EPR line shape in frozen glasses and in room-temperature aqueous solutions. 

Electron spin relaxation in aqueous solutions of Gd3+ chelates is too rapid to be observed at room temperature by the usual pulsed EPR methods, and must be studied by continuous wave (cw) techniques. Two EPR approaches have been used to study relaxation studies of the line shape of the cw EPR resonance of Gd3+ compounds in aqueous solution, and more direct measurement of Tle making use of Longitudinally Detected EPR (LODEPR) [70]. Currently, LODESR is available only at X-band, and the frequency dependence of relaxation is studied by following the frequency dependence of the cw EPR line shape, and especially of the peak-to-peak line width of the first derivative spectrum (ABpp). 

The EPR line shapes produced by powder samples was calculated in many papers1,7,8. The spectrum of powder sample is considered as the superposition of spectra of large number small single crystals with chaotic orientation with respect to direction of external magnetic field. The sum intensity in each point of absorption band is determined by both the number of particles with given orientation and transition probability for this orientation. 

The line-shape of a true (undistorted) EPR spectrum should be independent of the acquisition parameters, and therefore to assess spectral distortion one can compare spectra acquired with different parameters. Figure 15.6 illustrates the effect of modulation amplitude on EPR line-shape. The central line-width (peak-to-peak width AHpp = 1.6 G) remains unchanged when the modulation amplitude is increased from 0.5 to 1 G while at a modulation amplitude of 10 G, distortion and line-broadening (AHpp = 6.4 G) can be clearly observed. The main sources of spectral distortions are modulation amplitude, microwave power, and scanning rate (speed). These are discussed in the following sections. 

Figure 15.6 Effects of modulation amplitude on cw-EPR line-shape. Spectra shown were obtained on a Bruker EMX spectrometer equipped with a high-sensitive cavity. The sample was an aqueous solution of tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-l-oxyl) (12.5 fiM, 5 fil) placed in a round glass capillary (0.6 mm ID, 0.8 mm OD Vitrocom, Inc., Mountain Lakes, NJ) sealed at one end. Acquisition parameters listed in Table 15.1 were used, except that the modulation amplitude (M.A.) was varied as respectively indicated for each spectrum. Note that these spectra have not been normalized, and differences in the amplitude of the spectrum are due to the different modulation amplitudes used.

Fig. 21. EPR line shape of K2Mg, cMncF4 at 0 = 0°. Dashed and dashed-dot curves are theoretical curves for 1-D and 2-D systems. Full curves are Gaussian and Lorentzian theoretical curves1431...

One of the first methods used for investigating the iron-sulfur-cluster structure was reported by Cammack and Evans for FeS-A/FeS-B in photosystem I. The thylakoid polypeptides were unfolded in a 80% dimethyl sulfoxide solution containing guanidine-HCl and the EPR line shapes ofboth FeS-A and FeS-B were consistent with the [4Fe 4S] type. As FeS-X was not yet known at that time, its cluster structure was not taken into consideration. 

Type-2, or normal, Cu has undetectable absorption and the EPR line shape of the low-molecular-mass copper complexes (A >0.014Ocm ). Type-2 copper centers are present in copper-containing oxidases... 

In most cases, the biological macromolecules will be disordered in the sample tube, which means that the molecules in the spectrometer will have random orientation with respect to the external magnetic field Bq applied to the sample. With t q)ical sample volumes for EPR experiments of between 100 pi and 10 nl, and with protein concentrations in liquid solutions of 100 pmol/1, the sample consists of 10 -10 molecules with a statistical equal distribution of all possible orientations with respect to the magnetic field axis Bq. If these macromolecules are static in their orientation, as is the case for frozen samples, a sample with this random orientation distribution is called a powder sample. EPR Line shapes in such powder samples will typically be enormously broadened as compared with crystalline samples, where all molecules have the same orientation with respect to the magnetic-field axis, or as compared with liquid samples, where the molecules are reorient-... 

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