EPR continuous-wave

Kevan, L. and Bowman, M. K. (1990) Modern pulsed and continuous-wave EPR spectroscopy. Wiley, New York. 

Fig. 9. The continuous wave EPR spectrum of recombinant Rhodnius prolixus NPl-histamine at a microwave frequency of 9.338 GHz (trace 1) and field-sweep ESE spectra at 8.706 GHz (trace 2), 3.744 GHz (trace 3), 3.065 GHz (trace 4). Dashed arrows show the changes in magnetic fields corresponding to principalg -values at the different microwave frequencies. Inset trace 1, the primary ESE decay recorded at 8.706 GHz, Bo = 213 mT (the low-field turning point in the field-sweep ESE spectrum shown by trace 2 in the main panel). Inset trace 2, the primary ESE decay recorded at 3.065 GHz, Bo = 140.8 mT (the high-field turning point in the field-sweep ESE spectrum shown by trace 4 in the main panel). Reproduced with permission from Ref. (89).

The power satnration occurs when the rate of absorption of microwave exceeds the rate at which the system returns to eqnilibrinm. A spectral parameter, R1/2, is used to describe quantitatively the microwave power saturation profile. In the RNR tyrosyl radical case, the R1/2 values at four representative temperatnres are given in Table 3. The most straightforward interpretation for the easily saturated radical spectra with very small P j2 values, as seen in M. tuberculosis R2, is that the tyrosyl radical is minimally influenced in its relaxation by the di-ferric clnster. This finding is reverse in mouse and yeast R2 proteins. To obtain the precise distance information in a biological system, advanced techniques such as ESSEM would be more pertinent than the continuous-wave EPR spectroscopy. 

The physics of the continuous wave EPR experiment are summarised in the spin Hamiltonian... 

The Rh(trop2dach) complex (trop2dach = 1,4-bis(5//-dibenzo a,<7 -cyclohepten-5-yl)-l, 2-diamino-cyclohexane) was synthesized and its structure was computed29 and the spin density was found to be highly delocalized over the two trop moieties. The spin population was about 36% at the rhodium atom. The structure of the complex and the continuous-wave EPR spectrum of its frozen solution at X-band is depicted in... 

A key issue in the activity of these catalysts concerns the specific role of the Cu and Cr active sites which are expected to be located on the surface of the catalyst particles. Since both Cu(II) and Cr(III) are paramagnetic, EPR spectroscopy can be used to identify the nature of the heterometallic species within the catalyst matrix after different thermal and chemical treatments. However, a detailed picture of the local environment of tlie transition metal center camiot be obtained by conventional continuous wave EPR spectroscopy alone. These can, nevertlieless, be obtained by pulsed EPR methods, namely the electron spin echo envelope modulation (ESEEM) tecluiique. 

Fig. 2. Simulated X-band EPR spectra for a nitroxide powder pattern. Top Powder patterns for each hyperfine line and the relationship to the principal tensor values from the spin Hamiltonian. Middle Absorption pattern. Bottom First-derivative pattern as is usually seen in a continuous wave EPR spectrum.

Hanson GR, Gates KE, Noble CJ, Griffin M, Mitchell A, Benson S (2004) XSophe-Sophe-XeprView (R). A computer simulation software suite (v. 1.1.3) for the analysis of continuous wave EPR spectra. J Inorg Biochem 98 903-916... 

Fig. 13.38 The layout of a continuous-wave EPR spectrometer. A typical magnetic field is 0.3 T, which requires microwaves of frequency 9 GHz (wavelength 3 cm) for resonance.

Molecular Sophe currently contains only a single linewidth model for the simulation of continuous-wave EPR spectra, angular variation of g values, as described below ... 

Once a CW EPR Experiment has been added to the Explorer Tree, the CW EPR Experiment forms can be viewed by a left mouse click on the CW EPR Experiment Node in the Explorer Tree. The Continuous Wave EPR Experiment Form has Continuous Wave, Sophe, Spectra, and Configuration Tabs. The Sophe, Spectra, and Configuration Tabs are common to all experiments and will be dealt with separately. 

Thus, a variable temperature continuous-wave EPR (cwEPR) study can be employed to not only determine the magnitude of the exchange coupling constant and its sign, but also the spin state of each iron atom as the plots of I(Stot) versus temperature are sensitive to the spin states, for example. Figure 4. 

Most continuous-wave EPR (CW-EPR) systems operate at 9-10 GHz microwave frequency - that is, at X-band (Table 23.1). However, in order to distinguish between interactions that are dependent on the magnetic field from the field-independent one, multifrequency EPR needs to be applied [17]. The second most common band is the Q-band at 34 GHz, mainly because it provides a threefold larger Zeeman splitting while still allowing measurements for samples of a reasonable size (which can be easily handled). 

Figure 23.3 Simulated X-, Q-, and W-band continuous-wave EPR spectra of two radicals, (a) Characterized by g-factors differing by 0.0005 and no hyperfine interactions ...

Fig. 10 Comparison between theory (black dots) and experimental continuous wave EPR signal at 9,7 GHz (Ref. 66). (b) Calculated EPR spectra are seen to line up with the experimental spectrum at 9.7 GHz. Cancellation resonances are labelled 0,1,2,3,4,5,7 with arrows. Some transitions are labelled, e.g. "10 -> 9" corresponds to the transition from state 10> to state 9> as defined above in Fig. 9. Reprinted with permission from M. H. Mohammady, G. W. Morley and T. S, Monteiro, Physical Review Letters, 2010,105,067602. Gopyright 2010 the American Physical Society.

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