EPR spectral analysis

Proton transfer from alkane radical cations to alkane molecules results in the transformation of these cations into neutral alkyl radicals (the conjugate bases). The nature of these radicals is determined by the site of proton donation in the alkane radical cation. Information on the site of proton donation in the proton transfer from alkane radical cations to alkane molecules can thus be derived from EPR spectral analysis of the neutral alkyl radicals formed. To aid the reader in appreciating the results that are presented on this matter below and in understanding related spectra from the literature, a section on the characterization of neutral alkyl radicals by EPR spectroscopy in solid systems is included at this point. 

Summaries of the information content of EPR spectroscopic methods (in particular on nitroxide radicals) and the length scales of interest are given in Fig. 3. Focusing on one radical ( observer spin ), the standard method continuous wave (CW) EPR at any temperature and echo-detected (ED) EPR at low temperatures give valuable information on the fingerprint of the radical. This is mainly the electronic but can also be the geometric structure of the radical center. From CW EPR spectral analysis and/or simulations, rotational motion on the time scale 10 ps - 1 ps can be characterized qualitatively and quantitatively. Furthermore, in CW EPR, radicals also intrinsically report on their immediate (usually up to a few solvation layers, maximum up to 2 nm) chemical environment (e.g., polarity, proticity, etc.). 

In summary, the methods of EPR spectral analysis described here allow the determination of basic spectroscopic parameters of paramagnetic centers (components of the g and A tensors, line shape), and for gaining insight into their dynamical behavior. This in turn permits conclusions to be drawn on the structure and local properties of the host system vide infra). 

A potentially interesting aspect of the X-band (in contrast to Q-band) is the ready availability of parallel-mode resonators these types of spectra (S S S B) have parallel-mode spectra of intensity comparable to the normal-mode spectra (cf. Figure 12.7), and so parallel-mode EPR is an easy way to obtain an independent data set for spectral analysis. This interesting aspect of the intermediate-field case remains to be explored and developed. 

The study of oxo-Cr(V)-diols complexes formed by reduction of Cr(VI) by GSH in the presence of m-cyclopentanediol (8) or Iram-cyclopentanediol (9) expanded the EPR analysis of oxo-Cr(V)-diols species to five-membered ring 1,2-diols.66 Unlike the Cr(V) complexes formed with 6 and 7, the 1,2-cyclopentanediols 8 and 9 lead to oxo-Cr(V) complexes having very similar EPR spectral patterns (Fig. 6). The spectra are in both instances the sum of two EPR components, both split by the hyperfine interaction with nonequivalent carbinolic protons (Table II). 

This chapter first summarizes methods for attaching spin labels to nucleic acids, with a special focus on RNA (Section 1). Detailed protocols for these labeling methods have been described in a series of recent publications (Edwards and Sigurdsson, 2007 Qin et al, 2007 Schiemann et al, 2007), and will not be repeated here. Acquisition and processing of cw-EPR spectrum will then be described in detail (Section 2), followed by a brief discussion of spectral analysis (Section 3). Specific examples of using cw-EPR to study dynamics and interactions in RNA will subsequently be given (Section 4). 

Adam, W., Mock-Knobiauch, C., Saha-Moeiier, C. R., Herderich, M. Are Mn " Species Involved in Mn(Salen)-Catalyzed Jacobsen-Katsuki Epoxidations A Mechanistic Eiucidation of Their Formation and Reaction Modes by EPR Spectroscopy, Mass-Spectral Analysis, and Product Studies Chiorination versus Oxygen Transfer. J. Am. Chem. Soc. 2000, 122, 9685-9691. 

The spectra do not directly report on the dynamics of the labeled macromolecule as a whole but contain information on three types of motion (1) internal motion of the nitroxide about the chemical bonds of the linker (cf. Fig. 1, left), (2) motion of the site of attachment relative to the rest of the macromolecule (conformational flexibility), and (3) motion of the macromolecule as a whole. The internal motion of the label may be restricted by the environment, depending on the extent to which the molecular enviromnent engulfs the label. These three dynamic components significantly complicate the spectral analysis. However, a spectrum can often be approximated by a simple motional model to provide information on the properties of the macromolecule [32]. Temperature dependent experiments can adjust the contributions of the different types of motion to the motional properties reported by the EPR spectra [33]. 

A mechanistic elucidation of then-formation and reaction modes hy EPR spectroscopy, mass-spectral analysis, and product studies chlorination versus... 

Fe Q-band ENDOR study of the isotopically enriched Ni-C state of D. gigas and D. desulfuricans hydrogenases and Ni-B state of D. desulfuricans revealed a weak coupling between the Fe and the nickel atoms when the enzyme was in the Ni-A forms while no coupling was observed for the Ni-B form (186). A careful analysis of linewidth of Ni-A and Ni-B EPR signals detected in Fe enriched and nonenriched hydrogenase samples indicated that hyperfine interactions are lost in the spectral linewidth and, hence, nonde-tectable. 

From the reaction of photogenerated t-BuO radicals with silanes 48, in the cavity of the EPR spectrometer, the corresponding spiro cyclohexadienyl radicals are the only detectable species [22]. For n = 3, the intermediate radical 49 showed a spectral feature that suggests a chair-like arrangement and a chair-chair interconversion barrier of 18.4kJ/mol was obtained from a detailed line-shape analysis. 

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