Electron paramagnetic resonance detection

Henry, Y., Ducrocq, C., Drapier, J. C., Servent, D., Pellat, C., and Guissani, A. (1991). Nitric oxide, a biological effector. Electron paramagnetic resonance detection of nitrosyl-iron-protein complexes in whole cells. Eur. Biophys. J. 20, 1-15. 

Further support for this mechanism is derived by direct electron paramagnetic resonance detection of both Zr(III) intermediates, (r -Cp )2Zr(X)L and ( / -Cp)2Zr(R)L" . ... 

Frank HA and Violette CA (1989) MonomericbacteriochlorophyU is required for the triplet energy transfer between the primary donor and the carotenoid in photosynthetic bacterial reaction centers. Biochim Biophys Acta 976 222-232 Frank HA, Bolt JD, de B. Costa SM and Sauer K (1980) Electron paramagnetic resonance detection of carotenoid triplet states. J Am Chem Soc 102 4893 898 Frank HA, Machniki J and Felber M (1982a) Carotenoid triplet states in photosynthetic bacteria. Photochem Photobiol 35 713-718... 

Two types of EPR (electron paramagnetic resonance) detectable bistability have been detected during recent years. The first is related to the EPR spectrometer itself and not to the sample. The physical cause of the phenomenon is the non-linear behaviour of the sample to cavity coupling [26]. The width of the hysteresis loop was found to vary with the filling factor of the cavity. This type was detected by Giordano et al. [26] during studies on polypyrrole radical. 

Hgure 1 EPR spectra of electron transport particles (ETPs) from heart mitochondria from titration with NADH. (A) ETPs treated as other samples except that no reduclant was added (BHE) reduced with increased dose of NADH. (Reproduced with permission from Orme-Johnson N et al. (1974) Electron paramagnetic resonance-detectable electron acceptors in beef heart mitochondria. Journal of Biological Chemistry 249 1922-1939.)... 

Kim S S and Weissman S I 1976 Detection of transient electron paramagnetic resonance J. Magn. Reson. 24 167-9... 

Electron paramagnetic resonance spectroscopy (HER), also called electron spin resonance spectroscopy (ESR), may be used for direct detection and conformational and structural characterization of paramagnetic species. Good introductions to F.PR have been provided by Fischer8 and I.effler9 and most books on radical chemistry have a section on EPR. EPR detection limits arc dependent on radical structure and the signal complexity. However, with modern instrumentation, radical concentrations > 1 O 9 M can be detected and concentrations > I0"7 M can be reliably quantified. 

Electron spin is the basis of the experimental technique called electron paramagnetic resonance (EPR), which is used to study the structures and motions of molecules and ions that have unpaired electrons. This technique is based on detecting the energy needed to flip an electron between its two spin orientations. Like Stern and Gerlach s experiment, it works only with ions or molecules that have an unpaired electron. 

Shortly after, Doetschman and Hutchison reported the first example of a reactive carbene in the crystalline solid state, by preparing diphenylcarbene from diphenyldi-azomethane in mixed crystals with 1,1-diphenylethylene 84 (Scheme 7.23). When the mixed crystals were irradiated, carbene 85 was detected by electron paramagnetic resonance (EPR) and the disappearance of the signal was monitored to determine its kinetic behavior. Two reactions were shown to take place under topochemical... 

Up to date, several experimental techniques have been developed which are capable of detecting some of these particles under ordinary thermodynamic conditions. One can use these methods to keep track of transformations of the particles. For instance, it is relevant to mention here the method of electron paramagnetic resonance (EPR) with sensitivity of about 10 particles per cm [IJ. However, the above sensitivity is not sufficient to study physical and chemical processes developing in gaseous and liquid media (especially at the interface with solids). Moreover, this approach is not suitable if one is faced with detection of particles possessing the highest chemical activity, namely, free radicals and atoms. As for the detection of excited molecular or atom particles... 

Electron paramagnetic resonance (epr) spectroscopic methods are used for the detection and identification of species that have a nett electronic spin radicals, radical ions, etc. It is extremely sensitive, capable of detecting species down to concentration levels of 1 x 10 12 moles dm "3, and produces spectra that are distinctive and generally easily interpreted. Consequently, the technique has found extensive application in electrochemistry since the late 1950s. In order to understand epr, it may be helpful to review some fundamental concepts. 

A. Wennmalm, B. Lanne, and A.S. Petersson, Detection of endothelial-derived relaxing factor in human plasma in die basal state and following ischemia using electron-paramagnetic resonance spectrometry. Anal. Biochem. 187, 359-363 (1990). 

Various other techniques have been used to determine molybdenum, including adsorption voltammetry [510], electron-paramagnetic resonance spectrometry [512], and neutron activation analysis [513,514]. EPR spectrometry is carried out on the isoamyl alcohol soluble Mo(SCN)s complex and is capable of detecting 0.46 mg/1 molybdenum in seawater. Neutron activation is carried out on the /J-naphlhoin oxime [514] complex and the pyrrolidone dithiocar-bamate and diethyldithiocarbamate complex [513]. The neutron activation analysis method [514] was capable of determining down to 0.32 xg/l of molybdenum in seawater. 

Besides the electrically active complexes discussed above, there is indirect evidence for the existence of neutral complexes. In close analogy to the observations in silicon and several III-V materials it appears that hydrogen passivates deep and shallow acceptors. Because of the small concentrations of these neutral centers, all attempts to detect them directly with local vibrational mode (LVM) spectroscopy or electron paramagnetic resonance (EPR) have been unsuccessful. 

Electron paramagnetic resonance (EPR) spectroscopy is yet another diagnostic tool for the detection of isomorphous substitution of Ti. Its sensitivity is very high, and investigations can be performed with samples even with very low contents of paramagnetic species. The spectra and g parameters are sensitive to the local structure and associated molecular distortions. Hence, it is an ideal tool to characterize Ti in titanosilicates. Ti in the + 4 oxidation state in titanosilicates is diamagnetic and hence EPR-silent. Upon contacting with CO or H2 at elevated... 

Electron paramagnetic resonance (EPR) and NMR spectroscopy are quite similar in their basic principles and in experimental techniques. They detect different phenomena and thus yield different information. The major use of EPR spectroscopy is in the detection of free radicals which are uniquely characterised by their magnetic moment that arises from the presence of an unpaired electron. Measurement of a magnetic property of a material containing free radicals, like its magnetic susceptibility, provides the concentration of free radicals, but it lacks sensitivity and cannot reveal the structure of the radicals. Electron paramagnetic resonance spectroscopy is essentially free from these defects. 

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. 

---