Electron paramagnetic resonance correlations

One of the earliest reports of LO inhibition concerned the effects of ortho-dihydroxybenzene (catechol) derivatives on soybean 15-LO [58]. Lipophilic catechols, notably nordihydroguaiaretic acid (NDGA) (19), were more potent (10 /zM) than pyrocatechol itself. The inactivation was, under some conditions, irreversible, and was accompanied by oxidation of the phenolic compound. The orfAo-dihydroxyphenyl moiety was required for the best potency, and potency also correlated with overall lipophilicity of the inhibitor [61]. NDGA and other phenolic compounds have been shown by electron paramagnetic resonance spectroscopy to reduce the active-site iron from Fe(III) to Fe(II) [62] one-electron oxidation of the phenols occurs to yield detectable free radicals [63]. Electron-poor, less easily oxidized catechols form stable complexes with the active-site iron atom [64]. 

Deep state experiments measure carrier capture or emission rates, processes that are not sensitive to the microscopic structure (such as chemical composition, symmetry, or spin) of the defect. Therefore, the various techniques for analysis of deep states can at best only show a correlation with a particular impurity when used in conjunction with doping experiments. A definitive, unambiguous assignment is impossible without the aid of other experiments, such as high-resolution absorption or luminescence spectroscopy, or electron paramagnetic resonance (EPR). Unfortunately, these techniques are usually inapplicable to most deep levels. However, when absorption or luminescence lines are detectable and sharp, the symmetry of a defect can be deduced from Zeeman or stress experiments (see, for example, Ozeki et al. 1979b). In certain cases the energy of a transition is sensitive to the isotopic mass of an impurity, and use of isotopically enriched dopants can yield a positive chemical identification of a level. 

Toolson, E.C., White, T. R. and Glaunsinger, W. S. (1979). Electron paramagnetic resonance spectroscopy of spin-labelled cuticle of Centruroides sculpturatus (Scorpiones Buthidae) correlation with thermal effects on cuticular permeability. 

DPPH = 2,2-diphenyl-1-picrylhydrazyl ENDOR= electron-nuclear double resonance EPR = electron paramagnetic resonance ESE = electron spin echoes ESEEM = electron spin echo envelope modulation EFT = fast fourier transformations FWHM = fidl width at half maximum HYSCORE = hyperfine sublevel correlation nqi = nuclear quadrupole interaction TauD = taurme/aKG dioxygenase TWTA = traveling wave tube amphfier ZFS = zero field sphtting. 

Electron paramagnetic resonance (EPR) is also a technique whereby the local structure of an impurity ion may be inferred by correlating the geometry with the experimentally determined electronic properties of the center. The third order expression for the g values can be written in terms of the Jahn-Teller angular coordinate as... 

TT-Electron densities, correlation with proton chemical shift, of alkylthiazoles, 344 of phenylthiazoles, 345 Electronic charges, of thiazole, 31 of aminothiazoles, 44 of chlorothiazoles, 44 of methylthiazoles, 40 Electronic effects, of 5-substituents on quaternization, 390 Electronic paramagnetic resonance, 84 coupling constants for nitrothiazoles, 84 coupling constants for 2-thiazolyl radical, 84... 

Voltammetric data for ester reductions are available for several aromatic esters [51-54], and in particular cyclic voltammetry shows clearly that in the absence of proton donors reversible formation of anion radical occurs [51]. In dimethylfonnamide (DMF) solution the peak potential for reduction of methyl benzoate is —2.29 V (versus SCE) for comparison dimethyl terephthalate reduces at —1.68 V and phthalic anhydride at —1.25 V [4]. Half-wave potentials for reduction of aromatic carboxylate esters in an unbuffered solution of pH 7.2 are linearly correlated with cr values [51] electron-withdrawing substituents in the ring or alkoxy group shift Ei/o toward less negative potentials. Generally, esters seem to be more easily reducible than the parent carboxylic acids. Anion radicals of methyl, ethyl, and isopropyl benzoate have been detected by electron paramagnetic resonance (epr) spectroscopy upon cathodic reduction of these esters in acetonitrile-tetrapro-pylammonium perchlorate [52]. The anion radicals of several anhydrides, including phthalic anhydride, have similarly been studied [55]. 

The dimer analysis cannot readily be extended to correlated states in extended systems like solid CT complexes. The observation of small AEp via static susceptibility or electron paramagnetic resonance (epr) nevertheless suggest a paramagnetic ground state. The observed CT transition at AEct then supports correlated states when it greatly exceeds AEp and affords, through the last Eq. (4), an estimate for q when t is known. Face-to-face overlap of k complexes yields t 0.1-0.3 ev, with... 

Simatos et al. (1981) measured the mobility of a spin-label probe, TEMPO, a stable free radical commonly used for electron paramagnetic resonance (EPR) spectroscopy. She found that the probe showed no mobility below a critical that correlated to Wq. A critical a also existed at which the probe demonstrated a partitioning into a dissolved and a solid-like state. This critical a could represent the moisture content correlating to Tg, though this concept had not been introduced in foods at that time. The partitioning of a... 

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