Free radicals electron paramagnetic resonance studies

Extensive investigations on the catalytic mechanism of classical peroxidases resulted in a consensus model involving five different iron species [30, 31], These species are ferrous, ferric, Compound I, Compound II, and Compound III (Fig. 11.1). As discussed in Chap. 5, after the reaction of ground state (GS) Femporphyrin with H202, Compound I (Cl) is formed, a cationic oxob e,vpor-phyrin-based Ji-free radical. Electron paramagnetic resonance (EPR) studies established that, in peroxidases of classes I and III, the second oxidation equivalent in Cl is present as a porphyrin-based free radical [32, 33]. In peroxidases from fungal sources, electron abstraction from the protein results in the formation of a different species with the free radical based in a residue close to the porphyrin. 

Smaller B, Remko J R and Avery E C 1968 Electron paramagnetic resonance studies of transient free radicals produced by pulse radiolysis J. Chem. Rhys. 48 5174-81... 

Davydov, R., Kuprin, S. Graslund, A., and Ehrenberg, A. 1994. Electron paramagnetic resonance study of the mixed-valent diiron center in Escherichia coli ribonucleotide reductase produced by reduction of radical-free protein R2 at 77 K. Journal of the American Chemical Society 116 11120-11128. 

J. Kroh, B.C. Green, and J.W.J. Spinks, Electron paramagnetic resonance studies on free radicals produced by T(3-particles in frozen H2G and D2G media at liquid nitrogen temperature. Can. J. Chem. 40, 413-425 (1962). 

Electron Paramagnetic Resonance studies Analysis by electron paramagnetic resonance spectrometry (EPR) showed the electrophoresis fraction to contain stable free radicals. A strong EPR signal was obtained at the g factor value of 2.0035-0.0003. The involvement of free radicals in the Maillard reaction has previously been reported (16). Recently also Lessig and Baltes (17) reported the content of extremely stable free radicals in melanoids obtained from the reaction between glucose and 4-chloroaniline. 

Binet et al (2002) have undertaken an initial electron paramagnetic resonance study to examine the distribution of free radicals in Murchison and Orgueil macromolecular material. They suggest that there are radical-rich regions, which could represent regions of pristine interstellar organic matter preserved within the macromolecular material. 

Rex, R. W. (1960). Electron paramagnetic resonance studies on stable free radicals in lignins andbumic acids. Nature 188, 1185-1186. 

Using various free radical scavenging systems. Jolly et al. (1984), Burton (1985), Ambrosio et al. (1986), Bolli et al. (1987) and Bhatnagar (1995) have shown a link between myocardial ischaemia and reperfusion injury and the generation of various toxic free radical species. Sharma et al. (1994) in an electron paramagnetic resonance study found ascorbyl free radical as a real-time marker of free radical generation in briefly ischaemic and reper-fused dog hearts. Only Uraize et al. (1987) were unable to limit the size of myocardial necrosis after 40 min of ischaemia by superoxide dismutase. 

Connor, H.D., Gao, W., Nukina, S., Lemasters, J.J., Mason, R.P. and Thurman, KG. (1992). Evidence that free radicals are involved in graft failure following orthotopic liver transplantation in the rat an electron paramagnetic resonance spin trapping study. Transplantation 54, 199-204. 

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... 

Most stable ground-state molecules contain closed-shell electron configurations with a completely filled valence shell in which all molecular orbitals are doubly occupied or empty. Radicals, on the other hand, have an odd number of electrons and are therefore paramagnetic species. Electron paramagnetic resonance (EPR), sometimes called electron spin resonance (ESR), is a spectroscopic technique used to study species with one or more unpaired electrons, such as those found in free radicals, triplets (in the solid phase) and some inorganic complexes of transition-metal ions. 

McBride and co-workers have studied extensively the reactions of such free-radical precursors as azoalkanes and diacyl peroxides (246). By employing a variety of techniques, including X-ray structure analysis, electron paramagnetic resonance (EPR), and product studies, and comparing reactions in the crystal and in fluid and rigid solvents, they have been able to obtain extremely detailed pictures of the solid-state processes. We will describe here some of the types of lattice control they have elucidated, and the mechanisms that they suggest limit the efficacy of topochemical control. 

Buettner, G. R., KeUey, E. E., and Bums, C. P., 1993, Membrane hpid free radicals produced from LI 210 murine leukemia ceUs by photofrin photosensitization an electron paramagnetic resonance spin trapping study. Cancer Res. S3 3670-3673. 

Electron paramagnetic resonance spectroscopy (EPR) (also called electron spin resonance spectroscopy, ESR) has been scarcely applied in the field of art and art conservation. Some work can be found in which EPR is used as complementary technique to SEM-EDX, NMR, and mass spectrometry (MS) for studying free radicals occurring in polymerization, pyrolytic, oxidative, and other radical degradative processes in artwork, as well as in the characterization of varnishes and oleoresinous media [42]. 

In 1960 Rex (22) first reported the use of electron paramagnetic resonance spectrometry as a method for demonstrating the presence of stable organic free radicals in humic acid. We felt that this technique might provide useful information about the structure of humic acid which was not readily available by other physical methods, particularly if relations between EPR spectra and chemically modified humic acids could be demonstrated. Our preliminary studies (26) confirmed this presumption. 

Electron paramagnetic resonance (EPR) studies on pyridine solutions of bianthrone by Wasserman (41) suggest that the thermochromic form may have one unpaired electron localized on each half of the molecule. The structure proposed by Woodward and Wasserman (42) is VIII which they consider to be identical to the colored photochromic form. Klochkov (43) has warned that free radicals may be involved in decomposition processes occuring at high temperatures in studies of the bian-... 

Huttermann J, Ward JF, Myers LS Jr (1971) Electron spin resonance studies of free radicals in irradiated single crystals of 5-methylcytosine. Int J Radiat PhysChem 3 117-129 Huttermann J, Ohlmann J, Schaefer A, Gatzweiler W (1991) The polymorphism of a cytosine anion studied by electron paramagnetic resonance spectroscopy. Int J Radiat Biol 59 1297-1311 Hwang CT, Stumpf CL, Yu Y-Q, Kentamaa HI (1999) Intrinsic acidity and redox properties of the adenine radical cation. Int J Mass Spectrom 182/183 253-259 Ide H, Otsuki N, Nishimoto S, Kagiya T (1985) Photoreduction of thymine glycol sensitized by aromatic amines in aqueous solution. J Chem Soc Perkin Trans 2 1387-1392 Idris Ali KM, Scholes G (1980) Analysis of radiolysis products of aqueous uracil + N20 solutions. J Chem Soc Faraday Trans 176 449-456... 

Electron paramagnetic resonance (EPR) has been used extensively in studies of the mechanism of catalytic reactions. It has been used to identify free radicals and ion-radicals formed by chemisorbed species on catalytically active sites and to study the structure and distribution of paramagnetic catalytic sites such as those produced by transition metals or metal ions on a catalyst surface. EPR remains primarily aresearch tool for studying mechanisms of catalytic reactions. 

Electron spin resonance (ESR) or electron paramagnetic resonance (EPR) spectroscopy has developed at an outstanding pace since its discovery in 1945 (Zavoiskii 1945), so that at present the technique is very well understood in its many aspects. In wood chemistry, ESR has become an essential tool for the study of the structure and dynamics of molecular systems containing one or more unpaired electrons, i.e., free radicals. ESR has found applications as a highly sensitive tool for the detection and identification of free radical species in lignin and lignin model compounds (Steelink 1966, Kringstad and Lin 1970). A recent literature review of free radicals in wood chemistry is available (Simkovic 1986). 

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