Spin trapping, EPR

Spin trapping EPR technique and UV-Vis spectroscopy have been used (Polyakov et al. 2001b) to determine the relative rates of reaction of carotenoids with OOH radicals formed by the Fenton reaction in organic solvents. The Fe3+ species generated via the Fenton reaction... 

Polyakov, N. E., A. I. Kruppa et al. (2001b). Carotenoids as antioxidants Spin trapping, EPR and optical studies. Free Rad. Biol. Med. 31 43-52. 

Adam W, Arnold MA, Nau WM, Pischel U, Saha-Moller CR, Saha-Moller CR (2002a) A comparative photomechanistic study (spin trapping, EPR spectroscopy, transient kinetics, photoproducts) of nucleoside oxidation (dG and 8-oxo-dG) by triplet-excited acetophenones and by the radicals generated from a-oxy-substituted derivatives through Norrish-type I cleavage. J Am Chem Soc 124 3893-3904... 

Niehaus H, Hildenbrand K (2000) Continuous-flow and spin-trapping EPR studies on the reactions of cytidine induced by the sulfate radical-anion in aqueous solution. Evidence for an intermediate radical cation. J Chem Soc Perkin Trans 2 947-952 Niles JC, Burney S, Singh SP, Wishnok JS, Tannenbaum SR (1999) Peroxynitrie reaction products of 3, 5 -di-0-acetyl-8-oxo-7,8-dihydro-2 -deoxyguanosine. Proc Natl Acad Sci USA 96 11729-11734... 

Comparing the reactants and the products, the reactions are apparently nonredox processes. Using a spin-trapping EPR technique it was shown [114] that irradiation of the complexes leads to an alkyl radical formation (CH3 or C2Hj). The efficiency of the homolytic metal-carbon bond splitting depends on the electronic properties of the other axial ligand. The ostensibly non-redox photoinsertions are thus a product of two redox reactions. As far as the photoreactive excited state is concerned, the metal-carbon bond is either indirectly activated by a ir-nt excitation localized on the tetrapyrrole ring [112] or there is an... 

It should be pointed out that the nature of the primary photochemical step(s) is still obscured and can depend, even for the same complex, on experimental conditions. Thus, Fen,(Por)N3 converts under irradiation in the solid state at low temperatures [162,163] into FevN(Por) in some solution systems [133] the formation of azidoradicals N3 has been detected by spin-trapping EPR no information on the heterolytic splitting of the Fe-N3 bond yielding NJ anion has been described in the literature (for azido complexes of some other central atoms the photosubstitution of the coordinated N3 ligand is a dominant chemical deactivation mode [1]). In addition, at particular conditions, the... 

Solvent benzene, toluene, CH2C12 X, > 400 nm N evidenced by spin-trapping EPR... 

The synthesis and photochemistry of related rhodium(lll) bis(alkyl) complexes cA,cA-[Rh(R)2(I)(CO)(dmb)] (R = Me, Tr) have been studied. The complexes showed an IT (tv - r (dmb)) absorption band at ca. 295 nm. There was another band at ca. 370nm for both complexes, which was assigned to an XTCT (p(l)7r (dmb)) transition. Irradiation of both complexes in solution resulted in Rh-R bond homolysis, as evidenced by IR, UV-vis, and spin-trap EPR investigations. The photoreaction was proposed to occur after crossing from the XLCT state to the reactive SBLCT state. 

Hideg fi, Spetea C, Vass I. Singlet oxygen and free radical production during acceptor- and donor-induced photoinhibidon. Studies with spin trapping EPR spectroscopy. Biochim Biophys Acta 1994 1186 143-152. 

Federicamycin A (= NSC-305264) spontaneously forms an oxidised free radical with electron transfer to O2 (Hilton et al. 1986). The observed hyper-fine structure of this radical is consistent with one-electron oxidation of the quininoid group. After federicamycin A is exposed to O2, an electron paramagnetic resonance (EPR) signal is observed with axial symmetry with temperature and power saturation behaviour suggestive of 02 . Spin-trapping EPR studies demonstrated that the drug reduces O2 to 02 ". and H2O2 to HO. ... 

Adam, W., Marquardt, S., and Saha-Moller, C.R., Oxidative DNA damage in the photolysis of N-hydroxy-2-pyridone, a specific hydroxyl-radical source, Photochem. Photobiol, 70, 287-291, 1999. Adam, W., Hartung, J., Okamoto, H., Marquardt, S., Nau, W.M., Pischel, U., Saha-MoUer, C.R., and Spehar, K., Photochemistry of N-isopropoxy-substituted 2(lEf)-pyridone and 4-p-tolylthiaz-ole-2(3H)-thione alkoxyl-radical release (spin-trapping, EPR and transient spectroscopy) and its significance in the photooxidative induction of DNA strand breaks, /. Org. Chem., 67, 6041-6049, 2002. 

In spin trapping, radicals are trapped by reaction with a diamagnetic molecule to give a radical product.476 This feature (i.e. that the free spin is retained in the trapped product) distinguishes it from the other trapping methods. The technique involves EPR detection of the relatively stable radicals which result front the trapping of the more transient radicals. No product isolation or separation is required. The use of the technique in studies of polymerization is covered in reviews by Kamachi477 and Yamada ft a/.478... 

Many nitrones and nitroso-compounds have been exploited as spin traps in elucidating radical reaction mechanisms by EPR spectroscopy (Section 3.5.2.1). The initial adducts are nitroxides which can trap further radicals (Scheme 5.17). 

Carotenoid radical intermediates generated electrochemically, chemically, and photochemically in solutions, on oxide surfaces, and in mesoporous materials have been studied by a variety of advanced EPR techniques such as pulsed EPR, ESEEM, ENDOR, HYSCORE, and a multifrequency high-held EPR combined with EPR spin trapping and DFT calculations. EPR spectroscopy is a powerful tool to characterize carotenoid radicals to resolve -anisotropy (HF-EPR), anisotropic coupling constants due to a-protons (CW, pulsed ENDOR, HYSCORE), to determine distances between carotenoid radical and electron acceptor site (ESEEM, relaxation enhancement). 

Rosentel IK, Mosobba MM, Riesz P (1981) Sonolysis of perhalomethane as studied by EPR and spin trapping. J Magn Reson 45 359-361... 

Misik V, Miyoshi N, Riesz P (1995) EPR spin-trapping studies of the sonolysis of H2O/D2O mixtures probing the temperatures of cavitation regions. J Phys Chem 99 3605-3611... 

The above historical outline refers mainly to the EPR of transition ions. Key events in the development of radical bioEPR were the synthesis and binding to biomolecules of stable spin labels in 1965 in Stanford (e.g., Griffith and McConnell 1966) and the discovery of spin traps in the second half of the 1960s by the groups of M. Iwamura and N. Inamoto in Tokyo A. Mackor et al. in Amsterdam and E. G. Janzen and B. J. Blackburn in Athens, Georgia (e.g., Janzen 1971), and their subsequent application in biological systems by J. R. Harbour and J. R. Bolton in London, Ontario (Harbour and Bolton 1975). 

A spin trap is a diamagnetic compound that reacts with a radical by addition of the radical functionality typically to a double bond in the trap, thus forming a new radical that is more stable (better, less unstable) than the original radical. By far the most common class of spin traps are nitrone compounds that, upon addition of the primary radical, produce a stable aminoxyl radical (Figure 10.1). The compound DMPO is the paradigmatic spin trap it is readily available, widely used, and its EPR spectra are relatively easy to interpret. Some of its radical adducts have unpractically short lifetimes. 

From the EPR spectroscopist s viewpoint the spin-trap experiment is next to trivial the molecular mass of the radical adduct is small enough to guarantee the molecule to tumble sufficiently rapidly at ambient temperatures in aqueous solution to ensure complete averaging away of any anisotropy in the spin Hamiltonian ... 

Thus, superoxide itself is obviously too inert to be a direct initiator of lipid peroxidation. However, it may be converted into some reactive species in superoxide-dependent oxidative processes. It has been suggested that superoxide can initiate lipid peroxidation by reducing ferric into ferrous iron, which is able to catalyze the formation of free hydroxyl radicals via the Fenton reaction. The possibility of hydroxyl-initiated lipid peroxidation was considered in earlier studies. For example, Lai and Piette [8] identified hydroxyl radicals in NADPH-dependent microsomal lipid peroxidation by EPR spectroscopy using the spin-trapping agents DMPO and phenyl-tcrt-butylnitrone. They proposed that hydroxyl radicals are generated by the Fenton reaction between ferrous ions and hydrogen peroxide formed by the dismutation of superoxide. Later on, the formation of hydroxyl radicals was shown in the oxidation of NADPH catalyzed by microsomal NADPH-cytochrome P-450 reductase [9,10]. 

Recently, Batthyany et al. [133] pointed out that the reduction of cupric ions bound to apolipoprotein B-100 by endogenous LDL components might be an initiation step in copper-mediated LDL oxidation. They suggested that this reaction proceeds to form cuprous ion and the protein-tryptophanyl free radical the latter was identified on the basis of EPR spectrum with spin-trap 2-methyl-2-nitrosopropane. 

To study mechanisms C—E, it seems reasonable to employ both, electrochemical approaches and EPR-spectroscopy. It is important to be aware of the electrochemical properties of nitrones if used as spin traps for production of spin adducts (SA) is possible not only via homolytic process (C) but also via ionic processes shown in Scheme 2.77. In the case of (B), protonation can protect the... 

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