Spin trapping irradiation

One way to make the short-lived intermediates amenable to study is to increase their lifetime, usually by irradiation in the solid state and/or at very low temperatures. Then, the intermediates can be detected at the end of the irradiation by ESR or optical absorption spectroscopy. The ESR study of radicals in the solid state is done on single crystals, polycrystalline samples or frozen aqueous solution. In case of polycrystalline samples or frozen aqueous solution the identification of the radicals from the ESR spectra is difficult in many cases and, for better identification, the ESR experiment should be conducted on irradiated single crystals. Later, the method of spin trapping, developed for the liquid phase5, was extended to polycrystalline solids. In this technique the polycrystalline solids are /-irradiated and subsequently dissolved in a solution containing the spin trap. 

Chignell etal. intensely irradiated p-aminobenzoic acid with a xenon arc lamp and showed the formation of several free radicals detected by spin traps in conjunction with ESR spectroscopy. It was suggested that such radicals could cause lipid peroxidation or react via one of the excited forms of oxygen [42, 43],... 

Relatively simple spectra are obtained from spin adducts of the hindered nitroso-arenes, and these may be further refined by deuteration of the spin trap (Terabe et al., 1973). In spite of being substantially dimerized, even in dilute solution,6 nitrosodurene (ND) has two considerable advantages over MNP. Firstly, it is more reactive towards radical addition (Table 5, p. 33). Secondly, it is not sensitive to visible light, and even on ultraviolet irradiation any photodecomposition is apparently not a major source of nitroxides. 

Other complexities are revealed when frozen solutions of spin trap in methanol are irradiated, and the solution is then melted. The proportions of spin adducts are markedly dependent on radiolysis temperature. One contributory factor is undoubtedly the reaction of MeO with neighbouring methanol in the solid matrix, to produce HOCH2, before diffusion to reach spin-trap molecules is possible. 

Spin trapping by PBN has also been employed to detect radical formation in a photo-Kolbe reaction in which acetic acid is irradiated (A > 360 nm) in the presence of platinized titanium dioxide powder (Kraeutler et al, 1978). The nitroxide observed was considered to be (PBN—Me ), but the published spectrum clearly shows the presence of a second species spectral overlap might therefore be an alternative to solvent polarity as an explanation of the discrepancy between the observed splitting parameters and those previously reported for this species. Where poor resolution obtains, it is important that... 

The electrochemical behaviour of PBN +-cyanide ion is identical to that found in the two cases of inverted spin trapping described above, namely that attack at PBN + occurs via the softer carbon atom of CN". This contrasts with an observation of the cyano adduct to PBN formed by irradiation of Mo(CN) -... 

The formation of the trinitromethyl adduct of PBN by photolysis of PBN and tetranitromethane (Okhlobystina et al., 1975) is an unequivocal case of inverted spin trapping. These components give an orange-red CT complex in, for example, dichloromethane when this solution is irradiated by light which only can excite the CT complex (A > 430 nm) the spin adduct (N02)3C-PBN is formed via reaction (46) (Eberson et al., 1994b). This adduct is highly persistent. When the solution is acidified by —2% trifluoroacetic acid, irradiation does not lead to spin adduct formation owing to protonation of trinitromethanide ion. 

Much work conducted in low-temperature matrices has shown that the primary chemical process induced by y-irradiation is formation of electrons (e ) and positive holes (h+), the latter eventually leading to the formation of radical cations of the component(s) with the lowest ionization potential (Symons, 1997). This means that an added spin trap may be transformed into its radical cation by y-irradiation and thus create conditions for inverted spin trapping, as already described for PBN and DMPO above in experiments designed to study this aspect. 

The photochemical acceleration of a variety of other reactions involving Al(Por)R is observed, and as a result, the mechanism of this photochemical activation is important and was the subject of two studies. In the first, a spin trap (tributylnitrosobenzene) was added to Al(TPP)Et in the dark, and slow production of Et (trapped as ArN(Et)0 ) was observed by EPR spectroscopy. Upon irradiation the amount of this product increased and a signal corresponding to the Al—O... 

Finally superoxide radicals can also be generated photochemically in chloroplasts in the presence of ascorbate or of paraquat. The formation was demonstrated by spin trapping on illumination of spinach chloroplasts in the presence of oxygen and paraquat Superoxide radicals are formed, moreover, in the near-ultraviolet photooxidation of tryptophan, as indicated by the increase of the HjO production in the presence of SOD and on irradiation in aerated solutions of protoporphyrin at 400 nm and of melanin with light of 320—600nmas shown by spintrapping. 

Hedrick WR, Webb MD, Zimbrick JD (1982) Spin trapping of reactive uracilyl radicals produced by ionizing radiation in aqueous solutions. Int J Radiat Biol 41 435-442 Heelis PF, Deeble DJ, Kim S-T (1992) Splitting of cis-syn cyclobutane thymine-thymine dimers by radiolysis and its relevance to enzymatic photoreactivation. Int J Radiat Biol 62 137-143 Hems G, Eidinoff ML (1958) Effect of X-irradiation on aqueous solutions of adenosine diphosphate. Radiat Res 9 305-311... 

Simic M, Hayon E (1973) Comparison between the electron transfer reactions from free radicals and their corresponding peroxy radicals to quinones. Biochem Biophys Res Commun 50 364-369 Sommerfeld T (2001) Electron-induced chemistry of 5-chlorouracil. Chem Phys Chem 677-679 Spalletta RA, Bernhard WA (1982) Spin-trapping free radicals by solvating X-irradiated crystalline pyrimidines. Radiat Res 89 11-24... 

Zhang Z-Y, Kuwabara M, Yoshii G (1983) ESR and spin-trapping study of room-temperature radicals in y-irradiated polycrystalline pyrimidine nucleotides. Radiat Res 93 213-231... 

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... 

The cardiac agent API amiodarone was observed to deiodinate sequentially upon irradiation in deaerated ethanol to yield the mono iodo product and finally the des iodo product (Fig. 93) (136). Formation of aryl radicals during the de-iodination process was supported by a spin-trapping study. 

Irradiation of phenyliodonium salts lead to the formation of phenyl radicals. In the presence of C60 these radicals are efficiently trapped under formation of pheny-lated C6o derivatives, mainly the monoadduct. In reaction mixtures of C6o, phenyliodonium salts and spin traps like nitroso-tert-butane ( BuNO) or nitroso-durene (ND) no phenyl adducts with the spin traps could be observed after irradiation. This suggests that C6o is a more efficient scavenger for phenyl radicals than the spin traps [177], Other investigations yielded similar results, e.g., the photolysis of organomercury compounds in the presence of fullerenes leads to fullerene-derived radical adducts. These radical adducts can combine to form dimers that are thermally stable and accumulate in the samples [Eq. (7)] [178],... 

Electron spin resonance (ESR) has been used to show the formation of H02 radicals on UV-irradiated Ti02 at 77 K (11). Spin-trapping molecules... 

The method was developed for the liquid phase and was later extended to polycrystalline solids the latter are y-irradiated and subsequently dissolved in a solution containing the spin-trap. Most commonly it is aqueous solution and several spin-traps were used6. 

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