Spin/radical trap method

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... 

Another method of making the lifetime longer in the liquid phase is by adding compounds which, upon addition of radicals, produce long-lived radicals this method is called spin trapping. ... 

Absorption of a light quantum leads to an electron-hole pair Eq. (19). The electron reacts with an adsorbed oxygen molecule Eq. (20), and the hole semi-oxidizes a sulfide anion at the surface Eq. (21). Further oxidation of the sulfide anion occurs by O and O2 Eq. (22). The number of Cd ions formed equals that of the sulfate anions The oxidation of illuminated CdS powders was investigated by measuring the consumption and by detecting the superoxide radical,, by an ESR spin trapping method... 

Spin trapping methods were also used to show that when carotenoid-P-cyclodextrin 1 1 inclusion complex is formed (Polyakov et al. 2004), cyclodextrin does not prevent the reaction of carotenoids with Fe3+ ions but does reduce their scavenging rate toward OOH radicals. This implies that different sites of the carotenoid interact with free radicals and the Fe3+ ions. Presumably, the OOH radical attacks only the cyclohexene ring of the carotenoid. This indicates that the torus-shaped cyclodextrins, Scheme 9.6, protects the incorporated carotenoids from reactive oxygen species. Since cyclodextrins are widely used as carriers and stabilizers of dietary carotenoids, this demonstrates a mechanism for their safe delivery to the cell membrane before reaction with oxygen species occurs. 

Several other methods have been employed to access the conditions of bubble collapse. Misik et al. studied H20—D20 mixtures and through measurements with the use of spin traps, were able to determine the temperature from the relative rates of O—H and O—D cleavage [21]. They reported temperatures ranging from 2,000 to 4,000 K. Hart et al. developed a method based on the gas phase recombination of methyl radicals (MRR method), formed from the decomposition of methane [22]. They calculated temperatures of 2,000-2,800 K depending on the methane concentration. 

DNA breaks in human MCF-7 cells [65], Damaging effect of menadione was probably mediated by hydroxyl radicals as it was demonstrated by ESR spin-trapping method. The analogs of menadione 2-methylmethoxynaphthoquinone and 2-chloromethylnaphtho-quinone also stimulated DNA damage through the formation of superoxide and other free radicals [66]. Similar effects have been shown for hydroquinone, catechol, benzoquinone, and benzenetriol [67,68]. 

As shown above, addition of H202 into a solution of the complex caused a prompt color change into blue to give hydroxyl radical, which was detected with a spin-trapping method. The reaction was found therefore to be a Fenton-like reaction. 

Many of the early reports of spin-trapping experiments were focused on mechanistic investigations, and some of these feature in the early reviews (see p. 4). Unfortunately, it is in this application that inferences drawn may be most suspect. For example, the inability of the method to differentiate between radical trapping on the one hand, and a combination of nucleophile trapping with one-electron oxidation on the other, is a serious shortcoming. An early example of this was the tentative conclusion that acetoxyl radicals were spin-trapped by PBN competitively with their decarboxylation in reactions of lead tetraacetate. In view of the rapidity of the decarboxylation reaction, trapping of acetate ion and subsequent oxidation seems a likely alternative. 

Very recently, rate constants for scavenging of hydroxyl radicals by DMPO, and by the nitrone [18c], have been determined (Marriott et al., 1980) (see Table 5). As might be expected, the figures are close to the diffusion-controlled limit. The report of this work includes a concise and informative discussion of some of the difficulties with, and limitations of, the spin trapping method, especially where these relate to reactions involving hydroxyl radicals. 

The quite negative reduction potentials of spin traps (Table 2) make them less amenable to participation in the radical anion mechanism, as first established in the cathodic reduction of benzenediazonium salts at a controlled potential in the presence of PBN (Bard et al., 1974). In fact, the lower cathodic limit of the spin trapping method is set not by the nitrone but by the spin adduct formed. 

Thus there is little doubt that the hydroxyl radical, if generated by an unambiguous method such as pulse radiolysis, can be trapped by PBN or DMPO, even if the former has several deficiencies, among them low trapping efficiency and short half-life of HO-PBN. The problem in hydroxyl radical trapping thus rests with the possible competition from the nucleophilic addition-oxidation mechanism, as exemplified in reaction (69) for DMPO and Ox-Red as a general one-electron redox system, or the inverted spin trapping mechanism (70). The treatment to follow will mostly be limited to DMPO. 

In the preceding eqnation, the primary anion-radical gives the l-chloro-2,2,2-trifluoroethyl radical. In vivo, this radical was detected by the spin-trapping method (Poyer et al. 1981). Ahr et al. (1982) had presented additional evidence for the formation of the radical as an intermediate in halo-thane metabolism and identified l-chloro-2,2-difluoroethene as a product of radical stabilization. Metabolytic transformations of l-chloro-2,2-difluoroethene lead to acyl halides, which are relevant to halothane biotoxicity (Guengerich and Macdonald 1993). 

The method of spin traps is used to perform not only qualitative but also quantitative measurements. For quantitative determinations, the spin-trap method is applied if the rate constant of radical initiation is 10 -10 L mol s and trap concentration is not below 10 M (Freidlina et al. 1979). 

While using the trap method, one should take into account the oxidation properties of a trap with respect to radicals or other electron donors that are present in the system. Because spin traps can be electron donors themselves, their oxidation potentials should be more positive than that of the participants of the reaction under study. 

Kochany, J. and Bolton, J.R., Mechanism of photodegradation of aqueous organic pollutants. 2. Measurement of the primary rate constants for reaction of HO radicals with benzene and some halobenzenes using an EPR spin-trapping method following the photolysis of H202, Environ. Sci. Technol., 26(2), 262-265, 1992. 

Fig. 3.7. First derivative liquid-phase ESR spectra for nitroxide radicals formed by the spin-trapping method, (a) the CC13 adduct of PBN, showing hyperfine coupling to the 14N and H of the parent nitrone (b) the same species formed from l1-CClj radicals, showing the extra large doublet splitting from l3C (The /Mines are from an extraneous radical species.) (c) the hydroxyl radical adduct of DMPO, showing the characteristic 1 2 2 1 quartet, generated because of the fortuitous equality of the 1H and l4N splittings.

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