In spin trapping

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... 

The presence of /3-hydrogen in the nitroxide radical may lead to disproportionation reactions. In spin-trapping experiments, N-t-butyl-a-phenyl nitrone yields rather unstable spin adducts. This type of radical can be stabilized by coordination to Nin. The Ni11 complex with N-oxy-A-r-butyl-(2-pyridyl)phenylmethanamine (923) reveals a distorted octahedral geometry with antiferromagnetic interactions between the unpaired electrons of the metal ion and the radical spins.00... 

The electrostatic terms can be reasonably well handled in solvents of high dielectric constant, but problems are raised by some solvents of widespread use in spin trapping, for example dichloromethane ( ) = 8.9), chloroform (D = 4.8) and benzene (D = 2.3), in which the electrostatic terms calculated as above for acetonitrile become -24.8, -46 and —96 kcal mol-1, respectively. Already in dichloromethane the effective standard potential of Fe(CN)6 /Fe(CN)6- is increased by 1.08 V and in benzene by an absurdly high 4.2 V ... 

Eberson and Nilsson (1990). The benzoyl t-butyl nitroxide [9], often detected in spin trapping experiments under oxidizing conditions. 

DMPO is more difficult to oxidize than PBN by about 0.2 V (Table 1) and is therefore expected to engage in spin trapping via its radical cation with greater difficulty, as found for the 0sCl6-4-N02-PBN reaction. Only acetate ion, tetramethylsuccinimide ion and triethyl phosphite gave the corresponding adducts upon oxidation with TBPA + in dichloromethane in the presence of DMPO, whereas fluoride ion gave the hydroxyl adduct. The latter was probably formed from water available from the unavoidable hydration shell around fluoride ion in its tetrabutylammonium salt. 

Because nitroxyl radicals do not react with most organic functional groups, they have found wide application as radical traps, and in living free radical polymerizations. Nitroxyl radicals are also used as spin labels, and are formed in spin trapping by nitroso compounds and nitrones vide infra). 

The critical involvement of alkyl radicals has been established in spin-trapping experiments although deuterium incorporation seems to implicate the eventual protonation of an anionic intermediate... 

Another caveat in spin trapping with DMPO is that the DMPO/ OH could be formed from the reduction of DMPO-OOH. However, if this were to occur, the DMPO/ OH will be inhibited by SOD and not by catalase. 

The main drawback in the use of thiocarbonyl compounds as spin traps was represented by the fact that in most cases the resulting spin adducts either were as transient as the attacking radicals (aliphatic thioketones and dithioesters) or were characterized by very complex ESR spectra (thiobenzophenone and its derivatives). It was only after the introduction of thiobenzoyltriphenylsilane la that the use of thiocarbonyl compounds in spin trapping experiments acquired some practical value. 

This reaction is made use of in spin-trapping experiments with 5,5-dimethylpyrroline-N-oxide. 

Since these groups have been used extensively in spin-trapping experiments one would expect to observe readily nitroxides resulting from free radical intramolecular addition. Such an example has been reported by McConnell in the case of caryophyllene nitrosite. According to Aurich, oxidation of hydroxybenzotriazoles in aromatic solvents would involve opening of the ben-... 

In spin trapping experiments, relatively stable ESR-active compounds, the spin adducts, are formed by reaction of radicals with ESR-silent compounds, the spin traps, added to the smnpie. The most commonly used spin traps are nitroxides and nitrones, which form stabilized radicals by reaction with other radicals (23). Based on the characteristics of the spin adduct (e.g. hyperfine pattern, coupling constants, and g-value), an assignment of the radical in question is often possible. However, due to lack of specificity of the often-used nitroxides, like N-r-butyl-a-phenylnitro-ne (PBN), a valid verification of the radicals trapped depends on identification by tecimiques such as HPLC-MS. Despite the lack of spectral resolution, spin tr q>ping seems to be a promising technique for prediction of the oxidative susceptibility of dairy products (see later sections). 

Lipids. Most of the recently published EPR studies on lipid peroxidation have been concerned with developments in spin-trapping methodology e.g. radical-adduct extraction and separation by HPLC, - the use of novel spin traps and the reassignment of peroxyl adducts to alkoxyl adducts ) and are covered in Chapter 2 this volume by Davies. 

The radiolytic oxidation of glycine anion by OH was studied by pulse radiolysis CWTR EPR. Both aminomethyl radicals CH2NH2 and H2N-CH-CO2-, with a yield of 29% for -CHjNHj and 53% for H2N-CH-C02 , were identified. No EPR lines attributable to the aminyl radical HN-CH2-CO2 were directly detected. However, clear evidence for the presence of the aminyl radical was obtained in spin trapping experiments. The implications of the results of Hug et al are discussed in the context of the recently proposed scheme for the oxidation of glycine anions by Bonifacic et al (cf. also Section 2.2.1.5). 

Since the rate constants for this, and for related ( ), cyclizations are known these reactions can be used to determine the rates of reaction of primary alkyls with such diverse species as cupric ion (32, 37) and tert-butyl hypochlorite (38). However, there can be no doubt that some of the most interesting radical-molecule reactions are those involved in spin-trapping . This is a technique which has been used qualitatively for several years to detect and identify transient free-radicals (2U). Its quantitative use in mechanistic studies has been hampered by the paucity of data available for the rate constants, k, for the addition of radicals to the spin traps, T. 

Jones, J. W. Sasaki, T. Goodlett, D. R. Turecek, F. Electron capture in spin-trap capped peptides. An experimental example of ergodic dissociation in peptide cation-radicals. J. Am. Soc. Mass Spectrom. 2007, 18, 432-444. 

Cldment, J.-L Fordo, P. In Advances in Spin Trapping, Gilbert, B. C., Davies,... 

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