Spin adduct trapping

The initial radicals formed from the reaction of Ce(IV) ion and reductant systems can be trapped by MNP. The spin adducts of the initial radicals and MNP were observed by means of an ESR spectrometer. The structure of the initial radicals and the hypeiTme splitting constants of the spin adduct of the radical with MNP and tt-phenyl-N-/er/-butylnitrone (PBN) are compiled in Tables 5 and 6, respectively. 

Poyer, J.L., McCay, P.B., Lai, E.K., Janzen, E G. and Davis, E.R. (1980). Confirmation of assignment of the trich-loromethyl radical spin adduct detected by spin trapping during carbon tetrachloride metabolism in vim and in vivo. Biochem. Biophys. Res. Commun. 94, 1154-1160. 

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... 

When the lifetime of the radicals is very short and direct ESR detection is not an option, spin trapping is used to detect radicals at ambient temperatures. The method is based on the scavenging of radicals, P, by a spin trap, leading to the formation of a spin adduct with higher stability in most cases, this adduct is a nitroxide radical. 

Spin trapping experiments have been performed recently in a fuel cell inserted in the ESR resonator ("in situ" cell), using DMPO and a-(4-pyridyl-l-oxide)-N-ferf-butylnitrone (POBN) as the spin traps [78,82,83], These experiments allowed the separate examination of spin adducts at the anode and cathode sides. 

On the other hand, microsomes may also directly oxidize or reduce various substrates. As already mentioned, microsomal oxidation of carbon tetrachloride results in the formation of trichloromethyl free radical and the initiation of lipid peroxidation. The effect of carbon tetrachloride on microsomes has been widely studied in connection with its cytotoxic activity in humans and animals. It has been shown that CCI4 is reduced by cytochrome P-450. For example, by the use of spin-trapping technique, Albani et al. [38] demonstrated the formation of the CCI3 radical in rat liver microsomal fractions and in vivo in rats. McCay et al. [39] found that carbon tetrachloride metabolism to CC13 by rat liver accompanied by the formation of lipid dienyl and lipid peroxydienyl radicals. The incubation of carbon tetrachloride with liver cells resulted in the formation of the C02 free radical (identified as the PBN-CO2 radical spin adduct) in addition to trichoromethyl radical [40]. It was found that glutathione rather than dioxygen is needed for the formation of this additional free radical. The formation of trichloromethyl radical caused the inactivation of hepatic microsomal calcium pump [41]. 

Another approach to this problem is a search for the other more effective spin traps. Frejaville et al. [23] demonstrated that the half-life of spin-adduct of superoxide with 5-(diethoxyphosphoryl)-5-mcthyl-l -pyrrolinc-/V-oxide (DEMPO) is about tenfold longer than that of DMPO OOH. Despite a much more efficiency of this spin trap, its hydrophilic properties limit its use for superoxide detection in lipid membranes. Stolze et al. [24] studied the efficiency of some lipophilic derivatives of DEMPO in the reaction with superoxide. These authors demonstrated a higher stability of superoxide spin-adducts with 5-(di- -propoxypho-sphoryl)-5-methyl-1 -pyrrolinc-A -oxidc (DPPMPO) and 5-(di- -butoxyphosphoryl)-5-methyl-... 

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... 

In order to identify organic free - radicals present at quantifiable concentrations during the sonication of PCBs, we employed Electron Spin Resonance (ESR) with a spin trap, N-t-butyl-a-phenyl-nitrone (PBN). PBN reacts with the reactive free - radicals to form more stable spin-adducts, which are then detected by ESR. The ESR spectrum of a PBN spin adduct exhibits hyperfine coupling of the unpaired election with the 14N and the (3-H nuclei which leads to a triplet of doublets. The combination of the spin-adduct peak position and peak interval uniquely identifies the structure of a free-radical. 

This article concerns a simple expedient whereby short-lived reactive free radicals may be transformed into more persistent paramagnetic species, thus enabling esr techniques to be applied to systems in which the concentration of the reactive radical remains below normal detection limits. The principle is a simple one. It depends upon the addition to the reaction system of a small quantity of a diamagnetic substance (the spin-trap ) having a particularly high affinity for reactive radicals the product of this trapping reaction must be a particularly persistant free radical (the spin adduct ) whose concentration will build to readily detectable levels (>ca. 10—7—10-6 M). The general reaction is represented by equation (1). 

Spin-adduct spectra often reveal splittings to substituent atoms other than hydrogen. Indeed, since the spin-trapping technique provides a convenient route to many nitroxides containing structural features likely to be of spectroscopic interest, it has frequently been used to this end. Chlorine and bromine splittings, as well as those from fluorine, have been encountered and many nitroxide spectra have been reported in which there is splitting from a second nitrogen, from phosphorus, or even from a metal atom. 

The pre-eminent advantage of C-nitroso-compounds as spin traps is that in the spin adduct the scavenged radical is directly attached to the nitroxide nitrogen. Consequently, the esr spectrum of the spin adduct is likely to reveal splittings from magnetic nuclei in the trapped radical, and these will greatly facilitate its identification. A simple example is presented in Fig. 2, which shows the spectrum of the spin adduct of the methyl radical with 2-methyl-2-nitroso-... 

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. 

The esr parameters for a large number of alkyl aryl nitroxides have been collected recently, in part as a source of reference for spin trapping studies (Rockenbauer et al., 1978) the conformations of these nitroxides are also discussed. A selection of spectral parameters for spin adducts of the more important nitroso traps which have been discussed in this Section is presented in Table 2. 

The prime drawback with the nitrone traps, is the relative paucity of information immediately available from the spectra of the derived spin adducts. For example, with few exceptions the spin adducts of PBN [7] give six-line spectra with resolved splittings only from nitrogen and from the / proton. Identification of spin adducts must then be by careful comparison of the magnitude of the hyperfine splittings with those found for reference nitroxides... 

The spectra of spin adducts have been thoroughly examined in the preceding Sections, with emphasis on the ease with which structural information may be extracted. It has also been noted that some spin traps which are thermally or photochemically labile may give rise to nitroxides other than those deriving from the radical reaction under investigation. More seriously, there may be other possible sources of nitroxides in these systems which can cause (and almost certainly have caused) false mechanistic conclusions to be drawn. 

It is not uncommon to find the persistence of a spin adduct quantified in terms of half-life . This is a dangerous practice unless the experimental conditions are precisely defined, or it is known that the nitroxide decays by a unimolecular process. Decay may depend on reaction with a reducing agent present in the system, in which case the concentration of this species will influence the half-life. More commonly, decay will be second order (p. 5), in which case the time for disappearance of 50% of the spin adduct will show a profound dependence on its absolute concentration. The possibility of bimolecular association of nitroxides has been recognized for many years, but only very recently has it been suggested that this may be a complication under experimental conditions employed for spin trapping. Whilst the problem, which was encountered with the important [DMPO-HO ] system (Bullock et al., 1980), seems unlikely to be widespread, it is one which should always be borne in mind in quantitative studies. 

This simple expedient appears to have been employed first by Lunazzi et al. (1972), who compared the reactivities of ethylbenzene and a methylthiophene towards hydrogen abstraction by t-butoxyl radicals using MNP as trap (Scheme 3). Features in the spectra of the spin adducts of thienylmethyl and... 

Two possible approaches are indicated in Schemes 4 and 5. In the first, a reactive radical R> is spin-trapped in competition with its pseudo-first order reaction with a substrate SH, which occurs at a known rate to give RH and S. The growth of both spin-adducts (ST—R ) and (ST—S ) is monitored, and simple analysis leads to the trapping rate constant kT. In the second approach, R-does not react with a substrate, but undergoes unimolecular rearrangement or fragmentation at a known rate to give a new species R. This latter procedure... 

A more recent example is found in the work of Schmid and Ingold (1978), who used the rate of rearrangement (17) of 5-hexenyl radicals into cyclopentylmethyl radicals (R- and R - in Scheme 5) to time the spin trapping of primary alkyl radicals. In this system, both R and R are primary alkyl, and their spin adducts with several traps therefore have virtually indistinguishable spectra. This difficulty was circumvented by labelling C-l in the hex-5-enyl radical with 13C the unrearranged radical then gives spin... 

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