Applications of spin trapping

The last few years have seen numerous applications of spin trapping to biological systems, and in these the trapping of hydroxyl radicals has assumed some importance. This work has been confined almost exclusively to nitrone scavengers 4 the fact that the hydroxyl adduct [6] of DMPO is much more persistent than that [7] of the commonly used nitrone, benzylidene-t-butylamine-N-oxide ( phenyl t-butyl nitrone ,3 or PBN) [3], may be due to a fragmentation reaction, with subsequent oxidation of the cr-hydroxybenzyl radical, as shown. 

The trapping of hydroxyl radicals has also been of interest in connection with electrochemistry. Bard et al. (1974) initiated electrochemical applications of spin trapping and showed, for example, that the cathodic reduction of diazonium salts in the presence of PBN gives aryl-radical spin adducts. A route... 

Short-lived particulate- and vapor-phase free radicals that cannot be measured directly by ESR can be measured with the aid of spin traps. The first application of spin trapping to the study of free radicals in tobacco smoke was reported in 1971 by Bluhm et al. (349). MSS from commercial cigarettes, pipes, and cigars was bubbled through solutions of a-phenyl-A-tert-butylnitrone (PBN) in benzene (a spin trap). In each case, a doublet of triplets with a(N) = 1.376 and a(H) = 0.199... 

Although radicals may be measured directly in certain solid foods of low water activity, such as milk powder, indirect methods are required in liquid foods, such as emulsions and beer, because the radicals are very short lived (several seconds for peroxyl radicals). In solid samples, the ESR signals are, however, difficult to identify because of line broadening. In liquid samples, the use of artificial traps is required to form stable adducts that can be readily detected by ESR by reacting covalently with unstable radicals. Spin traps are usually nitroso compounds (e.g. tert-nitrosobutane), or nitrones (e.g. a-phe-nyl-ferf-butyl nitrone). The applications of spin traps are limited, however, because their efficiencies vary widely with different radicals. 

The main use of spin trapping is in identifying radical intermediates in organic and inorganic reactions. The reactions can be thermally, electro-chemically, or radiation induced. Probably the most important application of spin trapping is the study of radicals in biological systems. 

Various applications of spin trapping in both electrode oxidations and reductions have been reported. " The widely used spin traps fall into two classes of compounds, the nitrones and the nitroso compounds. The former are typified by PBN, which has the following structure ... 

In this section we will describe the fundamentals of ESR, the method of spin trapping, and the basics of ESR imaging. ESR fundamentals with emphasis on applications to polymeric systems have been described elsewhere [30-33]. 

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). 

The remainder of the present article is divided into two main sub-sections. The first sets out to emphasize those chemical and spectroscopic features which must be borne in mind when using the method of spin traps. The second takes a closer look at some of the applications of the method, and includes an account of the recent investigations of the kinetics of the trapping step itself. 

As data for the rates of spin-trapping reactions are accumulated, so it becomes possible to use the competition experiment in reverse , i.e. to determine rates of rearrangement, fragmentation, atom transfer, etc. which can compete with spin trapping. An attempt to estimate rates of decarbonylation of acyl radicals depended on this approach (Perkins and Roberts, 1973). Although the results obtained were intuitively reasonable, they depended on the assumption that the rate of scavenging of acyl radicals by MNP would be no different from that measured for the butoxycarbonyl radical. This still awaits experimental verification. Another application, reported recently, was to the rates of rearrangement (23) of a series of (o-(alkoxycarbonyl)-alkyl radicals... 

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. 

Earlier suggestions of the involvement of free radical intermediates have stimulated the application of spin chemistry methods to the investigation of the detailed mechanism of the photolysis of a-germyl ketones in either polar or nonpolar solvents, in the presence or in the absence of traps of element-centered free radicals. Of special interest are problems of the multiplicity of the reactive state, and the transformations of the element-centered free radicals in the bulk. 

The techniques and applications described in this chapter so far are restricted by the lifetime of the radical being investigated. If the radical has a particularly short lifetime, it may not be possible to observe it by direct in-situ methods. Instead, the technique of spin trapping may have to be used. The technique of spin trapping was first introduced by Janzen and Blackburn [104,105] short-lived free radicals react with a diamagnetic compound, e.g. a nitrone, to produce a relatively stable paramagnetic species. Where the spin trap is a nitrone, the corresponding nitroxide is found... 

Critical reviews of applications involving the design of spin-traps, advances in spin-labelling, paramagnetic centres on solid surfaces, exchange-coupled oligomers and radicals in flavoenzymes highlight the detailed and sophisticated information provided by EPR in areas of particular contemporary interest. 

Thornalley PJ (1986) Theory and biological applications of the electron spin resonance technique of spin trapping. Life Chemistry Reports 4 57-112. 

The application of RPR in the detection and quantification of species formed by spin-trapping the products of radical-monomer reactions is described in Section 3.5.2.1, The application of time-resolved F.PR spectroscopy to study intermolecular radical-alkene reactions in solution is mentioned in Section 3.5.1. 

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