Nitroso compounds, spin trapping

Nitrobenzene chloride, sulfonylperoxy radical reactions, 1035, 1036 2-Nitrobenzenesulfinylperoxy intermediate, superoxide reactions, 1034 Nitrogen-containing compound oxidation bis(trimethylsilyl) peroxide reactions, 802-3, 804 dioxiranes, 1151-5 primary aromatic amines, 1151 A-oxidation, 531-8, 539 Nitrohpids, hpid hydroperoxides, 952-4 Nitronate ions, dioxirane oxidation, 1152-3 Nitrosation, malondialdehyde, 667 Nitroso compounds, spin trapping, 664 Nitrotyrosine, peroxynitiite determination, 740-1... 

A diamagnetic compound (spin trap) is introduced into the radical producing system to give a relatively stable ESR-observable free radical (spin adduct). Typical spin traps are nitrone compounds such as phenyl-ZV-rcrt-butylnitrone (PEN) (8) and 5,5-dimethyl-1-pyrroline-/V-oxide (DMPO) (9) and nitroso compounds such as 2-methyl-2-nitro-sopropane (MNP) (10) and 2.4,6-tri-/crr-butylni-trosobenzene (TEN) (II)... 

Many nitrones and nitroso-compounds have been exploited as spin traps in elucidating radical reaction mechanisms by EPR spectroscopy (Section 3.5.2.1). The initial adducts are nitroxides which can trap further radicals (Scheme 5.17). 

Despite their short half-lives, it is possible to detect free radicals in biological tissues by the addition of nonradicals such as nitrones or nitroso compounds, which act as spin traps by forming relatively stable free radicals on reaction with the endogenous radical species. Utilizing the technique of electron spin resonance (e.s.r.) spectroscopy, we have demonstrated ROM generation by human rheumatoid synovium when subjected to cycles of hypoxia/normoxia in vitro. Using 3,5-dibromo-4-nitroso-benzenesulphonate (DBNBS) as a spin trap, a... 

Despite these interesting diversions, the vast majority of reports of investigations or applications of the spin-trapping technique depend on the use of C-nitroso-compounds or of nitrones the remainder of this review will be concerned exclusively with these two classes of scavenger. 

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-... 

Very shortly after the technique of spin trapping was first proposed as a versatile general method for probing free-radical reactions, it was pointed out by Forrester and Hepburn (1971) that both nitroso-compounds and nitrones are susceptible to nucleophilic attack. The resulting hydroxylamines are particularly sensitive to oxidation (14a,b) and this gives nitroxides Under many circumstances, the possibility of such a reaction is remote, but it should... 

Rate constants for spin trapping of alkyl radicals measured by the procedures outlined here, are collected with other spin-trapping rate data in Table 5. It will be seen that most nitrone and nitroso traps scavenge reactive radicals of diverse types with rate constants generally in the range 10s-10 1 mol-1 s l. Of the nitroso-compounds, the nitroso-aromatics (except for the very crowded TBN) are particularly reactive, whilst MBN and DMPO are the most reactive nitrones. Much of the data for spin trapping by nitrones has been accumulated by Janzen and his colleagues, who have discussed in a short review how steric and electronic factors influence these reactions (Janzen et ai, 1978). 

Nitroso compounds, nitrones, and other diamagnetic molecules are used as spin traps. Capturing radicals prodnced in the reaction, spin traps form the so-called spin adducts—stable nitroxyl radicals easily detectable by ESR spectroscopy. In other words, the progress of the reaction can easily be followed by an increasing intensity of the spin-adduct signal. By and large, the method of traps reveals radicals by the disappearance (or appearance) of the ESR signal. 

Spin traps. Nitroso compounds and nitrons (A-oxides) interact with radicals to form nitroxyl radicals (Scheme 4.22). 

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). 

Since only free radicals give an esr spectrum, the method can be used to detect the presence of radicals and to determine their concentration. Furthermore, information concerning the electron distribution (and hence the structure) of free radicals can be obtained from the splitting pattern of the esr spectrum (esr peaks are split by nearby protons).141 Fortunately (for the existence of most free radicals is very short), it is not necessary for a radical to be persistent for an esr spectrum to be obtained. Esr spectra have been observed for radicals with lifetimes considerably less than 1 sec. Failure to observe an esr spectrum does not prove that radicals are not involved, since the concentration may be too low for direct observation. In such cases the spin trapping technique can be used.142 In this technique a compound is added that is able to combine with very reactive radicals to produce more persistent radicals the new radicals can be observed by esr. The most important spin-trapping compounds are nitroso compounds, which react with radicals to give fairly stable nitroxide radicals 143 RN=0 + R —> RR N—O. 

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