Spin radical trapping agents

Electron-spin resonance (ESR) indicated free radicals in a brown polymer (mean ca 1 kDa by field-desorption MS) from a glucose-4-chloroaniline model system.185 Such free radicals (see Chapter 2) could be a source of visible colour. Exposure of the melanoidin to nitric oxide, a radical-trapping agent, diminished the ESR signal by 48% and changed the colour to red-brown. 

ESR studies on the initial free radicals were carried out by using MNP(2-methyl-2-nitrosopropane) or DMPO (5,5-dimethylpyrroline N-oxide) as the spin-trapping agent. The reactions are shown as ... 

Thus, superoxide itself is obviously too inert to be a direct initiator of lipid peroxidation. However, it may be converted into some reactive species in superoxide-dependent oxidative processes. It has been suggested that superoxide can initiate lipid peroxidation by reducing ferric into ferrous iron, which is able to catalyze the formation of free hydroxyl radicals via the Fenton reaction. The possibility of hydroxyl-initiated lipid peroxidation was considered in earlier studies. For example, Lai and Piette [8] identified hydroxyl radicals in NADPH-dependent microsomal lipid peroxidation by EPR spectroscopy using the spin-trapping agents DMPO and phenyl-tcrt-butylnitrone. They proposed that hydroxyl radicals are generated by the Fenton reaction between ferrous ions and hydrogen peroxide formed by the dismutation of superoxide. Later on, the formation of hydroxyl radicals was shown in the oxidation of NADPH catalyzed by microsomal NADPH-cytochrome P-450 reductase [9,10]. 

We have confirmed the dissociation manner of these compounds by means of the spin-trapping technique [167]. The radicals produced from 7 and 2-phenylethyl N,N-diethyldithiocarbamate (41) were trapped with 2,4,6-tri-ferf-butylnitrosobenzene (BNB) as a spin-trapping agent (Eq.44) [168] ... 

The sequence of events following the reaction of metMb with hydrogen peroxide has also been investigated through the use of spin trapping agents. Initial studies of this type with 5,5-dimethylpyrroline N-oxide (DMPO) led to the identification of Y103 as the primary site of DMPO adduct formation, a modification that was blocked by specific iodination of this residue 187). The identity of the radical trapped in this reaction... 

Nitric oxide has been difficult to detect with conventional spin-trapping agents. However, a new approach has been to use stable biradicals to trap nitric oxide (Fig. 19). The cheletrophic trap has two carbon centered radicals spaced the correct distance to catch nitric oxide and form a new ring (Korth et al.,... 

A practical examle of this kind of measurement is the study by Sato and Otsu of di-tert. -butyl peroxalate decomposition in p-xylene as solvent in the presence of vinyl monomer and 2-methyl-2-nitrosopropane (at room temperature the initiator is decomposed into Me3CO and C02). Due to repulsive forces, the spin-trapping agents reluctantly react with oxyl radicals. Therefore the following reactions mutually compete... 

Milatovic, D., Radic, Z., Zivin, M., Dettham, W-D. (2000a). Atypical effect of some spin trapping agents reversible inhibition of acetylcholinesterase. Free Radic. Biol. Med. 28 597-603. 

Camey, J.M., Floyd, R.A. (1991). Protection against oxidative damage to CNS by alpha-phenyl-tert-butylnitrone (PBN) and other spin-trapping agents a novel series of nonlipid free radical scavengers. J. Mol. Neurosci. 3 47-57. 

Mechanistic evidence for outer sphere SET is not easy to obtain for reactions which lead to conventional products. Instrumental techniques such as EPR or CIDNP often fail to show direct evidence for radical production because of short radical life-times [55]. Spin trapping agents [56] may interfere with the reaction from the start, especially since the widely employed nitroso compounds are good ligands for coordinatively unsaturated organometallics [57]. Even if rather j rsistent radicals are established via instrumental detection it is not always certain that they arise directly from electron exchange between the precursor molecules or that they are even related to the major reaction [58]. 

---