Spin-trap probes

Misik V, Miyoshi N, Riesz P (1995) EPR spin-trapping studies of the sonolysis of H2O/D2O mixtures probing the temperatures of cavitation regions. J Phys Chem 99 3605-3611... 

This technique can be used to measure the production of free radicals because the unpaired electron in a free radical has magnetic resonance. However, because the radicals are unstable, owing to their high chemical reactivity, the technique of spin-trapping is used. In this technique, the generated radicals react with a suitable probe, and the EPR spectra arising from the reaction of the probe with different radical species can then be identified. 

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... 

The value of the spin-trapping experiment as a qualitative (and perhaps as a quantitative) probe for radical reactions would be enhanced by two types of quantitative information. The first concerns rates of radical addition to the diamagnetic traps 9 the second involves the rates of decay of the various spin... 

In one of the earliest investigations of spin trapping, olefin polymerization was employed to demonstrate the utility of the method as a qualitative probe for free-radical reactions (Chalfont et al., 1968). The polymerization of styrene, initiated by t-butoxyl radicals, proved to be an excellent system with which to obtain spectra attributable to spin adducts with MNP (a) of the initiator radical... 

If any general conclusions can be drawn from this Section, they must be that, whilst spin trapping affords a useful and simple probe for radical production in radiation chemistry, the interpretation of spin-adduct formation may be less than straightforward, and drawing of quantitative conclusions may be dangerous. However, when similar conclusions can be reached from experiments with two or more different spin traps, they may be accepted with greater confidence. 

The name spin trapping was coined by Janzen, and derives from analogy with the use of stable nitroxyls as spin labels (or spin probes ) that provide spectroscopic information regarding their microscopic environment, a procedure pioneered by McConnell et alP ... 

Numerous studies have indicated that pro-inflammatory mediators (cytokines) are involved in the destruction of the insulin-producing p-cells of the pancreas in the development of type I diabetes. Tabatabaie et al. introduced cytokines and PBN into the pancreas of rats. The analysis of pancreatic extracts revealed that the cytokines stimulate the formation of lipid radicals. Radical generation did not occur in rats treated with streptozotocin, which destroys the P-cells.33 Evidence for the role of radicals in diabetes has also been provided by spin trapping studies in pancreatic homogenates, showing that streptozotocin, which is often used to induce the condition in laboratory animals, stimulates OH production.332 Other workers, using EPR to observe the decay of a spin probe in the abdomen of mice (at 1.2 GHz), have demonstrated that strep-... 

In studies of reaction pathways, nitrosyl radicals are frequently used as spin traps to provide evidence for free radical pathways. A caution in interpretation of these results is that the probe or products will interact with the transition metal complex in the reaction and affect the reactivity of the probe with the organic substrate or free radicals produced. A number of reactions of the stable free radicals RNO and R2NO with platinum(II) complexes have been carried out which show that such reactions must indeed be considered (equations 473-... 

Not all radicals will add to spin traps, and in some cases addition may be too slow to compete with other processes. Also in biological systems, it must be borne in mind that the spin-traps will divert the reactive radicals from their normal role. In fact, they act as radical scavengers. Scavengers are often used as probes of radical reactions, so it is useful to use spin-traps for this purpose as well as for detection by ESR spectroscopy. 

Figure 2. Detection of 02- in BAEC upon treatment with CsA. Left representative experiment of superoxide detection in supernatants of BAEC measured by electron spin resonance (ESR) using the superoxide-sensitive spin trap DMPO. The superoxide generator DMNQ was used as a positive control. Right flow cytometiy detection of 02 with die probe DHE. BAEC were preincubated for 1 hour with 5 pM DHE and then incubated for 2 hours with the indicated doses of CsA. Data are represented as mean intracellular fluorescence of ethidium (oxidized fluorescent form of DHE) (n = 7). p < 0.05 vs CsA vehicle (0 pM CsA).

Spin Probes and Spin Traps with a Spiroindolinic Structure... 

Use of Electron Spin Resonance Techniques. Electron spin resonance (ESR) studies have been used to examine both activity of antioxidants " and their location within the Uposome . Studies of antioxidant radicals via ESR provide data on the electron delocalization within the antioxidant, which can be correlated with antioxidant activity, although not always with very good agreement with inhibition studies . Spin traps have been themselves examined as potential antioxidants, and have been used to attempt to trap peroxyl species for study . However, trapped peroxyl species are not very stable and carbon-centered radicals have been preferentially trapped, even though in some studies other techniques (e.g. malondialdehyde/thiobarbituric acid, MDA/TEARS-technique) indicate the presence of peroxide species in the sample . Eremy s salt ((K+S03 )2N0 ) has been used in micellar systems to determine rate constants quantitatively for the antioxidants a-Toc and ascorbic acid and their derivatives, because it reacts with them in a way similar to peroxyl radicals and can be used as a spin probe in stop-flow ESR studies . ESR has also been used to monitor the loss of dPPH and galvinoxyl signal intensity... 

Nitroxides are persistent free radicals [1] which can often be isolated and handled as kinetically stable species. Nitroxides react rapidly with carbon free-radical intermediates [2] with well-characterized rate constants [3], and can thus be used as kinetic and mechanistic probes, as well as to trap carbon radicals in synthetic processes. They are easily oxidized or reduced, and thus have a rich redox chemistry that has been utilized for a variety of oxidations. As nitroxides have an unpaired electron, they are paramagnetic and thus ESR active, making them valuable as spin labels for biomolecules [4] and as spin traps for transient radicals [5]. In addition, nitroxides have been developed as organic ferromagnetic materials [6]. The synthesis of nitroxides has been reviewed in 1994 [7]. This review will focus on the synthetic applications of nitroxides. 

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