Radical stability spin traps

Many of these free radical intermediates have been detected directly with ESR. Others are too reactive to detect directly, but a method to stabilize these free radicals called spin trapping has proved successful. Spin trapping is a technique in which a short-lived reactive free radical (R ) combines with a diamagnetic molecule ( spin trap ) to form a more stable free radical ( radical adduct ) which can be detected by electron spin resonance ... 

Among all analytical methods, ESR is capable of directly detecting radicals. The main challenge is the short lifetime of hydroxyl (HO ) radicals. Without spin trap, HO radicals can be detected only by ESR at low temperatures (below -196°C) owing to their short lifetime (Bosnjakovic and Schlick 2004). HO radicals can be detected by ESR at -73°C. Elnorinated alkyl radicals can be detected after submerging Nafion/Ti -" in H O for 14 days at ambient temperature. It seems that the Nafion matrix can stabilize HO radicals. 

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... 

Spin trapping methods were also used to show that when carotenoid-P-cyclodextrin 1 1 inclusion complex is formed (Polyakov et al. 2004), cyclodextrin does not prevent the reaction of carotenoids with Fe3+ ions but does reduce their scavenging rate toward OOH radicals. This implies that different sites of the carotenoid interact with free radicals and the Fe3+ ions. Presumably, the OOH radical attacks only the cyclohexene ring of the carotenoid. This indicates that the torus-shaped cyclodextrins, Scheme 9.6, protects the incorporated carotenoids from reactive oxygen species. Since cyclodextrins are widely used as carriers and stabilizers of dietary carotenoids, this demonstrates a mechanism for their safe delivery to the cell membrane before reaction with oxygen species occurs. 

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 traps are usually not practical to stabilize radicals on biomacromolecules because the reactivity is too low, presumably due to steric hindrance. Spin traps are used to stabilize physiologically relevant radicals of relatively small size such as hydroxyl, superoxide, and carbon-based radicals on organic molecules, for example, lipids. 

The nature of the cleavage and the possible stability of sulfuranyl radicals formed after electron transfer could be advantageously revealed by ESR and the associated technique of spin trapping as well [247,248]. Spin traps such as f-butylphenylnitrone and nitrosodurene could contribute to demonstrate the structure of the leaving radical formed by cathodic decomposition of the sulfonium substrate. 

Radicals such as OH and OJ can be stabilized, and detected by epr, by spin-trapping with dmpo,... 

In the preceding eqnation, the primary anion-radical gives the l-chloro-2,2,2-trifluoroethyl radical. In vivo, this radical was detected by the spin-trapping method (Poyer et al. 1981). Ahr et al. (1982) had presented additional evidence for the formation of the radical as an intermediate in halo-thane metabolism and identified l-chloro-2,2-difluoroethene as a product of radical stabilization. Metabolytic transformations of l-chloro-2,2-difluoroethene lead to acyl halides, which are relevant to halothane biotoxicity (Guengerich and Macdonald 1993). 

Tripathi GNR (1998) Electron-transfer component in hydroxyl radical reactions observed by time resolved resonance Raman spectroscopy. J Am Chem Soc 120 4161-4166 TsaiT, Strauss R, Rosen GM (1999) Evaluation of various spin traps for the in vivo in situ detection of hydroxyl radical. J Chem Soc Perkin Trans 2 1759-1763 Tsay L-Y, Lee K-T, Liu T-Z (1998) Evidence for accelerated generation of OH radicals in experimental obstructive jaundice of rats. Free Rad Biol Med 24 732-737 Ulanski P, von Sonntag C (2000) Stability constants and decay of aqua-copper(lll) - a study by pulse radiolysis with conductometric detection. Eur J Inorg Chem 1211-1217 Veltwisch D, Janata E, Asmus K-D (1980) Primary processes in the reactions of OH radicals with sul-phoxides. J Chem Soc Perkin Trans 2 146-153... 

Spin traps come in basically two types nitroso compounds and nitrone compounds. Reactive free radicals react with the carbon of the nitrone functional group to form a radical adduct that always has a nitroxide group, which is an unusually stable type of free radical. Nitrones are the most useful spin traps for the in vivo detection of free radical metabolites because of the stability of the resulting radical adduct. However, identification of the parent radicals can be difficult because adducts derived from different radicals often have very similar EPR spectra. A comprehensive review of this area through 1992 has recently been published [48]. 

EPR spectra are essential for establishing reliable correlations between the formation of OH and/or OOH radicals and the catalytic activity. Because of their short lifetimes, these radical species can usually not be detected directly instead, they are detected after the stabilization with a spin trap such as a nitroxide. However, as a result of being trapped, they lose their reactivity, and consequently a relationship between their concentration and the conversion of the organic substrate cannot be determined. 

Other approaches to studying transient species involve some kind of stabilization. Freeze-quenching and chemical stabilization have been used. Freeze-quenching is quite common for macromolecular radicals and metal ions, whereas chemical stabilization (e.g., by spin trapping) is more usual for small radical species. 

The ways of getting around this problem involve increasing the lifetime of the radicals by some physical or chemical means. One such approach involves stabilizing the radicals by immobilization, for example, by freeze-quenching a reaction mixture [80]. The disadvantage of this method is that an immobilized radical is generally much harder to characterize and identify than one in fluid solution. Other approaches make use of the chemical reactivity of radicals, for example, their ability to add to the double bonds in nitrones and nitroso compounds. This has led to the development of the spin-trapping procedure [81,82], in which a transient radical is reacted with the... 

TABLE 10.2 Addition Rate Constants kj of Various Radicals to Monomers Reaction Rate Constants kr of These Radicals with Oxygen, an Amine, a Stabilizer (HQME Hydroquinone-Methyl Ether) and a Spin Trap (TEMPO 2,2,6,6 Tetramethylpiperidine N-Oxyl). 

---