Detection of free radicals

An extremely sensitive technique to detect the nature of radical pairs in a photochemical reaction, called chemically induced dynamic nuclear polarization (CIDNP), depends on the observation of an enhanced absorption in a nuclear magnetic resonance (NMR) spectrum of the sample irradiated in situ in the cavity of the NMR spectrometer. The background to and interpretation of CIDNP are discussed by Gilbert and Baggott (1991). 

Probably the main technique that has been used to detect free radical intermediates in photochemical reactions is the competitive reaction rate study in which various free radical scavengers are added to the sample during irradiation, and the rate of disappearance of drug and appearance of particular products is compared to 

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Another magnetic technique for the detection of free radicals uses an ordinary NMR instrument. It was discovered that if an NMR spectrum is taken during the course of a reaction, certain signals may be enhanced, either in a positive or negative... 

Globus MY, Busto R, Lin B, Schnippering H, Ginsberg MD. Detection of free radical activity during transient global ischemia and recirculation effects of intraischemic brain temperature modulation. J Neurochem 1995 65 1250-1256. 

Garlick, P.B., Davies, M.J., Hearse, D.J. and Slater, T.F. (1987). Direct detection of free radicals in the reperfused rat heart using electron spin resonance spectroscopy. Circ. Res. 61, 757-760. 

Evans, P.H., Morgan, L.G., Yano, E. and Urano, N. (1992a), Chemiluminescent detection of free radical generation by stimulated polymorphonuclear leukocytes in vitro effect of nickel compounds. In Nickel and Human Health Current Perspectives (eds. E. Nieboer and J.O. Nriagu) pp. 363-373. Wiley, New York. 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

Detection of free radicals can be performed using light absorption, luminescence, oxygen consumption, electrical conductivity, and enzyme activity measurements. A number of examples of relatively common systems are given in Table 1. 

Suzuki Y, Katagi T (2008) Novel fluorescence detection of free radicals generated in photolysis of fenvalerate. J Agric Food Chem 56 10811-10816... 

Electron paramagnetic resonance (EPR) and NMR spectroscopy are quite similar in their basic principles and in experimental techniques. They detect different phenomena and thus yield different information. The major use of EPR spectroscopy is in the detection of free radicals which are uniquely characterised by their magnetic moment that arises from the presence of an unpaired electron. Measurement of a magnetic property of a material containing free radicals, like its magnetic susceptibility, provides the concentration of free radicals, but it lacks sensitivity and cannot reveal the structure of the radicals. Electron paramagnetic resonance spectroscopy is essentially free from these defects. 

As is usually the case in the study of complicated reactions that involve a great many different species, more attention has been given to the analysis of reaction products and intermediates than to the problems of the investigation of the kinetics of possible elementary reaction steps. Analytical studies of the systems have been advanced by the development of techniques such as gas chromatography for the analysis of multicomponent systems and mass spectrometry for the detection of free radicals and other highly unstable species. Furthermore, since most... 

Ahmad FF, Cowan DL, Sun AY. 1987. Detection of free radical formation in various tissues after acute carbon tetrachloride administration in gerbil. Life Sci 40 2469-2475. 

Janzen EG, Towner RA, Haire DL. 1987. Detection of free radicals generated from the in vitro metabolism of carbon tetrachloride using improved esr spin trapping techniques. Free Radic Res Common 3 357-364. 

Another magnetic technique for the detection of free radicals uses an ordinary nmr instrument. It was discovered144 that if an nmr spectrum is taken during the course of a reaction, certain signals may be enhanced, either in a positive or negative direction others may be reduced. When this type of behavior, called chemically induced dynamic nuclear polarization145 (CIDNP), is found in the nmr spectrum of the product of a reaction, it means that at least a portion of that product was formed via the intermediacy of a free radical.146 For example, the question was raised whether radicals were intermediates in the exchange reaction between ethyl iodide and ethyllithium (2-39) ... 

Tomasi, A., Albano, E., Botti, B Vannini, V. (1987) Detection of free radical intermediates in the oxidative metabolism of carcinogenic hydrazine derivatives. Toxicol. Pathol., 15, 178-183... 

For the detection of free-radical-induced DNA damage and its repair, biochemical techniques are increasingly applied. It would exceed the allocated space to discuss these techniques here, and the reader is referred to the original literature. However, to assist biologists and biophysicists, some of the chemical techniques for measuring typical DNA lesions are discussed. Most of the kinetic data concerning the free-radical chemistry of DNA and its model systems has been obtained by pulse radiolysis. This technique is only available in a few laboratories worldwide. For this reason, it will be described in some detail here. 

In recent years, the sensitivity of the detection of free-radical-induced DNA damage has been considerably increased (for brief reviews, see Cadet and Wein-feld 1993 Cadet et al. 1997a, b, 1999b). There is often a problem with lesions that are refractory to an enzymatic excision (Dizdaroglu et al. 1978, and further examples are reported below), but it is expected that when enzymatic degradation techniques have been further developed, acid hydrolysis and its marked problems will no longer be required. 

EPR spectroscopy is also frequently used in polymer science. Applications involve the detection of free radicals in cross-linking, high-energy irradiation, photochemical degradation and oxidation, and mechanical fracture of polymer chains. 

Pryor WA, Prier DG, Church DF. Detection of free radicals from low-temperature ozone-olefin reaction by ESR spin trapping evidence that the radical precursor is a trioxide. J Am Chem Soc 1983 105 2883-2888. 

Detection of Free Radicals Directly in Toxicology. Research Using ESR... 

In recent years, modern instrumental methods have been developed to monitor lipid oxidation in biological samples, including dairy products. These include use of electron spin resonance (ESR) spectrometry, direct measurement of secondary oxidative products such as malonaldehyde, static and dynamic GC/MS methods. ESR spectrometry permits detection of free radicals formed in the very early stages of oxidation prior to the formation of peroxides. The method has been applied successfully to dairy products such as milk powders and processed cheese (Nielsen et al., 1997 Stapelfeldt... 

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]. 

Electron spin resonance (ESR) is the most direct method for detection of free radicals. Direct ESR, however, is not very sensitive, and radicals with short lifetimes do not build up to detectable levels of steady-state concentrations. The technique of spin trapping is a very effective approach to increase the lifetime of free radicals such as OH, O -, etc. [68], Spin trapping is a kinetic technique in which the transient radical reacts with a nitrone or nitroso spin trap to produce a more stable nitroxide spin adduct. 

Detection of Free Radicals in a Photoirradiated Lignin Model Compound... 

IR DETECTION OF FREE RADICALS AND MONITORING THEIR REACTIONS... 

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