Measurement of Free Radicals

Zweier, J.L., Flaherty, J.T. and Weisfeldt, M.L. (1987). Direct measurement of free radical generation following reperfusion of ischemic myocardium. Proc. Natl Acad. Sci. USA 84, 1404-1497. 

Nonaka, A., Manabe, T., Asano, N., Kyogoku, T., Imanishi, K., Tamura, K., Tobe, T., Sugiura, Y. and Makino, K. (1989a). Direct ESR measurement of free radicals in mouse pancreatic lesions. Int. J. Pancreatol. 5, 203-211. 

DPPH (l,l-diphenyl-2-picryhydrazyl) is a purple-colored stable free radical that is reduced to the yellow-colored diphenylpicrylhydrazine by free radicals. The DPPH assay measures one electron, such as hydrogen atom donating activity and hence provides a measure of free radical scavenging activity. This assay is suitable for the initial screening of multiple samples, such as plant extracts. Reaction mixtures containing test samples dissolved in DMSO and DPPH in absolute ethanol are incubated at 37°C for 30 min in a 96-well plate and absorbance measured at 515 nm. ... 

Accurate measurement of free-radical and molecular-product yields is important in radiation-chemistry studies on aqueous solutions, for these measurements enable quantitative predictions to be made regarding the extent of chemical changes during irradiations, and lead to an understanding of reaction mechanisms. Therefore, recent research has been directed toward the measurement of these yields, which are generally expressed as G values. An excellent account of the chemical methods used for determining G values... 

Free radical biosensors may help to investigate the role of O2 and NO as cellular messengers by the direct, real-time measuring of free radical production directly as cell signal and in relation to stimuli to which the cell is exposed. Two types of biosensors have been developed. The first exploits the highly specific reaction of superoxide dismutase [69], while the other type is based on the cytochrome c reduction by O2 [53-55,143,147]. A further alternative uses protoporphyrin IX adsorbed to carbon material [263]. Here, however, peroxide is a strong interferent. 

ESR (or electron paramagnetic resonance, EPR), has been used for the study of free radicals in polymers during the polymerization process or for the measurement of free radical formation upon the irradiation of the polymer with UV light or gamma rays. X-ray diffraction has been used for the determination of spatial arrangement of the atoms in a polymer and for the measurement of the degree of crystallinity. 

ESR, by itself, is considered a very specific, yet semiquan-tifiable, technique for the measurement of free radicals. It can be used in conjunction with other analytical techniques, such as gas chromatography, mass spectroscopy, and high performance liquid chromatography (HPLC) for the study of free radicals [Cranton and Frackelton (27A20)]. ESR used in conjunction with other analytical techniques is an emerging field. 

H. Rohrs. 1978. Measurements of free radicals in the atmosphere by matrix isolation and electron paramagnetic resonance. Pure Appl. Geophys. 116 530-536. 

Measurements of Free Radicals Reaction Rate Constants.120... 

This procedure (neglect of nuclear Zeeman term) is sometimes also adopted to obtain hyperfine coupling tensors from ESR measurements of free radicals. The method is, however, not suitable for the analysis of hyperfine structure due to a-H in jr-electron radicals of the type )Cc-H at X-band, and other cases where the anisotropic hyperfine coupling and the nuclear Zeeman energy are of comparable magnitudes, as discussed for case 3 below. 

L2.2g The investigation of reactions acting as sources and sinks o/HOx and RO2 radicals in the boundary layer in support of anticipated field measurements of free radical concentrations... 

Figure 4. Influetice of temperature and light exposure from fluorescent tubes (2000 be) on (a) signal intensity in the ESR spectrum as a measure of free radicals in processed cheese storedfor 15 months. The cheese was freeze-dried prior to recording of the spectra as shown in Figure 3. (b) secondary lipid oxidation products measured as TBARS (450 nm absorbance). Light ...

The first step, R6, converts the HALS initially added to the clearcoat, parent HALS, into inhibition cycle, R7 and R8, products. These reactions compete with R2 and R3 lowering the stationary radical concentration, which in turn lowers the hydroperoxide concentration and the photooxidation rate. The rate constants and radical concentrations are such that only a small fraction (—5%) of the HALS stabilizer is in the form of nitroxide. Although nitroxides are thermally stable, they are not pho-tolytically stable. Nitroxides absorb light, and excited-state nitroxides can abstract hydrogen atoms to initiate free-radical formation. These reactions have been discussed in detail. "Reactions R9 and RIO are important both for the nitroxide decay measurement of free radical formation and in limiting the ultimate effectiveness ofHALS.i° i5... 

Ridley MD, Jahan MS. Measurements of free radical in vitamin E-doped ultra-high molecular weight polyethylene dependence of materials processing and irradiation environments. Nucl Instr Meth Phys Res 2007 B 265 62-6. 

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