Detection of OH Radicals

in most OH-induced oxidations short-lived adducts must be considered as intermediates. A case in point in the realm of DNA free-radical chemistry is the oxidation of guanine. From the above, it is evident that OH, despite its high reduction potential, cannot be directly used for the study of one-electron oxidation reactions. However, one can make use of its high reduction potential by producing other reactive intermediates [e.g Tl(II) Chap. 10], which no longer undergo an addition to double bonds or H-abstraction. 

When one looks for methods to detect OH, one always has two keep in mind that these radicals are very reactive, and in the presence of substrates their steady-state concentrations are extremely low even at a high rate of OH production. The fact that OH only absorbs far out in the UV region (Hug 1981) is thus not the reason why an optical detection of OH is not feasible. Electron paramagnetic resonance (EPR) must also fail because of the extremely low steady-state concentrations that prevail in the presence of scavengers. The only possibility to detect their presence is by competition of a suitable OH probe that allows the identification of a characteristic product [probe product, reaction (41)]. When this reaction is carried out in a cellular environment, the reaction with the probe is in competition with all other cellular components which also readily react with OH [reaction (42)]. The concentration of the probe product is then given by Eq. (43), where [ OH ] is the total OH concentration that has been formed in this cellular environment and q is the yield of the probe product per OH that has reacted with the probe. 

Many detection systems are based on OH-induced hydroxylation of salicylate (see below). Salicylate, however, inhibits some enzymatic reactions that may 

Some systems that have been proposed as suitable OH probes (for reviews see Hageman et al. 1992 Kaur and Halliwell 1994,1996 Loft and Poulsen 1999 von Sonntag et al. 2000), and the principles on which they are based (and, if possible, their reliability) will be discussed in the following. 

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Kaur H, Halliwell B (1994) Detection of OH radicals by aromatic hydroxylation. Methods Enzymol 233 67-82... 

Nosaka Y, Ohtaka K, Ohguri N, Nosaka AY (2011) Detection of OH radicals generated in polymer electrolyte membranes of fuel cells. J Electrochem Soc 158 B430-B433... 

The reaction of OH with 3-methyl-butanone has been studied by Le Calvd et al. (1998). The measurements are given in table V-B-9. They used pulsed laser photolysis with LIF detection of OH radicals and found a small negative temperature dependence, with some evidence of curvature in the Arrhenius plot the rate coefficient was essentially independent of temperature above room temperature. Calvert et al. (2008) fitted the Arrhenius expression to the data from Le Calve et al. (1998), obtaining the relation k = 1.45x 10 exp(219/r)cm molecule" s for use in atmospheric models, with A (298) = 3.0 x 10 cm molecule" s In the absence of conhrmatory measurements the uncertainty is estimated to be 40%. 

The reaction of eq. 16.9 will regenerate the antioxidant Arj-OH at the expense of the antioxidant At2-OH. Despite the fact that such regeneration reactions are not simple electron transfer reactions, the rate of reactions like that of eq. 16.9 has been correlated with the E values for the respective Ar-0. Thermodynamic and kinetic effects have not been clearly separated for such hierarchies, but for a number of flavonoids the following pecking order was established in dimethyl formamid (DMF) by a combination of electrolysis for generating the a-tocopherol and the flavonoid phenoxyl radicals and electron spin resonance (ESR) spectroscopy for detection of these radicals (Jorgensen et al, 1999) ... 

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... 

Table 10.19. Pattern of OH-radical attack (in %) on H2Ura and some of its methyl derivatives. The OH-balance is sometimes significantly below 100%, partly due to the inefficiency of the method for the detection of oxidizing C(5)-yl radicals. (Schuchmann et al. 1984b)...

Several different methods have been used successfully to generate a detectable concentration of short-lived free radicals inside the resonant cavity. The simplest method is a microwave discharge in the flowing gas, located upstream of the resonant cavity discharges in water vapour, for example, yield readily detectable concentrations of OH radicals. Shorter lived species have been produced by atom abstraction reactions inside the cavity, for example, by mixing fluorine atoms, produced by a microwave discharge in CF4, with a suitable secondary gas. Reaction of F atoms with OCS, for example, produces detectable concentrations of the SF radical [4],... 

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