Aqueous solutions spin traps

The reaction of OH radicals with dimethyl sulfoxide in aqueous solution was studied already in 1964 by Norman and coworkers37 38. They used the system T1m-H202 to produce OH radicals and using ESR/rapid mixing techniques they were able to demonstrate elimination of a methyl radical during the OH induced oxidation. Further studies showed the formation of sulfmic radicals in this reaction either directly or by spin trapping experiments39-44. 

Makino K, Mosobba MM, Riesz P (1982) Chemical effects of ultrasound in aqueous solutions. Evidence for OH and H by spin-trapping. J Am Chem Soc 104 3537-3539... 

From the EPR spectroscopist s viewpoint the spin-trap experiment is next to trivial the molecular mass of the radical adduct is small enough to guarantee the molecule to tumble sufficiently rapidly at ambient temperatures in aqueous solution to ensure complete averaging away of any anisotropy in the spin Hamiltonian ... 

One way to make the short-lived intermediates amenable to study is to increase their lifetime, usually by irradiation in the solid state and/or at very low temperatures. Then, the intermediates can be detected at the end of the irradiation by ESR or optical absorption spectroscopy. The ESR study of radicals in the solid state is done on single crystals, polycrystalline samples or frozen aqueous solution. In case of polycrystalline samples or frozen aqueous solution the identification of the radicals from the ESR spectra is difficult in many cases and, for better identification, the ESR experiment should be conducted on irradiated single crystals. Later, the method of spin trapping, developed for the liquid phase5, was extended to polycrystalline solids. In this technique the polycrystalline solids are /-irradiated and subsequently dissolved in a solution containing the spin trap. 

For aqueous solutions, Sargent and Gardy (1976) have advocated the use of DMPO, which gives persistent spin adducts with hydroxyl radicals hydrogen-atom adducts arise by electron scavenging and subsequent protonation of DMPCK, but this can be minimized by incorporating N20 in the system, which traps electrons and raises the yield of hydroxyl radicals (28). 

An interesting extension of aqueous solution radiolysis involved solutions of sodium dodecyl sulphate in the presence of MNP. Spin adducts of secondary alkyl radicals were detected provided that the critical micelle concentration of the surfactant was exceeded. Whilst it was rather loosely concluded that there is a marked catalytic effect of micelles on the rates of reaction of radicals with nitroso spin traps , no single origin of this effect could be clearly identified (Bakalik and Thomas, 1977). 

If an aqueous solution saturated with oxygen is sonicated hydrogen peroxide formation occurs. This is due to hydroxyl ( OH) and hydroperoxyl ( OOH) radical recombination outside the cavitation bubble (Scheme 4.1). These radicals result from HjO and O2 homolytic cleavage inside the bubble and have been observed by spin trapping experiments (Scheme 4.1) [19]. 

Organic nitroxides (136a, 5), nitroparafins, and nitrolic acids (107) react with aqueous solutions of pentacyanocobaltate(II) salts to give alkyl pentacyanocobalt(III) nitroxide anion radicals, spin traps involving the formation of an N—Co bond (5). 

Combined EPR and spin-trapping studies showed that solvent vapor and ambient gases (e.g., air) decompose to atoms or free radicals in the gaseous bubble interior. Water vapor is thermally dissociated into OH radicals, H atoms, and O atoms. The latter interconvert with OH radicals at the high pressures in the cavity and recombine in the cooler interfacial region to form Oz and H2Oz. The power dependence in the sonolytic transformation of a phenolic aqueous solution was found to be the first order. 

Hedrick WR, Webb MD, Zimbrick JD (1982) Spin trapping of reactive uracilyl radicals produced by ionizing radiation in aqueous solutions. Int J Radiat Biol 41 435-442 Heelis PF, Deeble DJ, Kim S-T (1992) Splitting of cis-syn cyclobutane thymine-thymine dimers by radiolysis and its relevance to enzymatic photoreactivation. Int J Radiat Biol 62 137-143 Hems G, Eidinoff ML (1958) Effect of X-irradiation on aqueous solutions of adenosine diphosphate. Radiat Res 9 305-311... 

Hildenbrand K (1995) Spin-trapping studies of the reaction of the sulfate radical ion with N1-substituted pyrimidine bases. Comparison with continuous-flow electron paramagnetic resonance experiments.) Chem Soc Perkin Trans 2 2153-2162 Hildenbrand K, Schulte-Frohlinde D (1997) Time-resolved EPR studies on the reaction rates of per-oxyl radicals of polyfacrylic acid) and of calf thymus DNA with glutathione. Re-examination of a rate constant for DNA. Int J Radiat Biol 71 377-385 Hildenbrand K, Behrens G, Schulte-Frohlinde D, Herak JN (1989) Comparison of the reaction OH and S04- radicals with pyrimidine nucleosides. An electron spin resonance study in aqueous solution. J Chem Soc Perkin Trans 2 283-289... 

Mossoba MM, Rosenthal I, Riesz P (1981) ESR and spin-trapping studies of dihydropyrimidines, y-Radiolysis in the polycrystalline state and UV photolysis in aqueous solution. Int J Radiat Biol 40 541-552... 

Niehaus H, Hildenbrand K (2000) Continuous-flow and spin-trapping EPR studies on the reactions of cytidine induced by the sulfate radical-anion in aqueous solution. Evidence for an intermediate radical cation. J Chem Soc Perkin Trans 2 947-952 Niles JC, Burney S, Singh SP, Wishnok JS, Tannenbaum SR (1999) Peroxynitrie reaction products of 3, 5 -di-0-acetyl-8-oxo-7,8-dihydro-2 -deoxyguanosine. Proc Natl Acad Sci USA 96 11729-11734... 

Electron spin resonance (ESR) spectroscopy is of application to organic species containing unpaired electrons radicals, radical ions and triplet states, and is much more sensitive than NMR it is an extremely powerful tool in the field of radical chemistry (see Chapter 10). Highly unstable radicals can be generated in situ or, if necessary, trapped into solid matrices at very low temperatures. Examples of the application of this techniques include study of the formation of radical cations of methoxylated benzenes by reaction with different strong oxidants in aqueous solution [45], and the study of the photodissociation of N-trityl-anilines [46],... 

Riesz P, Kondo T, Carmichael AJ. Sonochemistry of acetone and acetonitrile in aqueous solutions. A spin-trapping study. Free Rad Res Comm 1993 19 S45-S53. 

The method was developed for the liquid phase and was later extended to polycrystalline solids the latter are y-irradiated and subsequently dissolved in a solution containing the spin-trap. Most commonly it is aqueous solution and several spin-traps were used6. 

Radicals formed from thiols and disulphides may play important roles in cellular function and malfunction, the RS radical, and RS-SR T radical anions being particularly implicated. The pKa values for many thiols, in the region of 9 to 10, are such that there are low, but possibly significant concentrations of RS- anions at pH.7. Thus RSH may act as an H-atom donor or, via RS-, as an electron donor. As with other radicals of this type, RS radicals have not been detected by ESR spectroscopy in aqueous solution, nor do they have intense absorption bands in the near-UV. However, they have a high affinity for RSH and RS- (Reactions [1.17]—[1.18]) which act as spin-traps provided they are present in fairly high concentrations. 

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