Clemens von Sonntag

OH-radical formation by ultrasound in aqueous solution--Part II: Terephthalate and Fricke dosimetry and the influence of various conditions on the sonolytic yield.

Terephthalate and Fricke dosimetry have been carried out to determine the sonolytic energy yields of the OH free radical and of its recombination product H2O2 in aqueous solutions under various operating conditions (nature of operating gas, power, frequency, temperature). For example, in the sonolysis of Ar-saturated terephthalate solutions at room temperature, a frequency of 321 kHz, and a power of 170 W kg-1, the total yield [G(.OH) + 2 G(H2O2)], equals 16 x 10(-10) mol J-1. This represents the total of .OH that reach the liquid phase from gas phase of the cavitating bubble. The higher the solute concentration, the lower the H2O2 production as more of the OH free radicals are scavenged, in competition with their recombination. Fricke dosimetry, in the absence and presence of Cu2+ ions, shows that the yield of H atom reaching the liquid phase is much lower, with G(H.) of the order of 3 x 10(-10) mol J-1. These sonolytic yields are smaller in solutions that are at the point of gas saturation, and increase to an optimum as the initial sonication-induced degassing and effervescence subsides. The probing of the sonic field has shown that the rate of sonolytic free-radical formation may vary across the sonicated volume depending on frequency and power input.

Radiation-Induced Strand Breaks in DNA: Chemical and Enzymatic Analysis of End Groups and Mechanistic Aspects

Publisher Summary This chapter reviews the results of chemical and enzymatic analyses of radiation-induced strand breaks together that allows proposing a detailed mechanism of DNA strand break formation by ionizing radiation. Chemical analysis enables one to describe radiation-induced reactions, and the final structure of the sugar moiety and enzymatic analysis determines the biochemical reactivity of the end groups on the 3’ or 5’ terminal of the strand break. Enzymatic methods may be more sensitive than chemical analysis, but it has become evident that both approaches are necessary for an understanding of the mechanisms of radiation-induced strand breakage in DNA. Exposure of DNA to ionizing radiation produces an interruption of the nucleotide strand that can be considered as an important lesion responsible for the inactivation of the biological function of DNA. The chapter describes an experimental work that leads to a detailed understanding of the chemical structure of radiation-induced strand breaks and to knowledge of the mechanisms of their formation. The application of different techniques such as pulse radiolysis, ESR spectroscopy, product analysis in one laboratory, and sedimentation and enzymatic analysis in another laboratory lead to comparable conclusions. The chapter describes the frequency of strand breaks under various radiation conditions for DNA in aqueous solution, in dry DNA, and in cells. The presence of intact hydroxyl or phosphate end groups on DNA strand breaks can be demonstrated by a variety of enzymes each of which reacts specifically with a certain end group on the 3’ terminal or on the 5’ terminal. The chapter also describes the details of the specificity of these enzymes and their reactivity with respect to radiation-induced strand breaks.

Free-Radical Reactions of Carbohydrates as Studied by Radiation Techniques

Publisher Summary This chapter discusses the free-radical reactions of carbohydrates as studied by radiation techniques. Radiation techniques are powerful tools for the study of free-radical reactions both in solution and in the solid state. In aqueous solution, the γ-radiolysis of the solvent water gives rise to OH radicals, solvated electrons, and H atoms as reactive species. While solvated electrons react slowly with carbohydrate, OH radicals and H atoms react quite readily with them, the former at nearly diffusion-controlled rates. Carbohydrate radicals undergo a number of elimination reactions, the most ubiquitous of which is the elimination of water. The elimination of HOR from β-alkoxy-α-hydroxyalkyl radicals has been reported for a number of compounds and under different conditions. Only the base-catalyzed elimination has been studied kinetically and it has been shown that its kinetics very much resemble those of the water elimination. Concepts related to the radical reactions of selected-compounds in aqueous solution are explained in the chapter, along with a discussion on the Radical reactions in crystalline carbohydrates.

[3] Kinetic factors that control the fate of thiyl radicals in cells

Publisher Summary A thiyl radical (RS) is produced when a thiol (RSH) loses the hydrogen atom from the S–H group, or loses an electron from sulfur, followed by a proton. The two processes are stoichiometrically equivalent and may be difficult to distinguish experimentally, because proton transfer reactions to solvent water are usually fast. Because the S–H bond strength is lower than that of many C–H bonds, numerous carbon-centered radicals are repaired by thiols by hydrogen (or electron/proton) donation. Thiols can also act as cellular antioxidants by electron transfer to oxidizing species, producing thiyl radicals. Radicals derived from DNA bases—such as those produced on reaction of guanine moieties with OH—are repaired (but not restituted) by thiols. Xenobiotic radicals produced by one-electron oxidation of drugs are often reactive toward thiols. The fate of thiyl radicals in cells reflects the kinetics of reactions that produce and remove them. The main experimental technique for measuring the kinetics of thiyl radical reactions is pulse radiolysis. This chapter discusses the kinetic factors controlling the reaction pathways of thiyl radicals in cells and the experimental problems in quantitation.

OH radical formation by ultrasound in aqueous solutions Part I: the chemistry underlying the terephthalate dosimeter

Abstract The terephthalate dosimeter is widely used in sonochemical studies. Details of the underlying chemistry have been elucidated using ionizing radiation techniques. Hydroxyl radicals were generated radiolytically in N 2 O-saturated aqueous solutions of terepthalate ions (10 −3 mol dm −3 ). The products were studied after γ-radiolysis and the kinetics were followed by pulse radiolysis. The OH radicals add rapidly to the ortho - and to a much lesser extent to the ipso -positions of the terephthalate ions. The resulting hydroxycyclohexadienyl radicals display a strong absorption at 350 nm ( e ≈4000 dm 3 mol −1 cm −1 ). They decay bimolecularly (2 k = 2.5 × 10 8 dm 3 mol −1 s −1 at pH 5 and 2 k = 4 × 10 7 dm 3 mol −1 s −1 at pH 10.2). They can be rapidly oxidized by IrCl 6 2− ( k = 7.7 × 10 7 dm 3 mol −1 s −1 ), yielding 2-hydroxyterephthalate ions in 84% yield ( G =4.9 × 10 −7 mol J −1 ). As further minor products 4-hydroxybenzoic acid ( G =0.2 × 10 −7 mol J −1 ) and 3-hydroxybenzoic acid ( G =0.1 × 10 −7 mol J −1 ) were also observed. When O 2 is used as oxidant the yield of 2-hydroxyterephthalic acid as much less [ G =2.1 × 10 −7 mol J −1 in N 2 OO 2 (4:1)-saturated solutions]. Pulse radiolysis revealed that O 2 adds to the hydroxycyclohexadienyl radicals with a rate constant k f =1.6×10 7 dm 3 mol −1 s −1 . The O 2 addistion is however, reversible ( k r =3.4 × 10 3 s −1 ), the equilibrium constant being K = 4800 dm 3 mol −1 . The hydroxycyclohexadienylperoxyl radicals undergo an HO 2 . elimination (leading to 2-hydroxyterephthalate ions) in competition with other (ring fragmentation) reactions. These reactions occur with an overall rate constant of 390s −1 . 2-Hydroxyterephthalate ions are readily detected by their fluorescence ( λ exc =315 nm, λ em =425 nm). Since other products do not interfere, this assay can be used for the determination of OH radical production not only in the radiolysis but also in the sonolysis of water.

The photochemistry of aqueous nitrate ion revisited

Aqueous nitrate solutions were photolysed at 254 nm in the absence of oxidizable additives, in the presence of methanol or propan-2-ol and oxygen and in the presence of cyclopentane under anaerobic conditions. The main nitrogen-containing products are nitrite and peroxynitrite. The quantum yields depend on the pH, nitrate concentration, nature of the additive and the light intensity. The intrinsic nitrite yield in alkaline solutions could not be determined directly because, under the conditions of the nitrite assay, the accompanying peroxynitrite decomposes to form nitrite and nitrate; it is smaller than the apparent nitrite yield. In the acidic (pH 4–7) range, the intrinsic nitrite quantum yield is equal to the apparent nitrite yield because there is no buildup of peroxynitrite under these conditions. The apparent nitrite quantum yield increases from 0.01 (no oxidizable additive) to approximately 0.03 (cyclopentane (millimolar range), oxygen free) to 0.06 (methanol (millimolar range), air saturated). At pH 13 and in the absence of oxidizable additives, the apparent nitrite quantum yield increases to about 0.1, whereas from material balance considerations the intrinsic nitrite quantum yield is estimated to be 0.06, twice the oxygen quantum yield of 0.03. Spectrophotometrically, peroxynitrite is detected in the alkaline range only, because its protonated form is unstable. In the absence of oxidizable additives, the quantum yield of peroxynitrite is about 0.1, i.e. only about two-thirds of the quantum yield in the presence of oxidizable additives. Mechanistic considerations on the basis of the pH dependence of the quantum yields of the products nitrite, peroxynitrite and oxygen, as well as their dependence on the kind of additive, indicate that the decisive factor of photolysis in the absence of additives is the formation of the nitric oxide peroxyl radical, ONOO, formed by reaction of peroxynitrite with the primarily generated OH radical. The decay of ONOO is the source of O2 in this system. Nitric oxide, NO, the other fragment of this decay reaction, reacts with nitrogen dioxide, which is one of the primarily formed intermediates. The latter reaction is one of the pathways to the product nitrite, particularly in the alkaline range. The formation of NO during photolysis has been verified by electron spin resonance (ESR) spectroscopic detection of the nitroxide 1,1,3,3-tetramethyl-isoindolin-2-oxyl, the NO adduct to 7,7,8,8-tetramethyl-o-quinodimethane. Of the three primary processes discussed in the literature, we conclude that reactions (1) and (2) occur with quantum yields of approximately 0.09 and 0.1 respectively NO3−+hv→NO2+O−(O−+H2O→OH+OH−)(1)NO3−+hv→ONOO−(2) It appears that none of the peroxynitrite anion is formed in a cage reaction through the recombination of the primary fragments from reaction (1). The primary process shown in reaction (3) is of relatively minor importance, with a quantum yield of no more than 0.001 NO3−+hv→NO2−+O(3) In the presence of methanol (or propan-2-ol) and oxygen under acidic conditions, formaldehyde (or acetone) is formed in an amount equivalent to nitrite via peroxyl radical reactions (quantum yield of approximately 0.06 for both alcohols). In the alkaline range, the apparent formaldehyde quantum yield decreases with increasing pH, while formic acid is produced in increasing amounts. The formation of formic acid is ascribed to the reaction of peroxynitrite anion with photolytically generated formaldehyde. The acetone quantum yield does not decrease with increasing pH over the whole alkaline pH range. In the presence of cyclopentane under oxygen-free conditions, apart from nitrite (and peroxynitrite when alkaline), the compounds nitrocyclopentane, cyclopentyl nitrate, cyclopentene, cyclopentanol and cyclopentanone are produced. The formation of the organic nitrogen compounds leads to an increase in the pH as photolysis proceeds. This pH shift is particularly pronounced in the neutral range.

Ozonolysis of phenols in aqueous solution

In the ozonolysis of phenol in aqueous solution at pH 3, 7 and 10 the following products were quantified: catechol, hydroquinone, 1,4-benzoquinone, cis,cis-muconic acid, H2O2, 2,4-dihydroxybiphenyl and 4,4-dihydroxybiphenyl. At pH 10, material balance (products vs. phenol consumption) is obtained. Singlet dioxygen, O2(1 delta g), and .OH are formed as short-lived intermediates. The precursor of the latter, O3.-, and a phenoxyl radical is suggested to arise from electron transfer from phenol/phenolate to ozone. Addition of .OH to phenol gives rise to dihydroxycyclohexadienyl radicals which add dioxygen and eliminate HO2. thereby forming catechol/hydroquinone. In competition and catalysed by H+ and OH-, the dihydroxycyclohexadienyl radical eliminates water yielding a phenoxyl radical. At pH 10, they readily oxidize catechol and hydroquinone. This reforms phenol (accounting for the low phenol consumption) and yields higher-oxidised products, eventually 1,4-benzoquinone. cis,cis-Muconic acid can be accounted for by the Criegee mechanism, while O2(1 delta g) is released on the way to (some of the) catechol and hydroquinone. Similar reactions proceed with hydroquinone (products: 1,4-benzoquinone, 2-hydroxy-1,4-benzoquinone and H2O2, with high yields of O2(1 delta g) and .OH) and with catechol (products: 2-hydroxy-1,4-benzoquinone, cis,cis-muconic acid, H2O2 with high yields of O2(1 delta g) and .OH). Material balance is not obtained for these two systems. Pentachlorophenolate, pentabromophenolate and 2,4,6-triiodophenolate ions give rise to halide ions, O2(1 delta g) (58%/48%/10%) and .OH (27%/2%/0%). It is suggested that together with O2(1 delta g) the corresponding ortho- and para-quinones plus a halide ion are formed. Further halide ion is released upon the hydrolysis of these and other products. For pentachlorophenolate the material balance with respect to the short-lived intermediates is 85%. With the bromo- and iodophenolates the O2(1 delta g) yields are substantially lowered, most likely due to release of triplet (ground state) dioxygen induced by the heavy atom effect.

Degradation of chlorotriazine pesticides by sulfate radicals and the influence of organic matter.

Atrazine, propazine, and terbuthylazine are chlorotriazine herbicides that have been frequently used in agriculture and thus are potential drinking water contaminants. Hydroxyl radicals produced by advanced oxidation processes can degrade these persistent compounds. These herbicides are also very reactive with sulfate radicals (2.2-3.5 × 10(9) M(-1) s(-1)). However, the dealkylated products of chlorotriazine pesticides are less reactive toward sulfate radicals (e.g., desethyl-desisopropyl-atrazine (DEDIA; 1.5 × 10(8) M(-1) s(-1))). The high reactivity of the herbicides is largely due to the ethyl or isopropyl group. For example, desisopropyl-atrazine (DIA) reacts quickly (k = 2 × 10(9) M(-1) s(-1)), whereas desethyl-atrazine (DEA) reacts more slowly (k = 9.6 × 10(8) M(-1) s(-1)). The tert-butyl group does not have a strong effect on reaction rate, as shown by the similar second order reaction rates between desethyl-terbuthylazine (DET; k = 3.6 × 10(8) M(-1) s(-1)) and DEDIA. Sulfate radicals degrade a significant proportion of atrazine (63%) via dealkylation, in which deethylation significantly dominates over deisopropylation (10:1). Sulfate and hydroxyl radicals react at an equally fast rate with atrazine (k (hydroxyl radical + atrazine) = 3 × 10(9) M(-1) s(-1)). However, sulfate and hydroxyl radicals differ considerably in their reaction rates with humic acids (k (sulfate radical + humic acids) = 6.8 × 10(3) L mgC(-1) s(-1) (mgC = mg carbon); k (hydroxyl radical + humic acids) = 1.4 × 10(4) L mgC(-1) s(-1)). Thus, in the presence of humic acids, atrazine is degraded more efficiently by sulfate radicals than by hydroxyl radicals.

HO2 ELIMINATION FROM α-HYDROXYALKYLPEROXYL RADICALS IN AQUEOUS SOLUTION

Abstract— In aqueous solutions α‐hydroxyalkylperoxyl radicals undergo a spontaneous and a base catalysed HO2 elimination. From kinetic deuterium isotope effects, temperature dependence, and the influence of solvent polarity it was concluded that the spontaneous reaction occurs via an HO2 elimination followed by the dissociation of the latter into H+ and O2‐. The rate constant of the spontaneous HO2 elimination increases with increasing methyl substitution in α‐position (k(CH2(OH)O2) < 10s‐1k(CH3CH(OH)O2) = 52s‐1k((CH3)2C(OH)O2) = 665 s‐1). The OH‐ catalysed reaction is somewhat below diffusion controlled. The mixture of peroxyl radicals derived from polyhydric alcohols eliminate HO2 at two different rates. Possible reasons for this behaviour are discussed. The mixture of the six peroxyl radicals derived from d‐glucose are observed to eliminate HO2 with at least three different rates. The fastest rate is attributed to the HO2 elimination from the peroxyl radical at C‐l (k > 7000s‐1). Because of the HO2 eliminations the peroxyl radicals derived from d‐glucose do not undergo a chain reaction in contrast to peroxyl radicals not containing an α‐OH group. In competition with the first order elimination reactions the α‐hydroxylalkylperoxyl radicals undergo a bimolecular decay. These reactions are briefly discussed.

Reactions of OH Radicals with Poly(U) in Deoxygenated Solutions: Sites of OH Radical Attack and the Kinetics of Base Release

Pulse radiolysis of N2O-saturated solutions of poly(U) in the presence of tetranitromethane showed that 81 per cent of the radicals formed are reducing in nature. Using data from other sources it has been estimated that 70 per cent of the OH radicals add to the base at C(5) and 23 per cent at C(6) while only 7 per cent abstract an H-atom from the sugar moiety. To a large extent the C(5) OH adduct radicals attack the sugar moiety of poly(U) thereby inducing strand breakage and base release. G (base release) = 2.9 can be subdivided into three components: (a) immediate (20 per cent), (b) fast (50 per cent) and (c) slow (30 per cent). The immediate base release must occur either during the free-radical stage or as a result of the rapid (t1/2 less than 4 min at 0 degree C) decomposition of a diamagnetic product. The fast and the slow processes are only readily observable at elevated temperatures, e.g. at 50 degrees C the half lives are 83 min and 26 h, respectively (Ea (fast) = 68 kJ mol-1, Ea (slow) = 89 kJ mol-1, A (fast) = 1.5 X 10(7) s-1, A (slow) = 1.9 X 10(9) s-1. It is concluded that there are three different types of sugar lesions giving rise to base release, structures for which are tentatively proposed.