C. von Sonntag

Advanced oxidation processes: mechanistic aspects

The reactive intermediate in Advanced Oxidation Processes (AOPs) is the *OH radical. It may be generated by various approaches such as the Fenton reaction (Fe2+/H2O2), photo-Fenton reaction (Fe3+/H2O2/hnu), UV/H2O2, peroxone reaction (O3/H2O2), O3/UV, O3/activated carbon, O3/dissolved organic carbon (DOC) of water matrix, ionizing radiation, vacuum UV, and ultrasound. The underlying reactions and *OH formation efficiencies are discussed. The key reactions of *OH radicals also addressed in this review.

Pulse radiolysis in model studies toward radiation processing

Abstract Using the pulse radiolysis technique, the OH-radical-induced reactions of poly(vinyl alcohol) PVAL, poly(acrylic acid) PAA, poly(methacrylic acid) PMA, and hyaluronic acid have been investigated in dilute aqueous solution. The reactions of the free-radical intermediates were followed by UV-spectroscopy and low-angle laser light-scattering; the scission of the charged polymers was also monitored by conductometry. For more detailed product studies, model systems such as 2,4-dihydroxypentane (for PVAL) and 2,4-dimethyl glutaric acid (for PAA) was also investigated. With PVA, OH-radicals react predominantly by abstraction of an H-atom in α-position to the hydroxyl group (70%). The observed bimolecular decay rate constant of the PVAL-radicals decreases with time. This has been interpreted as being due to an initially fast decay of proximate radicals and a decrease of the probability of such encounters with time. Intramolecular crosslinking (loop formation) predominates at high doses per pulse. In the presence of O2, peroxyl radicals are formed which in the case of the α-hydroxyperoxyl radicals can eliminate HO2-radicals in competition with bimolecular decay processes which lead to a fragmentation of the polymer. In PAA, radicals both in α-position (characterized by an absorption near 300 nm) and in β-position to the carboxylate groups are formed in an approximately 1:2 ratio. The lifetime of the radicals increases with increasing electrolytic dissociation of the polymer. The β-radicals undergo a slow (intra- as well as intermolecular) H-abstraction yielding α-radicals, in competition to crosslinking and scission reactions. In PMA only β-radicals are formed. Their fragmentation has been followed by conductometry. In hyaluronic acid, considerable fragmeentation is observed even in the absence of oxygen which, in fact, has some protective effect against this process. Thus free-radical attack on this important biopolymer makes it especially vulnerable with respect to a reduction of its viscosity, and in rheumatic diseases this effect may be the reason for their painfulnes.

The enhanced stability of the cross-linked hylan structure to hydroxyl (OH) radicals compared with the uncross-linked hyaluronan

Abstract A comparison has been made of the relative stabilities of hyaluronan and hylan to degradation by OH radicals produced by γ-irradiation of aqueous solutions in N2O, when G (yield per 100 eV) for OH radicals is 5.6 and H atoms 0.6. Using low angle light scattering and viscometric methods, the change in molecular weight of the polysaccharides was measured with increasing dose. From the yield/dose curves (expressed as breaks per molecule), the initial G value for hyaluronan degradation is ∼ 4. A further slow post-irradiation decrease in molecular weight is observed, which can be brought to completion by incubating the solutions for 1 h at 60°C. Thereafter, the G value for degradation is ∼ 6. A similar post-irradiation degradation was found for hylan. A technique using tetranitromethane (TNM) has been used to distinguish between two types of radicals formed on the hyaluronan backbone. Radicals of the 1-hydroxy-2-alkoxy type (C-2, C-4, C-2 and C3 of the glucuronic acid) would induce strand breakage by alkoxy elimination. For the equivalent alkoxy radical at C6 of the acetamido monosaccharide, ring opening would occur with formation of a hemi-acetal, leading also to strand breakage. The C-2 and C-3 radicals would eliminate water rather than produce breaks by β-alkoxy elimination. Thus three out of the initially formed radicals would produce breaks by β-alkoxy formation. These can be stabilised with TNM and distinguished. It is concluded that these are the radicals involved in the post-irradiation thermal degradation process. Comparison of hylan and hyaluronan is, therefore, most valid when this post-irradiation process has been completed. Therefore, all G values for degradation were measured after incubation for 1 h at 60°. This investigation establishes the greater stability of hylan (Gdegradation = 2) compared to hylan (Gdegradation = 6). Therefore, in an environment such as supplementation of an inflammed joint where OH radicals are released, hylan is able to retain its integrity as a viscoelastic macromolecule three times better than hyaluronan. Its potential as a viscosupplementation material, or as an inflammatory drug release matrix inserted within the joint is, therefore, greater than non-cross-linked hyaluronan.

Products and Kinetics of the OH-radical-induced Dealkylation of Atrazine

Hydroxyl radicals, generated radiolytically in N2O/O2-saturated solutions, yield in their reaction with atrazine equal amounts of deethylatrazine and acetaldehyde (40% of OH radical yield) and deisopropylatrazine and acetone (16%), respectively. The precursors of deethylatrazine and acetaldehyde is their Schiff base which hydrolyzes slowly (OH–-catalyzed: k = 5.2 dm3 mol–1 s–1). The hydrolysis of the Schiff base of deisopropylatrazine and acetone is too fast to be detected. In a pulse radiolysis experiment, the intermediate formed upon OH-radical attack (k = 3·109 dm3 mol–1 s–1) has a strong absorption at 440 nm. It decays in the presence of oxygen (k = 1.3·109 dm3 mol–1 s–1), and upon deprotonation [pKa(peroxyl radicals) ≈ 10.5] the peroxyl radicals thus-formed eliminate superoxide radicals (k = 2.9·105 s–1). s-Triazine itself reacts much more slowly with OH radicals (k = 9.7·107 dm3 mol–1 s–1). This can explain, why in the case of atrazine in comparison to other aromatic compounds, e.g. toluene, the addition of the OH radical to the ring (estimated at ca. 40%) is of relatively little importance as compared to an H-abstraction from (activated) positions of the side groups. Produkte und Kinetik der OH-radikalinduzierten Desalkylierung von Atrazin Hydroxylradikale wurden radiolytisch in N2O/O2-gesattigter Losung erzeugt. In ihrer Reaktion mit Atrazin liefern sie jeweils mit ubereinstimmenden Ausbeuten Desethylatrazin und Acetaldehyd (40% bezogen auf die Ausbeute der OH-Radikale) und Desisopropylatrazin und Aceton (16%). Der Vorlaufer von Desethylatrazin und Acetaldehyd ist deren Schiffsche Base, welche langsam hydrolysiert (OH–-katalysiert: k = 5.2 dm3 mol–1 s–1). Der Zerfall der Schiffschen Base aus Desisopropylatrazin und Aceton erfolgt zu schnell, um nachgewiesen werden zu konnen. In einem Pulsradiolyse-Experiment zeigt das Intermediat, welches nach Angriff des OH-Radikals gebildet wird (k = 3·109 dm3 mol–1 s–1), eine starke Absorption bei 440 nm. In Gegenwart von Sauerstoff verschwindet dieses Intermediat (k = 1.3·109 dm3 mol–1 s–1) und nach Deprotonierung [pKa(Peroxylradikale) ≈ 10.5] eliminieren die so gebildeten Peroxylradikale Superoxidradikale (k = 2.9·105 s–1). Der Grundkorper, s-Triazin, reagiert sehr viel langsamer mit OH-Radikalen (k = 9.7·107 dm3 mol–1 s–1). Dieses kann erklaren, warum im Falle des Atrazins im Vergleich zu anderen aromatischen Verbindungen, z.B. Toluol, die Addition des OH-Radikals an den Ring (geschatzt ca. 40%) von geringerer Bedeutung verglichen mit einer H-Abstraktion von (aktivierten) Positionen der Seitengruppen ist.

Phosphate ester cleavage in ribose-5-phosphate induced by OH radicals in deoxygenated aqueous solution. The effect of Fe(II) and Fe(III) ions.

The reaction of OH radicals and H atoms with ribose-5-phosphate (10(-2) M) in deoxygenated aqueous solution at room temperature (dose-rate 2-1 X 10(17) eV/ml-min, dose 5 X 10(18)-15 X 10(18) eV/ml) leads to the following dephosphorylation products (G-values): ribo-pentodialdose 1 (0-2), 2-hydroxy-4-oxoglutaraldehyde 2 (0-06), 5-deoxy-erythro-pentos-4-ulose 3 (0-1) and 3-oxoglutaraldehyde 4 (0-06). In addition, some minor phosphate free products (total G=0-09) are formed. G(inorganic phosphate) =1-3 and G(H2O2)=0-3. On the addition of 10(-3) M (Fe(III) ions, G (1) and G (3) increase to 0-6 and 0-4 respectively. In the presence of 10(-3) M Fe(II), G(1) and G(3) change to 0-4 and 0-8, respectively. The other dephosphorylation products are suppressed by the iron ions. G(1) also increases on the addition of increasing amounts of H2O2. Each product can be assigned a precursor radical formed by hydrogen abstraction from C-5, C-4 or C-3 of the ribose-5-phosphate molecule. Products 1 and 2 are formed by oxydative dephosphorylation of an alpha-phospho radical with preceeding H2O elimination for product 2. Elimination of H3PO4 from a beta-phospho radical leads to product 3; product 4 is formed by elimination of two molecules of H2O from its precursor radical and hydrolytic cleavage of an enol phosphate bond. Deuterium-labelling experiments and the effects of the iron ions and of H2O2 support the mechanisms proposed. The importance of the dephosphorylation mechanisms for the formation of strand breaks in DNA is discussed with special reference to the effects of the radiosensitizers.

Radiation Chemistry in the 1990s: Pressing Questions Relating to the Areas of Radiation Biology and Environmental Research

This preview discusses the possible future application of ionizing radiation in the study of the free-radical chemistry of some aqueous systems. With respect to the present state of knowledge in the area of chemistry related to radiobiology, ionizing-radiation damage to DNA will continue to be a major focus of radiobiological interest. It is noted that the purine-base, free-radical chemistry is still poorly understood, as is the chemistry of the direct effect of ionizing radiation on DNA. The role of transition metal ions, their interaction with hydrogen peroxide and the superoxide radical are areas of increasing interest, as are other reactions of the superoxide radical. The eventual chemical effects, on DNA, of the reactions of these species resemble those of the action of ionizing radiation and are therefore apt to be studied radiation-chemically. In contaminated water pollution control, procedures termed ‘advanced oxidation processes’ are gaining in importance. They are based on the action of the OH r...

The UV photolysis of liquid diethyl ether

Abstract In the 185 nm photolysis of liquid (O 2 -free) diethyl ether the following products (quantum yields) are formed: hydrogen (0·06 5 s). methane (0·0008), ethylene (0·09 5 ), ethane (0·12), propane (0·001 5 ), butane (0·07), acetaldehyde (0·06), ethanol (0·46), ethyl vinyl ether (0·09), sec-butyl ethyl ether (0·19 5 ), 1,1-diethoxyethane (⩽0·0003), and 2,3-diethoxybutane (0·06 5 ). From material balance calculations it has been concluded that the most important primary process is the homolytic scission of the CO bond into ethyl and ethoxy radicals ( ca . 70%). Fragmentation into molecules and cage reactions yield ethane and acetaldehyde ( ca . 10%), ethylene and ethanol ( ca . 8·5%) and hydrogen and ethyl vinyl ether ( ca . 11%). The scission of the CC bond is of minor importance (

Strahlenchemie von alkoholen—IX : Die UV-photolyse (λ = 185 nm) von methanol in flüssiger phase

Zusammenfassung Bei der Photolyse (λ = 185 nm) von flussigem Methanol entstehen Wasserstoff, Glykol, Formaldehyd und Methan sowie Spuren Athan. Die Quantenausbeuten (bezogen auf φ(H2) = 0·4 des Athanol -Aktinometers (5 mol/1 in Wasser)) betragen 0·83, 0·78, 0·058, 0·05 bzw. 0·002. Die Isotopenverteilung des bei der Photolyse von CH3OD entstehenden Wasserstoffs (85% HD) zeigt, dass in der flussigen Phase, ahnlich wie in der Gasphase,2 die Spaltung der OH-Bindung (1) der wichtigste Zerfallsprozess ist. CH3OH + hv (λ = 185 nm) → CH3O• + H• (1) In Mischungen mit Wasser, in denen das Wasser fast keinen Anteil der Strahlung absorbiert, werden die Quantenausbeuten der Produkte Wasserstoff, Glykol, Methan und Athan stark erniedrigt, wahrend die Formaldehydausbeute konstant bleibt. In 1 molarer Losung betragt φ(H2) = 0·42, φ(Glykol) = 0·32, φ(CH4) = 6·10−4. Athan ist nicht mehr nachweisbar.

Radiolysis of Aqueous Solutions of Nucleosides Halogenated at the Sugar Moiety

SummaryThe pulse radiolysis of aqueous solutions of nucleosides halogenated at the sugar moiety (2′-bromo-2′-deoxyuridine 4, 3′-deoxy-3′-iodothymidine 5, 5′-deoxy-5′-iodouridine 6) has been studied. G(Hal) were determined by conductometry varying the experimental conditions (pH, saturation with Ar, N2O or air, addition of t-butanol). The results indicate that solvated electrons both add to the nucleobases and eliminate halogen ions from the halogenated sugar moiety. In the case of 4 (and possibly of 5) the radical anion of the base transfers (k ≈ 105s−1) an electron to the sugar-bound halogen atom thus cleaving the C-Hal bond. In competition with this reaction there is a protonation of the radical anion of the base by protons and by water. For the latter reaction a rate constant of k = 5 × 103 M−1s−1 was estimated.Compound 4 has also been investigated by product analysis after 60-Co-γ-irradiation. In aerated solutions erythrose is formed with a G-value of 0·12. Its precursor radical is the 2′-radical gene...

SO.4--induced oxidation of 1,3,6-trimethyluracil and 1,3,5-trimethyluracil (1,3-dimethylthymine) by potassium peroxodisulphate in aqueous solution : an interesting contrast

In order to mimic the direct effect of ionizing radiation on DNA, deoxygenated aqueous solutions of potassium peroxodisulphate, tert-butanol and 1,3,6-trimethyluracil (1,3,6-Me3 U) or 1,3-dimethylthymine (1,3-Me2 T) were irradiated with 60Co gamma rays; the sulphate radical formed by the reaction of the solvated electron with peroxodisulphate oxidizes these pyrimidines. In the case of 1,3,6-Me3 U, a chain reaction results in the formation of sulphuric acid, the glycols (two thirds) and 1,3,6-trimethylisobarbituric acid (one third). Typically, at 5 x 10(-4) mol dm-3 1,3,6-Me3 U, 4 x 10(-2) mol dm-3 S2O8(2-) and 10(-2) mol dm-3 tert-BuOH with a dose-rate of 2.2 x 10(-3) Gy s-1, G(H+) is 220 x 10(-7) mol J-1. We believe that the sulphate radical adds to the 1,3,6-Me3 U and the adduct rapidly loses the sulphate dianion, giving rise to the 1,3,6-Me3 U radical cation. This reacts with water, yielding a proton and the reducing 1,3,6-Me3U C(5)-OH,C(6)-yl radical, which reacts with peroxodisulphate and so propagates the chain. In this oxidation process a carbocation is formed which can either react with water yielding the glycols, or deprotonate yielding the 1,3,6-trimethylisobarbituric acid. The 1,3-Me2 T system behaves differently. No chain reaction of any significance is induced. In the presence of oxygen an allyl-type radical can be trapped, as shown by the subsequent formation of 1,3-dimethyl-5-formyluracil (G = 2.1 x 10(-7) mol J-1) and 1,3-dimethyl-5-hydroxymethyluracil (G = 0.2 x 10(-7) mol J-1). As the corresponding products are not observed in the 1,3,6-Me3 U system, it is concluded that in contrast to the 1,3,6-Me3 U radical cation, the 1,3-Me2 T radical cation efficiently deprotonates (at C5-methyl), apart from also being able to react with water. In basic solution, OH- adds to the 1,3-Me2 T radical cation, thereby suppressing the formation of the allyl-type radical. Quantum-chemical model calculations on uracil, thymine and 6-methyluracil show why 1,3-Me2 T and 1,3,6-Me3 U should differ in their behaviour.(ABSTRACT TRUNCATED AT 250 WORDS)