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.

The basics of oxidants in water treatment. Part A: OH radical reactions.

The Advanced Oxidation Processes (AOPs) are based on the reactions of the highly reactive *OH radicals. The formation of *OH by the various AOPs and their ensuing reactions are reviewed.

Elimination of the musk fragrances galaxolide and tonalide from wastewater by ozonation and concomitant stripping.

Ozone reacts with the musk fragrances tonalide and galaxolide with rate constants of 8 M(-1)s(-1) and 140 M(-1)s(-1), respectively. In wastewater, ozone eliminates only the more reactive compound, galaxolide, in competition with its reaction with the wastewater matrix. As both compounds are also stripped in a bubble column, tonalide is also eliminated to some extent.

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 radical. Radiation techniques will provide the tools for understanding the underlying reactions better.

Topics in free radical-mediated DNA damage: purines and damage amplification-superoxic reactions-bleomycin, the incomplete radiomimetic.

Only a small percentage of the DNA damage set by ionizing radiation in the living cell manifests itself as lethal. It is now increasingly accepted that clustered lesions may constitute the kind of damage that the repair enzymes cannot adequately deal with. The question is raised as to whether damage amplification reactions (radical transfer reactions) may contribute to these clustered lesions, and examples of such damage amplification reactions are given. In one example a purine is involved. With 2'-deoxy adenosine and 2'-deoxy guanosine it is shown that these purine nucleosides undergo unexpected radical reactions. Evidence for the radical transfer from the purine to the sugar moiety is provided by the formation of the 5'-aldehydes. These products have been assayed with 2-thiobarbituric acid (TBA), a reagent commonly applied to the detection of malonaldehyde. TBA-reactive material has also been assayed in gamma-irradiated and bleomycin-treated DNA. In gamma-irradiated DNA, about one-third of this is free malonaldehyde, while the major part of the TBA-reactive material remains bound to the DNA. In contrast, bleomycin-treated DNA yields practically no free malonaldehyde, and the major TBA-reactive products are identified as the thymine and adenine base propenals. There is also further unidentified low-molecular-weight, TBA-reactive material. It is concluded that the drug must be involved not only in the first step (H-abstraction from the sugar moiety), but also in the subsequent free-radical reactions on the way to the base propenals.

Reaction of hydroxyl radicals with alkyl phosphates and the oxidation of phosphatoalkyl radicals by nitro compounds.

The rate constants for reactions of hydroxyl radicals with a number of alkyl phosphates have been determined by competition with KSCN. Hydroxyl radicals react with alkyl phosphates preferentially by H-abstraction at the alpha-position of the phosphate functions. The resulting alpha-phosphatoalkyl radicals are not very efficient one-electron reducing agents towards nitro compounds. They react with tetranitromethane (TNM) by addition to form adduct intermediates with absorption maxima at about 300 nm. The rate constants for decay of these TNM adducts to produce the nitroform anion (NF-) and the corresponding alpha-phosphato-alcohols have been determined by optical and/or conductance detection. The stability of these TNM adducts varies considerably with the chain length (methyl > ethyl > isopropyl) and number (trialkyl > dialkyl > monoalkyl) of the alkyl substituents. Additional formation of proton during or after the decay of the TNM adducts has been tentatively attributed to the hydrolysis of the alpha-phosphato-alcohols. Alpha-Phosphatoalkyl radicals derived from trimethyl, triethyl, triisopropyl, and diethyl phosphates react with p-nitroacetophenone (PNAP) very slowly (k < 5 x 10(7) dm3mol-1S-1) possibly forming adducts. One-electron reduction of PNAP by these radicals to PNAP.- was not observed under pulse radiolysis conditions. The rate constants for the reactions of .OH with glycerol 1-phosphate and glycerol 2-phosphate have been redetermined by competition with KSCN. Using the radical scavengers N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and TNM, the percentage of .OH attack at each carbon atom was obtained. Contrary to the simple alkyl phosphates described above, the alpha-position to the phosphate function is the least favoured (10-15% in glycerol 1-phosphate and 6% in glycerol 2-phosphate). These so-formed alpha-phosphatoalkyl radicals react with TNM also by forming adducts. The beta-phosphatoalkyl radicals in both cases eliminate inorganic phosphate on formation (k > 10(6)S-1). The gamma-phosphatoalkyl radical from glycerol 1-phosphate undergoes base-catalysed water elimination (kobs = 1.8 x 10(5)S-1 at pH 10.6) to give an oxidizing radical. Products in the gamma-radiolysis of N2O-saturated solutions of glycerol 1-phosphate and glycerol 2-phosphate have been identified and their yields determined. The mechanisms for their formation are discussed.

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)

Pulse radiolysis of the DNA-binding bisbenzimidazole derivatives Hoechst 33258 and 33342 in aqueous solutions.

PURPOSE: The DNA-minor-groove-ligands bisbenzimidazoles Hoechst 33258 and 33342 have been reported to protect against radiation-induced DNA-strand breakage. In order to elucidate the mechanisms of protection by these DNA-binding compounds, pulse radiolysis studies on the reactions of the OH radical, the solvated electron and the H atom with Hoechst as well as OH-radical-induced nucleotide radical quenching by free Hoechst (model level) was investigated. MATERIALS AND METHODS: The pulse radiolysis of Hoechst 33258 and 33342 was studied in N2O and N2O/O2-(4:1)-saturated aqueous solutions in the absence and presence of azide and bromide ions and nucleotides. RESULTS: In a fully scavenged system (3 x 10(-2) mol x dm(-3) t-butanol, N2O/O2-saturated), a transient is formed which in the presence of phosphate buffer is no longer observed. This is assigned metastable quinonoid forms of Hoechst (lambdamax(Hoechst) = 340; lambdamax(transient) = 370 nm) which is generated in protonation/ deprotonation reactions by H+/OH- formed during the pulse. To prevent their formation 10(-3) mol x dm(-3) phosphate buffer was added in all other experiments. The transient spectra formed upon OH-radical attack (k=9 x 10(9) dm3 x mol(-1) x s(-1)) indicate that a major part of the primary OH-adduct radicals undergo rapid transformation (k approximately 5 x 10(5) x s(-1)), attributed to water elimination yielding an N-centered radical. This intermediate, also generated by N3. (k = 4 x 10(9) dm3 mol(-1) x s(-1)), subsequently complexes with a Hoechst molecule [k = 8 x 10(8) dm3 x mol(-1) x s(-1) epsilon(440 nm) = 1.4 x 10(4) dm3 mol(-1) x cm(-1)]. The N-centered radical does not react with O2 (k < 5 x 10(5) dm3 mol(-1) x s(-1)), but reacts readily with the superoxide radical (k= 1.0 x 10(9) dm3 x mol(-1) x s(-1)). Hoechst reacts with the peroxyl radicals derived from uridine (k approximately 5 x 10(6) dm3 x mol(-1) x s(-1)) or 5'-UMP (k approximately 1 x 10(7) dm3 mol(-1) x (s-1)), but not with the less oxidizing (e.g. methylperoxyl radical) yielding intermediates whose spectral properties are similar to those of the N-centered radical. However, they decay at a much lower rate (2k approximately 1 x 10(8) dm3 mol(-1) x s(-1)) than the N-centered radicals generated by N3. (2k= 1.1 x 10(9) dm3 x mol(-1) s(-1)), and it has been suggested that these peroxyl radicals form adducts rather than undergoing electron transfer. The H atom (k= 7 x 10(9) dm3 x mol(-1) x s(-1)) and the solvated electron (k= 2.3 x 10(10) dm3 x mol(-1) x s(-1)) yield, albeit noticeably different, H-adduct radicals which also strongly absorb in the 440 nm region. The reduction potential of Hoechst 33258 has been determined electrochemically at 0.84-0.90 V vs. NHE at pH 6.8. CONCLUSION: Hoechst reacts fast only with strongly oxidizing radicals by electron transfer (e.g. with the adenine-and guanine-derived heteroatom-centered radicals), but also more slowly with nucleo-base-derived peroxyl radicals, here albeit via addition. This may have important implications with regard to its protection owing to DNA-radical quenching under oxic vs. anoxic conditions.

Electron transfer from nucleobase electron adducts to 5-bromouracil. Is guanine an ultimate sink for the electron in irradiated DNA?

Electron transfer to 5-bromouracil (5-BrU) from nucleobase (N) electron adducts (and their protonated forms) has been studied by product analysis and pulse radiolysis. When an electron is transferred to 5-BrU, the ensuing 5-BrU radical anion rapidly loses a bromide ion; the uracilyl radical thus formed reacts with added t-butanol, yielding uracil. From the uracil yields measured as the function of [N]/[5-BrU] after gamma-radiolysis of Ar-saturated solutions it is concluded that thymine and adenine electron adducts and their heteroatomprotonated forms transfer electrons quantitatively to 5-BrU. Like the electron adduct of adenine, those of cytosine and guanine are rapidly protonated by water. The (protonated) electron adduct of guanine does not transfer an electron to 5-BrU, and in the case of the (protonated) cytosine electron adduct only partial electron transfer is observed. The results can be modelled if the protonated electron adduct (protonated at N(3) or at the amino group) of cytosine, CH., which can transfer its electron to 5-BrU (k approximately 2 x 10(7) dm3 mol-1 s-1) is transformed in a slow tautomerization reaction (k approximately 2.5 x +/- 10(3) s-1) into another form C'H. (possibly protonated at C(6) or C(5)) which does not transfer an electron to 5-BrU. There is also electron transfer from the electron adduct of thymine to cytosine and guanine which serve as electron sinks. The rate constant of electron transfer from the thymine electron adduct to cytosine is about 250 times greater than that of the reverse reaction. The heteroatom-protonated electron-adduct of thymidine transfers an electron to 5-BrU more slowly (k = 2.3 x 10(7) dm3 mol-1 s-1) than the electron-adduct itself (k = 7.2 x 10(8) dm3 mol-1 s-1). Phosphate buffer-induced protonation of the electron-adduct of thymine at carbon (C(6)) prevents electron transfer to 5-BrU. Such phosphate catalysis is also observed as an intramolecular process (k approximately 2 x 10(4) s-1) with thymidine-5'-phosphate but not with the 3'-phosphate. Phosphate-induced protonation at carbon also reduces transfer efficiency for the electron adducts of dinucleoside phosphates such as dTpdT and dTpdA. The data raise the question whether in DNA the guanine moiety may act as the ultimate sink of the electron in competition with other processes such as protonation at C(6) of the thymine electron adduct.

Enhancement of radiation-induced base release from nucleosides in alkaline solution: essential role of the O.- radical.

The effect of pH on base release in the gamma-radiolysis of N2O-saturated solutions of a number of nucleosides (including uridine, 3-methyluridine, 2',3'-O-isopropylidene-uridine, and adenosine) has been investigated. For all these nucleotides, independent of the base or sugar moiety, base release is very low at pH below 10 (G approximately (0.3-0.7) x 10(-7) mol J-1), but increases drastically to G approximately (3-4) x 10(-7) mol J-1 at pH greater than or equal to 13. This phenomenon had already been previously reported and attributed to an OH(-)-induced transfer of a base radical into a sugar radical. However, it is now shown that at pH 12, where base release starts to increase, a lowering of the dose-rate does not affect the yield of free base. The increase in base release is accompanied by an overall reduction of chromophore loss of similar magnitude (with 2',3'-O-isopropylidene-uridine and 3-methyluridine), as well as by an increase in the yield of oxidizing radicals by a factor of 2 (with uridine). The measured rate constant of the reaction of .OH/O.- with the nucleosides is also pH-dependent, as .OH reacts faster than O.- with the nucleosides by a factor of 6-7. It is concluded that the increase in base release at high pH is caused by the increasing participation of O.-, which, unlike .OH, attacks the nucleosides preferentially at their sugar moieties, and is not due to an OH(-)-induced radical transfer from the base to the sugar moiety.

Superoxide radical reactions in aqueous solutions of pyrogallol and n-propyl gallate: the involvement of phenoxyl radicals. A pulse radiolysis study.

The reactions of O2-. in aqueous solutions of pyrogallol 1 and the antioxidant n-propyl gallate 2 have been studied. In both cases the initial reaction gives hydrogen peroxide and the corresponding phenoxyl radical (k(1 + O2-.) = 3.4 x 10(5), k(2 + O2-.) = 2.6 x 10(5) dm3 mol-1S-1). These phenoxyl radicals have been produced independently by reacting 1 and 2 with Br2-. and their spectra and first pKa values measured (pKa(phenoxyl radical from 1) = 5.1, pKa(phenoxyl radical from 2) = 4.1). It is necessary to correct the observed spectra for the contribution of the H-adducts, formed by the reaction of radiolytically produced H atoms with the substrates (k(1 + H) = 2.5 x 10(9), k(2 + H) = 3.8 x 10(9) dm3 mol-1 S-1). The H-adduct spectra are given. In the reactions of O2-. with the substrates the initial transient absorbances are characteristic of the phenoxyl radicals; however at longer times a new transient absorbing around 500 nm (epsilon congruent to 10(4) dm3 mol-1 cm-1) appears. This is believed to be the deprotonated hydroxy-orthoquinone, formed by the reaction of phenoxyl radicals with O2-. (k congruent to 1.5 x 10(8) dm3 mol-1 S-1, from kinetic curve-fitting). The absorbance due to the hydroxy-orthoquinones decays by first-order kinetics (1.6 x 10(2) in the case of 1 and 1.1 x 10(2) s-1 in the case of 2). This is thought to be mainly the result of the conversion of the hydroxy-orthoquinone into its hydrate. Similar experiments were carried out with catechol and ethyl protocatechuate. The chemistry appears to be similar to that of the pyrogallol derivatives. The rate constant for reaction of these compounds with O2-. is, however, only less than or equal to x 10(4) dm3 mol-1 s-1.

Generation and reactions of the disulphide radical anion derived from metallothionein: a pulse radiolytic study.

OH-radicals were generated by pulse radiolysis of aqueous solutions of rabbit (Zn,Cd)-metallothionein (MT). They react with MT mainly by forming a thiyl radical with a rate constant of 1.7 x 10(12) dm3 mol-1 s-1. The thiyl radical reacts rapidly but reversibly with a thiolate function to form RSSR.-: RS + RS- reversible RSSR.-. The kinetics of the formation and decay of this radical anion have been studied pulse radiolytically by monitoring the evolution of the optical absorption of RSSR.- at 450 nm. This process is mostly intermolecular, i.e. bimolecular in MT. In the absence of O2, RSSR.- decays bimolecularly: RSSR.(-)+RS.-->RSSR + RS-. In the presence of O2, RS. may be scavenged by O2 and thus the yield of RSSR.- decreases: RS.+O2 reversible RSOO.. Under these conditions RSSR.- decays by first-order kinetics: RSSR.(-)+O2-->RSSR + O2.-. The rate constants of these reactions have been determined at room temperature: k4 = 1.8 x 10(9) dm3 mol-1 s-1, k5 = 7 x 10(4) s-1, k10 = 9.2 x 10(8) dm3 mol-1 s-1, and k18 about 3 x 10(7) dm3 mol-1 s-1. From the dependence of the maximal absorbance at 450 nm on the thiolate concentration in the absence of oxygen, epsilon (RSSR.-) = 9 x 10(3) dm3 mol-1 cm-1 and the stability constant (K4/5) of 2.3 x 10(4) dm3 ml-1 was determined. K4/5 is in good agreement with that determined kinetically, k4/k5 = 2.6 x 10(4) dm3 mol-1. The stability constant K15/16 of the thiylperoxyl radical, RSOO., was determined to be 5.5 x 10(3) dm3 mol-1.

Bleomycin versus OH-radical-induced malonaldehydic-product formation in DNA.

PURPOSE: To compare the actions of bleomycin and ionizing radiation on DNA regarding the formation of malonaldehyde-like products. MATERIALS AND METHODS: Calf thymus DNA was treated with iron/bleomycin or gamma-radiation at pH 7. Products were analysed by HPLC. The thiobarbituric-acid reactivity of the samples was determined directly or after HPLC by post-column derivatization. ESI mass spectra were taken on-line following HPLC. RESULTS: Malonaldehyde and malonaldehyde-like products as detected by the sensitive 2-thiobarbituric acid (TBA) assay are formed in gamma-irradiated DNA and thymidine solutions as well as upon treatment of DNA with bleomycin/iron. In gamma-irradiated DNA solutions in the presence of oxygen, no base propenals were detected, and the major TBA-active product was malonaldehyde. In the gamma-radiolysis of thymidine, thymine propenal was formed only in traces (not more than 0.07 per cent of the OH-radical yield). Malonaldehyde was practically absent after treatment with bleomycin; three other TBA-active products were seen by HPLC which have been identified as the cytosine, thymine, and adenine propenals. Guanine propenal was not detected under our conditions. CONCLUSIONS: The absence of these base propenals upon gamma-radiolysis implies that although the initiating step of OH-radical and bleomycin action [i.e. H-abstraction at C(4')] may be the same, the bleomycin-iron complex must participate in subsequent steps en route to the base propenals. It is proposed that the bleomycin pathway may involve the interaction of the C(4')-peroxyl radical with the 'spent' bleomycin-iron complex by ligand exchange, under formation of a bleomycin-iron-peroxyl-radical complex, Blm(Fe4+,*OOR), which then decomposes by heterolysis into the alkoxy cation precursor +OR of the base propenal and reconstitution of the bleomycin-iron complex Blm(Fe,O)3+, i.e. gives rise to base propenal formation without the involvement of a C(4')-hydroperoxide.

Addition of e-aq and H atoms to hypoxanthine and inosine and the reactions of alpha-hydroxyalkyl radicals with purines. A pulse radiolysis and product analysis study.

The reactions of hydrated electrons e-aq with hypoxanthine and inosine were followed using pulse radiolysis methods. In a neutral solution the electron adduct of inosine is immediately protonated at the heteroatoms of the purine ring by water (k >> 2.5 x 10(6)s-1) to give In(N,O-H).. These N,O-protonated intermediates have a single absorption maximum at 300 nm. In basic solution the protonation of the electron adduct of inosine by water leads to other intermediate products with an absorption maximum at 350 nm. These intermediates are believed to be the C-protonated electron adducts of inosine (In(N,O-H).). In (N,O-H). and In(C-H). differ strongly in their ability to reduce p-nitroacetophenone (PNAP). In(N,O-H). are strong reductants and reduce PNAP quantitatively to PNAP.-. Based on the pH dependence of PNAP.- yields, two types of tautomers of In(C-H). could be distinguished. One of the tautomers can reduce PNAP, albeit with slower rate than In(N,O-H)., the other tautomer has no reducing properties. The latter is the one with the higher pKa and therefore thermodynamically more stable. The absorption spectrum of the intermediates produced in the reaction of e-aq with hypoxanthine at neutral pH is very similar to that of In(N,O-H). with a maximum at 300 nm. However, no build-up at 350 nm was observed in basic solution as in the case of the electron adduct of inosine. The reaction of H atoms with inosine produces in basic solution intermediate radicals with the same absorption spectrum as the C-protonated electron adducts of inosine. It is suggested that both the reactions of e-aq and H. with inosine in basic solution produce the same radical, namely the H-adduct of inosine (In(C-H)) with the highest pKa. alpha-Hydroxyalkyl radicals were found to react very slowly with purine bases and nucleosides in neutral to basic solutions. In acidic solution their reactivity increases and a number of rate constants were determined by pulse radiolysis measurements at pH 0.4. The intermediates from the reaction of 2-hydroxy-2-propyl radicals with inosine could be observed pulse spectrometrically in neutral and in basic solutions. In basic solution this reaction leads to intermediates with the same absorption maximum at 350 nm as that of the H-adduct of inosine. Furthermore, the yield of acetone was found to increase strongly in basic pH.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

The formation of phosphate end groups in the radiolysis of polynucleotides in aqueous solution.

The polynucleotides poly(U), poly(C), poly(A) and poly(G) have been gamma-irradiated in N2O- and N2O/O2 (4:1)-saturated aqueous solutions. Hydroxyl radicals from the radiolysis of water react with the polynucleotides thereby producing among other lesions strand breaks. Strand breakage is connected with the formation of phosphomonoester end groups. Such end groups have been determined by measuring inorganic phosphate after a three hour incubation at 37 degrees C with acid or alkaline phosphatase. In the absence of oxygen G(phosphomonoester end groups) (in units of mumol J-1) are 0.47 (poly(U)), 0.17 (poly(C)) and less than or equal to 0.04 (poly(A) and poly(G)). In the case of poly(U) and poly(C) on heating the sample for one hour at 95 degrees C prior to incubation with phosphatases the above values increased by 0.14 and 0.07 mumol J-1, resp., whereas such treatment of the purine polynucleotides still did not produce a measurable yield of phosphomonoester end groups. Comparing these values with G values for strand breakage taken from the literature, about two phosphomonoester end groups are formed per strand break in poly(U) while for poly(C) this ratio is about unity. The purine polynucleotides show very low yields of strand breakage in agreement with the negligible phosphomonoester yields. In the presence of oxygen G(phosphomonoester end groups) are 0.46 (poly(U)), 0.21 (poly(C)), and less than or equal to 0.04 (poly(A) and poly(G)). On heating, these values increase, most markedly for poly(U) and poly(C). This is possibly linked to the decomposition of unstable hydroperoxides which are formed in high yields in poly(U) and poly(C) (G = 0.7 and 0.19 mumol J-1, resp.).(ABSTRACT TRUNCATED AT 250 WORDS)

The kinetics of hydroxyl-radical-induced strand breakage of hyaluronic acid. A pulse radiolysis study using conductometry and laser-light-scattering.

Hydroxyl radicals were generated radiolytically in N2O- and N2O/O2(4:1)-saturated aqueous solutions of hyaluronic acid. The hydroxyl radicals react rapidly with hyaluronic acid mainly by abstracting carbon-bound H atoms. As a consequence of subsequent free-radical reactions, chain breakage occurs the kinetics of which has been followed using the pulse radiolysis technique. In the absence of oxygen, strand breakage was followed by the change in conductivity induced by the release of cationic counterions condensed at the surface of hyaluronic acid which is a polyanion consisting of subunits of glucuronic acid alternating with N-acetyl-glucosamine. It appears that strand breakage is not due to one single first-order process, however, the contributions of the different components cannot be adequately resolved. At pH 7 the overall half-life is 1.4 ms, in both acid and basic solutions the rate of free-radical induced strand breakage is accelerated (at pH 4.8, t1/2 = 0.6 ms; at pH 10, t1/2 = 0.18 ms). In the absence of oxygen there is no effect of dose rate on the kinetics of strand breakage. In the presence of oxygen in addition to conductometric detection, strand breakage was also followed by changes in low-angle laser light-scattering. These two techniques are complementary in that in this system the conductometry requires high doses per pulse while the light-scattering technique is best operated in the low-dose range. In the presence of oxygen a pronounced dose-rate effect is observed, e.g. at pH 9.7 after a dose of 9.4 Gy the overall half-time is approx. 0.5 s, while after a dose of 6.6 Gy the half-time is approx. 0.23 s. Both the yield and the rate of strand breakage increase with increasing pH, e.g. at pH 7 G(strand breaks) = 0.7 x 10(-7) mol J-1 and at pH 10.4, 4.8 x 10(-7) mol J-7. The radiolytic yields of CO2, H2O2, organic hydroperoxides, O2.- and oxygen consumption have been determined in gamma-irradiated N2O/O2(4:1)-saturated solutions of both hyaluronic acid and beta-cyclodextrin.