[en] Aqueous solutions of coumarin have been used to detect the hydroxyl radicals produced as a result of gamma radiolysis. Reverse-phase chromatography and fluorescence measurements show that the 7-hydroxycoumarin (umbelliferone) is produced. The hydroxycoumarin isomers of 3OH-, 4OH-, 5OH-, 6OH- and 8OH-coumarin are also produced although the fluorescence emission of these products is negligible compared to that of 7-hydroxycoumarin. The radiation chemical yield of 7-hydroxycoumarin has been studied in different gas conditions

[en] The hydroxyl radical is involved in a wide range of different fields, from oxidative stress to atmospheric chemistry. In addition to the study of oxidative damage in biological media, the hydroxyl radical detection allows to perform a dosimetry when it is produced by ionising radiation. The aims of this work have been double: - to improve the detection of the hydroxyl radical by the design of new probes - to improve knowledge on the reactive pathways in which the hydroxyl radical is involved. We have studied the coumarin molecule, as well as 6 derivatives that we have synthesised, as fluorescent probes of the hydroxyl radical. Firstly, fluorescence spectroscopy and HPLC chromatography have allowed the evaluation of the sensibility and selectivity of detection of the probes. Consequently to this study, two applications have been developed, concerning the determination of rate constants by competition kinetics and bidimensional dosimetry. Secondly, we have studied the reactivity of the hydroxyl radical through the regioselectivity of its addition on the aromatic cycle. This problem was addressed by the combined use of experimental methods such as time resolved kinetics and HPLC along with interpretation from classical and ab initio modelization. (author)

[en] The radiation damage of adenine base was studied by B3LYP and MP2 methods in the presence of hydroxyl radicals to probe the reactivities of five possible sites of an isolated adenine molecule. Both methods predict that the C8 site is the more vulnerable than the other sites. For its bonding covalently with the hydroxyl radicals, B3LYP predicts a barrierless pathway, while MP2 finds a transition state with an energy of 106.1 kJ/mol. For the hydroxylation at the C2 site, the barrier was calculated to be 165.3 kJ/mol using MP2 method. For the dehydrogenation reactions at five sites of adenine, B3LYP method predicts that the free energy barrier decreases in the order of H8 > H2 > HN62 > HN61 > HN9. (special topic — soft matter and biological physics)

[en] The photochemical role of Fe(III)-citrate complex is significant in natural waters due to its ubiquitous existence and excellent photoreactivity at near neutral pH. Although there are many reports on the photoinduced degradation of pollutants in the Fe(III)-citrate system, the optimum pH for its photoreactivity is yet not clearly understood. Here, for the first time, we demonstrated that the optimum pH was 5.5 for the photoproduction of ^•OH in the Fe(III)-citrate system via kinetics modeling based on the steady-state approximation. According to the experimental results, the ^•OH photoproduction increased with increasing pH until 5.5 and then decreased in Fe(III)-citrate solution, which agreed well with the prediction trend of kinetic modeling. The effect of the common ligand oxalate on the photoreactivity of Fe(III)-citrate system was also investigated. The addition of oxalate promoted the photoproduction of ^•OH in Fe(III)-citrate solutions, and the measured [^•OH]_ss increased with oxalate concentration under a fixed Fe(III)-to-citrate ratio. Little synergistic effect exists in Fe(III)-citrate-oxalate system at pH 4.0–5.5. In contrast, an appreciable synergistic effect was observed at near neutral pH (6.0–8.0). Higher oxalate-to-citrate ratio facilitated the synergistic effect. Furthermore, antifungal drug fluconazole could be removed efficiently in the Fe(III)-citrate-oxalate system. The photodegradation kinetics also verified the optimum pH of Fe(III)-citrate system and synergistic effect of oxalate. By LC-ESI-MS/MS analyses, the photoproducts of fluconazole in the Fe(III)-citrate-oxalate system were identified and the reaction mechanism involving hydroxylation substitution and subsequent cleavage of heterocyclic amine was proposed. These findings suggest that Fe(III)-citrate exhibits best photoreactivity at pH 5.5, and the coexistence of reactive ligands will enhance its photoreactivity at circumneutral pH, indicating potential application in wastewater treatment via addition of appropriate citrate and co-ligands.

[en] The ubiquity and photoreactivity of fluoroquinolone antibiotics (FQs) in surface waters urge new insights into their aqueous photochemical behavior. This study concerns the photochemistry of 6 FQs: ciprofloxacin, danofloxacin, levofloxacin, sarafloxacin, difloxacin and enrofloxacin. Methods were developed to calculate their solar direct photodegradation half-lives (t_d_,_E) and hydroxyl-radical oxidation half-lives (t_·_O_H_,_E) in sunlit surface waters. The t_d_,_E values range from 0.56 min to 28.8 min at 45° N latitude, whereas t_·_O_H_,_E ranges from 3.24 h to 33.6 h, suggesting that most FQs tend to undergo fast direct photolysis rather than hydroxyl-radical oxidation in surface waters. However, a case study for levofloxacin and sarafloxacin indicated that the hydroxyl-radical oxidation induced risky photochlorination and resulted in multi-degradation pathways, such as piperazinyl hydroxylation and clearage. Changes in the antibacterial activity of FQs caused by photodegradation in various waters were further examined using Escherichia coli, and it was found that the activity evolution depended on primary photodegradation pathways and products. Primary intermediates with intact FQ nuclei retained significant antibacterial activity. These results are important for assessing the fate and risk of FQs in surface waters. - Highlights: • It is first reported on hydroxyl-radical oxidation of 6 fluoroquinolone antibiotics. • Methods were developed to assess photolysis and oxidation fate in surface waters. • The neutral form reacted faster with hydroxyl radical than protonated forms. • The main oxidation intermediates and transformation pathways were clarified. • The antibacterial activity changes depend on dominant photolysis pathways

[en] The adenine—thymine base pair was studied in the presence of hydroxyl radicals in order to probe the hydrogen bond effect. The results show that the hydrogen bonds have little effect on the hydroxylation and dehydrogenation happened at the sites, which are not involved in a hydrogen bond, while at the sites involved in hydrogen bond formation in the base pair, the reaction becomes more difficult, both in view of the free energy barrier and the exothermicity. With a 6–311++G(d,p) level of description, both B3LYP and MP2 methods confirm that the C8 site of isolated adenine has the highest possibility to form covalent bond with the hydroxyl radicals, though with different energetics: B3LYP predicts a barrierless pathway, while MP2 finds a transition state with an energy of 106.1 kJ/mol. For the dehydrogenation reactions, B3LYP method predicts that the free energy barrier increases in the order of HN9 < HN61 < HN62 < H2 < H8. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

[en] The radiation-induced hydroxylation of benzonitrile, anisole, and fluorobenzene has been investigated. In absence of oxidizing agents (O₂, metal salts) only trace amounts of substituted phenols are formed. Some metal salts (Fe(CN)₆^3-, Fe³⁺, Cr₂O₇^2-) oxidize the intermediate hydroxycyclohexadienyl radicals and give a very high conversion of OH radicals to phenols, whose isomer distribution is dependent on the metal salts. With Cu²⁺ salts (Cu(ClO₄)₂, CuSO₄, CuCl₂) no oxidation of the hydroxycyanocyclohexadienyl radicals was observed. Oxidation with K₃Fe(CN)₆ leads in a quantitative way to the corresponding phenols. It was shown that in the hydroxylation of anisole, fluorobenzene, bromobenzene, toluene, benzonitrile, and nitrobenzene a linear relationship exists between log 2k/sub m//2k₀ + 2k/sub m/ + k/sub p/ and log k/sub p//k/sub m/ and sigma/sub M/ values. Electron-withdrawing substituents give a high percentage of meta hydroxylation, whereas electron-donating substituents give a low percentage of meta hydroxylation. As the difference in electron density between ortho and meta position increases the electrophilic OH radicals attack preferentially at the ortho and para positions. It was concluded that dehydration from the initially formed hydroxycyclohexadienyl radicals takes place only if the substituent is electron donating (+M effect), but not if the substituent is electron withdrawing

[en] The review addresses direct methane oxidation — an important fundamental problem, which has attracted much attention of researchers in recent years. Analysis of the available results on biomimetic and bio-inspired methane oxygenation has demonstrated that assimilating of the experience of Nature on oxidation of methane and other alkanes significantly enriches the arsenal of chemistry and can radically change the character of the entire chemical production, as well as enables the solution of many material, energetic and environmental problems. The bibliography includes 310 references.

[en] A method is described for the analysis of the natural ¹⁴CO concentration in the atmosphere. It is based on the separation of CO from ambient air, followed by the determination of its ¹⁴C-content by proportional gas counting employing a low volume counter. The method is described in detail. The concentration of ¹⁴CO at midlatitudes (51⁰N) exhibits a marked seasonal variation with a winter maximum of 25 +- 2 molecules/cm³ and a minimum of 11 +- 1 molecules/cm³ during summer. Measurements at different latitudes show a decrease of the ¹⁴CO concentration from the pole to the equator. Interpretations of the data are based on calculations with a 2-d-time dependent model. From the simultaneous examination of the ¹⁴CO and ¹²CO balance in the troposphere it is possible to derive the two most important parameters for the global cycle of atmospheric CO: the concentration of OH radicals in the troposphere - and thus the lifetime of CO - and the contribution of biological sources to the CO budget. From this work the concentration of OH radicals in the troposphere is estimated to average around 6.5 x 10⁵ molecules/cm³, biological sources are shown to contribute about 12.5 x 10¹⁴g ¹²CO per year to the CO budget. (orig.)

[en] In many biological systems, the role of O₂^- in hydroxylation and toxic processes, was assumed to be due to the formation of OH radicals. The Haber Weiss reaction (H₂O₂ + O₂^- → OH + OH^- + O₂) was suggested as the origin of this activity. In this study it is shown that this reaction pathway is too slow, and that probably OH is formed from the reaction of complexed superoxide with H₂O₂