Reactions of e aq' − H and OH with picolinic acid studied by pulse radiolysis

Rate constants for the reaction of the primary species of water radiolysis, viz. e _aq' ^– , H and OH, with picolinic acid have been determined at various pHs. The semi-reduced species exhibits _max at 305 and 350 nm. It is a strong reductant and has two pK_a values of 1 and 5.7. The OH adduct of this compound also exhibits two pK_a s at 2.1 and 5.2. H-atom reaction with picolinic acid gives rise to a mixture of species.     

Kinetics and spectral properties of electron and •OH adducts of dimethylpyridines: a pulse radiolysis study

The reactions of e_aq ^-, OH, O^- and SO^- ₄ with 2,4-, 2,6- and 3,5-dimethylpyridines have been investigated in aqueous solution by pulse radiolysis with optical detection. Both e_aq ^- and OH radicals have high reactivity toward these compounds with k = (4-8) × 10⁹ dm³ mol^-1 s^-1. The rates of O^- and SO₄ ^- reactions ((1-3) × 10⁹ dm³ mol^-1 s^-1) were lower compared to the rate observed with the OH radical. The transient absorption spectra obtained in the reaction of e_aq ^- with three isomers exhibited a weak broad band around 340-410 nm. The absorption maxima of the intermediates formed in the OH and SO₄ ^- reactions were centred around 320-330 nm ( = 2450-3500 dm³ mol^-1 cm^-1) with an additional broad peak in the range 460-520 nm which are attributed to the corresponding OH adducts. The spectra in the O^- reaction have absorption maxima between 300 and 320 nm and it reacts both by addition and H-abstraction from the CH₃ group. A reaction mechanism consistent with the observed results is proposed.     

Picosecond pulse radiolysis study of highly concentrated nitric acid solutions: formation mechanism of NO3• radical

The formation of nitrate radical, NO(3)(?), is observed for the first time directly by picosecond pulse radiolysis of highly concentrated nitric acid solutions. The experimental yield of NO(3)(-) ionization is deduced from the pulse-probe transient absorption measurements in the visible region where this radical absorbs. On the basis of the value of the extinction coefficient of nitrate radical at 640 nm equal to 1300 M cm(-1), the experimental yield of NO(3)(?) at 20 ps is found to be around 0.36 × 10(-7), 1.33 × 10(-7), and 2.85 × 10(-7) mol J(-1) for 1, 3.5, and 7 M nitric acid solutions, respectively. Relative to the dose absorbed by nitric acid by the direct effect, we find an unexpected high formation yield of the nitrate radical within the electron pulse. Therefore, we suggest that the trapping of the positive hole, H(2)O(?+), by NO(3)(-) also contributes to the formation of NO(3)(?) within the electron pulse. Moreover, after the pulse and within 4 ns, the beginning of the reaction of OH(?) radical with undissociated nitric acid is observed for the most concentrated nitric acid solution.     

Transient species of fullerenes studied with pulse radiolysis and laser photolysis

The excited triplet of C60 or C70 is generated via either direct excitation by laser light or energy transfer from excited states of solvent to C60 and C70. The cation radical of C60 is produced either via hole transfer from cation radical of CCl4 to C60 or via electron transfer from excited triplet of C60 to CCl4. C60 and C70 could be added to trichloromethyl radical to produce adduct radicals with different mechanisms.     

Reactivity of OH radicals with chlorobenzoic acids—A pulse radiolysis and steady-state radiolysis study

The reactions of OH radicals with 2-, 3-, 4-chlorobenzoic acids (ClBzA) and chlorobenzene (ClBz), k(OH+substrates)=(4.5?6.2)×10⁹ dm³ mol^?1 s^?1, have been studied by pulse radiolysis in N₂O saturated solutions. The absorption maxima of the OH-adducts were in the range of 320?340 nm. Their decay was according to a second-order reaction, 2k=(1?9)×10⁸ dm³ mol^?1 s^?1. In the presence of N₂O/O₂ the formation of peroxyl radicals was detectable for 2-, 4-ClBzA and ClBz, k(OH-adduct+O₂)=(2?4)×10⁷ dm³ mol^?1 s^?1, while this reaction for 3-ClBzA was too slow to be registered. In the presence of N₂O the degradation rates induced by gamma radiation were very similar for all chlorobenzoic acids, yet the chloride formation was distinctly higher for 3-ClBzA. In the presence of oxygen the initial degradation of 2-and 4-ClBzA equaled the OH-radical concentration, whereas in case of 3-ClBzA only ～60% of OH led to degradation. The order for the efficiency of dehalogenation was 4->2->3-ClBzA. Several primary radiolytic products could be detected by HPLC. To evaluate the toxicity of final products a bacterial bioluminescence test was carried out.     

Spectroscopic characterization of mechanisms of oxidation of Phe by SO_4~- radical: A pulse radiolysis study

By using time-resolved kinetic spectrophotometry and pulse radiolysis technique, the oxidation of Phe by SO4- radical has been investigated both in aqueous and water/acetonitrilemixed solutions. The results reveal that attack of the oxidizing SO4- radical on Phe leads directlyto the formation of Phe cation radical 3 with a strong absorption peak at 310 nm, then it proceeds in three competitive reactions via either hydroxylation, deprotonation or decarboxylation, which were found to be strongly dependent upon the ionization state of the substitutes -COOH and -NH2 and the nature of the solvents. Decarboxylation takes place only when the carboxyl group is deprotonated. At high pH deprotonation of Phe cation radical 3 is much easier to occur than that in neutral or acid solutions. Moreover, with addition of acetonitrile, deprotonation is more predominant than hydroxylation, whereas in aqueous solutions hydroxylation is much easier to occur.     

Reactions of tea polyphenol four constituents with hydroxyl radical: A pulse radiolytic study

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Reaction of carotenoids with CCl_3OO~· by using pulse radiolysis

CCl4 as a selective toxin to liver can be metabolized into the free radical 3CClg by cyto- chrome P450 through a reductive dehalogenation. In the presence of oxygen, 3CClg reacts rapidly with O2 to form CCl3OO·[1], which reacts with various biological molecules, including DNA bases, amino acids and lipids and leads to various types of tissue injury[2]. CCl3OO· is used as a model of alkyl peroxyl radicals because it can be generated in water/alcohol solution, in which sufficient solubi…     

A study of vitamin A radical cation generated by chemical and radiolytic oxidation

Electron transfer reaction between vitamin A (1) and tris(p-bromophenyl)aminium hexachloroautimonate (2) in dichloromethane (DCM) has been investigated by means of UV-VIS absorption and ESR spectroscopy. The title radical cation formed in the reaction was characterized by a new absorption band around 600 nm and a singlet unresolved ESR spectrum with g factor of 2.0038-2.0039 and line width of 20 G. Further studies indicated that ESR pattern and parameters of the radical cation generated by 7-irradiation of 1 in CFCl3 matrix at 77 K are consistent with that resulted in the chemical oxidation in DCM at ambient temperature.     

Photodissociation of pyrene dimer radical cation during the pulse radiolysis–laser flash photolysis combined method

Photodissociation of pyrene (Py) dimer radical cation (Py ₂ ^?+ ) giving pyrene radical cation (Py^?+) and Py and subsequent regeneration of Py ₂ ^?+ by association of Py^?+ and Py were directly observed during the pulse radiolysis–laser flash photolysis combined method at room temperature. When Py ₂ ^?+ was excited at the local excitation band with the 532-nm laser flash, the rapid growth and decay of monomeric Py^?+ were observed at 460 nm. The dissociation of Py ₂ ^?+ proceeded via a one-photon process to give the ground-state Py^?+(D₀) and Py in the quantum yield (Φ_diss) of (2.9 ± 0.9) × 10^?3. It was shown that Py^?+ decayed with a time constant of several tens of nanoseconds, indicating that the association of Py^?+ with Py regenerating Py ₂ ^?+ proceeds at a diffusion-controlled rate. The photodissociation proceeded from the lowest excited state of Py ₂ ^?+ , even when Py ₂ ^?+ was excited to the higher excited state. The difference between the Φ_diss value of Py ₂ ^?+ and that previously reported for naphthalene dimer radical cation (Np ₂ ^?+ ) is discussed.     

γ-ray radiolysis of dihydroxyurea in nitric acid and its radiolytic products

Journal of Radioanalytical and Nuclear Chemistry - Dihydroxyurea (DHU) is a new salt-free reducing agent applied for the separation of Pu and Np from U in spent fuel reprocessing. This paper...     

Antitumor mechanism of VP-16: A pulse radiolysis study

The antitumor mechanism of etoposide (VP-16) is investigated using pulse radiolysis technology. The oxidizing mechanism of VP-16 is studied by sodium persulfate, and the reaction rate constant is 4.04× 109 L· mol-1 · s-1. The electron-transfer between VP-16 and tyrosine is observed and the reaction rate constant is 1.1 - 108 L · mol-1· s-1.     

Effect of nitrous acid on γ-ray radiolysis and radiolytic products of N,N-dimethylhydroxylamine

Journal of Radioanalytical and Nuclear Chemistry - N, N-dimethylhydroxylamine (DMHA) is a new salt-free reductant used in spent fuel reprocessing, this paper reports the effect of HNO2 on...     

Dynamics of contact radical ion pairs C60—aromatic amines. A study by picosecond laser photolysis

Charge transfer complexes between C₆₀ and ternary aromatic amines (N,N,N,N-tetramethyl-p-phenylenediamine,p-methoxy-N,N-dimethylaniline,p-methyl-N,N-dimethylaniline,N,N-diethylaniline,N,N-dimethylaniline, and triphenylamine) were studied in chlorobenzene solutions. The lifetimes of the excited state with charge transfer in these complexes were measured by the method of picosecond laser photolysis. The dependence of the rate constant of the back electron transfer on G in the back electron transfer reaction with relaxation of the charge-transfer state exhibits the Marcus-inverted region.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1078–1089, June, 1995.The authors are grateful to D. V. Khudyakov and I. V. Rubtsov for help in experiments and to A. S. Lobach for the C₆₀ supplied.The work was financially supported by the Russian Foundation for Basic Research (Projects No. 93-03-05254 and 93-03-0217).     

The photocatalytic •OH production activity of g-C3N4 improved by the introduction of NO

The efficiency of photocatalytic pollutant removal largely depends on the ability of the photocatalytic system to produce hydroxyl radicals (^?OH). However, the capability of photocatalyst to produce ^?OH is not strong at present. Advancing the capacity of photocatalytic system to produce ^?OH has always been a tough problem and challenge in the field of environmental science. In this research, it was found that introducing nitric oxide (NO) into the graphitic carbon nitride (g-C₃N₄) photocatalytic system could memorably enhance the ability of producing ^?OH group. This study provides a new idea for improving the capacity of photocatalytic ^?OH production.     

Reactions of the OH adducts of cytosine and its derivatives with TEMPO and TEMPOL: A steady state γ-radiolysis and pulse radiolysis study

The reactions of the OH adducts of cytosine and its derivatives with nitroxides namely TEMPO and TEMPOL have been followed by steady state -radiolysis and pulse radiolysis. Pulse radiolysis studies show that the transient OH adducts of cytosine and its derivatives react with TEMPO and TEMPOL by electron transfer reaction and not by adduct formation as was observed in the case of thymine. The second order rate constants for these electron transfer reactions are of the order of 10⁷ - 10⁸ dm³ mol^-1 s^-1.     

From alcohols to sugars-how does the reactivity of alpha-hydroxyalkyl radicals change with structure? A quantitative examination by pulse radiolysis

Rate constants are reported for the 1-electron reduction of the azo dye Orange II in water (pH 7.0) by 10 different alpha-hydroxy radicals. The radicals were created by pulse radiolysis of aqueous solutions of the corresponding alcohol/sugar. The rate constants varied from 1 x 10(8) to 2.7 x 10(9) mol(-1) dm(3) s(-1) and radicals with beta-hydroxy groups had the lowest rate constant. The reaction was found to be controlled by the reduction potentials of the radicals, with steric influences having little effects. Good fits of the data were obtained using the Marcus equation with lambda =140 kJ/mol.     

Effect of temperature on O⊘ reactions and equilibria: A pulse radiolysis study

The pK_a of the hydroxyl radical was measured over the 20–80°C temperature range. At 20°C, the pK_a was 11.84 and fell to 10.81 at 80°C. The dissociation constant for the ozonide anion (O⊘₃⇌O₂+O⊘) was found to be 5.5 × 10^-7 mol dm^-3 at 20°C and 46.2 × 10^-7 mol dm^-3 at 70°C. The rate constants and activation energies for the reaction of O⊘ and OH with 2-propanol, methanol and 3-hexene-1,6-dicarboxylate ions have also been measured.     

Hydroxylation of 3-nitrotyrosine by hydroxyl radical

Hydroxylation of 3-nitrotyrosine (3-NT) and 3-NT containing peptide Gly-nitroTyr-Gly in aqueous solution by hydroxyl radical were investigated with gamma irradiation. The structures of the hydroxylated products were confirmed by electrospray ionization mass spectrometry and 1H NMR spectrometry. The reactivity of 3-nitrotyrosine has been investigated using density functional theory (DFT) calculation.     

Laser flash photolysis and pulse radiolysis study on chemical activity of VP-16 and podophyllotoxin

Time-resolved laser flash photolysis and pulse radiolysis have been used to study the chemical activity of podophyllotoxin(PPT) and etoposide(VP-16). The mechanism of photoioniza-tion of etoposide and podophyllotoxin has been confirmed and illustrated. It is demonstrated that VP-16 and PPT in aqueous solution can be photoionized at 248 nm to give hydrated electron and neutral radical resulting from rapid deprotonation of radical cation of VP-16 and PPT. The quantum yield for the photoionization of VP-16 and PPT with single-photo is 0.21 and 0.61, respectively. In addition, they can react with hydrated electron, hydrogen radical and hydroxyl radical. This will give chemists some advice on synthesizing new derivatives of podophyllotoxin in cancer treatment.