Pulse Radiolysis Of Aqueous Solutions Research Articles

Pathways of the Dissociative Electron Attachment Observed in 5- and 6-Azidomethyluracil Nucleosides: Nitrogen (N2) Elimination vs Azide Anion (N3-) Elimination.

5-Azidomethyl-2'-deoxyuridine (5-AmdU, 1) has been successfully employed for the metabolic labeling of DNA and fluorescent imaging of live cells. 5-AmdU also demonstrated significant radiosensitization in breast cancer cells via site-specific nitrogen-centered radical (π-aminyl (U-5-CH2-NH•), 2, and σ-iminyl (U-5-CH═N•), 3) formation. This work shows that these nitrogen-centered radicals are not formed via the reduction of the azido group in 6-azidomethyluridine (6-AmU, 4). Radical assignments were performed using electron spin resonance (ESR) in supercooled solutions, pulse radiolysis in aqueous solutions, and theoretical (DFT) calculations. Radiation-produced electron addition to 4 leads to the facile N3- loss, forming a stable neutral C-centered allylic radical (U-6-CH2•, 5) through dissociative electron attachment (DEA) via the transient negative ion, TNI (U-6-CH2-N3•-), in agreement with DFT calculations. In contrast, TNI (U-5-CH2-N3•-) of 1, via facile N2 loss (DEA) and protonation from the surrounding water, forms radical 2. Subsequently, 2 undergoes rapid H-atom abstraction from 1 and produces the metastable intermediate α-azidoalkyl radical (U-5-CH•-N3). U-5-CH•-N3 converts facilely to radical 3. N3- loss from U-6-CH2-N3•- is thermodynamically controlled, whereas N2 loss from U-5-CH2-N3•- is dictated by protonation from the surrounding waters and resonance conjugation of the azidomethyl side chain at C5 with the pyrimidine ring.

Modulation of the Directionality of Hole Transfer between the Base and the Sugar-Phosphate Backbone in DNA with the Number of Sulfur Atoms in the Phosphate Group.

This work shows that S atom substitution in phosphate controls the directionality of hole transfer processes between the base and sugar-phosphate backbone in DNA systems. The investigation combines synthesis, electron spin resonance (ESR) studies in supercooled homogeneous solution, pulse radiolysis in aqueous solution at ambient temperature, and density functional theory (DFT) calculations of in-house synthesized model compound dimethylphosphorothioate (DMTP(O-)═S) and nucleotide (5'-O-methoxyphosphorothioyl-2'-deoxyguanosine (G-P(O-)═S)). ESR investigations show that DMTP(O-)═S reacts with Cl2•- to form the σ2σ*1 adduct radical -P-S[Formula: see text]Cl, which subsequently reacts with DMTP(O-)═S to produce [-P-S[Formula: see text]S-P-]-. -P-S[Formula: see text]Cl in G-P(O-)═S undergoes hole transfer to Gua, forming the cation radical (G•+) via thermally activated hopping. However, pulse radiolysis measurements show that DMTP(O-)═S forms the thiyl radical (-P-S•) by one-electron oxidation, which did not produce [-P-S[Formula: see text]S-P-]-. Gua in G-P(O-)═S is oxidized unimolecularly by the -P-S• intermediate in the sub-picosecond range. DFT thermochemical calculations explain the differences in ESR and pulse radiolysis results obtained at different temperatures.

One Way Traffic: Base-to-Backbone Hole Transfer in Nucleoside Phosphorodithioate.

The directionality of the hole-transfer processes between DNA backbone and base was investigated by using phosphorodithioate [P(S- )=S] components. ESR spectroscopy in homogeneous frozen aqueous solutions and pulse radiolysis in aqueous solution at ambient temperature confirmed initial formation of G.+ -P(S- )=S. The ionization potential of G-P(S- )=S was calculated to be slightly lower than that of guanine in 5'-dGMP. Subsequent thermally activated hole transfer from G.+ to P(S- )=S led to dithiyl radical (P-2S. ) formation on the μs timescale. In parallel, ESR spectroscopy, pulse radiolysis, and density functional theory (DFT) calculations confirmed P-2S. formation in an abasic phosphorodithioate model compound. ESR investigations at low temperatures and higher G-P(S- )=S concentrations showed a bimolecular conversion of P-2S. to the σ2 -σ*1 -bonded dimer anion radical [-P-2S 2S-P-]- [ΔG (150 K, DFT)=-7.2 kcal mol-1 ]. However, [-P-2S 2S-P-]- formation was not observed by pulse radiolysis [ΔG° (298 K, DFT)=-1.4 kcal mol-1 ]. Neither P-2S. nor [-P-2S 2S-P-]- oxidized guanine base; only base-to-backbone hole transfer occurs in phosphorodithioate.

Radiation-induced reduction of quinoxalin-2-one derivatives in aqueous solutions

Quinoxaline-2-one derivatives have been proposed as potential drugs in treatments of various diseases since some of them showed a variety of pharmacological properties. The kinetics and spectral characteristics of the transients formed in the reactions of hydrated electrons (eaq−) with quinoxalin-2-(1H)-one (Q) and its methyl derivative, 3-methyl quinoxalin-2-(1H)-one (3-MeQ) were studied by pulse radiolysis in aqueous solutions at pH ranging from 5 to 14. The transient absorption spectra recorded in the reactions of (eaq−) with Q and 3-MeQ at pH 7 consisted of a broad, almost flat band in the range 390–450nm and were assigned to the respective protonated radical anions (QH∙/3-MeQH∙) at N4 atom in a pyrazin-2-one ring. On the other hand, the transient absorption spectra recorded in the reactions of (eaq−) with Q and 3-MeQ at pH 13 are characterized by a broad band with a much better pronounced maximum at λmax=390nm and higher intensity (in comparison to that at pH 7) and were assigned to the respective radical anions (Q∙−/3-MeQ∙−). Both forms are involved in the prototropic equilibrium with the pKa located at pH≥13.5. The rate constants of the reactions of (eaq−) with Q and 3-MeQ were found to be at pH 7 (2.6±0.1)×1010M−1s−1 and (2.1±0.1)×1010M−1s−1 and at pH 13 (1.6±0.1)×1010M−1s−1 and (1.3±0.1)×1010M−1s−1, respectively. Semi-empirical quantum mechanical calculations reproduce fairly well the spectral features of the experimental absorption spectra and show that protonated radical anions at nitrogen atom (N4) in both molecules are the most stable hydrogenated radicals.

Spectral and kinetic properties of radicals derived from oxidation of quinoxalin-2-one and its methyl derivative.

The kinetics and spectral characteristics of the transients formed in the reactions of •OH and •N3 with quinoxalin-2(1H)-one (Q), its methyl derivative, 3-methylquinoxalin-2(1H)-one (3-MeQ) and pyrazin-2-one (Pyr) were studied by pulse radiolysis in aqueous solutions at pH 7. The transient absorption spectra recorded in the reactions of •OH with Q and 3-MeQ consisted of an absorption band with λmax = 470 nm assigned to the OH-adducts on the benzene ring, and a second band with λmax = 390 nm (for Q) and 370 nm (for 3-MeQ) assigned, inter alia, to the N-centered radicals on a pyrazin-2-one ring. The rate constants of the reactions of •OH with Q and 3-MeQ were found to be in the interval (5.9–9.7) × 109 M–1·s–1 and were assigned to their addition to benzene and pyrazin-2-one rings and H-abstraction from the pyrazin-2-one nitrogen. In turn, the transient absorption spectrum observed in the reaction of •N3 exhibits an absorption band with λmax = 350 nm. This absorption was assigned to the N-centered radical on the Pyr ring formed after deprotonation of the respective radical cation resulting from one-electron oxidation of 3-MeQ. The rate constant of the reaction of •N3 with 3 MeQ was found to be (6.0 ± 0.5) × 109 M–1·s–1. Oxidation of 3-MeQ by •N3 and Pyr by •OH and •N3 confirms earlier spectral assignments. With the rate constant of the •OH radical with Pyr (k = 9.2 ± 0.2) × 109 M–1·s‒1, a primary distribution of the •OH attack was estimated nearly equal between benzene and pyrazin-2-one rings.

Radiation-induced pink nickel oligomeric clusters in water. Pulse radiolysis study.

γ-rays and pulse radiolysis of aqueous solutions of Ni(2+) ions in the presence of polyacrylate (PA(-)) and 2-propanol leads to the formation of metastable species absorbing at 540 nm that are ascribed to "pink" oligomeric clusters of a few nickel atoms only. The molar absorption coefficient is evaluated as ε540 nm = 3300 ± 300 L mol(-1) cm(-1) per Ni(0) atom. The successive steps from the reduction of Ni(2+) into Ni(+) ions to the formation of the pink clusters at 540 nm under conditions of complexation by PA(-) are investigated by pulse radiolysis. The yield of the formation of pink clusters increases markedly with the irradiation dose rate, demonstrating the occurrence of the disproportionation of the [Ni(+), PA(-)] complex after a single electron pulse. The reduction and nucleation mechanisms, including rate constants, in competition with the back oxidation by protons, particularly at low dose rate, are discussed.

ChemInform Abstract: Substituent Effects on the Spectral, Acid-Base, and Redox Properties of Indolyl Radicals: A Pulse Radiolysis Study.

Spectral and acid-base properties and reduction potentials of various substituted indolyl radicals were studied by pulse radiolysis in aqueous solutions at 20 o C. Except for the 5-methoxyindolyl and 5-carboxyindolyl radicals, the spectra of the substituted indolyl radicals resemble the previously published 320- and 520-nm spectra of the neutral and 330- and 580-nm spectra the cation indolyl and tryptophan radicals

Calculated and measured transient product yields in pulse radiolysis of aqueous solutions: Concentration dependence

Pulse radiolysis of aqueous solutions was modeled by using 54 equations for the reaction of water radiolysis intermediates with carefully selected rate coefficients. Yields of products formed in the hydrated electron+solute and hydroxyl radical+solute reactions were calculated and compared with the measured yields in wide concentration range. These reactions are in competition with the reactions of the water radiolysis intermediates with each other and with H 2O, H + and H 2O 2. An empirical equation was developed for the calculation of scavenged product yields that can be used in cases when due to low rate coefficient, low solubility or very high absorbance, low solute concentrations are applied and a considerable fraction of the water radiolysis intermediates does not react with the solute.

Reactions of reducing radicals with 2- and 3-nitroanilines in aqueous solutions: a pulse radiolysis study

Reactions of 2- and 3-nitro anilines (2- and 3-NA) with e aq − , H-atoms and one-electron reductants have been studied using pulse radiolysis in aqueous solutions. Reactions of e aq − were found to be quite fast with both 2-NA and 3-NA resulting in their corresponding semi-reduced species which are reducing in nature. Reduction potentials for 2-NA/2-Na•′ have been estimated to be approx. −0.56 Vvs. NHE and that for 3-NA/3-NA•− was found to be between −0.185 V and −0.45 Vvs. NHE. Semi-reduced 2-NA has main absorption peak at 300 nm with a shoulder in the 350 nm region and a broad weak band in the 470–500 nm region, whereas semi-reduced 3-NA possesses an absorption peak at 520 nm. Reducing radicals such as (CH3)2 C•OH and CO 2 •− reacted with 2-NA, producing semi-reduced species, whereas reactions of these radicals with 3-NA produced their corresponding radical-adduct species.

Reactions of Hydroxyl Radical with Salicylic Acid Derivatives: A Pulse Radiolysis Study

AbstractBased on pulse radiolysis of aqueous solutions of two salicylic acid derivatives including ethyl salicylate and benzylsalicylate the transient absorption spectra and the rate constants of hydroxyl radical with salicylic acid derivatives were determined for the first time. The results indicated that ethyl salicylate and benzylsalicylate have efficient activities to scavenge hydroxyl radical.

From Alcohols to SugarsHow Does the Reactivity of α-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.

Radical Cations of Phenyl‐Substituted Aziridines: What Are the Conditions for Ring Opening?

Radical cations were generated from different phenyl-substituted aziridines by pulse radiolysis in aqueous solution containing TlOH.+, N3. or SO4.- as oxidants or in n-butyl chloride, by 60Co gamma radiolysis in Freon matrices at 77 K, and in some cases by flash photolysis in aqueous solution. Depending on the substitution pattern of the aziridines, two different types of radical cations are formed: if the N atom carries a phenyl ring, the aziridine appears to retain its structure after oxidation and the resulting radical cation shows an intense band at 440-480 nm, similar to that of the radical cation of dimethylaniline. Conversely, if the N atom carries an alkyl substituent while a phenyl ring is attached to a C-atom of the aziridine, oxidation results in spontaneous ring opening to yield azomethine ylide radical cations which have broad absorptions in the 500-800 nm range. In aqueous solution the two types of radical cations are quenched by O2 with different rates, whereas in n-butyl chloride, the ring-closed aziridine radical cations are not quenchable by O2. The results of quantum chemical calculations confirm the assignment of these species and allow to rationalize the different effects that phenyl rings have if they are attached in different positions of aziridines. In the pulse radiolysis experiments in aqueous solution, the primary oxidants can also be observed, whereas in n-butyl chloride a transient at 325 nm remains unidentified. In the laser flash experiments, both types of radical cations were also observed.

Bimolecular Homolytic Substitution (SH2) Reactions with Hydrogen Atoms. Time-Resolved Electron Spin Resonance Detection in the Pulse Radiolysis of α-(Methylthio)acetamide

Pulse radiolysis of aqueous solutions of alpha-(methylthio)acetamide produced unexpectedly large quantities of acetamide radicals that were identified by time-resolved electron spin resonance (TRESR) spectroscopy. The pH dependence of the TRESR-measured radical yields, results from selective scavenging reactions, and density functional theory predictions of the reaction thermochemistry prove that bimolecular homolytic substitution, S(H)2, of the acetamide radical fragment by a H atom is the most likely formation pathway.

Deconvolution of Complex Absorption Spectra Recorded in the Pulse Radiolysis of Aqueous Solutions of 3,5-Dihydroxytolyene

The optical absorption spectra of short-lived species generated in aerated aqueous solutions of 3,5-dihydroxytoluene, a representative of alkylhydroxybenzenes, by the action of single electron beam pulses were recorded. The decay curves recorded at different wavelengths differ from each other, thus suggesting that the transient species react with one another and with water via a complex mechanism and making it possible to discriminate between individual bands characterizing each particular species in the overall spectrum. Two procedures for analyzing complex absorption spectra were employed: (i) with the use of spectra observed at different time delays after the electron pulse and (ii) with deconvolution into a sum of Gaussians.

Reaction Kinetics in the Ionic Liquid Methyltributylammonium Bis(trifluoromethylsulfonyl)imide. Pulse Radiolysis Study of 4-Mercaptobenzoic Acid

Hydrogen-abstraction reactions of various radicals with 4-mercaptobenzoic acid (MB) to produce the 4-carboxyphenylthiyl radical have been studied by pulse radiolysis in aqueous solutions and in the ionic liquid methyltributylammonium bis(trifluoromethylsulfonyl)imide (R4NNTf2). The rate constants in aqueous solutions are in the range of (1−3) × 108 L mol-1 s-1 for the reactions of MB with several alkyl radicals, are higher with reducing radicals (6.4 × 108 L mol-1 s-1 for CH3ĊHOH and 1.4 × 109 L mol-1 s-1 for (CH3)2ĊOH) and lower with oxidizing radicals (≤107 L mol-1 s-1 for •CH2COCH3). Because the bond dissociation energy for the S−H bond is much lower than that for the C−H bonds involved in these reactions, it appears that hydrogen abstraction from mercaptobenzoic acid is not controlled by the relative bond dissociation energies but rather by the electron density at the radical site through a polar transition state. The rate constants for similar reactions in alcohols are slightly lower than those in water, supporting a polar transition state. The rate constants in the ionic liquid are in the range of 107−108 L mol-1 s-1 and are essentially controlled by the diffusion rate; variations within this range appear to be due mainly to changes in viscosity. The •CF3 radical reacts slightly more slowly (3.6 × 106 L mol-1 s-1) with MB in the ionic liquid, in agreement with the low reactivity in water of radicals bearing electron-withdrawing groups.

Radiation-chemical effects in the near-field of a final disposal site: role of bromine on the radiolytic processes in NaCl-solutions

Summary The oxidation of Br− by Cl2 − is investigated by gamma pulse radiolysis in aqueous solutions of NaCl and NaBr. Depending on the ratio of the concentration of Br− to Cl−, the main product being observed is either Cl2 −, ClBr− or Br2 −. The mixed radical anion ClBr− exhibits a broad absorption band at 350 nm with ε350=9300 dm3 mol−1 cm−1. The rate constants of the equilibrium Cl2 − + Br−  ClBr−+Cl− are determined to be kf=4×109 dm3 mol−1 s−1 and kb=1.1×102 dm3 mol−1 s−1. The formation of the Cl3 − (λmax=220 nm), Cl2Br− (λmax=230 nm) and ClBr2 − (λmax=245 nm) ions in the radiation-chemical oxidation of Cl− and Br− ions in an aqueous solution was observed by pulse radiolysis, and its mechanisms of appearance and the equilibrium constants were determined.

A pulse radiolysis study of the oxidation of Br– by Cl2˙– in aqueous solution: formation and properties of ClBr˙–

The oxidation of Br− by Cl2˙− is investigated by pulse radiolysis in aqueous solutions of NaCl and NaBr. Depending on the ratio of the concentration of Br− to Cl−, the main product being observed is either Cl2˙−, ClBr˙− or Br2˙−. The mixed radical anion ClBr˙− exhibits a broad absorption band at 350 nm with e350 = 9300 dm3 mol−1 cm−1. The rate constants of the equilibrium Cl2˙− + Br− ⇌ ClBr˙− + Cl− are determined to be kf = 4 × 109 and kb = 1.1 × 102 dm3 mol−1 s−1.

Spectral, Kinetics, and Redox Studies on the Transients Formed on Pulse Radiolysis of Aqueous Solution of (4-Methylthiophenyl)methanol

Abstract Pulse radiolysis technique has been employed to investigate the nature and the redox properties of the transient species generated on radiolysis of aqueous solution of (4-methylthiophenyl)methanol (MTPM). Pulse radiolysis in 1,2-dichloroethane, reaction with specific one-electron oxidants (Cl2•−, SO4•−, Tl2+, Br•) and reaction of •OH radicals in acidic solution showed absorption bands at 320 and 545 nm; these are assigned to solute radical cation with positive charge on the benzene ring. OH-adduct was observed in neutral solution. The oxidation potential for MTPM/MTPM•+ couple is determined to be 1.55 ± 0.04 V vs NHE. eaq− reacts with a bimolecular rate constant of 1.5 × 109 dm3 mol−1 s−1 and the transient absorption bands (λmax = 320, 470 nm) are assigned to the solute radical anion. E0 value for MTPM/MTPM•− couple is determined to be −1.84 ± 0.04 V vs NHE. The redox properties of the transient species formed on reaction with •OH, •H, and O•− have been evaluated.

Equilibrium constant of the reaction ·OH + HNO3 ⇆ H2O + NO3. in aqueous solution

The equilibrium reaction ·OH + HNO3 ⇆ H2O + NO3. was investigated by pulse radiolysis in aqueous solutions. An estimate of the equilibrium constant was derived from the dependence of the optical absorbance due to NO3. radicals upon the concentration of HNO3. In addition, estimates of the forward and reverse rate constants were obtained from the dependence of the formation rate constant on the ratio of the activities of nitric acid and water. Studies were carried out at different values of the dose per pulse and at pH values of 1 and 0. The observations were modeled, taking into account all relevant reactions for the formation and decay of the nitrate radical. From these modeling studies the estimated rate and equilibrium constants were refined. The rate constant of the forward reaction is found to be (8.6 ± 1.3) × 107 L mol−1 s−1, the rate constant of the reverse reaction is found to be (3 ± 1) × 102 L mol−1 s−1, and the equilibrium constant Keq = (2.8 ± 0.4) × 105, all at zero ionic strength. From the latter value of Keq and taking E0(H+, .OH/H2O) = 2.72 V versus NHE, we calculate the reduction potentials E0(H+, NO3./HNO3) = (2.40 ± 0.01) V and E0(NO3./NO3−) = (2.48 ± 0.01) V. This value of the reduction potential for this couple leads to a Henry's law coefficient of KH = 0.018 mol L−1 bar−1 at 298 K for NO3.. This value suggests that the impact of NO3. on atmospheric droplets will be due solely to reactive uptake.

A new estimate of the [formula omitted] radical yield at early times in the radiolysis of liquid water

We have revisited earlier experiments on pulse radiolysis of aqueous solutions carried out at very high dose rates (intensities) up to 5×10 27 eV g −1 s −1 (∼10 12 Gy s −1). The ⋅ OH radical yield G ⋅ OH =4.6±0.25 molec./100 eV, derived from those studies, is here estimated to correspond to the time scale of ∼100 ps where, under these high irradiation-intensity conditions, spur overlap is assumed to be total. This ∼100 ps estimate is obtained with the aid of Monte Carlo simulation results of the nonhomogeneous chemical stage of water radiolysis. It is found that such a G ⋅ OH value at ∼100 ps compares well with the ⋅ OH yield values inferred from inverse Laplace transform analyses of ⋅ OH scavenger data and from Monte Carlo calculations, but strongly disagrees with direct measurements of G ⋅ OH (5.9±0.2 molec./100 eV at 200 ps).