γ-Radiolysis of 5-fluorouracil and 5-fluorouridine derivatives having sulfur-containing substituents

γ-Radiolysis reactions of eight 5-fluorouracil (5-FU) derivatives having sulfonyl group-containing substituents at the 1-position and five 5-fluorouridine (5-FUR) derivatives having thioureido group-containing substituents were studied under the conditions where hydrated electron (e_aq^?) and hydroxyl radical (HO·) become the principal reactive species. The 5-FU and 5-FUR derivatives were radiolyzed to give 5-FU and 5-FUR, respectively. The efficiency of the reactions depended upon the nature of reactive species and also upon the nature of substituents. The reactivity features of the γ-radiolysis reactions are discussed.     

Radical-induced destruction of diethyl phthalate in aqueous solution: kinetics, spectral properties, and degradation efficiencies studies

In this work, the reactions of ^?OH, e _aq ^? , and SO ₄ ^?? with diethyl phthalate (DEP) were investigated in aqueous solution by pulse radiolysis, and degradation efficiencies of DEP with ^?OH and e _aq ^? were evaluated in water using steady-state radiolysis as well. The absolute rate constants of ^?OH, e _aq ^? , and SO ₄ ^?? with DEP were determined as 2.3 × 10⁹, 1.0 × 10¹⁰, and 1.0 × 10⁸ M^?1 s^?1, respectively. The degradation efficiencies for the ^?OH and e _aq ^? reactions were 81 and 33 %, respectively. Transient absorption spectra were observed for the intermediate radicals produced by ^?OH, e _aq ^? , and SO ₄ ^?? reactions. The results suggested that e _aq ^? transferred to the ester group, resulting in the formation of DEP radical anions. In contrast, ^?OH and SO ₄ ^?? added predominantly to the aromatic ring of DEP, forming the corresponding ^?OH adducts. The fundamental mechanistic parameters and degradation efficiencies derived from these results were significant for evaluations and applications of advanced oxidation processes.     

Synthesis and γ-radiolysis of 2′-deoxy-5-fluorouridine and 5-fluorouridine derivatives

Seventeen compounds having a variety of substituents at the 3- and 5′-positions of 2′-deoxy-5-fluorouridine (5-FUdR) and 5-fluorouridine (5-FUR) were synthesized, and their γ-radiolysis in aqueous solutions were studied. The compounds having thioureido (RNHCSNH, R  H, PhCH₂, acyl) and thiocarbonylamino (XCSNH, X  PhCH₂S, PhO) groups at the 3-position of 5-FUdR were efficiently cleaved to give 5-FUdR with high G values upon γ-irradiation of their aqueous solutions. The active species for these cleavage reactions were hydrated electron (e⁻ _aq), H^• and HO^•. However, the compounds having a dimethylsulfoxyimino group at 3-position of 5-FUdR and 5-FUR afforded 5-FUdR and 5-FUR only under the radiolysis conditions where e⁻ _aq becomes a principal active species. The compound having a 2-benzoylthiazoylthiocarbonylamino group at the 3-position of 5-FUdR showed the highest reactivity toward HO^.. The mechanisms of these γ-radiolysis reactions are discussed. The examination of anticellular activities of γ-irradiated compounds having a thiocarbonylamino group at the 3-position of 5-FUdR toward murine Sarcoma 180 cells revealed that these compounds may be utilized as a candidate for a radiation-induced drug (RID).     

Photogeneration of superoxide and decarboxylated peptide radicals by carboquone, mitomycin C and streptonigrin. An electron spin resonance and spin trapping study

Visible light (405–615 nm) excitation of carboquone, mitomycin C, and streptonigrin dissolved in dimethylsulfoxide in the presence of oxygen generates superoxide anion radicals (O₂^?). The quantum yields for these reactions range from 4.2 times 10^?2 (carboquone, λ= 615 ± 10 nm) to 7.3 times 10^?6 (streptonigrin, λ=545 ± 10 nm). O₂^? radicals were spin trapped with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) and identified by electron spin resonance (ESR). The efficiency of DMPO to spin trap O₂^? in dimethylsulfoxide was determined and indicated that 91% of the O₂^? present in dimethylsulfoxide is trapped by DMPO. The oxidation of the photoexcited drug molecules occurs via a direct electron transfer to dissolved oxygen in solution. Ultraviolet irradiation (λ= 313 ± 10 nm) of the aminoquinone drug solutions (80% H₂O, 20% dimethylsulfoxide) in the presence of peptides results in the decarboxylation of the peptides. In this case the photoexcited drugs are reduced, abstracting an electron from the C-terminal carboxyl group of the peptides. The reaction is specific to the C-terminal amino acid of the peptide. The decarboxylated peptide radicals were spin trapped with 2-methyl-2-nitrosopropane (MNP) and identified by ESR.     

Rate constants for the reaction of hydrated electrons and hydroxyl radicals with acrylate monomers

The hydrated electron (e⁻_aq) and hydroxyl radical rate constants with 18 acrylate-, methacrylate-, crotonate-, fumarate- and maleate esters are discussed. The constants approach the diffusion-controlled limit. k(e⁻_aq) and k(OH) change in opposite direction; if k(e⁻_aq) is high then k(OH) is small. This tendency is connected with the nucleophilic character of e⁻_aq and the electrophilic character of OH, although the site of attack of e⁻_aq and OH is different: carbonyl versus vinyl group.     

Redox reactions of transient species formed during pulse radiolysis of 2-pyridine carboxaldehyde and 2-pyridine methanol in aqueous solutions

Rate constants for the reactions of e _aq ^? , H and OH radicals with 2-pyridine carboxaldehyde and 2-pyridine methanol have been determined by pulse radiolysis technique. Reactions of reducing radicals such as acetone ketyl radicals and CO₂ ^? with these compounds were also evaluated at various pHs. The species produced by the reaction of reducing radicals with these solutes was a strong reductant itself. While pyridinyl were produced in the case of 2-pyridine methanol, one-electron reduction of 2-pyridine carboxaldehyde led to the formation of PyCHOH radical. The one-electron reduction potential of PyCHOH radicals was estimated by establishing an equilibrium with MV⁺ radical cations to be ?0.6V vs NHE. OH radical reaction with 2-pyridine carboxaldehyde gave an OH adduct, while in the case of 2-pyridine methanol, OH radicals reacted partly by H-abstraction from the ?CH₂OH group. SO₄ ^? radical reaction with 2-pyridine carboxaldehyde produced a species which was reducing in nature. The rate constants for the reaction of e _aq ^? and OH radicals are compared with similar values obtained in the case of other 2-pyridine derivatives to see if there is any electron-inductive effect.     

PHOTOLYSIS MECHANISM OF AQUEOUS TYROSINE AND TYROSYL PEPTIDES*

Abstract— Laser flash photolysis at 265 nm has been employed for measuring the initial hydrated electron (e^-_aq) and p-alanylphenoxyl radical (Tyr) in aqueous Tyr, small Tyr peptides and R Nase A. The results indicate that monophotonic photolysis not involving the fluorescent or triplet states is the principal initial process. Equivalent yields of e and Tyr were found in all cases except Tyr, where the Tyr yield was 60% higher than e^-_aq attributed to splitting of the phenolic bond. Computer analysis of e^-_aq and Tyr decays for Tyr indicates the importance of electron-radical recombination in competition with electron scavenging and bimolecular radical-radical reactions. Evidence for intramolecular electron migration has been obtained in cystinyl-bis-Tyr.     

The kinetics of radical reactions in the radiolysis of aqueous solutions of organic nitro compounds

Pulsed radiolysis and computer simulation of gamma radiolytic decomposition of organic nitrates in aqueous solutions were performed to determine the rate constants for reactions with the participation of intermediates determining the mechanism of the process. 2,4,6-Trinitrotoluene, 2,4-dinitrotoluene, and cyclic nitramine, cyclotrimethylene-trinitramine, were used as substrates. The bimolecular rate constants for the reactions of hydrated electrons (e _aq ^? ) and hydroxyl radicals (^?OH) with the substrates and constants for the recombination of electron adducts and carbon-centered radicals (the products of the detachment of the H atom from the nitro compound molecule by the OH radical) were determined by direct measurements with the use of high-speed spectrophotometry. Computer simulation of the reaction scheme was used to estimate the rate constant for significant reactions, monomolecular forward and back reactions of electron adducts and electron transfer to molecular oxygen, and refine the rate constant for the reaction of e _aq ^? with tert-butanol.     

DAMAGE TO URACIL- AND ADENINE-CONTAINING BASES, NUCLEOSIDES, NUCLEOTIDES AND POLYNUCLEOTIDES: QUANTUM YIELDS ON IRRADIATION AT 193 AND 254 NM

Abstract Photoreactions, such as base release and decomposition of the base moiety, induced by either 20 ns laser pulses at 193 nm or continuous 254 nm irradiation, were studied for a series of uracil and adenine derivatives in neutral aqueous solution. The quantum yield of chromophore loss (φ) depends significantly on the nature of the nucleic acid constituent and the saturating gas (Ar, N₂O or O₂). In the case of polynucleotides the destruction of nucleotides was measured by high-performance liquid chromatography after hydrolysis; the quantum yields (φ) are comparable to those of chromophore loss or larger. The φ_cl and aφ_dn of 0.04–0.1 for poly(U) and poly(dU), obtained for both wavelengths of irradiation, are due to processes originating from the lowest excited singlet state, i.e. formation of photohydrates and photodimers, and a second part from photoionization using λ_irr= 193 nm. Irradiation at 193 nm effectively splits pyrimidine dimers and thus reverts them into monomers. The quantum yield for release of undamaged bases (φ_br) from nucleosides, nucleotides and polynucleotides upon irradiation at 254 nm is typically φ_br= (0.1–1) × 10^?4 Breakage of the N-glycosidic bond is significantly more efficient for λ_irr=193 nm, e.g. φ_br= 1.1 × 10^?3, 0.8 × 10^?3, 4.3 × 10^?3 and 0.5 × 10^?3 for poly(A), poly(dA), poly(U) and poly(dU) in Ar-saturated solution, respectively. Enhanced φ values for λ_irr= 193 nm, essentially for adenine and its derivatives, are caused by photo-processes that are initiated by photoionization.     

Redox reactions of 2-hydroxy pyridine: A pulse radiolysis study

Rate constants for reactions of 2-pyridinol with one electron reductants, such ase _aq ⁻ and H atoms and one-electron oxidants, viz. OH, N₃, Br ₂ ⁻ , C1 ₂ ⁻ and O⁻ have been determined at different pH values using the pulse radiolysis technique. From the corrected absorption spectra of the product transient species, the extinction coefficients of these species at their respective absorption maxima have been determined. The kinetics of decay of these transients have been investigated. ThepK _a values of transients formed bye _aq ⁻ and OH radical reactions have been estimated to be 7.6 and 3.5 respectively. Rate constants for electron transfer from semireduced 2-pyridinol to different electron acceptors have been determined.     

Kinetic and spectral properties of the intermediates formed in the pulse radiolysis of 2-mercaptobenzimidazole

The reactions of primary species of water radiolysis such as e^- _aq, H* and *OH, and some specific one electron reductants and oxidants with 2-mercaptobenzimidazole have been studied at various pHs. *OH radical reaction with MBZ at pH 7 gave a transient species having absorption maxima (λ_max) at 330 and 590 nm. The transient species (pK_a = 3.6) was found to be neutral at this pH and was a mild oxidant. The initial transient species formed by the reaction of MBZ with e^{- aq} at pH 7 and with H atom at pH 0 were found to react with the parent molecule to form another transient species which has an absorption spectrum similar to that obtained by *OH radical reaction with λ_max at 590 nm. However the reaction is not quantitative. The kinetic, spectral, acid-base and redox properties of the transient species are reported.     

Residual photocurrent at a mercury electrode

The residual photocurrent observed at negative potentials with mercury in contact with slightly alkaline 1 M KCl solution has been reinvestigated and found to be caused partly by the reaction of e^?_aq with traces of unidentified electron acceptors and partly by reaction with the solvent. Such a solution can be freed from electron-accepting impurities by irradiation with light of shorter wavelength than 270 nm if SO^2?₃ is present at low concentration. The time constant for the homogeneous decay of e^?_aq at pH ca. 8.8 is then raised to about 400 μs. This slow decay is due to reaction with the solvent as the decay time constant can be increased considerably by raising the pH of the solution. In unsteady state experiments with purified 1 M KCl solutions of high pH containing no added scavenger for e^?_aq the current connected with electron emission and e^?_aq diffusion towards the electrode and the bulk of the solution, is cancelled out at ca. ?1.4 V vs. SCE by a thermal non-faradaic current component when 360 nm light is employed. When CO₂ is present at high concentration a similar cancellation is observed at lower pH at ca. ?1.1 V vs. SCE, suggesting a value for the diffusion coefficient of the radical-ion CO^?₂ about 3.3 times smaller than that of e^?_aq.     

New Mechanistic Aspects of the Fenton Reaction

The kinetics of the Fenton reaction was studied in detail. A second reaction step in the presence of excess H₂O₂ is attributed to formation of the complex Fe^III(^?O₂H)_aq. Therefore, the reaction of Fe(H₂O)₆²⁺ with Fe^III(^?O₂H)_aq in the presence of Fe^II to form Fe^III_aq (k=(7.7±1.5)×10⁵ M ^?1 s^?1) may contribute to the overall Fenton reaction, and could account for some of the debate in the literature concerning its detailed mechanism. If this is correct for LFe^III(^?O₂H)_aq also, then it might be of significant biological importance. The activation parameters ΔH^≠, ΔS^≠, and ΔV^≠ for the Fenton reaction were measured under various experimental conditions, and are used in the mechanistic interpretation.     

Solvated positron chemistry. II. The reaction of hydrated positrons with Cl−, Br− and I− ions

The reaction of the hydrated positron, e_aq⁺ with Cl^?, Br^?, and I^? ions in aqueous solutions was studied by means of positron The measured angular correlation curves for [Cl^?, e⁺], [Br^?, e⁺, and [I^?, e⁺] bound states were in good agreement with th Because of this agreement and the fact that the calculated positron wavefunctions penetrate far outside the X^? ions in the [X^?, e⁺] sta propose that a bubble is formed around the [X^?, e⁺] state, similar to the Ps bubble found in nearly all liquids. F^?ions did not react w Preliminary results showed that CN^? ions react with e_aq⁺ while OH^?ions are non reactive. The rate constants were 3.9 × 10¹⁰ M^?1 s^?1, 4.4 × 10¹⁰ M^?1 s^?1, and 6.3 × 10¹⁰ M^?1 s^?1 for Cl^?, Br^?, and I^?, respectively, at low (? 0.03 M) X^? concentrations. A 25% decrease in the rate constant caused by the addition of 1 M ethanol to the I^? solutions was i The influence of halide ions on the positronium (Ps) yields in pure water was studied by use of lifetime measurements. The Cl^?, Br^?, and I^? ions reduced the Ps yields at low concentrations (? 0.03 M), while F^? ions only reduced the Ps-yield However, the Ps yields saturated (e.g. at ≈ 21% ortho-Ps yield in the Cl^? case) at higher concentrations. This saturation and the high-concentration effects-in the angular correlation results were interpreted as caused by rather complicated spur effects, wh It is proposed that spur electrons may pick off the positron from the [X^?, e⁺ states with an efficiency which depends on the structure of the     

eaq− hydration energy and photoemission threshold potentials for mercury in contact with aqueous electrolytes

The meaning of the “red limit” potential in photoemission experiments is discussed. For mercury in contact with aqueous electrolytes, the energy of a photoemitted electron at the “red limit” is 0.6 eV higher than the solvation energy of e_aq^?. This difference is attributed to the solvent reorganization energy contribution to the hydration energy of e_aq^?.     

Thermodynamic Studies of Bi-Univalent Electrolytes. VI. Thermodynamics of Cadmium Acetate from E. M. F. and Calorimetric Measurements

E. M. F. of the Cell, Cd-Hg (2-phase)/CdAc₂(m), Hg₂Ac₂(s)/Hg was measured at 20°, 25°, 30° and 40°C. The standard e. m. f. of the cell, Cd/CdAc₃(m), Hg₂Ac₂(c)/Hg was evaluated as E°=1.1500?11.09×10^?4T+1.06×10^?8T² The thermodynamic data of the reaction, Cd(c) + Hg₂Ac₂(c)=2Hg(l)+Cd⁺⁺(aq)+2Ac^?(aq) at 25°C were estimated as ΔF°=?42,139, ΔH°=?48,698 cal mole^?1 and ΔS°=?22.0 cal deg^?1 mole^?1 at 25°C. The thermodynamic data for the formation of Hg₂Ac₂(s) were evaluated as ΔF_f°=?202.3, ΔH_f°=?154.5 Kcal mole^?1 and S°=72.9 cal deg^?1 mole^?1. From measurements of the heats of solution of CdAc₂·2H₂O in aqueous solution, the relative partial molal enthalpies of cadmium acetate in aqueous solution were estimated.     

Kinetics of the radiation chemical reactions of tervalent and quadrivalent actinides and lanthanides in carbonate solutions

The reactions of the ions of tervalent and quadrivalent actinides and lanthanides with hydrated electrons e_aq ^– and CO₃ ^– radicals in concentrated carbonate solutions have been studied by microsecond pulse radiolysis, using spectrophotometric recording of short-lived particles. It is shown that the rate of the reactions of e_aq ^– with carbonato complexes of Ce(IV), Pu(IV), and Np(IV) is controlled by diffusion. The carbonato complex of Eu(III) reacts with e_aq ^–appreciably more slowly. A linear relationship is obtained between the logarithm of the rate constant for the reactions of CO₃ ^– with the carbonato complexes of Am(III), Ce(III), and Pu(III) and the redox potential of the complexes. The rate of the reaction of CO₃ ^– with Np(III) in carbonate solutions is controlled by diffusion.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 28–32, January, 1990.     

The scavenging of the precursors of the solvated electron from the positron lifetime spectroscopy and the doppler broadening of annihilation line shape technique

The electron scavenging properties of aqueous solutions of two series of solutes are investigated, using the positron as a probe. For a better interpretation of the data, both lifetime spectroscopy and the Doppler broadening of annihilation line shape technique are used. All solutes inhibit the positronium (Ps) formation, by the scavenging of electrons. The first series consists of the halate ions, that should follow the Hunt linear relation between the rate constant for reaction with the solvated electrons, k_(e^?aq+S), and that for its precursor(s), 1/C₃₇. The Ps inhibition constants, k, are 0.14, 1.44 and 3.45 M^?1 for ClO^?₃, BrO^?₃ and IO^?₃, respectively. This sequence is quantitatively consistent with that of the respective k_(e^?eq+S). The second series includes the SeO⁼₄, Te(OH)₆ and BrO^?₄ species, and the Ps inhibition constants are 5.62, 10.5 and 14.3 M^?1 respectively. These values are much higher than expected from the k_(e^?aq+S) constants, on basis of the Hunt relation, in agreement with previous results from pulse radiolysis experimets.     

Kinetics of hydrated electron reactions with phosphate anions: a laser photolysis study

The decay kinetics of hydrated electron (e_aq ⁻) formed upon photolysis of aqueous solutions of sodium pyrene-1,3,6,8-tetrasulfonate at λ = 337 nm in the presence of phosphate anions (up to 2 mol L⁻¹) was studied by nanosecond laser-pulse photolysis in a wide range of pH (3.5–10) and ionic strength (I, up to 2 mol L⁻¹) values. At high pH values, where the HPO₄ ²⁻ ions dominate, the e_aq ⁻ decay kinetics depends only slightly on phosphate concentration (rate constant for the reaction is at most 2·10⁵ L mol⁻¹ s⁻¹). The H₂PO₄ ⁻ ions react with e_aq ⁻ at a rate constant of 2.8·10⁶ L mol⁻¹ s⁻¹ (I = 0), which increases linearly with the parameter in accordance with the Debye-Hückel theory. The rate constant for quenching of e_aq ⁻ by H₃PO₄ at pH ≤ 4 decreases linearly with the parameter due to the secondary salt effect and equals 1.6·10⁹ L mol⁻¹ s⁻¹ at I = 0. The logarithm of the rate constant for quenching of e_aq ⁻ by phosphates is linearly related to the number of the O-H bonds in the phosphate molecule. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1277–1280, July, 2007.     

Muonium atoms compared to hydrogen atoms and hydrated electrons through reactions with nitrous oxide and 2-propanol

N₂O and 2-propanol constitute a pair of solutes with completely opposite relative reactivities towards H and e^-_aq. It is observed in the present study that muonium atoms (Mu) fall directly between H and e^-_aq for both of these reactions. Aspects of mass-dependent processes and kinetic isotope effects are discussed with regard to these findings.