Pulse radiolysis study of one-electron oxidation of thionine in aqueous solutions

One-electron oxidation of thionine has been studied using specific oxidizing radicals such as Cl⁻Tl(II) and N₃ generated by pulse radiolysis of aqueous solutions. The semioxidized thionine exhibited threepK’s indicating four conjugate acid-base forms. N₃ radicals were found to be less efficient in oxidizing thionine as compared to Cl ₂ ⁻ , Tl²⁺ and Tl(OH)⁺. The rate constants for electron abstraction from thionine by Cl ₂ ⁻ , Tl²⁺, Tl(OH)⁺, Tl(OH)₂ and N₃ were evaluated. The spectra of different protonated forms of semioxidized thionine and the extinction coefficients at λ_max are presented. Reaction of OH radicals with thionine gave transient products whose spectra and acid-base properties were different from those of semioxidized thionine. The rate constant for formation of the product transient agrees well with competition kinetic value for reaction of OH with thionine reported earlier.     

Pulse radiolysis study of initiation and propagation in radiation induced polymerisation of cyclohexyl methacrylate

Initiation and individual propagation rate constants for the first few steps are determined for e^-_aq, OH radical and H atom reactions with cyclohexyl methacrylate as the monomer using the technique of pulse radiolysis. Though the rate constants for initiation are higher, the propagation rate constants for the transient species formed by reaction of OH radical and H atom are lower compared to that by reaction of e^-_aq with the monomer. It is also observed that in the case of e^-_aq reaction the first propagation step is about an order of magnitude faster than the second propagation step and propagation through radical-anionic form is much faster than its radical counterpart.     

Pulse radiolysis study of redox reactions of safranine T in sodium dodecylsulphate (SDS) micellar medium

Reaction of hydrated electrons with safranine T (SF⁺), a phenazine dye useful as sensitizer in photogalvanic cell and the transient semireduced species formed by this reaction have been studied in SDS micellar medium using the technique of pulse radiolysis. Thee _aq ⁻ reaction with SF⁺ in the micellar environment was only marginally slower (5.1 × 10⁹ dm³ mol⁻¹ s⁻¹) as compared to that in homogeneous aqueous medium (2.2 × 10¹⁰ dm³ mol⁻¹ s⁻¹) explicable on the basis of our finding that although a large fraction of the dye gets localized near the micelle Stern layer where the molecule experiences a dielectric.constant of ≈40, a small but significant concentration of the dye exists in the aqueous bulk as charge pair complex with the anionic surfactant monomer (association constant for the formation of the complex being 2.8 × 10⁴ dm³ mol⁻¹). The transient semireduced absorption band observed in the micellar medium showed a red shift of ≈ 50 nm and also the decay of the transient, which was very fast with 2k = 1 × 10⁹ dm³ mol⁻¹ s⁻¹ in aqueous medium, was stable in the SDS micellar medium over a few tens of milliseconds suggesting that the radical is incorporated deeper than the parent molecule in the SDS micelle. The effect of this stability on the photogalvanic conversion needs to be examined.     

Reactions of oxidizing radicals with 2- and 3-aminopyridines: a pulse radiolysis study

Reactions of OH radicals and some one-electron oxidants with 2-aminopyridine (2-AmPy) and 3-aminopyridine (3-AmPy) were studied in aqueous solutions using pulse radiolysis technique. The OH adduct of 2-AmPy at pH 9 has an absorption maximum at 360 nm along with a weak absorption band in the visible region and was found to be reactive with oxygen. The rate constant for its reaction with O₂ was determined to be 1.0×10⁸ dm³ mol⁻¹ s⁻¹. At pH 4 also, the OH adduct of 2-AmPy has an absorption band at 360 nm. However, there are differences in the absorption at other wavelengths. From the plot of ΔOD vs. pH at 340 nm, the pK_a of the OH adduct was determined to be 6.5. Among the specific oxidants, only SO₄^−√ radicals were able to oxidize 2-AmPy. In the case of 3-aminopyridine (3-AmPy), the transient species formed by OH radical reaction at pH 9 has an absorption maximum at 410 nm with shoulder bands on both the sides. Its absorption spectrum at pH 4 was different indicating the existence of a pK value for the OH adduct. pK_a of 3-AmPy-OH radical adduct species was evaluated to be 5.7. This adduct species was also found to be reactive with oxygen (k=7.6×10⁶ dm³ mol⁻¹ s⁻¹). Specific one-electron oxidants like N₃^√, Br₂^−√ C₂^−√ and SO₄^−√ were able to oxidize 3-AmPy indicating that it is easier to oxidize 3-AmPy as compared to 2-AmPy.     

Pulse radiolysis of 2-[(methacryloyloxy)ethyl]trimethylammonium chloride in aqueous solution

The early stages of the polymerization of 2-[(methacryloyloxy)ethyl]trimethyl ammonium chloride (MADQUAT) in aqueous solution were studied by pulse radiolysis. The rate constant of the reaction of this monomer with OH√ and hydrated electron was measured as 1.5×10¹⁰ and 2.2×10¹⁰ dm³mol⁻¹s⁻¹, respectively. The absorption spectrum of the OH√ adduct to MADQUAT molecule appeared to have a maximum at 290 nm. Its decay depends on the monomer concentration. The shape and maximum of the absorption spectrum of electron adduct to the monomer depend on pH of the solution. In acidic solution, radical anion is formed. It subsequently undergoes reversible protonation giving spectrum with a maximum below 260 nm. In basic solution the spectrum has a maximum at 290 nm. In this case irreversible protonation takes place at the β-carbon atom, giving an -centred carbon radical.     

Pulse radiolysis studies of poly(methyl methacrylate) containing pyrene

A pulse radiolysis study of poly(methyl methacrylate) in the presence of pyrene has been carried out in the temperature range 100–295 K. The concentration of pyrene was changed from 10⁻³ to 10⁻¹ mol dm⁻³. The absorption/emission spectra and kinetics of solute excited states and solute radical ions were investigated. It was found that pyrene excited states were formed as a result of their radical ion recombination in a time scale up to seconds. The decay of solute radical ions was influenced by photobleaching and can be described by a time-dependent rate constant. The activation energy of Py ions decay was temperature dependent and was equal to 35.7 and 1.2 kJ/mol for temperatures >T_γ and <T_γ, respectively, where T_γ ∼ 175 K represented the transition temperature responsible for γ-relaxation. The reaction mechanism was proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1209–1215, 1998     

A pulse radiolysis study of oil/water microemulsions

Absorption spectrum and the yield of e_aq⁻ determined in quaternary benzene/water and dodecane/water microemulsions were found to be identical with those in pure water. This indicates that the excess electrons produced in the oil droplets cross the oil/water interface and appear in the aqueous phase as hydrated electrons. On the contrary, it was found that the OH radical yield measured by converting into (SCN)₂^.− was directly proportional to the water content and there was no contribution from the oil phase. The e_aq⁻ decay in aerated microemulsion showed that the [O₂] in the aqueous phase was decreasing with continuous pulse irradiation and the estimated G(–O₂) in the oil was higher than in water. However, peroxides were predominantly formed in the aqueous phase and almost not in the oil. Rate constants for the addition of e_aq⁻ and OH to benzene in the benzene/water microemulsion and the following bimolecular decay of cyclohexadienyl and hydroxycyclohexadienyl radicals agree well with those reported in aqueous solutions.     

One-electron oxidation of toluidine blue. A pulse radiolysis study

The one-electron oxidation of toluidine blue by specific oxidising radicals such as Cl ₂ ⁻ , Tl(II), N₃, Br ₂ ⁻ etc. has been studied by employing the pulse radiolysis technique. The Br ₂ ⁻ radical was found to be less efficient in oxidising toluidine blue as compared to Cl ₂ ⁻ ., Tl⁺² and N₃. The semioxidised species exhibited only onepK _a indicating the presence of two conjugate acid-base forms whose spectral and kinetic features were evaluated. Reaction of OH radicals with the dye gave rise to a transient species which exhibited spectral and kinetic features different from that of the semioxidised species indicating that the mode of reaction of OH is different.     

Pulse radiolysis study of ion-species effects on the solvated electron in alkylammonium ionic liquids

The spectra and kinetic behavior of solvated electrons (e_sol⁻) in alkyl ammonium ionic liquids (ILs), i.e. N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEMMA-TFSI), N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEMMA-BF₄), N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI), N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13-TFSI), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P13-TFSI), and N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P14-TFSI) were investigated by the pulse radiolysis method. The e_sol⁻ in each of the ammonium ILs has an absorption peak at 1100 nm, with molar absorption coefficients of 1.5–2.3×10⁴ dm³ mol⁻¹ cm⁻¹. The e_sol⁻ decayed by first order with a rate constant of 1.4–6.4×10⁶ s⁻¹. The reaction rate constant of the solvated electron with pyrene (Py) was 1.5–3.5×10⁸ dm³ mol⁻¹ s⁻¹ in the various ILs. These values were about one order of magnitude higher than the diffusion-controlled limits calculated from measured viscosities. The radiolytic yields (G-value) of the e_sol⁻ were 0.8–1.7×10⁻⁷ mol J⁻¹. The formation rate constant of e_sol⁻ in DEMMA-TFSI was 3.9×10¹⁰ s⁻¹. The dry electron (e_dry⁻) in DEMMA-TFSI reacts with Py with a rate constant of 7.9×10¹¹ dm³ mol⁻¹ s⁻¹, three orders of magnitude higher than that of the e_sol⁻ reactions. The G-value of the e_sol⁻ in the picosecond time region is 1.2×10⁻⁷ mol J⁻¹. The capture of e_dry⁻ by scavengers was found to be very fast in ILs.     

Pulse radiolysis study on the free radical scavenging activities of puerarin

Free radical theory of biology and medicine suggests that free radicals are involved in the happening of many chronic diseases including cancers and cardiovascular diseases, and natural antioxidants play positive effect in the prevention of such diseases[1]. This theory leads to much interest in the free radical scavenging activities of the pharmacologically active components in herbal medicines. Puerarin (fig. 1), a compound of isoflavone, is isolated from Chinese herbal medicine radix puera…     

A pulse radiolysis study of free radicals formed by one-electron oxidation of the antimalarial drug pyronaridine

Free radicals from one-electron oxidation of the antimalarial drug pyronaridine have been studied by pulse radiolysis. The results show that pyronaridine is readily oxidised to an intermediate semi-iminoquine radical by inorganic and organic free radicals, including those derived from tryptophan and acetaminophen. The pyronaridine radical is rapidly reduced by both ascorbate and caffeic acid. The results indicate that the one-electron reduction potential of the pyronaridine radical at neutral pH lies between those of acetaminophen (707 mV) and caffeic acid (534 mV). The pyronaridine radical decays to produce the iminoquinone, detected by electrospray mass spectrometry, in a second-order process that density functional theory (DFT) calculations (UB3LYP/6-31+G*) suggest is a disproportionation reaction. Important calculated dimensions of pyronaridine, its phenoxyl and aminyl radical, as well as the iminoquinone, are presented. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Pulse radiolysis study of I− oxidation with radical anions Cl2 ⋅− in an aqueous solution

Radiation-chemical transformations of chloride solutions in the presence of iodide additives were studied by pulse radiolysis. Radical anion Cl₂ ^⋅− oxidize I⁻ ion, while in the secondary reactions Cl₂ reacts with I⁻ to form a mixed trihalide ion ICl₂ ⁻. A reaction model that satisfactorily describes the experimental data was proposed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1336–1340, June, 2005.     

Pulse radiolysis study on the mechanisms of reactions of CCl_3OO radical with quercetin, rutin and epigallocatechin gallate

The mechanisms of reactions between CCI3OO radical and quercetin, rutin and epigal-locatechin gallate (EGCG) have been studied using pulse radiolytic technique. It is suggested that the electron transfer reaction is the main reaction between CCI3OO radical and rutin, EGCG, but there are two main pathways for the reaction of CCI3OO" radical with quercetin, one is the electron transfer reaction, the other is addition reaction. The reaction rate constants were determined. It is proved that quercetin and rutin are better CCI3OO radical scavengers than EGCG.     

Effect of electron-withdrawing power of the substituted group on?OH radical reaction with substituted aryl sulphides: A pulse radiolysis study

In neutral aqueous solution of (phenylthio)acetic acid, hydroxyl radical is observed to react with a bimolecular rate constant of 7.2 × 10^-1 dm³ mol⁻s⁻ and the transient absorption bands are assigned to^•OH radical addition to benzene and sulphur with a rough estimated values of 50 and 40% respectively. The reaction of the^•OH radical with diphenyl sulphide (k = 4.3 × 10⁸ dm³ mol⁻¹ s⁻¹) is observed to take place with formation of solute radical cation, OH-adduct at sulphur and benzene with estimated values of about 12, 28 and 60% respectively. The transient absorption bands observed on reaction of^•OH radical, in neutral aqueous solution of 4-(methylthio)phenyl acetic acid, are assigned to solute radical cation (λ_max = 550 and 730 nm), OH-adduct at sulphur (λ_max = 360 nm) and addition at benzene ring (λ_max = 320 nm). The fraction of^•OH radical reacting to form solute radical cation is observed to depend on the electron-withdrawing power of substituted group. In acidic solutions, depending on the concentration of acid and electron-withdrawing power, solute radical cation is the only transient species formed on reaction of^•OH radical with the sulphides studied.     

Pulse radiolysis study on the initial processes of radiation-induced cationic polymerization of styrene and α-methylstyrene

Initial processes of radiation-induced cationic polymerization of styrene and α-methylstyrene have been studied by means of microsecond pulse radiolysis. For styrene, absorption bands caused by the monomer cation radical St⁺? appear at 630 and 350 nm in a mixture of isopentane and n-butyl chloride at about ?165°C. In parallel with the decay of St⁺?, three absorption bands appear in the near-infrared (IR) region, and at 600 and 450 nm. The IR and 600 nm bands are assigned to the associated dimer cation radical St₂⁺?, and the 450 nm band to the bonded dimer cation radical St-St⁺?. The kinetic behavior of these species shows that reaction of St⁺? with styrene monomer forms both St₂⁺? and St-St⁺?. With the decay of St-St⁺?, another absorption band appears at 340 nm, and the lifetime of this band is relatively long. The 340 nm band may be due to carbonium ions of the growing polystyrene. For α-methylstyrene, the monomer cation radical (at 690 and 350 nm), the associated dimer cation radical (in the near-IR region and at 620 nm) and the bonded dimer cation radical (at 480 nm) behave in a manner similar to that of the corresponding styrene species. The absorption band caused by carbonium ions of growing poly(α-methylstyrene) appears at 340 nm.     

Reaction of H with H2O2 as observed by optical absorption of perhydroxyl radicals or aliphatic alcohol radicals and of OH with H2O2. A pulse radiolysis study

Two new procedures were employed for studying the reaction of hydrogen atoms with hydrogen peroxide. The absorption in the UV-range was observed either for an acidic aqueous solution containing only hydrogen peroxide or for a similar solution but also containing an aliphatic alcohol. From the increase in absorption of various alcohol radicals, a rate constant of 3.5×10⁷ dm³ mol⁻¹ s⁻¹ was determined. In addition, the rate constant for the reaction of hydroxyl radicals with hydrogen peroxide was determined to be 3.0×10⁷ dm³ mol⁻¹ s⁻¹.     

Pulse radiolysis study of trapped electron in MgSO4.7H2O single crystal

The single crystal of MgSO₄.7H₂O is very interesting to study the structure and kinetics of trapped electron in solids, because the hydrated salt has many H₂O molecules in the crystal lattice like water. The pulse radiolysis study of the trapped electrons in MgSO₄.7H₂O single crystal was carried out based on an electron linear accelerator. Two optical absorption peaks of the trapped electron in MgSO₄.7H₂O single crystal were observed: one is at the wavelength of 600 nm with a short lifetime of 81.4 ns; another is at 400 nm with a long lifetime of 1.06 μs. The kinetics of the trapped electron in the crystal was also compared with that in the aqueous solution.     

ESR study of low-temperature radiolysis and photolysis of substitutedN′-furfurylidenebenzhydrazides

Low-temperature (77 K) γ- and UV irradiation of substitutedN′-furfurylidenebenzhydrazides produces paramagnetic particles in radiation-chemical yields of (0.05–0.5)/100 eV and quantum yields of 4·10⁻⁵–10⁻³, respectively. ESR study showed that hydrazyl radicals and HC·O and N·O₂ are the main products of radiolysis and photolysis, and the latter decay upon heating of the sample to 190 K, whereas the hydrazyl radicals survive up to 423 K. Further heating results in thermodestruction of the hydrazides, and the ESR spectrum exhibits only a singlet with splitting at 1 mT, which is characteristic of polyconjugated compounds. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2078–2082, November, 1999.