Pulse Radiolysis Of Aqueous Solutions Research Articles

Singlet oxygen quenching and the redox properties of hydroxycinnamic acids

The singlet oxygen quenching rate constants (k q) for a range of hydroxycinnamic acids in acetonitrile and D 2O solutions were measured using time resolved near infrared phosphorescence in order to establish their antioxidant activity. The magnitude of k q observed depends on both the nature of the substituent groups and solvent polarity. The variations in k q depend on the energy of the hydroxycinnamic acid/molecular oxygen charge transfer states, (O 2 δ−...HCA δ+). In D 2O the values of k q range from 4 × 10 7 M −1 s −1 to 4 × 10 6 M −1 s −1 for caffeic acid and o-coumaric acid respectively. In acetonitrile, the charge transfer energy levels are raised and this is reflected in lower singlet oxygen quenching rate constants with a k q value of 5 × 10 6 M −1 s −1 for caffeic acid. The phenoxyl radical spectra derived from the hydroxycinnamic acids were determined using pulse radiolysis of aqueous solutions and the reduction potentials were found to range from 534 to 596 mV. A linear correlation is observed between reduction potential, and hence free energy for electron transfer, and log k q. These correlations suggest a charge transfer mechanism for the quenching of singlet oxygen by the hydroxycinnamic acids.

Investigations on the nature and redox properties of the transients formed on pulse radiolysis of aqueous solutions of 2-(phenylthio)ethanol

The radical cation of 2-(phenylthio)ethanol (PTE)+, generated on reaction of specific one-electron oxidants Cl2-, Tl2+, SO4-, CCl3OO, Br2-, by pulse radiolysis in aqueous solutions of PTE exhibits absorption bands at 315 and 530 nm. Pulse radiolysis of PTE in 1,2-dichloroethane also produced a similar transient absorption spectrum. The hydroxyl radicals are observed to react with a bimolecular rate constant of 7.5×109 d mol-1 s-1 and form absorption bands at 300, 365 and 530 nm. While the 530 nm band decayed by first order kinetics with k=2.1×104 s-1, other bands showed mixed kinetics. O- reacts exclusively by H atom abstraction forming a transient absorption band in 290–330 nm region, H reacts both by H abstraction and H-adduct formation. Based on these studies, OH radicals are inferred to react by electron transfer, H abstraction and OH-adduct formation. The radical cation reacts with electron donors, I-, N3-, with a high rate constant value. In neutral solutions, eaq- reacts with a bimolecular rate constant of 7.1×108 d mol-1 s-1 and the transient absorption band at 360 nm (ε=1.4×103 d mol-1 cm-1) is assigned to H-adduct formed on protonation of radical anion (pKa=7.9). In basic solutions, the radical anion has very small absorption at 360 nm (ε=0.6×103 d mol-1 cm-1). The radical anion formed on reaction of eaq- with PTE at pH 12 is able to transfer an electron to MV2+ whereas at neutral pH, electron transfer is not possible. The reduction potential for the PTE/PTE- couple is determined to be -1.23 V.

Oxidation of α-pinene by OH•, Cl• and Cl2 •− radicals Error! Reference source not found

The rate constants of the reactions of the radicals OH• and Cl2•− with α-pinene determined by pulse radiolysis in aqueous solution are given. The reaction of Cl• with α-pinene has been studied by laser photolyis in CCl4 solution. Furthermore, the rate constants of the reactions of the formed organic radicals with oxygen as well as those of the deactivation of all radicals were determined.

Transients and stable radical from the deamination of α-alanine

The objects of investigation were single crystals of L-α-alanine, in which radical anion CH3 C˙H CO 2 − has been formed by radiation induced deamination of alanine. Previously, this stable radical has been spectrally identified (λmax, e=1100 M−1·cm−1), and its characteristics have found to be identical with characteristics of the same radical obtained by pulse radiolysis in aqueous solution. The mechanism of radical formation in the solid state is not known. Time resolved pulse radiolysis of single crystal alanine has shown more complicated way of the formation of the same radical in solid state than in aqueous solution. The electrons abstracted from the solid alanine molecule neutralise positive of zwitter-ion alanine. Ammonia is leaving the reaction-complex in time of milliseconds, leaving the stable radical anion.

Cobalt Porphyrin Catalyzed Reduction of CO2. Radiation Chemical, Photochemical, and Electrochemical Studies

Several cobalt porphyrins (CoP) have been reduced by radiation chemical, photochemical, and electrochemical methods, in aqueous and organic solvents. In aqueous solutions, the CoIP state is stable at high pH but is shorter lived in neutral and acidic solutions. Stable CoIP is also observed in organic solvents and is unreactive toward CO2. One-electron reduction of CoIP leads to formation of a species that is observed as a transient intermediate by pulse radiolysis in aqueous solutions and as a stable product following reduction by Na in tetrahydrofuran solutions. The spectrum of this species is not the characteristic spectrum of a metalloporphyrin π-radical anion and is ascribed to Co0P. This species binds and reduces CO2. Catalytic formation of CO and HCO2- is confirmed by photochemical experiments in acetonitrile solutions containing triethylamine as a reductive quencher. Catalytic reduction of CO2 is also confirmed by cyclic voltammetry in acetonitrile and butyronitrile solutions and is shown to occur ...

Reaction of the superoxide radical with the N-centered radical derived from N-acetyltryptophan methyl ester

Hydroxyl radicals, solvated electrons and H atoms are generated by pulse radiolysis in aqueous solutions of N-acetyltryptophan methyl ester (AM-Trp). The solvated electrons are converted with N2O into further OH radicals and the latter with azide into N3 radicals which oxidize AM-Trp to its N-centered radical (AM-TrpN˙). It is characterized by a strong absorption at 510 nm (e = 1830 dm3 mol–1 cm–1). The bimolecular decay of the radicals (2k = 7.3 × 108 dm3 mol–1 s–1) is not affected by O2 [k(AM-TrpN˙ + O2) < 105 dm3 mol–1; 2 × 103 dm3 mol–1 s–1 deduced from other data]. In the presence of O2, and when the majority of the OH radicals are converted with formate into superoxide radicals, O2˙–, decay of the AM-TrpN˙ radicals follows first-order kinetics [k(AM-TrpN˙ + O2˙–) = 1.2 × 109 dm3 mol–1 s–1]. In O2-saturated azide-containing solutions [G(AM-TrpN˙) = 2.9 × 10–7 mol J–1; G(O2˙–) = 3.3 × 10–7 mol J–1] AM-Trp is consumed with a G value of (2.9 ± 0.5) × 10–7 mol J–1, i.e. a restitution of AM-Trp by electron transfer from O2˙– to AM-TrpN˙, although thermodynamically possible [E7(O2˙–) = –0.33 V; E7(TrpN˙) = +1.0 V], must be of very little importance compared to an addition. This has been supported by a product study. The major products are the corresponding N-formylkynurenine (G = 1.4 × 10–7 mol J–1) and two hydroperoxides (total G = 0.7 × 10–7 mol J–1) which to a large extent convert upon standing at room temperature into 1-acetyl-2-methoxycarbonyl-3-hydroxy-1,2,3,8,8-hexahydropyrroloindole (AM-HIP). The same products and product ratios are also formed, when singlet oxygen (from the irradiation of Rose Bengal with visible light) is reacted with AM-Trp suggesting that these two processes lead to the same (short-lived) hydroperoxidic intermediate.

Antioxidant Potential of Theaflavins. A Pulse Radiolysis Study

Spectral, acid−base, and redox properties of theaflavin radicals were studied by pulse radiolysis in aqueous solutions. Theaflavin radicals are generated by the azide radical one-electron oxidation of theaflavin, theflavin gallates A and B, and theaflavin digallate. Being relatively strong transient oxidant, N3• oxidizes more than one phenolic site in the complex polyphenols. The resulting mixture of phenoxyl radicals transforms via an intramolecular electron transfer to the hydroxycycloheptenone radical, with apparently lowest reduction potential. The neutral hydroxycycloheptenone radical is more aromatic than the parent compound, which reflects in high rate constant of the formation of the radical. The rate of the reaction of theaflavin with the superoxide radical at pH 7, k = 1 × 107 M-1 s-1, is an order of magnitude higher than that with epigallocatechin gallate (EGCG), k = 7.3 × 105 M-1 s-1, in spite of higher reduction potential of the theaflavin radicals (E7 = 0.53 V vs E7 = 0.44 V for EGCG). Even ...

Generation and Reactivity of Aromatic Thioether Radical Cations in Aqueous Solution As Studied by Pulse Radiolysis

The radical cations of thioanisole (1), p-methylthioanisole (2), and the benzyl phenyl sulfides 3−5 have been produced by pulse radiolysis in aqueous solutions, using SO4•- or Tl2+ as oxidizing species. The radical cations 1•+−5•+ exhibit very similar UV spectra, with strong absorptions between 300−350 and 500−600 nm. In contrast to aliphatic thioether radical cations, 1•+−5•+ do not undergo dimerization (via formation of a three-electron bond with the parent thioethers). In the absence of bases, 1•+ is a long-lived species with a lifetime >30 ms, whereas 3•+, 4•+, and 5•+ decay rapidly by both C−S bond and C−H bond cleavage with kC-H = 1.3 × 103 s-1 and kC-S = 1.3 × 103 s-1 for 3•+and kC-H = 0.95 × 103 s-1 and kC-S = 2.65 × 103 s-1 for 4•+. In the presence of OH- or HPO42-, also 1•+ undergoes a deprotonation process, with a rate larger than those of the benzyl phenyl sulfide radical cations. For example, the rate constant for the OH--induced deprotonation is 3.4 × 107 M-1 s-1 for 1•+and 9.5 × 106 M-1 s-1 for 3•+. Thioanisole radical cation 1•+ was also produced by reduction of thioanisole sulfoxide. Under these conditions, it was possible to study the reaction of 1•+ with a number of nucleophiles or electron donors. It was found that 1•+ reacts with I-, N3-, PhS-, PhSH, Br-, and SCN- by an electron transfer mechanism, producing the oxidized form of the nucleophile. This reaction is diffusion controlled with the first four nucleophiles, which are more easily oxidized than 1 (E° < 1.45 V); slower reactions were observed with SCN- (E° = 1.62 V) and with Br- (E° = 1.96 V). NO3- (E° = 2.3V) is unreactive (k < 106 M-1 s-1).

Oxidation of Ferrous and Ferrocyanide Ions by Peroxyl Radicals

Alkylperoxyl and arylperoxyl radicals were produced by pulse radiolysis in aqueous solutions, and their reactions with ferrous and ferrocyanide ions were studied by kinetic spectrophotometry. Oxidation of Fe(CN)64- took place with rate constants that varied from <1 × 105 to 5 × 107 L mol-1 s-1, depending on the electron-withdrawing effects of the substituents on the peroxyl radical and presumably reflecting variations in reduction potential of the peroxyl radical, as expected for outer-sphere electron transfer. Oxidation of Feaq2+, on the other hand, took place by an inner-sphere mechanism controlled by the rate of dissociative interchange of the water ligand. The rate constants were nearly the same for all the peroxyl radicals examined (k = (0.5−1.1) × 106 L mol-1 s-1) and involved the formation of a transient intermediate, RO2-Fe3+, which later decomposed to yield Feaq3+. The decomposition was accelerated by H+ and by Fe2+. The proposed mechanism is a modification of a previously suggested reaction scheme.

Interaction of melanin with carbon- and oxygen-centered radicals from methanol and ethanol

The interaction of dopa-melanin (DM) and cysteinyldopa-melanin (CDM) with carbon- and oxygen-centered radicals generated by benzophenone-photosensitized hydrogen abstraction from ethanol, or by pulse radiolysis of aqueous solutions of methanol and ethanol, is reported. Photosensitized formation of carbon-centered radicals and their interaction with melanin was monitored by electron paramagnetic resonance (EPR) spin trapping using DMPO, and via the melanin free radical signal itself. In the pulse radiolysis experiments, the interaction of DM or CDM with hydroxymethyl, hydroxyethyl, and the corresponding methanol peroxyl radical was monitored by recording time-dependent changes of the melanin absorbance at selected wavelengths. The data indicate that both melanins are good scavengers of carbon-centered radicals, with corresponding rate constants in the range of 10 7 to 10 8 M −1 s −1. Significantly, compared to DM, CDM is also an exceptionally efficient scavenger of oxygen-centered radicals derived from methanol with corresponding rate constants of 2.7 × 10 4 and 2 × 10 6, M −1 s −1 for DM and CDM, respectively. The results are discussed with reference to the potential role of melanin in protecting the integrity of melanosomes by inhibiting peroxidation of lipid components of the organelle membrane.

Heterocyclic thiols as antioxidants: Why Ovothiol C is a better antioxidant than ergothioneine

Spectral, acid-base, and redox properties of 4-mercaptoimidazoles were investigated by pulse radiolysis in aqueous solutions. Thiyl radicals of 1-methyl-5-ethyl-4-mercaptoimidazole (MEMI) have weak absorption band at 330 nm, ϵ = 300 ± 60 M−1 cm−1. Because the ionic strength variation from 0.01 to 0.1 M in the pH range from 3 to 14 does not influence the rate constant of the radical decay, it is concluded that the MEMI thiyl radical is neutral. At pH 7, the reduction potential of the MEMI radical, E7 = 0.45 V, is lower than E7 = 0.48 V of the Trolox C radical, which means that MEMI may restitute vitamin E under physiological conditions (assuming similar reduction potentials of Trolox C and vitamin E radicals). Because pKa = 10.3 of the SH group in MEMI is lower than pKa = 11.9 of the OH group of Trolox C, the redox equilibrium with Trolox C is reversed at pH 13, and E13(MEMI-radical) = 0.29 ± 0.04 V is determined against E13(Trolox C - radical) = 0.19 V. In contrast to extraordinary electron donating properties, MEMI is only a moderately good H-atom donor. k(·CH3 + MEMI) = (1.5 ± 0.3) × 105 M−1 s−1 in neutral media is considerably lower than k(·CH3 + GSH) = 5 × 107 M−1 s−1, which is explained by the zwitterionic structure of MEMI. The ability of MEMI to act as antioxidant in biological systems is further demonstrated by its ability to efficiently scavenge superoxide and linoleate peroxyl radicals. On the basis of the two-fold increase in the rate of inhibition of radiation-induced oxidation of linoleic acid by MEMI and vitamin E (1:1) as compared with that by MEMI alone, it is concluded that MEMI shows synergism with vitamin E.

Generation and reactions of the disulphide radical anion derived from metallothionein: a pulse radiolytic study.

OH-radicals were generated by pulse radiolysis of aqueous solutions of rabbit (Zn,Cd)-metallothionein (MT). They react with MT mainly by forming a thiyl radical with a rate constant of 1.7 x 10(12) dm3 mol-1 s-1. The thiyl radical reacts rapidly but reversibly with a thiolate function to form RSSR.-: RS + RS- reversible RSSR.-. The kinetics of the formation and decay of this radical anion have been studied pulse radiolytically by monitoring the evolution of the optical absorption of RSSR.- at 450 nm. This process is mostly intermolecular, i.e. bimolecular in MT. In the absence of O2, RSSR.- decays bimolecularly: RSSR.(-)+RS.-->RSSR + RS-. In the presence of O2, RS. may be scavenged by O2 and thus the yield of RSSR.- decreases: RS.+O2 reversible RSOO.. Under these conditions RSSR.- decays by first-order kinetics: RSSR.(-)+O2-->RSSR + O2.-. The rate constants of these reactions have been determined at room temperature: k4 = 1.8 x 10(9) dm3 mol-1 s-1, k5 = 7 x 10(4) s-1, k10 = 9.2 x 10(8) dm3 mol-1 s-1, and k18 about 3 x 10(7) dm3 mol-1 s-1. From the dependence of the maximal absorbance at 450 nm on the thiolate concentration in the absence of oxygen, epsilon (RSSR.-) = 9 x 10(3) dm3 mol-1 cm-1 and the stability constant (K4/5) of 2.3 x 10(4) dm3 ml-1 was determined. K4/5 is in good agreement with that determined kinetically, k4/k5 = 2.6 x 10(4) dm3 mol-1. The stability constant K15/16 of the thiylperoxyl radical, RSOO., was determined to be 5.5 x 10(3) dm3 mol-1.

Pulse radiolysis of aqueous solutions of sodium tetraphenylborate

In previous work on the 60Co γ-radiolysis of aqueous tetraphenylborate (TPB -) solutions carried out in this laboratory, it was found that several organic products, including benzene, phenol and biphenyl, are produced with substantial yield. However, the reaction mechanism was not established. In the present study, reactions initiated by OḢ radicals, N 3 radicals and e - aq in aqueous TPB - solutions were studied by pulse radiolysis using a 600 keV Febetron electron accelerator. The lack of reactivity between TPB - and e - aq was demonstrated by directly monitoring the transient optical absorbance of e - aq. Concerning the reaction with O Ḣ, two schemes were considered: (1) electron transfer from B(C 6H 5) - 4 to O Ḣ; or (2) O Ḣ addition to B(C 6H 5) - 4. Comparison of observed transient absorption spectra with expectations based on two different schemes suggests that O Ḣ addition is the dominant reaction pathway under conditions of N 2O saturation, with an experimentally determined second-order rate constant of 6.2×10 9M -1s -1. A mechanism based on an initial first-order self-decomposition of the O Ḣ adduct, (C 6H 5) 3BC 6H 5OH -· with a measured rate constant of 4×10 4s -1 is proposed. Kinetic modeling on the proposed mechanistic scheme gives good agreement with our experimental results.

Interaction of phenolic antioxidants and hydroxyl radicals

Based on pulse radiolysis of aqueous solutions of four phenolic antioxidants including green tea polyphenols, quercetin, caffeic acid and sinapic acid the rate constants for reactions of OH and the antioxidants were determined. And green tea polyphenols and quercetin are the strongest antioxidants.

Pulse radiolysis of aqueous solutions of ethyl acrylate and hydroxy ethyl acrylate

Ethyl- and hydroxy ethyl acrylate show high reactivities with hydrated electron and hydroxyl radical intermediates of water radiolysis. The electron adduct reversibly protonate with pK values of 5.7 and 7.3. The adducts may take part in irreversible protonation at the β carbon atom forming α-carboxyl alkyl radicals. Same type of radical forms in reaction of acrylates with OH: at low concentration the adduct mainly disappear in self termination reactions. Above 5 mmol dm -1 the signals showed the startup of oligomerization.

A pulse radiolysis study of the reaction of ozone with Cl ∸2 in aqueous solutions

Yields of . ̇ OH radicals were determined, with the use of the Cl ∸ 2 radical, in perchloric acid solutions (1.0 mM to 1.0 M) saturated with oxygen, nitrous oxide or nitrous oxide and ozone. The Cl ∸ 2 radical, produced by pulse radiolysis in aqueous solutions containing variable amounts of NaCl and HClO 4/HCl, reacts with added ozone at k(O 3 + Cl ∸ 2) = (9.0 ± 0.3) × 10 7 dm 3 mol -1 s -1. Its molar absorbance at its 340 nm peak is ϵ = (9580 ± 100) dm 3 mol -1 cm -1. G(Cl ∸ 2) = (6.4 ± 0.1) in N 2O saturated 0.1 mol dm -3 HClO 4 containing 0.01 mol dm -3 NaCl and 0.4–6.3 mmol dm -3 ozone. The conversion of the primary reducing radicals (e - aq, H) to . ̇ OH radicals by ozone, will facilitate studies of hypervalent oxidation states of metal ions in acid solutions where G(H) > G( . ̇ OH).

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

Laser flash photolysis and pulse radiolysis of aqueous solutions of the fluoroxysulfate ion, SO4F-

The fluoroxysulfate ion, SO{sub 4}F{sup {minus}}, is the only known ionic hypofluorite. It is a very strong oxidant capable of oxidizing water, but the rate of its reaction with water is sufficiently low to allow studies of its chemistry in aqueous solution. Aqueous fluoroxysulfate appears to react primarily as a 1-electron oxidant, and an F atom transfer mechanism has been proposed to account for many of its reactions. The present work is a study of the laser flash photolysis and pulse radiolysis of aqueous solutions of SO{sub 4}F{sup {minus}}.

The brookhaven accelerator test facility

The first set of pulse radiolysis apparatus in China with microsecond time resolution was built up at Beijing Radiation Center(Institute of Low Energy Nuclear Physics, Beijing Normal University). The radiation source is a 5-Mev s-band scanning Linac which is mainly used as a radiator for doing radiation processing technology studies in this laboratory. In order to use this Linac as a radiation source in pulse radiolysis, some special designs of electronic, mechanical, optical and sample-holding system were taken. Several experiments on pulse radiolysis of aqueous solution have been done successfully proving out that this apparatus is very useful in our attempt to combine the industrial applications of radiation with the basic research of radiation chemistry in this laboratory.

Pulse radiolysis investigations of one-electron reduction of terephthalonitrile in aqueous solutions

Pulse radiolysis of aqueous solutions of terephthalonitrile (TPN) was carried out to study the redox properties of anions of TPN, by determining the rate constants for the reaction of TPN with specific one-electron reducing species formed on reaction of OH radicals with methyl,ethyl,isopropyl alcohols,tetrahydrofuran and cytosine. Formation of anions of TPN, which absorb at wavelength 340 nm, was followed. From the graph of rate constants and maximum absorbance vs reduction potential of reducing species, reduction potential of TPN was found out to be —0.85 V vs NHE. Reduction of thionine (TH +) and methyl viologen (MV 2+) with the help of radical anions of TPN was carried out to establish its high reduction potential.