Pulse radiolysis studies on the reactions of 3-pyridinol with oxidizing radicals

Reaction rate constants for the reactions of 3-pyridinol with oxidizing radicals viz. OH, N₃, Br ₂ ^– , Cl ₂ ^– , SO ₄ ^– and O^– have been determined in aqueous solutions at different, pH's. Absorption spectra of the product transient species have been recorded in the 320–600 nm region. In the alkaline region (pH 13) the N₃ reaction product decays in two steps and O^– does not bring about one-electron oxidation. Similarly, at neutral pH, SO ₄ ^– does not cause selective one-electron oxidation of 3-pyridinol.     

Theoretical studies on the reactions of hydroxyl radicals with trimethylsilane and tetramethylsilane

The multiple-channel reactions OH + SiH(CH₃)₃ → products (R1) and the single-channel reaction OH + Si(CH₃)₄ → Si(CH₃)₃CH₂ + H₂O (R2) have been studied by means of the direct dynamics method at the BMC-CCSD//MP2/6-311+G(2d,2p) level. The optimized geometries, frequencies and minimum energy path are all obtained at the MP2/6-311+G(2d,2p) levels, and energy information is further refined by the BMC-CCSD (single-point) level. The rate constants for every reaction channels are calculated by canonical variational transition states theory (CVT) with small-curvature tunneling (SCT) contributions over the temperature range 200–2,000 K. The theoretical total rate constants are in good agreement with the available experimental data, and the three-parameter expression k ₁ = 2.53×10⁻²¹ T ^3.14 exp(1, 352.86/T), k ₂ = 6.00 × 10⁻¹⁹ T ^2.54 exp(−106.11/T) (in unit of cm³ molecule⁻¹ s⁻¹) over the temperature range 200–2,000 K are given. Our calculations indicate that at the low temperature range, for reaction R1, H-abstraction is favored for the SiH group, while the abstraction from the CH₃ group is a minor channel. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Pulse radiolysis and cyclic voltammetry studies of redox properties of phenothiazine radicals

One-electron transfer equilibria between seven phenothiazines were characterized by pulse radiolysis, producing radical-cations via oxidation by Br₂^·− or (SCN)₂^·− radicals. The reduction potentials of the phenothiazine radicals were determined by cyclic voltammetry. As an independent check, the redox equilibrium between one phenothiazine and the redox indicator ABTS was investigated. The data establish phenothiazines as useful indicators for radical redox properties. However, there are potential problems of aggregation, additional reactions with Br⁻/Br₂^·− and reactivity of the radicals towards buffers or other nucleophiles.     

Pulse radiolysis studies on galactose oxidase

Single-Cu-containing galactose oxidase in the GOase(semi) state (Cu(II), no Tyr(*) radical) reacts with pulse radiolysis generated formate radicals CO(2)(*-) to give an intermediate UV-vis spectrum assigned as RSSR(*-), peak at 450 nm (epsilon = 8100 M(-1) cm(-1)). From a detailed kinetic analysis at 450 nm, pH 7.0, the following steps have been identified. First the strongly reducing CO(2)(*-) (-1.9V) reduces GOase(semi) (k(0) > or = 6.5 x 10(8) M(-1) s(-1)) to a species GOase(semi)(*-). This is followed by biphasic reactions (i) GOase(semi)(*-) + GOase(semi) (k(1) = 1.6 x 10(7) M(-1) s(-1)) to give GOase(semi) + P(*-) and (ii) P(*-) + GOase(semi) (k(2) = 6.7 x 10(6) M(-1) s(-1)) to give GOase(semi)RSSR(*-). There are no significant absorbance changes for the formation of GOase(semi)(*-) and P(*-), which are Cu(I) (or related) species. However, GOase(semi)RSSR(*-) has an absorption spectrum which differs significantly from that of GOase(semi). The 450 nm peak is characteristic of an RSSR(*-) radical with two cysteines in close sequence proximity and is here assigned to Cys515-Cys518, which is at the GOase surface and 10.2 A from the Cu. On chemical modification of the RSSR group with HSPO(3)(2-) to give RSSPO(3)H(-) and RS(-), absorbance changes are approximately 50% of those previously observed. The decay of RSSR(*-) (0.17 s(-1)) results in the formation of GOase(red). No RSSR(*-) formation is observed in the reaction of GOase(semi) Tyr495Phe with CO(2)(*-), and a single process giving GOase(red)Tyr495Phe occurs. Similarly in the reaction of GOase(ox) with CO(2)(*-), a single-stage reaction gives GOase(semi).     

Pulse radiolysis studies of styrene

We have applied the pulse radiolysis technique of studying short-lived, radiation-produced intermediates to a study of pure, dry liquid styrene. We have observed at least three distinct species. The most rapidly decaying species (τ1/2 = 4 μsec., λ_max ≈ 370 mμ) exhibits an apparent first-order decay which is slowed down in the presence of dry oxygen, occurs too rapidly to be measured in the presence of water, and has a temperature coefficient of approximately 1 kcal./mole between 25 and 45°C. We have tentatively identified the species as the styryl anion with a G value for formation of approximately 0.15. A second species (τ1/2 = 220 μsec. λ_max ≈ 320–330 mμ) exhibits a first-order decay which appears to be independent of oxygen and water. From its spectrum, we have tentatively identified it as the styryl radical. The third species (λ_max ≈ 310 and 320 mμ) is relatively stable in the dark, but is radily photolyzed by the analyzing light of the usual experimental set-up for pulse radiolysis studies. The formation of this species appears to be independent of the water and oxygen content of the styrene.     

Reaction of hydroxyl radicals with azacytosines: a pulse radiolysis and theoretical study

Pulse radiolysis and density functional theory (DFT) calculations at B3LYP/6-31+G(d,p) level have been carried out to probe the reaction of the water-derived hydroxyl radicals (*OH) with 5-azacytosine (5Ac) and 5-azacytidine (5Acyd) at near neutral and basic pH. A low percentage of nitrogen-centered oxidizing radicals, and a high percentage of non-oxidizing carbon-centered radicals were identified based on the reaction of transient intermediates with 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate), ABTS2-. Theoretical calculations suggests that the N3 atom in 5Ac is the most reactive center as it is the main contributor of HOMO, whereas C5 atom is the prime donor for the HOMO of cytosine (Cyt) where the major addition site is C5. The order of stability of the adduct species were found to be C6-OH_5Ac*>C4-OH_5Ac*>N3-OH_5Ac*>N5-OH_5Ac* both in the gaseous and solution phase (using the PCM model) respectively due to the additions of *OH at C6, C4, N3, and N5 atoms. These additions occur in direct manner, without the intervention of any precursor complex formation. The possibility of a 1,2-hydrogen shift from the C6 to N5 in the nitrogen-centered C6-OH_5Ac* radical is considered in order to account for the experimental observation of the high yield of non-oxidizing radicals, and found that such a conversion requires activation energy of about 32 kcal/mol, and hence this possibility is ruled out. The hydrogen abstraction reactions were assumed to occur from precursor complexes (hydrogen bonded complexes represented as S1, S2, S3, and S4) resulted from the electrostatic interactions of the lone pairs on the N3, N5, and O8 atoms with the incoming *OH radical. It was found that the conversion of these precursor complexes to their respective transition states has ample barrier heights, and it persists even when the effect of solvent is considered. It was also found that the formation of precursor complexes itself is highly endergonic in solution phase. Hence, the abstraction reactions will not occur in the present case. Finally, the time dependent density functional theory (TDDFT) calculations predicted an absorption maximum of 292 nm for the N3-OH_5Ac* adduct, which is close to the experimentally observed spectral maxima at 290 nm. Hence, it is assumed that the addition to the most reactive center N3, which results the N3-OH_5Ac* radical, occurs via a kinetically driven process.     

Gas phase reactions of hydroxyl radicals arenes

At elevated temperatures. OH is found to abstract H from benzene derivatives; in air, phenols are then generated via reversible addition of ArOO· to arene.     

Pulse radiolytic investigation of the reaction of hydroxyl radicals with gentamycin

Hydroxyl radicals were generated radiolytically in N₂O-saturated aqueous solutions of the aminoglycoside antibiotic, gentamycin. Using the pulse radiolysis technique, the rate constant of OH radicals with gentamycin determined was 1.2·10⁹ dm³·mol⁻¹·s⁻¹. Upon^.OH attack a transient species with an absorption maximum at 270 nm is observed which decays by second-order kinetics within the solute concentration range of 3.2·10⁻⁵ to 1·10⁻³mol·dm⁻³. Transient species undergoes transformation to a permanent product absorbing between 260 and 340 nm with maximum absorption at 300 nm. Rate constant of the reaction of bimolecular decay of gentamycin radicals, k (Gen^.+Gen^.) was found to be ≈ 1.4·10⁷ dm³·mol⁻¹·s⁻¹.     

ESR studies on hydroxyl radical reactions with glycine

Radicals H₂N? ?H? COOH and H₂N? ?H? COO^? which have different ESR. spectra are observed during the reaction of hydroxyl radicals with glycine in aqueous solution. Rapid and reversible exchange between the different dissociated radicals is induced by addition of phosphoric acid. The pH dependence of the ESR. spectra yields the pK value and rate constants for proton transfer reactions between the radicals and phosphoric acid.     

Pulse radiolysis study of the reactions of catechins with nitrogen dioxide

Nitrogen dioxide (^?NO₂), one of the oxidizing radicals formed in vivo is suspected to play a role in various pathophysiological processes. The reactions of ^?NO₂ with dietary catechins, the group of flavonoids present in high amounts in green tea and red wine, have been investigated by pulse radiolysis method. The kinetics of the reaction of ^?NO₂ with gallic acid have been also studied for comparison. The spectra of transient intermediates are presented. The rate constants of the reaction of ^?NO₂ with catechin, epigallocatechin, epigallocatechin gallate and gallic acid determined by the competition method with 2,2’-azinobis-(3-ethylbenzthiazoline-6-sulfonate) at pH 7.0 and room temperature have been found to be 0.9, 1.0, 2.3 and 0.5×10⁸ M^?1 s^?1, respectively. The values for catechins are among the highest reported for the reactions of ^?NO₂ with non-radical compounds.     

Polar transition states in reactions of aromatic hydrocarbons with hydroxyl radicals

The mechanism of formation of radical adducts in the radical hydroxylation of aromatic hydrocarbons has been investigated. The analysis of correlations between the logarithms of the rate constants of the radical hydroxylation of substituted benzenes, naphthalenes, and condensed hydrocarbons in aqueous solutions and the ⁺ constants of the substituents, the ionization potentials, the localization energies of the arenes, and also the quantum-chemical calculations of the surfaces of potential energies of the hydroxylation of benzene and naphthalene leads to the conclusion that an ion pair in the excited state, formed by the cation radical of the hydrocarbon and the hydroxyl anion, can serve as the model for the transition states of the above-mentioned reactions. A decrease in the ionization potential of the arene leads to an increase in the coulombic interaction in this pair, to its stabilization, and to an increase in the reaction rate.Deceased.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 1, pp. 96–99, January–February, 1986.     

Pulse radiolysis studies of the reactions of bromine atoms and dimethyl sulfoxide–bromine atom complexes with alcohols

Dimethylsulfoxide (DMSO)–Br complexes were generated by pulse radiolysis of DMSO/bromomethane mixtures and the formation mechanism and spectral characteristics of the formed complexes were investigated in detail. The rate constant for the reaction of bromine atoms with DMSO and the extinction coefficient of the complex were obtained to be 4.6×10⁹ M⁻¹ s⁻¹ and 6300 M⁻¹ cm⁻¹ at the absorption maximum of 430 nm. Rate constants for the reaction of bromine atoms with a series of alcohols were determined in CBrCl₃ solutions applying a competitive kinetic method using the DMSO–Br complex as the reference system. The obtained rate constants were ∼10⁸ M⁻¹ s⁻¹, one or two orders larger than those reported for highly polar solvents. Rate constants of DMSO–Br complexes with alcohols were determined to be ∼ 10⁷ M⁻¹ s⁻¹. A comparison of the reactivities of Br atoms and DMSO–Br complexes with those of chlorine atoms and chlorine atom complexes which are ascribed to hydrogen abstracting reactants strongly indicates that hydrogen abstraction from alcohols is not the rate determining step in the case of Br atoms and DMSO–Br complexes.     

Pulse radiolysis investigations on the reactions of primary radiolytic species of water with atropine

On pulse radiolysis of N₂O saturated aqueous solutions of atropine, an optical absorption band (_max at 320 nm,e=2.81·10³ dm³·mol^–1·cm^–1) was observed, which is assigned to the product of reaction of OH radicals with the solute. This absorption decayed following second order kinetics with a rate constant of 4.5·10⁸ dm³·mol^–1·s^–1. The rate constant for the reaction of OH radicals with atropine as estimated by following the build-up kinetics is 2.7·10⁹ dm³·mol^–1·s^–1. The H atoms also reacted with this compound to produce a transient absorption band behaving similarly to the one observed in the case of reaction with OH radicals. The transient species formed in both cases is assigned to a radical derived by H atom abstraction by H/OH radicals from the parent compound. This radical was unreactive towards 2-mercaptoethanol. e _aq ^– was found to react with atropine forming a transient band with _max at 310 nm (=3.55·10³ dm³·mol^–1). Its decay was also second order with a rate constant of 1.64·10⁹ dm³·mol^–1·s^–1. The bimolecular rate constant for the reaction of e _aq ^– with atropine as estimated from the decay of e _aq ^– absorption at 720 nm is 3.9·10⁹ dm³·mol^–1·s^–1. Specific one-electron oxidizing and reducing agents (such as Cl ₂ ^– , Tl²⁺, SO ₄ ^– and (CH₃)₂COH, CO ₂ ^– , respectively) failed to oxidize or reduce this compound in aqoues solutions. The radical anion of atropine formed by its reaction with e _aq ^– was found to reduce thionine and methyl viologen with bimolecular rate constant of 3.8·10⁹ and 3.2·10⁹ dm³·mol^–1·s^–1, respectively.     

Pulse radiolysis study of reactions of alkyl and alkylperoxy radicals originating from methyl tert-butyl ether in the gas phase

UV spectra and kinetics for the reactions of alkyl and alkylperoxy radicals from methyl tert-butyl ether (MTBE) were studied in 1 atm of SF₆ by the pulse radiolysis-UV absorption technique. UV spectra for the radical mixtures were quantified from 215 to 340 nm. At 240 nm. σ_R = (2.6 ± 0.4) × 10⁻¹⁸ cm² molecule⁻¹ and σ_RO2 = (4.1 ± 0.6) × 10⁻¹⁸ cm² molecule⁻¹ (base e). The rate constant for the self-reaction of the alkyl radicals is (2.5 ± 1.1) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. The rate constants for reaction of the alkyl radicals with molecular oxygen and the alkylperoxy radicals with NO and NO₂ are (9.1 ± 1.5) × 10⁻¹³, (4.3 ± 1.6) × 10⁻¹² and (1.2 ± 0.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, respectively. The rate constants given above refer to reaction at the tert-butyl side of the molecule.     

Pulse radiolysis studies of short-lived species in solid amino acids as precursors of radicals and detected by ESR

The aim of the study was to bring closer solid state radiation chemistry and ESR spectroscopy by looking for precursors of free radicals which give ESR signals. It has been performed using time-resolved spectrophotometry (pulse radiolysis of the solid state) and diffuse reflection spectrophotometry. Alanine has been especially considered as the most investigated amino acid, important for radiation dosimetry. Absorption of the transient (Λ maximum at 460 nm) is identified as the species during deamination. The stable absorption spectrum with the Λ maximum at 345 nm is due to the same radical as the one detected by ESR. Other amino acids: valine, threonine, glutamine and arginine show similar behaviour: microsecond spectrum of the intermediate appears always at longer wavelenghts. The transient spectrum changes into stable absorption in UV of a lower wavelenght. Along with the optical spectrum, the ESR spectrum appears, of similar stability. Also, other features indicate that the same radical is responsible for both the electronic and ESR spectrum. Some amino acids, like methionine give intensive transient absorption in the microsecond range but no ESR signal, after completion of consecutive fast reactions. In that case any optical absorption is due to the stable product of radiolysis, i.e. compounds with paired electrons only.     

Influence of 1,4-dihydropyridine derivatives on the generation of hydroxyl radicals

It was shown that a series of 1,4-dihydropyridine (1,4-DHP) derivatives in the NAD-H-Cu²⁺ -H₂O₂ system inhibit the formation of the hydroxyl radical (HO), while derivatives of 1,4-DHP with electron-donor substituents in the molecule are themselves capable of generating HO in the presence of Cu²⁺ and H₂O₂.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 519–521, April, 1992.     

Fast kinetics of the reactions of hydroxyl radicals with nitrone spin traps

The technique of spin trapping with nitrone spin traps nas gained wide acceptance as a method for estimating·OH yields in ESR studies. In our study, fast optical kinetic techniques applied to a series of these traps (PBN, 2-PyBN, 3-PyBN, 4-PyBN, 3-PyOBN and 4-PyOBN) reveal relaxation spectra that indicate two absorption maxima with different time constants, with all except 4-PyOBN showing second order behavior. These two spectral regions show different kinetics. Thus, two reaction sites are indicated, only one of which is necessarily a measure of initial · OH when ESR methods are used. One other trap (DMPO) after · OH reaction decays in one mode suggesting that its final product might be useful as a measure of initial · OH. Also, our ESR evidence shows that OH detection can be improved significantly by spin trapping -hydroxyalkyl radicals formed by · OH attack on alcohols.     

4-氯酚稀水溶液的脉冲辐解研究

利用脉冲电子束进行了多种条件下4－氯酚稀水溶液的脉冲辐解研究,对其瞬态光谱中的主要吸收峰作了归属,并初步考察了这些瞬态物种的生长、衰减等行为,研究表明,·OH基与4－氯酚在碱性条件下反应生成氯代酚氧基,速率常数为4.14×10^9L/(mol·s),在酸性条件下要经过OH-adducts;H原子与4-氯酚反应生成H-adducts的速率常为2.0×10^9L/(mol·s),产物可通过双分子二级反应逐步脱氯;eaq^-可直接从4-氯酚分子夺氯,反应速率常数为1.82×10^9/L(mol·s)。     

Pulse radiolysis studies of 4,7-phenanthroline(II)

The one-electron reduction of 4,7-phenanthroline (P) in aqueous solutions at neutral pH has been further studied by pulse radiolysis. The spectral and kinetic properties of the transient formed due to the reaction of 4,7-phenanthroline with hydrated electron were investigated. The transient absorption spectrum obtained 5μs after the pulse exhibits a broad band with a λ_max at 420 nm. The λ_max is 10 nm blue shift compared with the absorption spectrum obtained at pH 2.9 where the reactant was the protonated form. The bimolecular'rate constant of the reaction of 4,7-phenanthroline with hydrated electron was 0etermined to be (2.2±0.1)×10¹⁰ dm³ mol⁻¹ s⁻¹. It was found that the decay of the transient was mainly following a first-order kinetics. The first-order decay rate constant was determined to be (1.25±0.1)×10⁴s⁻¹.