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.     

One-electron oxidation of glutathione by azide radicals in neutral medium: A gamma and pulse radiolysis study

The selective interactions of azide radicals with glutathione (GSH) have been quantitatively studied in buffered neutral aqueous solutions using γ and pulse radiolysis. The sulfur centered GS̽ and GSSG⨪ radicals are produced in pulse and γ radiolysis. Kinetic experiments and simulation allowed to estimate the rate constant of N̽₃ with GSH which has been found to be equal to (9.5 ± 0.5) × 10⁶M⁻¹ s⁻¹ at pH 7. In steady state radiolysis, we have found GSSG as the final product formed with an initial G value of 2.9 × 10⁻⁷ mol J⁻¹.     

One-electron oxidations of ferrocenes: A pulse radiolysis study

Using the pulse radiolysis technique we have studied the oxidation by various inorganic radicals of four water soluble ferrocene derivatives, hydroxyethyl, dimethylaminomethyl, monocarboxylic acid and dicarboxylic acid. We report the second order rate constants for these reactions, the stabilities and spectral properties of the ferrocinium products, and the electrochemically determined ferrocinium/ferrocene redox potentials. We also present preliminary estimates of tyrosine and tryptophan radical redox potentials obtained with the dicarboxylic acid ferrocene derivative as reference, and we discuss the relationship between redox potential differences and the reactivities of the ferrocenes with the inorganic radicals.     

One-electron oxidation of the iron(II) complex of 1,10-phenanthroline-5,6-quinone A pulse radiolysis study

One-electron oxidation of the ferrous tris-PQ complex, a model for lipoxygenase, was attempted using oxidants such as •OH, N^•₃, Br^•-₂, Tl²⁺ and TlOH⁺. •OH was found to react with the complex with a bimolecular rate constant of (3.9±0.6)x10⁹dm³mol^-1s^-1, a rate which is not very dissimilar to that for the reaction with the ligand PQ. However the product of the reaction was found to be a OH-adduct rather than a cation radical. No reaction was found to occur with N^•₃ or Br^•-₂. Both Tl²⁺ and TlOH⁺ reacted with the complex to form its oxidised species with rate constants of (7.0±1)x10⁸dm³mol^-1s^-1 and (4.0±0.8)x10⁸dm³mol^-1s^-1, respectively. From a comparison of the rate constants and the transient spectra it was concluded that the centre of oxidation is the ligand rather than the metal.     

Reactions of halogenated organic peroxyl radicals with various purine derivatives,tyrosine, and thymine: A pulse radiolysis study

Absolute rate constants for the one electron oxidation of guanine, guanosine, uric acid, xanthine, hypoxanthine, tyrosine, and thymine by various halogenated peroxyl radicals in aqueous solutions have been determined using the technique of pulse radiolysis. Roughly, linear correlations have been observed between the logarithm of these rate constants and Taft's inductive parameter (σ*) for the radicals. However, the rate constants for the radical CBr₃O are slightly higher than those for CCl₃O for most of these compounds. © John Wiley & Sons, Inc.     

One-electron reduction of thionine studied by pulse radiolysis

One-electron reduction of thionine has been studied by using the technique of nanosecond pulse radiolysis and kinetic spectrophotometry. H,e _aq ⁻ as well as radicals derived from methanol, ethanol, isopropanol, THF, dioxane andt-butanol by H atom abstraction were used as reductants. The rate constants for the transfer of electrons from these radicalts to thionine were directly determined from the pseudo first-order formation rates of the product, semithionine and the one-electron reduction potential of thionine estimated. The absorption spectrum of semithionine in its different conjugate acid-base forms was found to be in agreement with previously reported spectra and the decay of the species was second order. By monitoring transient absorbance changes as a function of pH, twopK _a values were observed and, based on the effect of ionic strength on the second-order decay constants of the species were assigned to the equilibria described.     

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.     

One-electron oxidation and demetalation of metalloporphyrins and chlorophyll a in dichloroethane solutions as studied by pulse radiolysis

Formation of cation radicals by pulse radiolysis of metalloporphyrins and chlorophyll a in 1,2-dichloroethane is reported Demetalation of the metalloporphyrin by radiolytically produced HC1 is also observed. Rate constants for demetalation of ZnTPP and Chl a are 1 1 × 10⁸ and ≈ 3 × 10⁸ M^?1 S^?1. Oxidation of Chl a by ZnTPP⁺ has a rate constant of ≈ 4× 10⁹ M^?1S^?1.     

Direct optical observation of the formation of some aliphatic alcohol radicals. A pulse radiolysis study

The kinetics of the reactions of hydroxyl radicals and hydrogen atoms with some aliphatic alcohols in aqueous solutions were studied using pulse radiolysis. Based on the increase in optical absorption in the UV region, the rate constants for the reaction of hydroxyl radicals and hydrogen atoms with methanol, ethanol, 2-propanol ort-butyl alcohol were determined to be 9.0 × 10⁸, 2.2 × 10⁹, 2.0 × 10⁹,6.2×l0⁸ and 1.1 × 10⁶, 1.8 × 10⁷, 5.3 × 10⁷, 2.3 × 10⁵ dm³ mol⁻¹ s⁻¹ respectively. The bimolecular decay rate constants for the alcohol radicals produced in methanol and ethanol were evaluated to be 2.4 × 10⁹ and 1.5 × 10⁹ dm³ mol⁻¹s⁻¹. The values observed are in fairly good agreement with those reported earlier.     

One-electron reduction of 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin: a pulse radiolysis study

Geldanamycin, a benzoquinone ansamycin antibiotic, is a natural product inhibitor of Hsp90 with potent and broad anticancer properties but with unacceptable levels of hepatotoxicity. Consequently, numerous structural analogs, which differ only in their 17-substituent, have been synthesized including the water-soluble and less toxic 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17-DMAG, Alvespimycin). It has been suggested that the different hepatotoxicity reflects the redox active properties of the quinone moiety. The present pulse radiolysis study was aimed at studying the one-electron reduction of 17-DMAG. The UV-visible spectrum of the semiquinone radical, its pK(a), and the second-order rate constants for the reactions of 17-DMAG with CO(2)(?-) and (CH(3))(2)C(?)OH have been obtained. The reduction potential of 17-DMAG has been determined to be -194 ± 6 mV (vs NHE) using oxygen, 1,4-naphthoquinone, and menadione as electron acceptors. This reduction potential is lower than that of O(2) demonstrating that thermodynamically the semiquinone radical can reduce O(2) to superoxide, particularly since the concentration of O(2) is expected to exceed that of the drug in cells and tissues.     

Study of baicalin scavenging hydroxyethyl peroxyl radicals by radiolysis of aerated ethanol-baicalin system

The reaction of baicalin (β- -glucopyranosiduronic acid, 5,6-dihydroxy-oxo-2-phenyl-4H-1-benzopyran-7-yl) scavenging hydroxyethyl peroxyl radicals (RO₂^.) was studied with the aid of radiolysis of aerated ethanol. Two main stable products were separated by reverse HPLC and their possible molecular structures were derived from their UV, IR and FAB-MS spectra. The dependence of G(H₂O₂), G(CH₃CHO) and G(-baicalin) on the concentration of baicalin showed that one baicalin molecule could inhibit the formation of one H₂O₂ molecule and two CH₃CHO molecules. A possible reaction mechanism between baicalin and RO₂^. radical was suggested.     

Kinetic and redox characteristics of phenoxyl radicals of eugenol and isoeugenol: A pulse radiolysis study

The phenoxyl radicals of eugenol (EgOH) and isoeugenol (iEgOH) were generated by the specific one‐electron oxidant N₃^· using pulse radiolysis technique, and were characterized by their absorption spectra, decay and formation kinetics, and one‐electron reduction potential (E₇¹) values. Reactivities of eugenol phenoxyl radical with the biologically important molecule, trolox C (analogue of vitamin E, α‐tocopheral), were determined. Reactions of OH with these phenols were studied at different pHs and suitable mechanisms for these reactions were suggested. Scavenging abilities of the phenols toward highly damaging Br^·, NO₂^·, and CCl₃O₂^· radicals were evaluated. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 17–23, 2000     

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.     

A pulse radiolysis study of the reactivity of neptunoyl ions relative to inorganic free radicals

Conclusions 1. Pulse radiolysis was used to find the rate constants for the reactions of OH, HSO₄, NO₃, and Cl₂ radicals with neptunoyl ions.Change in the NO₃ and H⁺ ion concentrations do not affect the term k[NO₃ + NpO₂ ⁺], while k[Cl₂ ^– + NpO₂ ⁺] increases with increasing chloride concentration due to the formation of neptunoyl ion chlorocomplexes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 456–458, February, 1986.     

Carbon- and nitrogen-centered radicals produced from l-lysine by radiation-induced oxidation: A pulse radiolysis study

Radical species generated from the reactions of a basic amino acid, l-lysine (Lys), with hydroxyl radicals (OH) and sulfate radical anion () have been detected by the method of pulse radiolysis. On the basis of electron transfer reactivities toward tetranitromethane (TNM), it was demonstrated that reducing carbon-centered radicals are generated as a result of hydrogen abstraction from CH₂ of Lys with a G-value of 1.9 × 10⁻⁷ mol J⁻¹. On the other hand, direct oxidation of l-Lys by formed a transient species with different spectroscopic properties, most likely, the ε-N-centered Lys radical.     

Spectroscopic properties of anion radicals studied using pulse radiolysis

The anion radicals of benzophenone, amino derivatives of benzophenone, fluorenone, furil and antharcene have been generated by pulse radiolysis in acetonitrile solution and their spectroscopic and kinetic properties have been characterized. The G-value for generation of anion radicals in acetonitrile has been measured to be 1.01 ± 0.1 per 100 eV. We have also investigated the electron-transfer reactions from anion radicals of one kind of solutes to the ground state of another solute. This study has demonstrated that pulse radiolysis can be a useful technique for generation and characterization of anion radicals and for studying electron-transfer reactions using acetonitrile as solvent.     

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.     

Reactions of H atoms and OH radicals with ascorbic acid: A pulse radiolysis Fourier transform ESR study

Using pulse radiolysis, free radicals of ascorbic acid were generated by reactions of the primary radicals H and OH in acidic and basic aqueous solutions. The formation and the decay of several radicals of ascorbic acid were detected by time resolved Fourier transform electron spin resonance within a time interval of 100 ns to 1 ms. The rate constant of addition of H atoms to ascorbic acid (1.3·10⁸ dm³· mol⁻¹·s⁻¹) was directly determined by the change of line width of the low field line of the H atom in the presence of ascorbic acid. The addition of OH radicals to ascorbic acid results in different radical structures, detected by highly resolved Fourier transform ESR spectra.     

Spectroscopic characterization of mechanisms of oxidation of Phe by SO_4~- radical: A pulse radiolysis study

By using time-resolved kinetic spectrophotometry and pulse radiolysis technique, the oxidation of Phe by SO4- radical has been investigated both in aqueous and water/acetonitrilemixed solutions. The results reveal that attack of the oxidizing SO4- radical on Phe leads directlyto the formation of Phe cation radical 3 with a strong absorption peak at 310 nm, then it proceeds in three competitive reactions via either hydroxylation, deprotonation or decarboxylation, which were found to be strongly dependent upon the ionization state of the substitutes -COOH and -NH2 and the nature of the solvents. Decarboxylation takes place only when the carboxyl group is deprotonated. At high pH deprotonation of Phe cation radical 3 is much easier to occur than that in neutral or acid solutions. Moreover, with addition of acetonitrile, deprotonation is more predominant than hydroxylation, whereas in aqueous solutions hydroxylation is much easier to occur.     

A study of radicals arising in the low-temperature radiolysis of thiophene and some of its derivatives

EPR was used to investigate the radicals formed by radiolysis of furan, thiophene, and some of their substitution products having the general formula C₄H₃RS, where R = Cl, Br, COOH, C(CH₃)₃, and of di-tert-butylthiophene. By analyzing the EPR spectra it is shown that radicals formed by radiolysis of furan, thiophene, their alkyl substitution products, and 2-thiophene carboxylic acid arise in primary radiation-chemical events with splitting of the C-H bond in both ring and side chain. Characteristic of the radiolysis of these compounds is formation of secondary radicals by addition of hydrogen atoms at the double bond of the heteroaromatic ring. The values of the hyperfine splittings in the spectra of the secondary radicals are, for protons of methylene group 32 e [positions 2(5)] or 40 e [positions 3(4)], and 13 e for protons of the thiophene ring. In radiolysis of 2-chlorothiophene and 3-bromothiophene the main products are radicals with the unpaired electron localized on the sulfur atom.