Showing papers on "Radical ion published in 1997"
TL;DR: This study introduces a simple direct antioxidant assay, based on the reduction of the ABTS, and compares it with the myoglobin/ABTS, establishing that the antioxidants studied to date in the latter assay act by scavenging the AB TS.
Abstract: (1997). Factors Influencing the Antioxidant Activity Determined by the ABTS•+ Radical Cation Assay. Free Radical Research: Vol. 26, No. 3, pp. 195-199.
655 citations
TL;DR: The current lineup of popular density functional theories, in particular those based on Becke's exchange functionals, fail to predict a correct dissociation behavior in radical ions where charge and spin must be separated (model: H2•+) or where both must be localized on one fragment (Model: He2•+) as discussed by the authors.
Abstract: The current lineup of popular density functional theories, in particular those based on Becke's exchange functionals, fail to predict a correct dissociation behavior in radical ions where charge and spin must be separated (model: H2•+) or where both must be localized on one fragment (model: He2•+). The repercussions of this on the location of certain transition states on radical ion potential energy surfaces are pointed out.
305 citations
TL;DR: Although there were some discernible trends in carotenoid reactivity for individual radicals, rate constants varied by no greater than a factor of 2.5, the mechanism and rate of scavenging is strongly dependent on the nature of the oxidising radical species but much less dependent onThe carotanoid structure.
275 citations
TL;DR: Physical, chemical, and spectroscopic methods indicate that the quinone is associated with the DNA by end-capping rather than by intercalation, and migration of a radical cation through the duplex DNA from its point of generation near the anthraquinone toits point of reaction at the GG step.
Abstract: A series of duplex DNA structures was prepared that incorporate an anthraquinone group linked covalently to the 5‘-terminus of one strand and two GG steps separated by three base pairs in the complementary strand. Physical, chemical, and spectroscopic methods indicate that the quinone is associated with the DNA by end-capping rather than by intercalation. Irradiation of the linked anthraquinone leads to piperidine-requiring strand cleavage predominantly at the 5‘-G of the GG steps. The 5‘-G of the distal GG step is more than 40 A from the anthraquinone group. This long-range DNA damage is attributed to migration of a radical cation (hole) through the duplex DNA from its point of generation near the anthraquinone to its point of reaction at the GG step. This migration is not interrupted by imposition of a gap caused by an abasic site. However, the incorporation of 8-OxoG into the DNA duplex blocks migration by introducing a deep, reactive trap for the radical cation.
273 citations
TL;DR: Guanine-specific modification of both single- and double-stranded oligodeoxynucleotides via the autoxidation of sulfite is shown to be catalyzed by [NiCR]2+, which represents the first demonstration of DNA damage by sulfite and nickel(II) complexes.
Abstract: Guanine-specific modification of both single- and double-stranded oligodeoxynucleotides via the autoxidation of sulfite is shown to be catalyzed by [NiCR]2+ (where CR = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[1131]heptadeca-1(17),2,11,13,15-pentaene) and [NiKGH-nh2]+ (where KGH = lysylglycylhistidine) In the latter case, the nickel complex is proposed to act as a catalyst in three separate steps of sulfur oxide chemistry Oxidative damage of guanines led to strand scission after piperidine treatment The observed reactivity represents the first demonstration of DNA damage by sulfite and nickel(II) complexes Importantly, these reactions were conducted using sulfite concentrations relevant to levels known to be cytotoxic Mechanistic studies suggest the importance of both monoperoxysulfate and sulfate radical anion in the observed DNA damage Evidence for formation of guanine radical cation as the initial product of DNA oxidation was found by comparison of the sequence dependence of guanine reactivity i
115 citations
TL;DR: The reaction between the radical cation derived from 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) and phenols follows a kinetic law given by with stoichiometric coefficients between one and two as discussed by the authors.
Abstract: The reaction between the radical cation derived from 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and phenols follows a kinetic law given by with stoichiometric coefficients between one and two. The rate constant is almost unrelated to the structure of the phenol, while the number of ABTS radicals scavenged by each phenol molecule increases with para-substitution. These results are explained in terms of a fast, reversible electron transfer followed by the self-combination of the phenoxy radicals and/or their reaction with another ABTS derived radical action. The relative rate of these processes determine the value of the stoichiometric coefficient. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 219–224, 1997.
103 citations
TL;DR: In this article, high-level ab initio molecular orbital calculations have been carried out for the phenyl cation (1), phenyl radical (2), and phenyl anion (3).
Abstract: High-level ab initio molecular orbital calculations have been carried out for the phenyl cation (1), the phenyl radical (2), and the phenyl anion (3). Our best estimate for the heat of formation (Δ...
95 citations
90 citations
TL;DR: Two organosilver(I) compounds of pylene and perylene presented here exhibit an unusual tetra-η2-coordination, bridging four metal centers and leading to W-type sandwich polymeric chains and sheets.
Abstract: Two organosilver(I) compounds of pylene and perylene presented here exhibit an unusual tetra-η2-coordination, bridging four metal centers and leading to W-type sandwich polymeric chains and sheets. Both compounds, show typical radical cation ESR signals upon irradiation with light or iodination.
86 citations
TL;DR: In this article, the ground-state proton affinity of eight common matrix-assisted laser desorption/ionization (MALDI) matrices was determined. And the [M'+'H]+ ion was generated by methane chemical ionization, trapped and isolated in a Fourier transform ion cyclotron resonance mass spectrometer, allowed to cool for 5 s and reacted with reference compounds of known proton affinities.
Abstract: Protonated molecules of analytes in matrix-assisted laser desorption/ionization (MALDI) are frequently the most intense ions observed, especially when the concentration of alkali metal cations is low. Examination of the laser desorption mass spectra of MALDI matrices usually shows the presence of both molecular radical ions M+• and [M + H]+ ions. With some matrices, the intensity of the [M + H]+ ion is greater than that of the molecular radical ion, e.g. with 2,5-dihydroxybenzoic acid. A logical source for the ions of protonated analyte in MALDI is proton donation from the [M + H]+ ions of the matrix, but donation could also occur from the radical molecular ions. A knowledge of the proton affinities of the common MALDI matrices might be helpful in understanding why some matrices are ‘hotter’ than others and lead to more post-source as well as prompt decay. The ground-state proton affinity of eight common MALDI matrices were determined. For each matrix, the [M + H]+ ion was generated by methane chemical ionization, trapped and isolated in a Fourier transform ion cyclotron resonance mass spectrometer, allowed to cool for 5 s and reacted with reference compounds of known proton affinities. In some cases, the matrix proton affinities are low enough that proton transfer can occur from the ground state [M + H]+ ion to MALDI analytes; in other cases, the matrix proton affinities are so high that some other mechanism for proton transfer is required. © 1997 John Wiley & Sons, Ltd.
83 citations
TL;DR: In this article, the formation of radical anions in the gas phase by various pathways, such as (dissociative) electron attachment and bimolecular reactions, is reviewed together with the thermochemical properties of these ions and the related neutral species.
Abstract: The research concerned with the ion/molecule chemistry of radical anions is reviewed together with the thermochemical properties of these ions and the related neutral species. Attention has been given to the formation of radical anions in the gas phase by various pathways, such as (dissociative) electron attachment and bimolecular reactions. A brief discussion of the electron affinity of the neutral species related to a given radical anion has been included, because this parameter is important for the discussion of the finding that an HA˙ radical is often more acidic than the related H2A molecule. In addition, the thermochemical properties form the basis for a discussion of the various processes that may occur in reactions of radical anions with organic molecules. In particular, electron transfer, proton transfer, and hydrogen atom abstraction reactions have been described for selected radical anions. Other processes that have been reviewed include SN2 substitution and an attack on a carbonyl function in an organic molecule. The interplay between these latter two processes has been described for the reactions of some halogen-substituted carbene radical anions with the methyl ester of trifluoroacetic acid and dimethyl carbonate. The structural characterization of radical anions, distonic radical anions in particular, is summarized with an emphasis on methods such as isomer-specific ion/molecule reactions and collision-induced dissociation or charge-reversal processes. © 1998 John Wiley & Sons, Inc., Mass Spectrom Rev 16, 181–200, 1997
TL;DR: In this paper, a liquid phase tert-butylation of anthracene, naphthalene and thianthrene was carried out at atmospheric pressure and moderate temperatures (below 100°C) in the presence of a series of large pore zeolites and mesoporous aluminosilicates.
Abstract: Liquid phase tert-butylation of anthracene, naphthalene and thianthrene to afford 2-tert-butyl plus 2,6- and 2,7-di tert-butyl derivatives were carried out at atmospheric pressure and moderate temperatures (below 100°C) in the presence of a series of large pore zeolites and mesoporous aluminosilicates including HY-100 (Si/Al 2.6), HY-D1 (Si/Al 2.75), HY-D2 (Si/Al 15), Hβ, HMor and MCM-41. The activity increases from MCM-41 and Mor to β and Y zeolites, the latter being the most active catalysts. In addition, HY zeolites are much more efficient as catalysts than p-toluenesulfonic acid under the same reaction conditions. As alkylating reagent tert-butyl alcohol was employed in isooctane or carbon tetrachloride. It was found that isooctane itself is a powerful and convenient reagent for the tert-butylation. Mechanistic studies using radical scavengers indicates that in the isooctane alkylation tert-butyl cation is the sole intermediate involved. In the case of thianthrene, the concomitant formation of the thianthrenium radical cation during the alkylation reaction has been observed.
TL;DR: In this article, the synthesis and photochemical characterization of two porphyrin-fullerene dyads and a carotenobuckminster fullerene are reviewed.
Abstract: The synthesis and photochemical characterization of two porphyrin-fullerene dyads, two zinc porphyrin-fullerene dyads, and a carotenobuckminsterfullerene are reviewed. In these molecules, the fullerene first excited singlet state may be formed by direct excitation or by singlet-singlet energy transfer from the attached pigment. In polar solvents, the dominant singlet-state decay pathway is photoinduced electron transfer to yield the pigment radical cation and fullerene radical anion. This charge-separated state has a long lifetime relative to the time constant for charge separation. In toluene, in cases where photoinduced electron transfer is slow for thermodynamic reasons, the fullerene singlet state decays by intersystem crossing, and the resulting triplet energy is partitioned between the components of the dyad according to their triplet energies. The results suggest that fullerenes can be valuable components of photochemically active multicomponent molecular systems.
TL;DR: The hypothesis that the peroxidase can act in an oxidase cycle involving ferrous enzyme and compound III, with superoxide as a product, supports the hypothesis that in proteins where the tryptophanyl radical exists as a stable species it must either have its reactivity modified by the protein environment or be inaccessible to oxygen.
TL;DR: Ion abundance vs potential profiles offer additional evidence to corroborate previously proposed reaction mechanisms for pseudodimer sulfonium ion generation from diphenyl sulfide, as well as pyridine addition to DPA radical cation also in methylene chloride at the platinum electrode.
Abstract: A three-electrode electrochemical cell coupled to an electrospray (ES) mass spectrometer has been developed and employed for on-line linear sweep voltammetry-ES mass spectrometry and electrochemical reaction studies. Generated in this type of electrochemistry/mass spectrometry (EC/MS) experiment are ion abundance vs potential profiles that provide molecular weight and structural information pertaining to intermediates and products of anodic oxidation of diphenyl sulfide, anodic pyridation of 9,10-diphenylanthracene, and reduction of nitrobenzene. Anodic oxidation of diphenyl sulfide in methylene chloride media at the platinum electrode yielded diphenyl sulfoxide and pseudodimer sulfonium ion along with products of further oxidation. The obtained ion abundance vs potential profiles offer additional evidence to corroborate previously proposed reaction mechanisms for pseudodimer sulfonium ion generation from diphenyl sulfide, as well as pyridine addition to DPA radical cation also in methylene chloride at th...
TL;DR: In this paper, the reaction of Ac and Ac•+ with its radical cation (Ac•+) is studied at the CCSD(T)/cc-pVTZ/QCISD/6-31G* level of theory and by the B3LYP/6 -31G * density functional method.
Abstract: The reaction of acetylene (Ac) with its radical cation (Ac•+) is studied at the CCSD(T)/cc-pVTZ//QCISD/6-31G* level of theory and by the B3LYP/6-31G* density functional method. Contrary to earlier claims, we find that vertical ionization of the neutral acetylene dimer leads to a bound state which relaxes to a T-shaped ion−molecule complex 8.3 kcal/mol below the separated fragments. Once Ac and Ac•+ have approached to less than an ≈3 A center to center distance, spin and charge begin to delocalize and the complex collapses smoothly to the cyclobutadiene radical cation (CB) via a linear complex (LC1) and/or a cyclopropenylcarbene cation (CC). Rearrangements to other stable C4H4•+ isomers require H shifts which are promoted by localization of the positive charge on one C atom. From LC1 this leads directly to the transition state LC2 for formation of a pivotal intermediate, H2CCCHCH•+ (methyleneallene, MA), which in turn collapses with Ea ≈ 3 kcal/mol to the methylenecyclopropene radical cation (MCP), the mos...
TL;DR: Out‐of‐phase electron spin echo envelope modulation (ESEEM) spectroscopy was used to determine the distance between the primary donor radical cation P680 +· and the quinone acceptor radical anion QA −· in iron‐depleted photosystem II in membrane fragments from spinach that are deprived of the water oxidizing complex.
TL;DR: The photoinduced oxidation of benzyl phenyl sulfides 1-3 has been investigated in MeCN using 9,10-dicyanoanthracene as sensitizer in the presence of O2 as discussed by the authors.
TL;DR: In this paper, a possible mechanism for the formation of cysteine, cysteic acid and partially oxidized cystine species in keratin following exposure to UVC radiation at 254 nm is proposed.
Abstract: A possible mechanism for the formation of cysteine, cysteic acid and partially oxidized cystine species in keratin following exposure to UVC radiation at 254 nm is proposed. The primary photolysis products are the radical ions RSSR'' and RSSR'-, and it is suggested that partially oxidized cystine species are formed from reactions of the radical cation only. The mechanisms m is supported by Fourier transform IR (FT-IR) and optical reflectance spectroscopy of UVC-treated wool exposed to air, oxygen and nitrogen atmospheres, and by previous electron spin resonance (ESR) studies on disulphides exposed to both UV and ionizing radiation. The mechanism supports the Symons assignment for the sulphur radical “species X” as [RS(SR)SR]', rather than RSS'. Exposure of wool to less energetic UV radiation produces different products via a quenching mechanism involving the formation of the radical anion RSSR'- only, which precludes the formation of oxidized cystine intermediates. The formation of S-sulphonate residues at longer wavelengths suggests that a competing singlet oxygen process may also occur.
TL;DR: In this paper, it was shown that in the radical cations of 3,4,5-(MeO) 3 PhCH 2 SPh (4 ) and 2,4.6(MeO), 3 Ph CH 2 Sph (5 ) the positive charge is not localized on the sulfur atom, but in the benzylic aromatic ring.
TL;DR: In this paper, the structure and redox chemistry of the benzo(1,2-d:4,5-d')bis(1 2,3-dithiazole) framework is discussed in the light of the results of ab initio calculations.
Abstract: The condensation of diaminobenzenedithiol with sulfur monochloride leads to the chloride salt of the radical cation of 3,6-dichlorobenzo(1,2-d:4,5-d')bis(1,2,3-dithiazole), dichloro-(BB-123-DTA)(Cl), which can be reduced to neutral dichloro-(BB-123-DTA) with triphenylantimony. A similar condensation with selenium tetrachloride leads, upon reduction, to the corresponding bis(1,2,3-thiaselenazole) dichloro-(BB-123-TSA). The crystal and molecular structures of both compounds have been determined by X-ray diffraction. Both compounds, which are formally antiaromatic 16﷿-systems, exhibit internal bond lengths consistent with a quinoid formulation. The radical cations of both rings have been characterized by ESR spectroscopy; for dichloro-(BB-123-DTA) + g ) 2.0114 and aN ) 0.201 mT, while for dichloro-(BB-123-TSA) + g ) 2.021 and aN ) 0.44 mT. Further oxidation of both rings affords the corresponding dications, both of which have been characterized crystallographically as their AlCl 4 - salts. The structural features of these compounds are consistent with those expected for dithiazolylium (or thiaselenazolylium) derivatives. The structure and redox chemistry of the benzo(1,2-d:4,5-d')bis(1,2,3-dithiazole) framework is discussed in the light of the results of ab initio calculations.
TL;DR: In this paper, the yields, lifetimes and absorption spectra of rhodamine 6G dyes have been obtained using pulse radiolysis and laser flash photolysis.
Abstract: The techniques of pulse radiolysis and laser flash photolysis have been used to obtain both novel and improved data on the yields, lifetimes and absorption spectra of the triplet state, radical anion and radical cation of a number of rhodamine dyes. These parameters have also been measured for the widely-studied rhodamine 6G and are compared with previous data obtained using different techniques. In view of the importance of rhodamine dyes in laser action, in photobiology and in other biological processes, the results of this study provide key data on the photophysical and free radical properties of this important class of dyes.
TL;DR: In this article, the thermal stability of substituted styrenes embedded within the cavities of the large pore zeolite NaY leads to the formation of the corresponding styrene radical cation.
Abstract: Laser flash photolysis of a series of substituted styrenes embedded within the cavities of the large pore zeolite NaY leads to the formation of the corresponding styrene radical cation. The reactivity and spectra of these radical cations embedded within NaY are examined and compared to the reactivity of the same radical cations in solution. It is found that for the highly reactive parent styrene radical cation the zeolite framework provides a strong stabilizing effect. For the 4-methoxy-substituted styrene radical cation the zeolite framework plays less of a role in stabilizing the radical cation as compared to the reactivity of the same radical cation in acetonitrile solution. Rigorous analysis of the thermal stability of 4-methoxystyrene, 4-methylstyrene, and anethole in the zeolite micropores was carried out using two sources of NaY zeolite (Aldrich and The PQ Corporation). It was found that the thermal stability was surprisingly dependent on the source of the NaY zeolite. 4-Methoxystyrene, 4-methylsty...
TL;DR: In this paper, a C8-OH radical adduct is postulated as the intermediate after reaction with persulfate and hydroxyl radicals, and leads to the final product after further oxidation.
Abstract: The oxidation of caffeine with persulfate and hydroxyl radicals in
aqueous solution has been studied by EPR spectroscopy and HPLC analysis.
In both cases the formation of 1,3,7-trimethyluric acid is observed as
the main final product. A C8–OH radical adduct is postulated as
the intermediate after reaction with OH˙, and leads to the final
product after further oxidation. This radical is too short-lived to be
observed by EPR. After oxidation of caffeine with
SO4˙- its radical cation is detected
by EPR. This radical reacts with water to produce the above mentioned
C8–OH radical adduct after deprotonation. In the presence of
dibasic phosphate a spectrum attributed to the C8-phosphate radical
adduct is observed. This radical results from the nucleophilic attack of
the buffer on the radical cation of caffeine. Hydrolysis and oxidation
of the phosphate radical intermediate results in the formation of
1,3,7-trimethyluric acid.
TL;DR: In this paper, a study of ground-state recovery of perylene radical cation (Pe•+), perylene radicals anion, and anthraquinone radical anion (AQ•-) is reported.
Abstract: A study of the dynamics of ground-state recovery of the perylene radical cation (Pe•+), of perylene radical anion (Pe•-), and of anthraquinone radical anion (AQ•-) is reported. In boric acid glass, the excited-state lifetime of Pe•+ is 35 ± 3 ps, while in concentrated sulfuric acid, it is smaller than 15 ps, the time resolution of the experimental setup. The excited-state lifetime of Pe•+, Pe•-, and AQ•- generated by photoinduced intermolecular electron-transfer reaction in MeCN is shorter than 15 ps. In the case of Pe•-, the uncomplete ground-state recovery is ascribed to the occurrence of electron photoejection. The free ion yield in the intermolecular electron-transfer reaction between 9,10-dicyanoanthracene (DCA) and two electron acceptors was measured in a two-pulse experiment, where the second pulse excited the ensuing DCA•-. This excitation has no influence on the magnitude of the free ion yield, indicating a short excited-state lifetime of DCA•-* relative to the time scale of back electron transfe...
TL;DR: The catalytic cycle of heme peroxidases involves two reactive states, compound I and compound II, and both reactions exhibit similar reorganization energies, and the different reactivities of the two enzyme states can be ascribed to a higher apparent rate of activationless electron‐transfer in the compound I reactions.
TL;DR: In this paper, a product study of the photooxidation of the aromatic sulfides 1−5 sensitized by TiO2 in MeCN, in the presence of Ag2SO4, has been carried out.
Abstract: A product study of the photooxidation of the aromatic sulfides 1−5 sensitized by TiO2 in MeCN, in the presence of Ag2SO4, has been carried out. With benzyl phenyl sulfide (1), 4-methoxybenzyl phenyl sulfide (2), and phenethyl phenyl sulfide (4), the photooxidation led mainly to aldehydic products, benzaldehyde, 4-methoxybenzaldehyde, and phenylacetaldehyde, respectively. With benzyl 4-methoxyphenyl sulfide (3), the major product was the corresponding sulfoxide. Diphenyl sulfoxide was the only product observed in the oxidation of diphenyl sulfide (5). These results suggest α-C−H deprotonation as the major reaction path for the radical cations of 1, 2, and 4, and an active role in this respect of the oxygenated basic sites present on the TiO2 surface is envisaged. An interaction of these oxygenated centers with the sulfur atom of the adsorbed radical cation is also suggested to explain the remarkable finding that substantial amounts of sulfoxide are observed with 3 and 5, in spite of the fact that the photo...
TL;DR: In this paper, the authors used hybrid density functional methods B3LYP and B3PW91 isodesmic reactions involving the well-known fragmentation of the benzene radical cation to derive the dissociation energies for the fragmentations of naphthalene, anthracene, and phenanthrene radical cations.
Abstract: The energetics of acetylene loss from anthracene and phenanthrene radical cations and the relative stabilities of the six possible C12H8•+ fragments have been investigated using the hybrid density functional methods B3LYP and B3PW91 Isodesmic reactions involving the well-known fragmentation of the benzene radical cation were employed to derive the dissociation energies for the fragmentations of naphthalene, anthracene, and phenanthrene radical cations CCSD(T) calculations were also carried out for the acetylene loss from benzene and naphthalene radical cations The B3LYP and B3PW91 methods appear to bracket the CCSD(T) result The result for the naphthalene radical cation agrees with the previous suggestion that the benzocyclobutadiene radical cation is the most plausible product of acetylene loss (Chem Phys 1995, 191, 165) Computational results for anthracene and phenanthrene will be discussed in the light of experimental data Acetylene losses from anthracene and phenanthrene radical cations will b
TL;DR: Time-resolved detection by absorption spectroscopy of decay of the lycopene radical cation, of formation of the tocopheroxyl radicals, and of bleaching of Lycopene has shown that alpha-tocopherol is able to reduce the lyCopene radicalcation and thereby partially regenerate lycopenes on a ms timescale.
Abstract: Laser flash photolysis of lycopene in homogeneous chloroform solution together with tocopherol homolopes results in rapid formation of the lycopene radical cation and slower formation of tocopheroxyl radicals. Time-resolved detection by absorption spectroscopy of decay of the lycopene radical cation, of formation of the tocopheroxyl radicals, and of bleaching of lycopene has shown that a-tocopherol is able to reduce the lycopene radical cation and thereby partially regenerate lycopene on a ms timescale. In contrast, lycopene is able to reduce the δ-tocopheroxyl radical, whereas an equilibrium exists between the lycopene radical cation and β- or γ-tocopherol. The relative stability of these antioxidant radicals is hence: a-tocopheroxyl > lycopene radical cation ≈ β-tocopheroxyl - γ-tocopheroxyl > S-toco-pheroxyl.