Showing papers on "Radical ion published in 1996"

On the Enhanced Stability of the Guanine−Cytosine Base-Pair Radical Cation

Abstract: Ab initio (UHF/6-31G*) and density functional (Becke3LYP/D95*) calculations have been used to investigate the structures and stabilities of the radical cations of the DNA bases and base pairs. The calculated structures of the base pairs show excellent agreement with crystallographic data. The most easily oxidizable base, guanine, forms a particularly stable radical cation base pair with cytosine, so that the calculated adiabatic ionization potential for the guanine−cytosine hydrogen-bonded complex is about 0.75 eV lower than that of guanine itself. UBecke3LYP/D95*//UHF/6-31G* calculations show that the shift of the central hydrogen-bonded proton at N1 of guanine to N3 of cytosine is only slightly endothermic (+1.6 kcal mol-1). The product of the corresponding proton shift in the adenine−thymine system is unfavorable by +14.1 kcal mol-1. These results suggest that the guanine−cytosine radical cation represents even more of a thermodynamic sink in oxidized DNA than might be concluded from the ionization pot...

Melatonin and structurally-related, endogenous indoles act as potent electron donors and radical scavengers in vitro.

Abstract: SummaryThe radical scavenging properties of melatonin, structurally-related indoles and known antioxidants were investigated in kinetic competition studies using the specific radical trapping reagent 2,2′-azino-bis(3-ethylbenz-thiazoline-6-sulfonic acid) (ABTS). In the presence of highly reactive radicals, ABTS is oxidized to the stable thiazoline cation radical, ABTS*+ which, due to its intense green color, can be measured photometrically at 420 nm absorbance. The indoles melatonin, 5-methoxytryptophol, 5-methoxyindole acetic acid and 5-methoxytryptamine as well as the phenolic and thiolic antioxidants ascorbic acid, Trolox, and glutathione inhibited ABTS cation radical formation and catalyzed ABTS radical cation reduction. Melatonin was the most potent radical scavenger and electron donor when compared with the methoxylated indole analogs and the other antioxidants tested. Melatonin, the methoxylated indole analogs and the other antioxidants tested acted as potent electron donors which scavenged initiat...

Radical Pair and Triplet State Dynamics of a Photosynthetic Reaction-Center Model Embedded in Isotropic Media and Liquid Crystals

Abstract: The electron spin dynamics associated with intramolecular electron transfer in a photosynthetic model system, which consists of a linear structure of the type A-B-C, is described. In this structure, donor A is a p-methoxyaniline, chromophore B is a 4-amino-1,8-naphthalimide, and acceptor C is a 1,4:5,8-naphthalenediimide. This supramolecular electron donor−acceptor array was isotropically oriented in toluene, and anisotropically oriented in liquid crystal matrices, and studied by time-resolved electron paramagnetic resonance spectroscopy. Photoexcitation of B results in a two-step electron transfer to yield the radical ion pair, A•+-B-C•-. Charge recombination within A•+-B-C•- produces a molecular triplet state, A-B-3*C, which exhibits the unique spin-polarized electron paramagnetic resonance signal that has been observed only in photosynthetic reaction-center proteins.

Ultrafast Photoinduced Electron Transfer in a Chlorophyll-Based Triad: Vibrationally Hot Ion Pair Intermediates and Dynamic Solvent Effects

Abstract: We report ultrafast transient absorption studies of photoinduced electron transfer in the triad molecule zinc methyl 131-desoxopyropheophorbide a−pyromellitimide−1,8:4,5-naphthalenediimide (ZC−PI−NI) in solution. The absorption spectra of the radical anions of PI and NI possess narrow and well-separated absorption bands which permit the direct observation of both the intermediate and final charge-separated states. Selective optical excitation of the ZC donor results in the formation of the intermediate charge-separated state, ZC+−PI-−NI, in less than 2 ps in nonpolar solvents. The PI radical ion within the ZC+−PI-−NI intermediate is vibrationally excited as illustrated by time-dependent changes in the band shape of its transient absorption spectrum. The rate of the initial charge separation reaction forming ZC+−PI-−NI and the subsequent charge shift reaction to form the final state, ZC+−PI−NI-, as well as the appearance of the vibrationally excited intermediate are all highly solvent dependent even for so...

Radical Cations of Sterically Hindered Phenols as Intermediates in Radiation-Induced Electron Transfer Processes

Abstract: The radical cations of several preferably sterically hindered phenols have been observed through the radiation-induced electron transfer from the phenol (ArOH) to the parent cations of nonpolar solvents (n- and iso-butyl chloride, cyclohexane, freon-113). For p-methoxyphenol and its 2,6-di-tert-butyl-substituted derivative, ArOH•+ has been characterized after radiolysis at 77 K by EPR and optical absorption spectroscopies. Their broad optical absorption bands at λ = 430 and <310 nm agree well with those seen at room temperature after pulse radiolysis of the phenol solutions in n-butyl chloride. Generally, also the other phenols studied show similar absorption spectra, but the bands are not so pronounced as in the case of the p-methoxy-substituted ones. Electron transfer from the phenols to n-butyl chloride radical cation proceeds with a rate constant of (1.5−2.0) × 1010 dm3 mol-1 s-1 at 293 K. In the case of phenols containing a further functional group distant from the phenol aromatic ring (phenyl acryla...

Reactivity of Radical Cations. Effect of Radical Cation and Alkene Structure on the Absolute Rate Constants of Radical Cation Mediated Cycloaddition Reactions1

Abstract: Absolute rate constants for the reactions of styrene, 4-methylstyrene, 4-methoxystyrene, and β-methyl-4-methoxystyrene radical cations with a series of alkenes, dienes, and enol ethers have been measured by laser flash photolysis. The measured rate constants correspond to either addition or electron transfer reactions, with the latter predominating when the oxidation potential of the alkene is lower than that of the styrene. The measured rate constants for the diene additions provide some of the first absolute kinetic data for the initial step in the synthetically-useful radical cation mediated Diels−Alder reaction. The addition reactions are sensitive to steric and electronic effects on both the radical cation and the alkene or diene. For example, the reactivity of the radical cations follows the general trend of 4-H > 4-CH3 > 4-CH3O > 4-CH3O-β-CH3. The effects of alkyl substitution on the relative reactivity of alkenes toward styrene radical cations may be summarized as 1,2-dialkyl < 2-alkyl < trialkyl ...

Hydroxylation of phenol over Sn-silicalite-1 molecular sieve: solvent effects

Abstract: Tin-silicalite-1 (MFI) prepared by hydrothermal synthesis has been used as catalyst in the hydroxylation of phenol with aqueous H2O2. Isolated Sn4+ ions which are probably attached to the defect silanols are active in this reaction. At optimum conditions, a H2O2 efficiency of 70% and a ortho to para product ratio of 1.6 have been achieved. The solvent used has a strong influence on the activity. The UV-Vis spectral studies indicate that acetone and acetonitrile probably coordinate strongly with Sn4+ centers preventing the solvolysis of the SiOSn units and the formation of peroxo intermediate. Methanol probably causes the cleavage of SiOSn bond to form SiOH and SnOMe species. Water was found to be an efficient solvent. The formation of EPR active radical ion in presence of H2O2 and H2O and its attenuation in presence of acetone, acetonitrile and methanol support the above conclusion.

Observation of the Hammick Intermediate: Reduction of the Pyridine-2-ylid Ion in the Gas Phase†

Abstract: Azacyclohexatriene-2-ylidene (1), the 2-isomer of pyridine (2), has been generated by one-electron reduction of the corresponding radical cation in neutralization−reionization mass spectrometric experiments. The experimental finding that this molecule is stable on the microsecond time scale is in agreement with results of quantum chemical calculations that indicate both 1 and its radical cation, 1•+, correspond to minima on the C5H5N and C5H5N•+ potential energy surfaces. The calculations predict that 1 is less stable than pyridine, 2, by 50 and 49 kcal/mol (MP2/6-31G** and CASSCF-MP2/6-31G**, respectively) or 47 kcal/mol (B3LYP/6-31G**), whereas the radical cations 1•+ and 2•+ are much closer in energy. The ylid ion 1•+ is predicted to be 6 and 7 kcal/mol lower in energy than 2•+ at the MP2 and CASSCF-MP2/6-31G** levels, respectively, and 1 kcal/mol higher according to the hybrid density functional theory. Calculations also suggest that facile isomerization of the ions is prohibited by an energy barrier,...

Photochemical energy conversion in a helical oligoproline assembly.

Abstract: A general method is described for constructing a helical oligoproline assembly having a spatially ordered array of functional sites protruding from a proline-II helix. Three different redox-active carboxylic acids were coupled to the side chain of cis-4-amino-L-proline. These redox modules were incorporated through solid-phase peptide synthesis into a 13-residue helical oligoproline assembly bearing in linear array a phenothiazine electron donor, a tris(bipyridine)ruthenium(II) chromophore, and an anthraquinone electron acceptor. Upon transient 460-nm irradiation in acetonitrile, this peptide triad formed with 53% efficiency an excited state containing a phenothiazine radical cation and an anthraquinone radical anion. This light-induced redox-separated state had a lifetime of 175 ns and stored 1.65 eV of energy.

Distinguishing features of indolyl radical and radical cation : implications for tryptophan radical studies

Abstract: Tryptophan radicals or radical cations, currently believed to participate in electron transfer in cytochrome c peroxidase, DNA photolyase, galactose oxidase, and perhaps ribonucleotide reductase, a...

Charged Phenyl Radicals

Abstract: The chemical properties of phenyl radicals with different chemically inert charged substituents in the ortho, meta, and para positions were examined in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. The radicals were generated by replacing a chlorine, bromine, or iodine atom in a radical cation of dihalobenzene with a nucleophile and by cleaving the remaining iodine or bromine atom by collision-activated dissociation. The radicals' structures were characterized by ion−molecule and dissociation reactions and by comparison to the reactivity of isomeric reference ions. Ab initio molecular orbital calculations (ROMP2/6-31G*//ROHF/6-31G* + ZPE) carried out for the 2-, 3-, and 4-dehydrophenylsulfonium ions suggest that these three species are nearly equal in energy and significantly less stable than the isomeric thiophenol radical cation. Most of the charge density is localized on the substituent in the charged phenyl radicals examined computationally. The odd-spin density at th...

Density Functional Theory Study of Vibrational Spectra of Anthracene Neutral and Radical Cation

Abstract: Ab initio Hartree-Fock and Becke 3-Lee-Yang-Parr (B3LYP) density functional theory calculations using 6-31G* basis set were carried out to study the vibrational spectra of anthracene neutral (h10 and d10) and radical cation (h10). We report results of the fundamental vibrational frequencies obtained on the basis of the calculations. The assignments of fundamentals show a one-to-one correspondence between the observed and calculated fundamentals.

Oxidative Functionalization of Adamantane and Some of Its Derivatives in Solution

Abstract: 1,2,4,5-Benzenetetracarbonitrile (TCB) is irradiated in the presence of adamantane (1) and some of its derivatives. The singlet excited state of TCB is a strong oxidant, and there is various evidence, including time-resolved spectroscopy, to prove that SET from the alkane to TCB1* takes place and yields the corresponding radical ions. The adamantane radical cation deprotonates from the bridgehead position, and the resulting radical couples with TCB-•. Deprotonation via the radical cation occurs with a number of substituted adamantanes and remains the exclusive or predominating reaction also with derivatives containing a potential electrofugal group, such as one of the following carbocations: t-Bu, CH2OMe, CH2OH (notable here is that C−H deprotonation is more efficient than O−H deprotonation). A carboxy group is lost more efficiently than a proton, however. In contrast, detaching of such cations is the main process when the radical cations of substituted adamantanes is produced anodically. This different ...

Vibrational Spectra of N,N-Dimethylaniline and Its Radical Cation. An Interpretation Based on Quantum Chemical Calculations

Abstract: The resonance Raman spectrum of the radical cation of N,N-dimethylaniline (DMA) has been remeasured, and different types of ab initio calculations have been performed to interpret the vibrational spectra of this species and the deuteriated isotopomers previously studied by Poizat et al. (J. Chem. Phys. 1989, 90, 4697). Density functional methods (UBLYP/6-31G*) give results superior to those of Hartree−Fock calculations for the vibrational frequencies of the radical cation of DMA. The same level of theory was also successfully applied to the vibrational spectrum of aniline. CASSCF calculations were used to study the potential energy surfaces of the lowest excited states of the radical cation of DMA, allowing the estimation of resonance Raman intensities. For the neutral DMA conventional HF/6-31G* calculations allow a straightforward interpretation of the infrared and Raman spectra. The calculations lead to revision of some of the previous empirical assignments.

Mechanism of Sulfoxide Formation through Reaction of Sulfur Radical Cation Complexes with Superoxide or Hydroxide Ion in Oxygenated Aqueous Solution

Abstract: We have characterized and quantified several pathways which transform aliphatic sulfur radical cations into sulfoxides in aqueous solution. Sulfur radical cations were produced photochemically via ...

The reaction between ABTS radical cation and antioxidants and its use to evaluate the antioxidant status of serum samples.

Abstract: The 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical cation can be generated by incubation of ABTS and 2,2'-azo-bis(2- amidinopropane) at 45 degrees C. The ABTS radical cation is stable for several minutes at room temperature and reacts quantitatively and instantaneously with several antioxidants, such as Trolox, ascorbic acid, uric acid, cysteine, glutathione and bilirubin. In contrast, the ABTS radical cation reacts slowly with albumin. When serum is added to a solution of the ABTS radical cation, the bleaching of the radical follows biphasic kinetics, with a fast decay followed by a slow decay that takes place within several minutes. The fast decay is primarily due to uric acid, while the slow decay is related to the protein content of the sample. We propose that this procedure can provide an independent and simultaneous evaluation of the low molecular weight and protein antioxidants present in biological samples such as serum.

Gas‐Phase Experiments Aimed at Probing the Existence of the Elusive Water Oxide Molecule

Abstract: Collisional activation, charge reversal, and six different neutralization - reionization mass spectrometric experiments with [H2,O2]·+ radical cations and [H2,O2]·- radical anions were performed in order to probe the predicated existence of neutral water oxide, H2OO, the long sought after tautomer of hydrogen peroxide, HOOH. The experiments together with ab initio calculations indicate that H2OO is a local minimum on the [H2,O2] potential-energy surface, and the elusive molecule seems to be formed as a transient upon neutralization of the corresponding radical cation H2OO·+ in the gas phase.

Kinetics of parallel electron transfer from beta-carotene to phenoxyl radical and adduct formation between phenoxyl radical and beta-carotene.

Abstract: Phenoxyl radicals generated by laser flash photolysis were found to react with beta-carotene with concomitant beta-carotene bleaching in two parallel reactions with similar rates: (i) formation of a beta-carotene adduct with a (pseudo) first order rate constant of 1-15 x 10(4) s-1 with absorption maximum around 800 nm, and (ii) formation of a beta-carotene radical cation with a (pseudo) first order rate constant of 2-3 x 10(4) s-1 with absorption maximum around 920 nm Both beta-carotene radicals decay on a similar time scale and have virtually disappeared after 100 ms, the beta-carotene adduct by a second order process Oxygen had no effect on beta-carotene bleaching or radical formation and decay The reduction of phenoxyl radicals by beta-carotene may prove important for an understanding of how beta-carotene acts as an antioxidant

Time-resolved resonance Raman spectroscopic studies on the radical anions of menaquinone and naphthoquinone

Abstract: Quinones and their radical ion intermediates have been much studied by vibrational spectroscopy to understand their structure−function relationships in various biological processes. In this paper, we present a comprehensive analysis of vibrational spectra in the structure-sensitive region of both the naphthoquinone (NQ) and 2-methyl-1,4-naphthoquinone (MQ, menaquinone) radical anions using time-resolved resonance Raman and ab initio studies. Specific vibrational mode assignments have been made to all the vibrational frequencies recorded in the experiment. It is observed that the carbonyl and CC stretching frequencies show considerable coupling in NQ and MQ radical anions. Further, the asymmetric substitution present in MQ with respect to NQ shows important signatures in the radical anion spectrum. It is concluded that assignments of vibrational frequencies of asymmetrically substituted quinones must take into consideration the influence of asymmetry on structure and reactivity.

Reaction of cytochrome P450 with cumene hydroperoxide: ESR spin-trapping evidence for the homolytic scission of the peroxide O-O bond by ferric cytochrome P450 1A2.

Abstract: ESR spin trapping was used to investigate the reaction of rabbit cytochrome P450 (P450) 1A2 with cumene hydroperoxide. Cumene hydroperoxide-derived peroxyl, alkoxyl, and carboncentered radicals were formed and trapped during the reaction. The relative contributions of each radical adduct to the composite ESR spectrum were influenced by the concentration of the spin trap. Computer simulation of the experimental data obtained at various 5,5-dimethyl1-pyrroline N-oxide (DMPO) concentrations was used to quantitate the contributions of each radical adduct to the composite ESR spectrum. The alkoxyl radical was the initial radical produced during the reaction. Experiments with 2-methyl-2-nitrosopropane identified the carbon-centered adducts as those of the methyl radical, hydroxymethyl radical, and a secondary carbon-centered radical. The reaction did not require NADPH-cytochrome P450 reductase or NADPH. It is concluded that the reaction involves the initial homolytic scission of the peroxide O-O bond to produce the cumoxyl radical. Methyl radicals were produced from the ‚-scission of the cumoxyl radical. The peroxyl adduct was not observed in the absence of molecular oxygen. We conclude that the DMPO peroxyl radical adduct detected in the presence of oxygen was due to the methylperoxyl radical formed by the reaction of the methyl radical with oxygen. At a higher P450 concentration, a protein-derived radical adduct was also detected.

Sensitized photo-oxidation of sulfur-containing amino acids and peptides in aqueous solution

Abstract: Qualitative and quantitative studies of the photo-oxidation of sulfur-containing amino acids and methionine-containing dipeptides and tripeptides in aqueous solution sensitized by 4-carboxybenzophenone (CB) are reviewed. The mechanism of the photo-oxidation reaction was investigated using the techniques of flash photolysis, steady state photolysis and pulse radiolysis. The rate constants for quenching of the CB triplet by twelve sulfur-containing amino acids and six methionine-containing peptides were determined to be in the range 10 8 –10 9 M −1 s −1 for both neutral and alkaline solutions. The amino acids varied in structure, having different numbers of COCH and NH 2 terminal groups and their sulfur atom at different locations relative to the terminal groups. The methionine-containing peptides were MetGly, GlyMet, MetMet MetGlyGly, GlyMetGly and GlyGlyMet. Time-resolved transient spectra accompanying the quenching events were assigned to the triplet states of CB, ketyl radicals of CB, radical anions of CB and radical anions of CB and radical cations derived from the amino acids and peptides. The radical cations identified were intermolecularly (S.·.S) + -bonded cations, intramolecularly (S.·.N) + -bonded cations and an intramolecularly (S.·.S) + -bonded radical cation that was observed in experiments with MetMet. The quantum yields of the transients and their kinetics of formation and decay were measured by flash photolysis. The quantum yields of CO 2 formation were determined by steady state photolysis. Electron transfer from the sulfur atom to the triplet state of the ketone was found to be a primary photochemical step. A detailed mechanism of the CB-sensitized photo-oxidation of sulfur-containing amino acids and methionine-containing peptides, including primary and secondary photoreactions, is proposed and discussed. Within the mechanism, contrasting behavior between the peptides and amino acids as quenchers is emphasized.

Design of a ruthenium-cytochrome c derivative to measure electron transfer to the radical cation and oxyferryl heme in cytochrome c peroxidase.

Abstract: A new ruthenium-labeled cytochrome c derivative was designed to measure the actual rate of electron transfer to the Trp-191 radical cation and the oxyferryl heme in cytochrome c peroxidase compound I {CMPI(FeIVO,R•+)}. The H39C,C102T variant of yeast iso-1-cytochrome c was labeled at the single cysteine residue with a tris(bipyridyl)ruthenium(II) reagent to form Ru-39-Cc. This derivative has the same reactivity with CMPI as native yCc measured by stopped-flow spectroscopy, indicating that the ruthenium group does not interfere with the interaction between the two proteins. Laser excitation of the 1:1 Ru-39-Cc−CMPI complex in low ionic strength buffer (2 mM phosphate, pH 7) resulted in electron transfer from RuII* to heme c FeIII with a rate constant of 5 × 105 s-1, followed by electron transfer from heme c FeII to the Trp-191 indolyl radical cation in CMPI(FeIVO,R•+) with a rate constant of keta = 2 × 106 s-1. A subsequent laser flash led to electron transfer from heme c to the oxyferryl heme in CMPII(FeI...

Generation of semiquinone radical anion and reactive oxygen (1O2, O2•− and •OH) during the photosensitization of a water-soluble perylenequinone derivative

Abstract: 5(or 8)-sulfonated hypocrellin B (5-SO 3 H-HB) is a water-soluble perylenequinone derivative. Electron spin resonance studies in combination with chemical methods show that 5-SO 3 H-HB is an active photosensitizer. Visible-light illumination of 5-SO 3 H-HB solutions generates a semiquinone radical anion, singlet oxygen, a superoxide radical anion, a hydroxyl radical and hydrogen peroxide. In anaerobic solution, the semiquinone radical anion is predominately photoproduced, via self-electron transfer between the excited and ground states of 5-SO 3 H-HB. In aerobic solution, singlet oxygen is generated in the photosensitization of 5-SO 3 H-HB. In addition, a superoxide radical anion is also generated from 5-SO 3 H-HB on illumination in aerobic solution, via the reduction of oxygen by the semiquinone radical anion. In aqueous solution, the superoxide radical anion formed can rapidly transformed to a hydroxyl radical via the Fenton reaction and via the reaction of the semiquinone radical anion with H 2 O 2 . Based on these results, the pathways for the photodynamic action of 5-SO 3 H-HB have been proposed.

Semiconductor-Catalysed Photoaddition: γ,δ-Unsaturated Amines from Cyclopentene and Schiff Bases

Abstract: Irradiation of methanolic cadmium sulfide suspensions in the presence of Schiff bases and cyclopentene opens an easy access to new γ,δ-unsaturated amines. Their formation can be described as a kind of paired photoelectrolysis induced by the photogenerated electron-hole pair trapped at reactive surface sites of CdS. At the reductive site the imine is converted to an α-aminobenzyl radical by a proton-coupled electron transfer, while at the oxidative site cyclopentene is transformed to the cyclopentenyl radical through deprotonation of the intermediate radical cation. C–C coupling of these radicals affords the addition products. Competitive dimerisation of the radicals produces hydrodimers of the imines and dehydrodimers of the olefin, as demonstrated in the case of 1-phenylcyclohexene. Using optical active imines, we found no significant diastereoselectivity. In the absence of an olefin the alcoholic solvent was added to the imine. The molecular structure of one hydrodimer was resolved by X-ray analysis. When CdS/Pt was used as photocatalyst, hydrolysis products of the imine were formed, suggesting involvement of an intermediate OH radical.