Showing papers on "Radical ion published in 2003"

Oxidative damage to DNA: formation, measurement and biochemical features.

Abstract: Emphasis is placed in the first part of this survey on mechanistic aspects of the formation of 8-oxo-7,8-dihydroguanine (8-oxoGua) as the result of exposure to z.rad;OH radical, one-electron oxidants and singlet oxygen (1O(2)) oxidation. It was found that 8-oxoGua, which is generated by either hydration of the guanine radical cation or .OH addition at C8 of the imidazole ring, is a preferential target for further reactions with 1O(2) and one-electron oxidants, including the highly oxidizing oxyl-type guanine radical. Interestingly, tandem base lesions that involve 8-oxoGua and a vicinal formylamine residue were found to be generated within DNA as the result of a single .OH radical hit. The likely mechanism of formation of the latter lesions involves the transient generation of 5-(6)-peroxy-6-(5)-hydroxy-5,6-dihydropyrimidyl radicals that may add to the C8 of a vicinal guanine base before undergoing rearrangement. Another major topic which is addressed deals with recent developments in the measurement of oxidative base damage to cellular DNA. This was mostly achieved using the accurate and highly specific HPLC method coupled with the tandem mass spectrometry detection technique. Interestingly, optimized conditions of DNA extraction and subsequent work-up allow the accurate measurement of 11 modified nucleosides and bases within cellular DNA upon exposure to oxidizing agents including UVA and ionizing radiations. Finally, recently available data on the substrate specificity of DNA repair enzymes belonging to the base excision and nucleotide excision pathways are briefly reviewed. For this purpose modified oligonucleotides in which cyclopurine, and cyclopyrimidine nucleosides were site-specifically inserted were synthesized.

Direct observation of guanine radical cation deprotonation in duplex DNA using pulse radiolysis.

Abstract: The dynamics of one-electron oxidation of guanine (G) base mononucleotide and that in DNA have been investigated by pulse radiolysis. The radical cation (G+.) of deoxyguanosine (dG), produced by oxidation with SO4-., rapidly deprotonates to form the neutral G radical (G(−H).) with a rate constant of 1.8 × 107 s-1 at pH 7.0, as judged from transient spectroscopy. With experiments using different double-stranded oligonucleotides containing G, GG, and GGG sequences, the absorbance increases at 625 nm, characteristic of formation of the G(−H)., were found to consist of two phases. The rate constants of the faster (∼1.3 × 107 s-1) and slower phases (∼3.0 × 106 s-1) were similar for the different oligonucleotides. On the other hand, in the oligonucleotide containing G located at the 5‘- and 3‘-terminal positions, only the faster phase was seen. These results suggest that the lifetime of the radical cation of the G:C base pair (GC+.), depending on its location in the DNA chain, is longer than that of free dG. In...

Electronic structures, vibrational spectra, and revised assignment of aniline and its radical cation: Theoretical study

Abstract: Comprehensive studies of the molecular and electronic structures, vibrational frequencies, and infrared and Raman intensities of the aniline radical cation, C6H5NH2+ have been performed by using the unrestricted density functional (UB3LYP) and second-order Moller–Plesset (UMP2) methods with the extended 6-311++G(df,pd) basis set. For comparison, analogous calculations were carried out for the closed-shell neutral aniline. The studies provided detailed insight into the bonding changes that take place in aniline upon ionization. The natural bond orbital (NBO) analysis has revealed that the pπ-radical conjugative interactions are of prime importance in stabilizing the planar, quinoid-type structure of the aniline radical cation. It is shown that the natural charges calculated for aniline are consistent with the chemical properties of this molecule (an ortho- and para-directing power of the NH2 group in electrophilic substitutions), whereas Mulliken charges are not reliable. The theoretical vibrational frequencies of aniline, calculated by the B3LYP method, show excellent agreement with the available experimental data. In contrast, the MP2 method is deficient in predicting the frequencies of several modes in aniline, despite the use of the extended basis set in calculations. The frequencies of aniline radical cation, calculated at the UB3LYP/6-311++G(df,pd) level, are in very good agreement with the recently reported experimental data from zero kinetic energy photoelectron and infrared depletion spectroscopic studies. The clear- cut assignment of the IR and Raman spectra of the investigated molecules has been made on the basis of the calculated potential energy distributions. Several bands in the spectra have been reassigned. It is shown that ionization of aniline can be easily identified by the appearance of the very strong band at about 1490 cm−1, in the Raman spectrum. The redshift of the N–H stretching frequencies and the blueshift of the C–H stretching frequencies are observed in aniline, upon ionization. As revealed by NBO analysis, the frequency shifts can be correlated with the increase of electron density (ED) on the antibonding orbitals (σNH*) and decrease of ED on σCH*, respectively. These effects are associated with a weakening of N–H bonds and strengthening of C–H bonds in the aniline radical cation. The simulated theoretical Raman and infrared spectra of aniline and its radical cation, reported in this work, can be used in further spectroscopic studies of their van der Waals clusters and hydrogen bonded complexes.

Electron Transfer and Singlet Oxygen Mechanisms in the Photooxygenation of Dibutyl Sulfide and Thioanisole in MeCN Sensitized by N-Methylquinolinium Tetrafluoborate and 9,10-Dicyanoanthracene. The Probable Involvement of a Thiadioxirane Intermediate in Electron Transfer Photooxygenations

Abstract: Photooxygenations of PhSMe and Bu2S sensitized by N-methylquinolinium (NMQ+) and 9,10-dicyanoanthracene (DCA) in O2-saturated MeCN have been investigated by laser and steady-state photolysis. Laser photolysis experiments showed that excited NMQ+ promotes the efficient formation of sulfide radical cations with both substrates either in the presence or in absence of a cosensitizer (toluene). In contrast, excited DCA promotes the formation of radical ions with PhSMe, but not with Bu2S. To observe radical ions with the latter substrate, the presence of a cosensitizer (biphenyl) was necessary. With Bu2S, only the dimeric form of the radical cation, (Bu2S)2+•, was observed, while the absorptions of both PhSMe+• and (PhSMe)2+• were present in the PhSMe time-resolved spectra. The decay of the radical cations followed second-order kinetics, which in the presence of O2, was attributed to the reaction of the radical cation (presumably in the monomeric form) with O2-• generated in the reaction between NMQ• or DCA-• a...

First Radical Cation Salt of Paramagnetic Transition Metal Complex Containing TTF as Ligand, [CuII(hfac)2(TTF-py)2](PF6)·2CH2Cl2 (hfac = Hexafluoroacetylacetonate and TTF-py = 4-(2-Tetrathiafulvalenyl-ethenyl)pyridine)

Abstract: The preparation, X-ray crystal structure, EPR data, and magnetic measurement of [Cu(II)(hfac)(2)(TTF-py)(2)](PF(6)).2CH(2)Cl(2), a novel material where the conducting and the localized spin systems are covalently linked through conjugated bridges, are reported. The partial oxidation of the TTF-type organic donor ligand yielded the first radical cation salt of a paramagnetic transition metal complex. Moreover, this compound shows a mixed valence state at the unimolecular level, and additionally, the arrangement of the molecules in the crystal structure revealed the presence of isolated mixed valence TTF dimers.

Electrogenerated Chemiluminescence. 70. The Application of ECL to Determine Electrode Potentials of Tri-n-propylamine, Its Radical Cation, and Intermediate Free Radical in MeCN/Benzene Solutions†

Abstract: We report here electrogenerated chemiluminescence (ECL) observed upon oxidation of aromatic hydrocarbons with different potentials for oxidation to the radical cations (D+•) and tri-n-propylamine (TPrA) in MeCN/benzene solutions. In this system, with TPrA as a coreactant, ECL is generated by reaction of D+• with the free radical intermediate produced upon oxidation of TPrA. ECL was observed from three out of the eight compounds studied. On the basis of the energetic criterion for ECL, the potential for oxidation of the intermediate free radical, Pr2N(CH•)Et, was found to be about −1.7 V vs SCE. By studying the fluorescence quenching of several polycyclic aromatic hydrocarbons (PAHs) by TPrA, the standard potential for oxidation of TPrA to TPrA+• was estimated as 0.9 V vs SCE.

Direct Measurement of Singlet-Triplet Splitting within Rodlike Photogenerated Radical Ion Pairs Using Magnetic Field Effects: Estimation of the Electronic Coupling for Charge Recombination

Abstract: Determining the electronic coupling matrix element, V, for an electron transfer reaction is challenging both experimentally and theoretically. The magnitude of the singlet−triplet splitting (spin−spin exchange interaction), 2J, within a radical ion pair (RP) is directly related to the sum of the squares of the matrix elements that couple the RP state to the ground state and to other energetically proximate excited and ionic states. Each term in this sum is weighted by the reciprocal of the energy gap between the RP state and the particular state to which it is coupled. We present here a series of intramolecular triads with linear, rodlike structures that undergo very efficient two-step electron transfer following direct excitation of a 4-(N-piperidinyl)naphthalene-1,8-dicarboximide (6ANI) chromophore. Attachment of a p-methoxyaniline (MeOAn) donor by means of the piperazine bridge and naphthalene-1,8:4,5-bis(dicarboximide) (NI) or pyromellitimide (PI) acceptors, either directly or through a 2,5-dimethylph...

Time-Dependent Density Functional Study of the Electronic Excited States of Polycyclic Aromatic Hydrocarbon Radical Ions

Abstract: A uniform, comprehensive theoretical interpretation of spectroscopic data is presented for 51 radical ion species of polycyclic aromatic hydrocarbons (PAHs) with the aid of (Tamm−Dancoff) time-dependent density functional theory (TDDFT). TDDFT is capable of predicting the transition energies to the low-lying excited states of PAH ions with quantitative accuracy (the standard deviation from experimental results being less than 0.3 eV) and their intensity patterns qualitatively correctly. The accuracy is hardly affected by the sizes of PAH ions (azulene through dinaphthocoronene), the types of transitions (Koopmans or satellite transitions), the types of orbitals involved (π* ← π, π* ← σ, or σ* ← π transitions), the types of ions (cations or anions), or other geometrical or electronic perturbations (nonplanarity, sp3 carbons, or heterocyclic or nonbenzenoid rings).

A phenol–imidazole pro-ligand that can exist as a phenoxyl radical, alone and when complexed to copper(II) and zinc(II)

Abstract: A new N,O-bidentate, phenol–imidazole pro-ligand 2′-(4′,6′-di-tert-butylhydroxyphenyl)-4,5-diphenyl imidazole (LH) has been designed, synthesised, and characterised. LH possesses no readily oxidisable position (other than the phenol) and involves o- and p-substituents on the phenol ring that prevent radical coupling reactions. LH undergoes a reversible one-electron oxidation to generate the corresponding [LH]˙+ radical cation that possesses phenoxyl radical character. The unusual reversibility of the [LH]/[LH]˙+ redox couple is attributed, at least in part, to a stabilisation of [LH]˙+ by intramolecular O–H⋯N hydrogen bonding. The compounds [CuL2] (1) and [ZnL2] (2) have been synthesised and characterised structurally, spectroscopically, and electrochemically. The crystal structures of 1·4DMF, 1·3MeOH, and 2·2.5MeCN·0.3CH2Cl2 have been determined and each shown to possess an N2O2-coordination sphere, the geometry of which varies with the nature of the metal and the nature of the co-crystallised solvent. 1 and 2 each undergo two, reversible, ligand-based, one-electron oxidations, to form, firstly, [M(L)(L˙)]+ and secondly [M(L˙)2]2+. The [M(L)(L˙)]+ (M = Cu, Zn) cations have been generated by both electrochemical and chemical oxidation and their [ML2][BF4] salts isolated as air-stable, dark green, crystalline solids. The UV/vis, EPR, and magnetic characteristics of these compounds are consistent with each cation involving an MII (M = Cu or Zn) centre bound to a phenoxide (L−) and a phenoxyl radical (L˙). The structural information obtained by a determination of the crystal structures of [CuL2][BF4]·2CH2Cl2 and [ZnL2][BF4]·2CH2Cl2·0.75pentane fully supports this interpretation. For each of these salts, there is a clear indication that the coordinated phenoxyl radical is involved in intramolecular π–π stacking interactions that parallel those in galactose oxidase.

Reaction of carotenoids and ferric chloride: Equilibria, isomerization, and products

Abstract: In the oxidation of carotenoids, ethyl all-trans-8‘-apo-β-caroten-8‘-oate and all-trans-β-carotene, with ferric chloride, several equilibria occur between Fe3+, Fe2+, Cl-, the neutral carotenoid, and its radical cation and dication. The radical cation and dication were found to abstract an electron from Fe2+. Isomerization of carotenoids occurs during the oxidation. In the presence of air, a stable product is formed in high yield during the oxidation. 1H NMR, LC-MS, and optical studies show that this product is the 5,8-peroxide of the starting material. A mechanism for the formation of this compound is proposed.

Photoinduced Electron Transfer in “Two-Point” Bound Supramolecular Triads Composed of N,N-Dimethylaminophenyl-Fullerene-Pyridine Coordinated to Zinc Porphyrin

Abstract: Stable with defined distance and orientation, self-assembled supramolecular triads composed of N,Ndimethylaminophenylfullerene-pyridine bound to zinc porphyrins by a newly developed “two-point” binding strategy are reported. For this, zinc porphyrin was derivatized to bear “hydrogen-bonding” functionalities, carboxylic acid or amide groups, and C60 was functionalized to bear a ligating group, pyridine, and a second electron donor, N,N-dimethylaminophenyl group. The supramolecular triads formed by self-assembly of the zinc porphyrin and fullerene derivatives via the two-point binding method were characterized by spectroscopic and electrochemical techniques and were modeled by using ab initio computational methods. Evidence for axial coordination of the pyridine entity to the zinc metal center and H-bonding between the pyrrolidine ring nitrogen and the pendant carboxylic acid or amide groups was obtained. In the supramolecular triads, the second electron donor, N,N-dimethylaminophenyl, promotes efficient charge separation upon excitation of the zinc porphyrin to yield the radical ion pairs. The radical ion pairs thus generated undergo slow charge recombination to yield relatively long-lived (30-40 ns) charge-separated states.

Mapping the influence of molecular structure on rates of electron transfer using direct measurements of the electron spin-spin exchange interaction.

Abstract: The spin−spin exchange interaction, 2J, in a radical ion pair produced by a photoinduced electron transfer reaction can provide a direct measure of the electronic coupling matrix element, V, for the subsequent charge recombination reaction. We have developed a series of dyad and triad donor−acceptor molecules in which 2J is measured directly as a function of incremental changes in their structures. In the dyads the chromophoric electron donors 4-(N-pyrrolidinyl)- and 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, 5ANI and 6ANI, respectively, and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked to the meta positions of a phenyl spacer to yield 5ANI−Ph−NI and 6ANI−Ph−NI. In the triads the same structure is used, except that the piperidine in 6ANI is replaced by a piperazine in which a para-X−phenyl, where X = H, F, Cl, MeO, and Me2N, is attached to the N‘ nitrogen to form a para-X-aniline (XAn) donor to give XAn−6ANI−Ph−NI. Photoexcitation yields the respective 5ANI+−Ph−NI-, 6ANI+−Ph−NI-, ...

Oligothiophene radical cations: Polaron structure in hybrid DFT and MP2 calculations

Abstract: We investigate the geometric and electronic structure of radical cations in oligothiophene chains by performing hybrid DFT (BHandHLYP) calculations on chains with up to 13 thiophene rings and ab initio MP2 calculations. The BHandHLYP-optimized structures indicate a self-localization of charge, spin, and geometric distortion around the middle of the chain, i.e., the formation of a polaron even in the case of the longer chains. This result contrasts with previous pure DFT findings that indicate complete delocalization of the radical cation over the whole chain. The MP2-optimized structure of octathiophene, an oligomer long enough to allow for possible polaron formation, confirms the presence of a well-localized polaron structure, similar to that obtained at the Hartree–Fock level. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

First estimation of C4-H bond dissociation energies of NADH and its radical cation in aqueous solution.

Abstract: The heterolytic and homolytic C4-H bond dissociation energies of NADH and its radical cation (NADH*+) in aqueous solution were estimated according to the reaction of NADH with N,N,N',N'-tetramethyl-p-phenylenediamine radical cation perchlorate (TMPA*+) in aqueous solution. The results show that the values of the heterolytic and homolytic C4-H bond dissociation energies of NADH in aqueous solution are 53.6 and 79.3 kcal/mol, respectively; the values of the heterolytic and homolytic C4-H bond dissociation energies of NADH*+*+ in aqueous solution are 5.1 and 36.3 kcal/mol, respectively, which, to our knowledge, is first reported. This energetic information disclosed in the present work should be believed to furnish hints to the understanding of the mechanisms for the redox interconversions of coenzyme couple NADH/NAD+ in vivo.

Oxidation of the two beta-carotene molecules in the photosystem II reaction center.

Abstract: We present a spectroscopic characterization of the two nonequivalent beta-carotene molecules in the photosystem II reaction center. Their electronic and vibrational properties exhibit significant differences, reflecting a somewhat different configuration for these two cofactors. Both carotenoid molecules are redox-active and can be oxidized by illumination of the reaction centers in the presence of an electron acceptor. The radical cation species show similar differences in their spectroscopic properties. The results are discussed in terms of the structure and unusual function of these carotenoids. In addition, the attribution of resonance Raman spectra of photosystem II preparations excited in the range 800-900 nm is discussed. Although contributions of chlorophyll cations cannot be formally ruled out, our results demonstrate that these spectra mainly arise from the cation radical species of the two carotenoids present in photosystem II reaction centers.

Photoinduced electron transfer for pyrromethene dyes

Abstract: The photophysics and electrochemistry of pyrromethene dyes (PM), an important class of laser dyes, have been investigated. Selected dyes were found to be moderately good electron donors. Diffusion-limited fluorescence quenching (ca. 6 × 109 M-1 s-1) is observed for the dyes having half-wave oxidation potentials <1.07 V (vs SCE) in the presence of an electron acceptor, such as methyl viologen or pyromellitic dianhydride. For pyrromethene 567, the dye radical cation and triplet state, along with the pyromellitic dianhydride radical anion and the methyl viologen radical cation, were identified using transient absorption spectroscopy. The dye radical cation, which displays a sharp absorption band at 410 nm, is found to be insensitive toward oxygen. Secondary electron transfer involving the reduction of oxygen and producing superoxide radical anion is suggested. Oxygen plays a critical role in PM dye photochemistry as it induces intersystem crossing to generate the dye triplet state and gives rise to products ...

Mechanisms of electron-transfer oxidation of NADH analogues and chemiluminescence. Detection of the keto and enol radical cations.

Abstract: The radical cation of an NADH analogue (BNAH: 1-benzyl-1,4-dihydronicotinamide) has been successfully detected as the transient absorption and ESR spectra in the thermal electron transfer from BNAH to Fe(bpy)(3)(3+) (bpy = 2,2'-bipyridine) and Ru(bpy)(3)(3+). The ESR spectra of the radical cations of BNAH and the dideuterated compound (BNAH-4,4'-d(2)) indicate that the observed radical cation is the keto form rather than the enol form in the tautomerization. The deprotonation rate and the kinetic isotope effects of the keto form of BNAH(*)(+) were determined from the kinetic analysis of the electron-transfer reactions. In the case of electron transfer from BNAH to Ru(bpy)(3)(3+), the chemiluminescence due to Ru(bpy)(3)(2+) was observed in the second electron-transfer step from BNA(*), produced by the deprotonation of the keto form of BNAH(*)(+), to Ru(bpy)(3)(3+). The observation of chemiluminescence due to Ru(bpy)(3)(2+) provides compelling evidence that the Marcus inverted region is observed even for such an intermolecular electron-transfer reaction. When BNAH is replaced by 4-tert-butylated BNAH (4-t-BuBNAH), no chemiluminescence due to Ru(bpy)(3)(2+) has been observed in the electron transfer from 4-t-BuBNAH to Ru(bpy)(3)(3+). This is ascribed to the facile C-C bond cleavage in 4-t-BuBNAH(*)(+). In the laser flash photolysis of a deaerated MeCN solution of BNAH and CHBr(3), the transient absorption spectrum of the enol form of BNAH(*)(+) was detected instead of the keto form of BNAH(*)(+), and the enol form was tautomerized to the keto form. The rate of intramolecular proton transfer in the enol form to produce the keto form of BNAH(*)(+) was determined from the decay of the absorption band due to the enol form and the rise in the absorption band due to the keto form. The kinetic isotope effects were observed for the intramolecular proton-transfer process in the keto form to produce the enol form.

Subpicosecond Pulse Radiolysis Study of Geminate Ion Recombination in Liquid Benzene

Abstract: Subpico- and picosecond pulse radiolyses were carried out to investigate geminate ion recombination in neat benzene. The fast decay of dimer radical cation corresponding to geminate ion recombination with an electron was directly observed for the first time. The characteristic lifetime and average initial distance between a thermalized electron and its parent cation were estimated to be 1.2 ps and 3.2 nm.

Mass-Selected Ion Mobility Studies of the Isomerization of the Benzene Radical Cation and Binding Energy of the Benzene Dimer Cation. Separation of Isomeric Ions by Dimer Formation

Abstract: A mass-selected, ion mobility drift-cell technique has been used to study the isomerization of the benzene radical cation generated by electron impact (EI) ionization. Evidence is presented for the generation of both the benzene and fulvene cations, with a lower limit for the barrier of isomerization Ea(benzene+• → fulvene+•) estimated as Ea > 1.6 eV. The reduced mobilities of the benzene and fulvene cations are nearly similar, making it difficult to separate the two ions in pure helium. However, because of the large difference in the bonding of the benzene and fulvene cations with neutral benzene, the two isomeric ions are readily separated in the presence of neutral benzene in the drift cell. This suggests that isomer ions with similar mobilities can be separated on the basis of the degree of their associations with their neutral precursor in the drift cell. This separation concept allows us to discover the “hidden isomerization” process responsible for the generation of the fulvene cations via EI ioniz...

Decatungstate photocatalyzed electron-transfer reactions of alkenes. Interception of the geminate radical ion pair by oxygen

Abstract: A combination of steady-state and time-resolved techniques has been used to investigate the mechanism of the decatungstate (W 1 0 O 3 2 4-) photocatalyzed oxygenation of cyclohexene, and of a series of hexene isomers, as well as of their saturated counterparts, in acetonitrile. It is demonstrated that (i) alkenes react by both charge-transfer and hydrogen-atom-transfer mechanisms, the former being dominant in each case; (ii) alkanes exclusively react by hydrogen-atom abstraction; (iii) a previously unknown reaction pathway, resulting from interception of the geminate radical ion pair by molecular oxygen, and leading to quantitative formation of hydroperoxides without decatungstate reduction, can significantly contribute to the overall reaction; (iv) no back-electron transfer occurs prior to separation of the geminate pair or its interception by O 2 ; (v) the catalytic activity is strongly reduced in the presence of olefins, due to the formation of very long-lived complexes involving both one- and two-electron-reduced decatungstate; and (vi) the decatungstate anion undergoes strong precomplexation with the reactive solvent acetonitrile.

N−H···π Interactions in Indole···Benzene-h6,d6 and Indole···Benzene-h6,d6 Radical Cation Complexes. Mass Analyzed Threshold Ionization Experiments and Correlated ab Initio Quantum Chemical Calculations

Abstract: Indole−benzene complexes in the neutral and cationic forms were investigated by resonance enhanced two-photon ionization (REMPI) and mass analyzed threshold ionization (MATI) experiments and nonemp...

Use of electrospray ionization mass spectrometry for the investigation of radical cation chain reactions in solution: detection of transient radical cations

Abstract: Electrospray ionization mass spectrometry (ESI-MS) coupled to a microreactor is an excellent tool for the investigation of reactions in solution. Here, we report the first results of our investigations into preparatively interesting electron-transfer-initiated chain reactions in solution which proceed via radical cations as reactive intermediates. The tris(p-bromophenyl)aminium hexachloroantimonate (1)-mediated [2+2] cycloaddition of trans-anethole (2) to give 1,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane (3) was investigated. The reaction proceeds as a radical cation chain reaction via transient intermediates 2 ●+ and 3 ●+ that could be detected and characterized unambiguously directly in the reacting solution by ESI-MS/MS. The identity of the intermediates was confirmed by comparison with authentic MS/MS spectra of 2 ●+ and 3 ●+ obtained by atmospheric pressure chemical ionization mass spectrometry (APCI-MS). In addition, substrate and product can be monitored easily in the reacting solution by APCI-MS.

Base sequence effects in radical cation migration in duplex DNA: support for the polaron-like hopping model.

Abstract: A series of anthraquinone-linked (AQ) duplex DNA oligomers were prepared and investigated. Irradiation of the AQ injects a radical cation into the DNA. The radical cation migrates through the DNA and reacts selectively at GG steps, which leads to strand cleavage after treatment with piperidine. The oligomers investigated in this work were selected to assess the effect on long-distance charge transport of placing a T base (or bases) in a strand of repeating purine bases. With notable exceptions, the amount of strand scission decreases with the distance between the AQ and the GG step. The results are consistent only with models for long-distance transport, such as thermally activated polaron-like hopping, that incorporate radical cation delocalization over two or more adjacent bases.

Lignin peroxidase-catalyzed oxidation of nonphenolic trimeric lignin model compounds: fragmentation reactions in the intermediate radical cations.

Abstract: The H2O2-promoted oxidations of the two nonphenolic β-O-aryl lignin model trimers 1 and 2, catalyzed by lignin peroxidase (LiP) at pH = 3.5, have been studied. The results have been compared with those obtained in the oxidation of 1 and 2 with the genuine one-electron oxidant potassium 12-tungstocobalt(III)ate. These models present a different substitution pattern of the three aromatic rings, and by one-electron oxidation, they form radical cations with the positive charge, which is localized in the dialkoxylated ring as also evidenced by a pulse radiolysis study. Both the oxidations with the enzymatic and with the chemical systems lead to the formation of products deriving from the cleavage of C−C and C−H bonds in a β position with respect to the radical cation with the charge residing in the dialkoxylated ring (3,4-dimethoxybenzaldehyde (5) and a trimeric ketone 6 in the oxidation of 1 and a dimeric aldehyde 8 and a trimeric ketone 9 in the oxidation of 2). These products are accompanied by a dimeric al...

Using spin dynamics of covalently linked radical ion pairs to probe the impact of structural and energetic changes on charge recombination.

Abstract: We have synthesized a series of structurally related, covalently linked electron donor-acceptor triads having highly restricted conformations to study the effects of radical ion pair (RP) structure, energetics, and solvation on charge recombination. The chromophoric electron acceptor in these triads is a 4-aminonaphthalene-1,8-dicarboximide (6ANI), in which the 4-amine nitrogen atom is part of a piperazine ring. The second nitrogen atom of the piperazine ring is part of a para-substituted aniline donor, where the para substituents are X = H, OMe, and NMe(2). The imide group of 6ANI is linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) electron acceptor across a phenyl spacer in a meta relationship. The triads undergo two-step photoinduced electron transfer to yield their respective XAn(*)(+)-6ANI-Ph-NI(*)(-) RP states, which undergo radical pair intersystem crossing followed by charge recombination to yield (3)NI. Time-resolved electron paramagnetic resonance experiments on the spin-polarized RPs and triplet states carried out in toluene and in E-7, a mixture of nematic liquid crystals (LCs), show that for all three triads, the XAn(*)(+)-6ANI-Ph-NI(*)(-) RPs are correlated radical pairs and directly yield values of the spin-spin exchange interaction, J, and the dipolar interaction, D. The values of J are all about -1 mT and show that the LC environment most likely enforces the chair conformation at the piperazine ring, for which the RP distance is larger than that for the corresponding boat conformation. The values of D yield effective RP distances that agree well with those calculated earlier from the spin distributions of the radical ions. Within the LC, changing the temperature shows that the CR mechanism can be changed significantly as the energy levels of the RPs change relative to that of the recombination triplet.

Mechanism of the oxidation of aromatic sulfides catalysed by a water soluble iron porphyrin

Abstract: The oxygen atom transfer–electron transfer (ET) mechanistic dichotomy has been investigated in the oxidation of a number of aryl sulfides by H2O2 in acidic (pH 3) aqueous medium catalysed by the water soluble iron(III) porphyrin 5,10,15,20-tetraphenyl-21H,23H-porphine-p,p′,p″,p‴-tetrasulfonic acid iron(III) chloride (FeTPPSCl). Under these reaction conditions, the iron–oxo complex porphyrin radical cation, P+˙ Fe(IV)O, should be the active oxidant. When the oxidation of a series of para-X substituted phenyl alkyl sulfides (X = OCH3, CH3, H, Br, CN) was studied the corresponding sulfoxides were the only observed product and the reaction yields as well as the reactivity were little influenced by the nature of X as well as by the bulkiness of the alkyl group. Labelling experiments using H218O or H218O2 clearly indicated that the oxygen atom in the sulfoxides comes exclusively from the oxidant. Moreover, no fragmentation products were observed in the oxidation of a benzyl phenyl sulfide whose radical cation is expected to undergo cleavage of the β C–H and C–S bonds. These results would seem to suggest a direct oxygen atom transfer from the iron–oxo complex to the sulfide. However, competitive experiments between thioanisole (E° = 1.49 V vs. NHE in H2O) and N,N-dimethylaniline (E° = 0.97 V vs. NHE in H2O) resulted in exclusive N-demethylation, whereas the oxidation of N-methylphenothiazine (10, E° = 0.95 V vs. NHE in CH3CN) and N,N-dimethyl-4-methylthioaniline (11, E° = 0.65 V vs. NHE in H2O) produced the corresponding sulfoxide with complete oxygen incorporation from the oxidant. Since an ET mechanism must certainly hold in the reactions of 10 and 11, the oxygen incorporation experiments indicate that the intermediate radical cation, once formed, has to react with PFe(IV)O (the reduced form of the iron–oxo complex which is formed by the ET step) in a fast oxygen rebound. Thus, an ET step followed by a fast oxygen rebound is also suggested for the other sulfides investigated in this work.

Dynamics of Photoinduced Electron‐Transfer Processes in Fullerene‐Based Dyads: Effects of Varying the Donor Strength

Abstract: Two classes of fullerene-based donor-bridge-acceptor (D-B-A) systems containing donors of varying oxidation potentials have been synthesized These systems include fullerenes linked to heteroaromatic donor groups (phenothiazine/phenoxazine) as well as substituted anilines (p-anisidine/p-toluidine) In contrast to the model compound, an efficient intramolecular electron transfer is observed from the fullerene singlet excited state in polar solvents An increase in the rate constant and quantum yield of charge separation (kcs and phi cs) has been observed for both classes of dyads, with decrease in the oxidation potentials of the donor groups This observation indicates that the rates of the forward electron transfer fall in the normal region of the Marcus curve The long-lived charge separation enabled the characterization of electron transfer products, namely, the radical cation of the donor and radical anion of the pyrrolidinofullerene, by using nanosecond transient absorption spectroscopy The small reorganization energy (lambda) of C60 coupled with large negative free energy changes (-delta G degree) for the back electron transfer places the back electron process in the inverted region of Marcus curve, thereby stabilizing the electron transfer products

Positive charge carriers on isolated chains of MEH-PPV with broken conjugation: optical absorption and mobility

Abstract: Pulse radiolysis of oxygen-saturated dilute solutions of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene]s in benzene showed charge-transfer from the solvent radical cation to the polymer, yielding positive charge carriers (radical-cations or "holes") on the isolated chains. The charge-transfer reactions are diffusion controlled with (time-dependent) rate coefficients that are determined by the conformation adopted by the polymer chains. The radical cation of the fully conjugated polymer exhibited an absorption maximum at 1.32 eV. The absorption maximum shifted to higher energies with decreasing conjugated fraction, in a way consistent with thermal equilibration of the positive charge over the conjugated segments of different length. The mobility of the positive charge along the fully conjugated MEH-PPV chains was determined to be 0.42 cm 2 V - 1 s - 1 . The introduction of conjugated breaks decreased the charge mobility strongly, which is attributed to the disruption of the π conjugation and/or the conformational disorder caused by the flexibility of the chains at the sites of conjugation breaks.