Showing papers on "Radical ion published in 2008"

Fenton-like reaction catalyzed by the rare earth inner transition metal cerium.

Abstract: Cerium (Ce) is a rare earth metal that is not known to have any biological role. Cerium oxide materials of several sizes and shapes have been developed in recent years as a scaffold for catalysts. Indeed even cerium oxide nanoparticles themselves have displayed catalytic activities and antioxidant properties in tissue culture and animal models. Because of ceria's ability to cycle between the +3 and +4 states at oxygen vacancy sites, we investigated whether cerium metal would catalyze a Fenton-like reaction with hydrogen peroxide. Indeed, cerium chloride did exhibit radical production in the presence of hydrogen peroxide, as assessed by relaxation of supercoiled plasmid DNA. Radical production in this reaction was also followed by production of radical cation of 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS). Radical scavengers and spin traps were capable of competing with ABTS for radicals produced in this cerium dependent Fenton-like reaction. Electron paramagnetic resonance experiments reveal both hydroxyl radical and superoxide anion in a reaction containing cerium and hydrogen peroxide. Based on these results we propose that cerium is capable of redox-cycling with peroxide to generate damaging oxygen radicals.

Discrete Photopatternable π-Conjugated Oligomers for Electrochromic Devices

Abstract: Three discrete oligomeric systems including an all-thiophene (T6) system, a thiophene/phenylene (TPTTPT) system, and a thiophene/EDOT/phenylene (TPEEPT) system have been constructed and characterized with emphasis on structural, optical, electrochemical, and spectroelectrochemical properties. For all three chromophores, the radical cation, the dication, and the π-dimer have been identified and characterized. EPR spectroscopy reveals that the radical cations of TPTTPT and TPEEPT have g values of 2.008−2.012 and peak-to-peak widths in the range 4.2−5.3 G. Formation of the radical cation takes place at a lower potential for TPEEPT than for TPTTPT and T6, whereas subsequent oxidation to the dication occurs more easily for TPTTPT than for TPEEPT and T6. We ascribe this observation to more localized charges in the oxidized species of TPEEPT, which is supported by our finding that the radical cation of TPEEPT is less prone to undergo π-dimerization than the radical cations of TPTTPT and T6. All the oxidized spec...

Intramolecular Anodic Olefin Coupling Reactions: The Use of a Nitrogen Trapping Group

Abstract: Anodic olefin coupling reactions using a tosylamine trapping group have been studied. The cyclizations are favored by the use of a less-polar radical cation and more basic reaction conditions. The most significant factor for obtaining good yields of cyclic product is the use of the more basic reaction conditions. The cyclizations allow for the rapid synthesis of substituted proline derivatives.

Cu(II)-mediated generation of triarylamine radical cations and their dimerization. an easy route to tetraarylbenzidines.

Abstract: Triphenylamine (TPA) derivatives react with Cu2+ in acetonitrile to give TPA radical cations which undergo dimerization and deprotonation reactions to yield tetraphenylbenzidines (TPB). Synthetic utility of this reaction is demonstrated using several triphenylamine derivatives, and yields in excess of 80% are obtained in most cases. Involvement of the amine radical cations in these reactions was confirmed by ESR and absorption spectroscopic studies. A mechanism consistent with all observations is proposed. This study also revealed a very good correlation between the free energy change for radical cation formation and product yields.

Clarification of the oxidation state of cobalt corroles in heterogeneous and homogeneous catalytic reduction of dioxygen.

Abstract: Co(III) corroles were investigated as efficient catalysts for the reduction of dioxygen in the presence of perchloric acid in both heterogeneous and homogeneous systems. The investigated compounds are (5,10,15-tris(pentafluorophenyl)corrole)cobalt (TPFCor)Co, (10-pentafluorophenyl-5,15-dimesitylcorrole)cobalt (F 5PhMes 2Cor)Co, and (5,10,15-trismesitylcorrole)cobalt (Mes 3Cor)Co, all of which contain bulky substituents at the three meso positions of the corrole macrocycle. Cyclic voltammetry and rotating ring-disk electrode voltammetry were used to examine the catalytic activity of the compounds when adsorbed on the surface of a graphite electrode in the presence of 1.0 M perchloric acid, and this data is compared to results for the homogeneous catalytic reduction of O 2 in benzonitrile containing 10 (-2) M HClO 4. The corroles were also investigated as to their redox properties in nonaqueous media. A reversible one-electron oxidation occurs at E 1/2 values between 0.42 and 0.89 V versus SCE depending upon the solvent and number of fluorine substituents on the compounds, and this is followed by a second reversible one-electron abstraction at E 1/2 = 0.86 to 1.18 V in CH 2Cl 2, THF, or PhCN. Two reductions of each corrole are also observed in the three solvents. A linear relationship is observed between E 1/2 for oxidation or reduction and the number of electron-withdrawing fluorine groups on the compounds, and the magnitude of the substituent effect is compared to what is observed in the case of tetraphenylporphyrins containing meso -substituted C 6F 5 substituents. The electrochemically generated forms of the corrole can exist with Co(I), Co(II), or Co(IV) central metal ions, and the site of the electron-transfer in each oxidation or reduction of the initial Co(III) complex was examined by UV-vis spectroelectrochemistry. ESR characterization was also used to characterize singly oxidized (F 5PhMes 2Cor)Co, which is unambiguously assigned as a Co(III) radical cation rather than the expected Co(IV) corrole with an unoxidized macrocyclic ring.

An exploration of mechanisms for the transformation of 8-oxoguanine to guanidinohydantoin and spiroiminodihydantoin by density functional theory.

Abstract: The potential energy surface for formation of 2-amino-5-hydroxy-7,9-dihydropurine-6,8-dione (5-OH-OG), guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp) from 8-oxoguanine (8-oxoG) has been mapped out using B3LYP density functional theory, the aug-cc-pVTZ and 6-31+G(d,p) basis sets and the IEF-polarizable continuum model (PCM) solvation model. Three pathways for formation of 5-OH-OG from 8-oxoG were evaluated: (A) stepwise loss of two electrons and two protons to form the quinonoid intermediate 2-amino-7,9-dihydro-purine-6,8-dione (8-oxoG(ox)) followed by hydration; (B) stepwise loss of two electrons and one proton and net addition of hydroxide, in which the key step is nucleophilic addition to the 8-oxoG radical cation; and (C) stepwise loss of one electron and one proton and addition of hydroxyl radical to the 8-oxoG radical cation. The data suggest that all three pathways are energetically feasible mechanisms for the formation of 5-OH-OG, however, Pathway A may be kinetically favored over Pathway B. Although lower in energy, Pathway C may be of limited biological significance since it depends on the local concentration of hydroxyl radical. Pathways for hydrolysis and decarboxylation of 5-OH-OG to form Gh via either a carboxylic acid or substituted carbamic acid intermediate have been evaluated with the result that cleavage of the N1-C6 bond is clearly favored over that of the C5-C6 bond. Formation of Sp from 5-OH-OG via stepwise proton transfer and acyl migration or ring opening followed by proton transfer and ring closure have also been explored and suggest that deprotonation of the hydroxyl group facilitates a 1,2 acyl shift. Results of the calculations are consistent with experimental studies showing dependence of the Gh/Sp product ratio on pH. Under neutral and basic conditions, the data predict that formation of Sp is kinetically favored over the pathways for formation of Gh. Under acidic conditions, Gh is predicted to be the kinetically favored product.

Probing the Compound I-like reactivity of a bare high-valent oxo iron porphyrin complex: the oxidation of tertiary amines.

Abstract: The mechanisms of oxidative N-dealkylation of amines by heme enzymes including peroxidases and cytochromes P450 and by functional models for the active Compound I species have long been studied. A debated issue has concerned in particular the character of the primary step initiating the oxidation sequence, either a hydrogen atom transfer (HAT) or an electron transfer (ET) event, facing problems such as the possible contribution of multiple oxidants and complex environmental effects. In the present study, an oxo iron(IV) porphyrin radical cation intermediate 1, [(TPFPP)*+ Fe(IV)=O]+ (TPFPP = meso-tetrakis (pentafluorophenyl)porphinato dianion), functional model of Compound I, has been produced as a bare species. The gas-phase reaction with amines (A) studied by ESI-FT-ICR mass spectrometry has revealed for the first time the elementary steps and the ionic intermediates involved in the oxidative activation. Ionic products are formed involving ET (A*+, the amine radical cation), formal hydride transfer (HT) from the amine ([A(-H)]+, an iminium ion), and oxygen atom transfer (OAT) to the amine (A(O), likely a carbinolamine product), whereas an ionic product involving a net initial HAT event is never observed. The reaction appears to be initiated by an ET event for the majority of the tested amines which included tertiary aliphatic and aromatic amines as well as a cyclic and a secondary amine. For a series of N,N-dimethylanilines the reaction efficiency for the ET activated pathways was found to correlate with the ionization energy of the amine. A stepwise pathway accounts for the C-H bond activation resulting in the formal HT product, namely a primary ET process forming A*+, which is deprotonated at the alpha-C-H bond forming an N-methyl-N-arylaminomethyl radical, A(-H)*, readily oxidized to the iminium ion, [A(-H)]+. The kinetic isotope effect (KIE) for proton transfer (PT) increases as the acidity of the amine radical cation increases and the PT reaction to the base, the ferryl group of (TPFPP)Fe(IV)=O, approaches thermoneutrality. The ET reaction displayed by 1 with gaseous N,N-dimethylaniline finds a counterpart in the ET reactivity of FeO+, reportedly a potent oxidant in the gas phase, and with the barrierless ET process for a model (P)*+ Fe(IV)=O species (where P is the porphine dianion) as found by theoretical calculations. Finally, the remarkable OAT reactivity of 1 with C6F5N(CH3)2 may hint to a mechanism along a route of diverse spin multiplicity.

A Polymeric Pseudorotaxane Constructed from Cucurbituril and Aniline, and Stabilization of Its Radical Cation

Abstract: Polymeric (pseudo)rotaxanes constructed through the encapsulation of covalent polymers, especially p-conjugated polymers, by macrocyclic molecules have attracted increasing interest in the fields of both supramolecular chemistry and polymer chemistry because of their fascinating properties, which are different from those of the original covalent polymers. The early investigations on such polymeric (pseudo)rotaxanes usually involved cyclodextrins or crown ethers. The corresponding cucurbit[n]uril (CB[n], n is usually 6–8) systems have also recently been investigated as a consequence of their considerable binding abilities towards many cationic guests. The first attempts to construct main-chain cucurbituril-threaded polymers were performed by Kim and co-workers as well as by Tuncel and Steinke. Recently, Garcia and co-workers synthesized a polypseudorotaxane consisting of the conjugated polymer poly(phenylene vinylene) threaded through multiple cucurbit[7]uril (CB[7]) molecules, and investigated its enhanced luminescenece properties. However, many studies have been performed on conducting polymers to explore their potential applications in material chemistry and electrochemistry. Among the wide variety of conducting polymers known, polyaniline (PANI) is widely regarded as one of the eminent species because of its high conductivity (up to 1000 Scm ) and its wide applications in electronic and optical materials. In general, numerous radical cations exist in the emeraldine state, that is, the conductive doped form, of PANI, and the stabilization of these radical cations is very important to the stability and conductivity of PANI. The research groups of Goux, Han, and Dawn reported, respectively, the stabilization of PANI radical cations by modulation of the pH value, modification with propylthiosulfonate groups, and complexation with DNA. Recently, Anderson and co-workers reported the stabilization of a radical cation in a cucurbituril/oligoaniline rotaxane: they found that complexation of an oligoaniline with CB[7] could stabilize its radical cation. However, studies on the threading of CB[7]s on long PANI chains and the resulting stabilization of the radical cation have so far not been reported to the best of our knowledge. Herein, we report the construction of a polypseudorotaxane 1 by threading a PANI chain through numerous CB[7] cavities. We have also used microscopy, EPR, and cyclic voltammetry to investigate the morphology and stabilization of PANI radical cations. The results showed that, compared with free PANI, the CB[7]/PANI polypseudorotaxane had higher water solubility and the radical cation had greater stability as a result of the complexation with CB[7]. These effects will allow the further application of PANI-based supramolecular assemblies in many fields of material chemistry. The good host–guest complexation ability of cucurbiturils with aniline derivatives enabled polypseudorotaxane 1 to be conveniently prepared by a polycondensation reaction of aniline in the presence of CB[7] (Scheme 1) by using a

Reaction mechanism of apocarotenoid oxygenase (ACO): a DFT study.

Abstract: The mechanism of the oxidative cleavage catalyzed by apocarotenoid oxygenase (ACO) was studied by using a quantum chemical (DFT: B3 LYP) method. Based on the available crystal structure, relatively large models of the unusual active-site region, in which a ferrous ion is coordinated by four histidines and no negatively charged ligand, were selected and used in the computational investigation of the reaction mechanism. The results suggest that binding of dioxygen to the ferrous ion in the active site promotes one-electron oxidation of carotenoid leading to a substrate radical cation and a Fe-bound superoxide radical. Recombination of the two radicals, which can be realized in at least two different ways, yields a reactive peroxo species that subsequently evolves into either a dioxetane or an epoxide intermediate. The former easily decays into the final aldehyde products, whereas the oxidation of the epoxide to the proper products of the reaction requires involvement of a water molecule. The calculated activation barriers favor the dioxetane mechanism, yet the mechanism involving the epoxide intermediate cannot be ruled out.

Synthesis and Photoinduced Electron‐Transfer Properties of Phthalocyanine–[60]Fullerene Conjugates

Abstract: A series of three novel ZnPc-C60 conjugates (Pc=phthalocyanine) 1 a-c bearing different spacers (single, double, and triple bond) between the two electroactive moieties was synthesized and compared to that of ZnPc-C60 conjugate 2, in which the two electroactive moieties are linked directly. The synthetic strategy- towards the preparation of 1 a-c- involved palladium-catalyzed cross-coupling reactions over a monoiodophthalocyanine precursor 4 to introduce the corresponding spacer, and subsequent dipolar cycloaddition reaction to C60. Detailed photophysical investigations of 1 a-c and 2 prompted an intramolecular electron transfer that evolves from the photoexcited ZnPc to the electron-accepting C60. In particular, with the help of femtosecond laser photolysis charge separation was indeed confirmed as the major deactivation channel. Complementary time-dependent density functional calculations supported the spectral assignment, namely, the spectral identity of the ZnPc(*+) radical cation and the C60 (*-) radical anion as seen in the differential absorption spectra. The lifetimes of the correspondingly formed radical ion-pair states depend markedly on the solvent polarity: they increase as polarity decreases. Similarly, although to a lesser extent, the nature of the linker impacts the lifetime of the radical ion-pair states. In general, the lifetimes of these states tend to be shortest in the system that lacks any spacer at all (2), whereas the longest lifetimes were found in the system that carries the triple-bond spacer (1 a).

Kinetics and mechanisms of chlorine dioxide oxidation of tryptophan.

Abstract: The reactions of aqueous ClO2 (*) and tryptophan (Trp) are investigated by stopped-flow kinetics, and the products are identified by high-performance liquid chromatography (HPLC) coupled with electrospray ionization mass spectrometry and by ion chromatography. The rates of ClO2 (*) loss increase from pH 3 to 5, are essentially constant from pH 5 to 7, and increase from pH 7 to 10. The reactions are first-order in Trp with variable order in ClO2 (*). Below pH 5.0, the reactions are second- or mixed-order in [ClO2 (*)], depending on the chlorite concentration. Above pH 5.0, the reactions are first-order in [ClO2 (*)] in the absence of added chlorite. At pH 7.0, the Trp reaction with ClO2 (*) is first-order in each reactant with a second-order rate constant of 3.4 x 10(4) M(-1) s(-1) at 25.0 degrees C. In the proposed mechanism, the initial reaction is a one-electron oxidation to form a tryptophyl radical cation and chlorite ion. The radical cation deprotonates to form a neutral tryptophyl radical that combines rapidly with a second ClO 2 (*) to give an observable, short-lived adduct ( k obs = 48 s(-1)) with proposed C(H)-OClO bonding. This adduct decays to give HOCl in a three-electron oxidation. The overall reaction consumes two ClO2 (*) per Trp and forms ClO2- and HOCl. This corresponds to a four-electron oxidation. Decay of the tryptophyl-OClO adduct at pH 6.4 gives five initial products that are observed after 2 min and are separated by HPLC with elution times that vary from 4 to 17 min (with an eluent of 6.3% CH 3OH and 0.1% CH 3COOH). Each of these products is characterized by mass spectrometry and UV-vis spectroscopy. One initial product with a molecular weight of 236 decays within 47 min to yield the most stable product, N-formylkynurenine (NFK), which also has a molecular weight of 236. Other products also are observed and examined.

Electron-transfer-catalyzed dimerization of trans-anethole: detection of the distonic tetramethylene radical cation intermediate by extractive electrospray ionization mass spectrometry.

Abstract: The desolvating droplets of EESI were used as microreactor for the mass spectrometric study of short-lived transients of reactions in condensed liquid phase in the millisecond time region. The electron-transfer-catalyzed dimerization of trans-anethole was investigated and the intermediates in the radical cation chain reaction were isolated and characterized by MS−MS. The distonic tetramethylene radical cation 3o•+ was detected as intermediate, and its cyclization to give the cyclobutane radical cation 3c•+ was observed giving strong evidence that the reaction takes place stepwise.

An allylic ketyl radical intermediate in clostridial amino-acid fermentation.

Abstract: The human pathogenic bacterium Clostridium difficile thrives by the fermentation of l-leucine to ammonia, CO(2), 3-methylbutanoate and 4-methylpentanoate under anaerobic conditions The reductive branch to 4-methylpentanoate proceeds by means of the dehydration of (R)-2-hydroxy-4-methylpentanoyl-CoA to 4-methylpent-2-enoyl-CoA, which is chemically the most demanding step Ketyl radicals have been proposed to mediate this reaction catalysed by an iron-sulphur-cluster-containing dehydratase, which requires activation by ATP-dependent electron transfer from a second iron-sulphur protein functionally similar to the iron protein of nitrogenase Here we identify a kinetically competent product-related allylic ketyl radical bound to the enzyme by electron paramagnetic resonance spectroscopy employing isotope-labelled (R)-2-hydroxy-4-methylpentanoyl-CoA species We also found that the enzyme generated the stabilized pentadienoyl ketyl radical from the substrate analogue 2-hydroxypent-4-enoyl-CoA, supporting the proposed mechanism Our results imply that also other 2-hydroxyacyl-CoA dehydratases and the related benzoyl-CoA reductases-present in anaerobically living bacteria-employ ketyl radical intermediates The absence of radical generators such as coenzyme B12, S-adenosylmethionine or oxygen makes these enzymes unprecedented in biochemistry

Direct observation of mixed-valence and radical cation dimer states of tetrathiafulvalene in solution at room temperature: association and dissociation of molecular clip dimers under oxidative control.

Abstract: We have observed the mixed-valence and radical cation dimer states of a glycoluril-based molecular clip with tetrathiafulvalene (TTF) sidewalls at low concentration (1 mM) at room temperature This molecular clip has four consecutive anodic steps in its cyclic voltammogram, which suggests a sequential oxidation of these TTF sidewalls to generate species existing in several distinct charge states: neutral monomers, mixed-valence dimers, radical cation dimers, and fully oxidized tetracationic monomers The observation of characteristic NIR spectroscopic absorption bands at approximately 1650 and 830 nm in spectroelectrochemistry experiments supports the presence of intermediary mixed-valence and radical cation dimers, respectively, during the oxidation process The stacking of four TTF radical cations in the dimer led to the appearance of a charge-transfer band at approximately 946 nm Nanoelectrospray ionization mass spectrometry was used to verify the tricationic state and confirm the existence of other different charged dimers during the oxidation of the molecular clip

Formation of Methanofullerene Cation in Bulk Heterojunction Polymer Solar Cells Studied by Transient Absorption Spectroscopy

Abstract: Photogenerated charge carriers for blend films of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been investigated by transient absorption spectroscopy. The blend film with a low PCBM fraction ( 30 wt %) exhibits a major absorption band at ∼900 nm, which is characteristic of the PCBM radical cation. For identification of charge carriers, the absorption spectrum and molar absorption coefficient of each charged species have been evaluated separately using various combinations of electron donor and acceptor materials. Consequently, the MDMO-PPV hole polaron has been found to have a broad absorption at ∼950 nm and the PCBM radical anion and cation show a distinct absorption at 1020 and 890 nm, respectively. On the basis of these absorption spectra, the transient spectra observed for the blend films have been simulated. The spectrum for a low PCBM fraction is well reproduced by superposition of the absorption spectra of the MDMO-PPV hole polaron and PCBM radical anion. On the other hand, the spectrum for a high PCBM fraction is well reproduced by superposition of the absorption spectra of the MDMO-PPV hole polaron, PCBM radical anion, and PCBM radical cation, which indicates that the PCBM radical cation is formed in the blend films with PCBM at a high concentration. Possible mechanisms for the formation of the PCBM radical cation in the blend are also discussed.

Direct observation of the preference of hole transfer over electron transfer for radical ion pair recombination in donor-bridge-acceptor molecules.

Abstract: Understanding how the electronic structures of electron donor−bridge−acceptor (D−B−A) molecules influence the lifetimes of radical ion pairs (RPs) photogenerated within them (D+•−B−A-•) is critical to designing and developing molecular systems for solar energy conversion. A general question that often arises is whether the HOMOs or LUMOs of D, B, and A within D+•−B−A-• are primarily involved in charge recombination. We have developed a new series of D−B−A molecules consisting of a 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) electron donor linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor via a series of Phn oligomers, where n = 1−4, to give DMJ-An-Phn-NI. The photoexcited charge transfer state of DMJ-An acts as a high-potential photoreductant to rapidly and nearly quantitatively transfer an electron across the Phn bridge to produce a spin-coherent singlet RP 1(DMJ+•-An-Phn-NI-•). Subsequent radical pair intersystem crossing yields 3(DMJ+•-An-Phn-NI-•). Charge recombination within the ...

Metal bacteriochlorins which act as dual singlet oxygen and superoxide generators.

Abstract: A series of stable free-base, Zn(II) and Pd(II) bacteriochlorins containing a fused six- or five-member diketo- or imide ring have been synthesized as good candidates for photodynamic therapy sensitizers, and their electrochemical, photophysical, and photochemical properties were examined. Photoexcitation of the palladium bacteriochlorin affords the triplet excited state without fluorescence emission, resulting in formation of singlet oxygen with a high quantum yield due to the heavy atom effect of palladium. Electrochemical studies revealed that the zinc bacteriochlorin has the smallest HOMO-LUMO gap of the investigated compounds, and this value is significantly lower than the triplet excited-state energy of the compound in benzonitrile. Such a small HOMO-LUMO gap of the zinc bacteriochlorin enables intermolecular photoinduced electron transfer from the triplet excited state to the ground state to produce both the radical cation and the radical anion. The radical anion thus produced can transfer an electron to molecular oxygen to produce superoxide anion which was detected by electron spin resonance. The same photosensitizer can also act as an efficient singlet oxygen generator. Thus, the same zinc bacteriochlorin can function as a sensitizer with a dual role in that it produces both singlet oxygen and superoxide anion in an aprotic solvent (benzonitrile).

Factors controlling lifetimes of photoinduced charge-separated states of fullerene-donor molecular systems

Abstract: Lifetimes of the photoinduced charge-separated states for composite molecular systems of covalently bonded fullerenes with electron donors are usually very long compared with those of the flat electron-acceptor molecules with functional groups such as keton and cyano-groups. In order to confirm such long-lived charge-separated states, it is very important to carefully identify the transient radical ion pairs by observing both the radical anions and the radical cations in the same time. However, in general, assignments of the transient species are not easy, because the absorption bands overlap with those of other species such as short lived S1-states and long-lived T1-states. In this review, we selected reliable data of the dyads studied mainly by the transient absorption spectral methods in the wide wavelength regions (UV–vis–NIR) and wide time regions (picosecond, nanosecond, microsecond, and millisecond). The lifetimes of the charge-separated states evaluated at room temperature are summarized in order to reveal the factors controlling the lifetimes of photoinduced charge-separated states of fullerene-donor molecular systems. In most cases, the rate parameters and efficiencies for photoinduced charge-separation and charge-recombination processes can be reasonably interpreted by the concepts based on the Marcus theory; some Marcus parameters were experimentally evaluated by temperature dependency of the rate parameters. In addition, spin-multiplicity of the charge-separation precursors and generated radical ion pair may play important roles. As a whole, selections of the kinds of the electron-donors, lengths of the bridges, solvent polarities, which strongly affect the photoinduced electron transfer processes, are all important to achieve the long lifetimes of the charge-separated states.

Selective one-electron oxidation of duplex DNA oligomers: reaction at thymines

Abstract: The one-electron oxidation of duplex DNA generates a nucleobase radical cation (electron “hole”) that migrates long distances by a hopping mechanism. The radical cation reacts irreversibly with H2O or O2 to form oxidation products (damaged bases). In normal DNA (containing the four common DNA bases), reaction occurs most frequently at guanine. However, in DNA duplexes that do not contain guanine (i.e., those comprised exclusively of A/T base pairs), we discovered that reaction occurs primarily at thymine and gives products resulting from oxidation of the T-C5 methyl group and from addition to its C5–C6 double bond. This surprising result shows that it is the relative reactivity, not the stability, of a nucleobase radical cation that determines the nature of the products formed from oxidation of DNA. A mechanism for reaction is proposed whereby a thymine radical cation may either lose a proton from its methyl group or H2O/O2 may add across its double bond. In the latter case, addition may initiate a tandem reaction that converts both thymines of a TT step to oxidation products.

Tunneling in C−H Oxidation Reactions by an Oxoiron(IV) Porphyrin Radical Cation: Direct Measurements of Very Large H/D Kinetic Isotope Effects

Abstract: Rate constants for oxidations of benzyl alcohol-d0 and -d7 by oxoiron(IV) tetramesitylporphyrin radical cation perchlorate in acetonitrile were measured in single turnover kinetic studies. The kinetic isotope effect (kH/kD) increased from 28 at 23 °C to 360 at −30 °C due to extensive hydrogen atom tunneling that was analyzed in terms of a parabolic energy barrier to tunneling. Similarly, large KIE values were found for oxidations of ethylbenzene-d0 and -d10 at room temperature. The large KIE values are a function of the porphyrin identity, and porphyrins containing electron-withdrawing groups display normal KIEs. KIEs found under catalytic turnover conditions are somewhat smaller than those obtained in single turnover reactions. The results should serve as benchmarks for computational studies of C−H oxidations by porphyrin and heme-iron-oxo systems.

Structure of the Observable Histidine Radical Cation in the Gas Phase: A Captodative α‐Radical Ion

Abstract: Protein-based radicals play crucial roles in some of the greatest biosynthetic challenges in nature, including photosynthesis and substrate oxidation. Radical centers have been located on aromatic and sulfur-containing amino acid residues, as well as glycine residues. Invariably these charged or neutral radical species are generated through involvement of an adjacent metal cofactor. The positions of charge and spin in the radical cations are paramount for reactivity modulation and proton-coupled electron transfer, but obtaining structural details is difficult even for the simplest models. 2] Experiments in vacuo permit the investigation of intrinsic properties of radical cations in the absence of a reactivity-modulating environment. Radical cations of amino acids and peptides have been produced in vacuo by one-electron transfer in collision-induced dissociations (CIDs) of a ternary complex system comprising copper(II), an auxiliary ligand, and the amino acid or peptide. Such ternary complexes are efficiently generated by electrospray ionization, and probed downstream by using mass spectrometry (MS). Under appropriate conditions, CID of the complex yields the radical cation of the amino acid or peptide that can be isolated and trapped for spectroscopic interrogation. Herein, we report the first infrared multiple photon dissociation (IRMPD) spectroscopic experiments on a prototypical amino acid radical cation, HisC, and its ternary complex ion. In a recent article, Ke et al. showed that, by judicious choice of the auxiliary ligand, HisC of different stabilities are formed through CID of the ternary complex ion. In particular, the use of 2,2’:6’,2’’-terpyridine (tpy) as the ligand leads primarily to a HisC that is stable on the MS timescale and can be isolated and fragmented at a subsequent MS stage; by contrast, employing acetone as the ligand results in a metastable HisC and only its fragment ions are observed. Furthermore, the former, relatively stable HisC fragments by losing a water molecule to give [b1-H]C + and then CO to give [a1-H]C , whereas the latter, metastable HisC dissociates spontaneously by losing first CO2 to give the 4-ethaniminoimidazole radical cation, which then loses methanimine to give the 4-methyleneimidazole radical cation. Density functional theory (DFT) calculations at the (unrestricted) UB3LYP/6-311 + + G(d,p) level of theory predicted five low-energy HisC structures. Scheme 1 shows these structures with additional, new information on the barriers against their interconversions (see Figures S2 and S3 in the Supporting Information for details). Ke et al. postulated that the stable and metastable HisC are His5 (the structure at the global minimum) and His2, respectively. His5 is a captodative aradical ion that differs from the canonical His1 structure in having the a-CH hydrogen migrated to the imino nitrogen of the imidazole ring; His2 is best described as a 4-ethaniminoimidazole radical cation solvated by CO2. His2–His5 are all unconventional structures, and experimental verification of the HisC structure is highly desirable for confirmation of the key roles played by spin and charge delocalization in HisC stabilization. Figure 1 compares the experimental IRMPD spectrum collected for HisC with the DFT-predicted IR spectra of His1–His5. It is apparent that only one predicted IR spectrum, that of His5, resembles the measured IRMPD spectrum. In particular, His5 is the only isomer predicted to exhibit two bands, 1596 and 1653 cm , which are assigned as NH2 scissoring and C=O stretching, respectively, that match the 1606 and 1666 cm 1 bands in the IRMPD spectrum. The lack of a strong band at around 1780–1790 cm 1 in the IRMPD spectrum rules out the presence of a significant fraction of His3 and His4. Similarly, His1 can be ruled out by the presence of the doublet, 1606 and 1666 cm , and the absence of spectroscopic details in the region of 1077– 1320 cm . His2 can be eliminated by the absence of peaks at around 810–820 cm 1 and by the low endothermicity against loss of the solvating CO2 (5 kcalmol ). We interpret the excellent match between the experimental IRMPD spectrum and the predicted IR spectrum of His5 to indicate that His5 is the only abundant species present. This degree of selectivity is feasible as His5 is positioned at the bottom of a deep well on the potential-energy surface of HisC. The barriers against His5 converting into the other His isomers and dissociating into [b1-H]C + are high (Scheme 1), [*] Dr. J. Zhao, Dr. C.-K. Siu, Y. Ke, Dr. U. H. Verkerk, Prof. A. C. Hopkinson, Prof. K. W. M. Siu Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street Toronto, ON M3J 1P3 (Canada) E-mail: kwmsiu@yorku.ca

Response: Why had long-lived electron-transfer states of donor-substituted 10-methylacridinium ions been overlooked? Formation of the dimer radical cations detected in the near-IR region.

Abstract: Photoexcitation of an acetonitrile solution of the 9-(1-naphthyl)-10-methylacridinium ion results in formation of the electron-transfer state that forms the long-lived dimer radical cation with the acridinium ion, which was clearly detected as the transient absorption spectrum in the near-IR region.

Variations of diffusion coefficients of redox active molecules in room temperature ionic liquids upon electron transfer.

Abstract: In ionic liquids, the diffusion coefficients of a redox couple vary considerably between the neutral and radical ion forms of the molecule. For a reduction, the inequality of the diffusion coefficients is characterized by the ratio gamma = D(red)/D(ox), where D(red) and D(ox) are the diffusion coefficients of the electrogenerated radical anion and of the corresponding neutral molecule, respectively. In this work, measurements of gamma have been performed by scanning electrochemical microscopy (SECM) in transient feedback mode, in three different room temperature ionic liquids (RTILs) sharing the same anion and with a series of nitro-derivative compounds taken as a test family. The smallest gamma ratios were determined in an imidazolium-based RTIL and with the charge of the radical anion localized on the nitro group. Conversely, gamma tends to unity when the radical anion is fully delocalized or when the nitro group is sterically protected by bulky substituents. The gamma ratios, standard potentials of the redox couple measured in RTILs, and those observed in a classical organic solvent were compared for the investigated family of compounds. The stabilization energies approximately follow the gamma ratios in a given RTIL but change considerably between ionic liquids with the nature of the cation.

Oxidation of guanine in G, GG, and GGG sequence contexts by aromatic pyrenyl radical cations and carbonate radical anions: relationship between kinetics and distribution of alkali-labile lesions.

Abstract: Oxidatively generated DNA damage induced by the aromatic radical cation of the pyrene derivative 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (BPT), and by carbonate radicals anions, was monitored from the initial one-electron transfer, or hole injection step, to the formation of hot alkali-labile chemical end-products monitored by gel electrophoresis. The fractions of BPT molecules bound to double-stranded 20−35-mer oligonucleotides with noncontiguous guanines G and grouped as contiguous GG and GGG sequences were determined by a fluorescence quenching method. Utilizing intense nanosecond 355 nm Nd:YAG laser pulses, the DNA-bound BPT molecules were photoionized to BPT•+ radicals by a consecutive two-photon ionization mechanism. The BPT•+ radicals thus generated within the duplexes selectively oxidize guanine by intraduplex electron-transfer reactions, and the rate constants of these reactions follow the trend 5‘-..GGG.. > 5‘-..GG.. > 5‘-..G... In the case of CO3•- radicals, the oxidation of guanine occur...

Pulse radiolysis studies on reactions of hydroxyl radicals with selenocystine derivatives.

Abstract: Reactions of hydroxyl radicals (•OH) with selenocystine (SeCys) and two of its analogues, diselenodipropionic acid (SeP) and selenocystamine (SeA), have been studied in aqueous solutions at pHs of 1, 7, and 10 using the pulse radiolysis technique coupled with absorption detection. All of these diselenides react with •OH radicals with rate constants of ∼1010 M-1 s-1, producing diselenide radical cations (∼1−5 μs after the pulse), with an absorption maximum at 560 nm, by elimination of H2O or OH- from hydroxyl radical adducts. Assignment of the 560 nm band to the diselenide radical cation was made by comparing the transient spectra with those produced upon reaction of diselenides with specific one-electron oxidants, Cl2•- (pH 1) and Br2•- radicals (pHs of 7 and 10). SeP having a carboxylic acid functionality showed quantitative conversion of hydroxyl radical adducts to radical cations. The compounds SeCys and SeA, having an amino functional group, in addition to the radical cations, produced a new transient...