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Intramolecular Electron Transfer (intramolecular + electron_transfer)

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Chemistry	100%

Selected Abstracts

Synthesis and Characterization of Radical Cations Derived from Mono- and Biferrocenyl-Substituted 2-Aza-1,3-butadienes: A Study of the Influence of an Asymmetric and Oxidizable Bridge on Intramolecular Electron Transfer

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 12 2005
Vega Lloveras
Abstract The synthesis and study of structural and electronic properties of mono-ferrocenyl ,-conjugated complexes 5a,d, whose electronic characteristics have been systematically varied by introducing an electron-donating or electron-withdrawing substituent either at the 1-position or at the 4-position of the 2-aza-1,3-butadiene moiety linked to the ferrocenyl unit, are presented. The structural and electronic properties of the homobimetallic complex 5f, with two ferrocene units linked through the asymmetric and oxidizable 2-aza-1,3-butadiene bridge, is also reported. The crystal structures of complexes 5b, 5d, and 5f show a large degree of conjugation in this family of compounds. Complexes 5 show a rich electrochemical behavior due both to the oxidation of ferrocenyl units and the 2-aza-1,3-butadiene bridge, as revealed by cyclic voltammetry. Radical cations 5+· were prepared from 5 by coulometric oxidations following their generation by absorption spectroscopy. The electronic properties of all reported neutral and oxidized ,-conjugated complexes have been investigated by means of UV/Vis,near-IR, EPR and 57Fe Mössbauer spectroscopy. The detailed study of mono-oxidized species 5a+·,5f+· has permitted the determination of the influence of an asymmetric bridge with an electroactive character on the intramolecular electron transfer (IET) phenomenon, thus demonstrating that the 2-aza-1,3-butadiene bridge promotes the IET between the two metallic units of 5f+· through two different pathways. The experimental data and conclusions are supported by DFT computations (B3LYP/3-21G*) and time-dependent DFT methods. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

Synthesis and Photophysical Properties of C60 -carbazole Adducts

CHINESE JOURNAL OF CHEMISTRY, Issue 9 2001
Gui Yin
Abstract Three C60 -carbazole adducts have been synthesized by 1, 3-dipolar cycloaddition reaction. Intramolecular energy/electron transfer from carbazole to C60 was observed by steady-state absorption and fluorescence spectra. The fluorescence spectra of these adducts were similar to each other and dependent on the excitation wavelength and solvent. [source]

Synthesis and Characterization of Radical Cations Derived from Mono- and Biferrocenyl-Substituted 2-Aza-1,3-butadienes: A Study of the Influence of an Asymmetric and Oxidizable Bridge on Intramolecular Electron Transfer

Re-evaluation of intramolecular long-range electron transfer between tyrosine and tryptophan in lysozymes

FEBS JOURNAL, Issue 17 2003
Evidence for the participation of other residues
One-electron oxidation of six different c-type lysozymes from hen egg white, turkey egg white, human milk, horse milk, camel stomach and tortoise was studied by gamma- and pulse-radiolysis. In the first step, one tryptophan side chain is oxidized to indolyl free radical, which is produced quantitatively. As shown already, the indolyl radical subsequently oxidizes a tyrosine side chain to the phenoxy radical in an intramolecular reaction. However this reaction is not total and its stoichiometry depends on the protein. Rate constants also vary between proteins, from 120·s,1 to 1000·s,1 at pH 7.0 and room temperature [extremes are hen and turkey egg white (120·s,1) and human milk (1000·s,1)]. In hen and turkey egg white lysozymes we show that another reactive site is the Asn103,Gly104 peptidic bond, which gets broken radiolytically. Tryptic digestion followed by HPLC separation and identification of the peptides was performed for nonirradiated and irradiated hen lysozyme. Fluorescence spectra of the peptides indicate that Trp108 and/or 111 remain oxidized and that Tyr20 and 53 give bityrosine. Tyr23 appears not to be involved in the process. Thus new features of long-range intramolecular electron transfer in proteins appear: it is only partial and other groups are involved which are silent in pulse radiolysis. [source]

Photoinduced intracomplex electron transfer between cytochrome c oxidase and TUPS-modified cytochrome c

FEBS JOURNAL, Issue 18 2000
Alexander Kotlyar
A novel method for initiating intramolecular electron transfer in cytochrome c oxidase is reported. The method is based upon photoreduction of cytochrome c labeled with thiouredopyrene-3,6,8-trisulfonate in complex with cytochrome oxidase. The thiouredopyrene-3,6,8-trisulfonate-labeled cytochrome c was prepared by incubating the thiol reactive form of the dye with yeast iso-1-cytochrome c, containing a single cysteine residue. Laser pulse excitation of a stoichiometrical complex between thiouredopyrene-3,6,8-trisulfonate-cytochrome c and bovine heart cytochrome oxidase at low ionic strength resulted in the reduction of cytochrome c by the excited form of thiouredopyrene-3,6,8-trisulfonate and subsequent intramolecular electron transfer from the reduced cytochrome c to cytochrome oxidase. The maximum efficiency by a single laser pulse resulted in the reduction of ,,17% of cytochrome a, and was achieved only at a 1 : 1 ratio of cytochrome c to cytochrome oxidase. At higher cytochrome c to cytochrome oxidase ratios the heme a reduction was strongly suppressed. [source]

Synthesis of Cyclic Peptides by Photochemical Decarboxylation of N -Phthaloyl Peptides in Aqueous Solution

HELVETICA CHIMICA ACTA, Issue 12 2002

The synthesis of a variety of cyclic peptides from N -phthaloyl-protected di-, tri-, tetra-, and pentapeptides with different aminocarboxylic acid tethers by photodecarboxylation , initiated by intramolecular electron transfer , has been explored in aqueous media. The progress and the chemoselectivity of the follow-up processes after CO2 extrusion were traced by the respective pH/time-profiles, as well as by the overall change in pH after completion of the reaction. The competition between cyclization and simple oxidative decarboxylation depends on spacer length and geometry, H-bonding interaction between the electron accepting phthalimide CO groups and amide H-atoms, as well as the geometric reorganization coupled with the radical combination step and the formation of the lactam rings. With progressing reaction, hydrolysis of the phthalimide chromophore becomes an increasingly important side reaction due to the constant increase in pH. The use of phosphate-buffered aqueous media consequently improved the cyclization yields. The ground-state interactions between amide groups and the terminal COO, group with the imide CO groups were studied for the model system [N -(phthaloyl)glycyl]sarcosine (1) by NMR spectroscopy where the amide (E/Z)-equilibrium depends on the presence of carboxylate vs. free carboxylic acid, demonstrating the role of H-bonding and metal coordination. [source]

Synthesis of 5- tert -butyl-1-(3- tert -butyldimethylsiloxy)phenyl-4,4-dimethyl-2,6,7-trioxabicyclo[3.2.0]heptanes and their fluoride-induced chemiluminescent decomposition: effect of a phenolic electron donor on the CIEEL decay rate in aprotic polar solvent

LUMINESCENCE: THE JOURNAL OF BIOLOGICAL AND CHEMICAL LUMINESCENCE, Issue 5 2002
Masakatsu Matsumoto
Abstract Four bicyclic dioxetanes bearing a phenolic substituent, 3- tert -butyldimethylsiloxy-4-chlorophenyl (3a), 5- tert -butyldimethylsiloxy-4-chloro-2-ethylphenyl (3b), 5- tert -butyldimethylsiloxy-2-ethylphenyl (3c), and 3- tert -butyldimethylsiloxy-4-ethylphenyl (3d), were synthesized. All dioxetanes 3a,3d gave intense blue light on treatment with tetrabutylammonium fluoride (TBAF) in DMSO or acetonitrile. Kinetic study on the fluoride-induced CIEEL decay of these dioxetanes 3a,3d and the parent dioxetane 2b revealed that the para -substitution with chlorine on the phenolic moiety of dioxetane increases free energy of activation (,G,), while the para -substitution with ethyl on the aryl decreases ,G,. On the other hand, substitution with an ethyl at the ortho -position instead of the para -position was found to increase ,G, and to suppress the CIEEL decay. This fact is attributed to the steric factor of the ortho -ethyl group which would prevent the aromatic ring from rotating freely around the axis joined to the peroxide ring, and supports the suggestion for a CIEEL-active dioxetane bearing a phenolic moiety that an intramolecular electron transfer occurs preferentially from the phenolic donor to O,O of the dioxetane ring, when the aromatic ring lies in a certain conformation(s). Copyright © 2002 John Wiley & Sons, Ltd. [source]

Picosecond and femtosecond X-ray absorption spectroscopy of molecular systems

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2010
Majed Chergui
The need to visualize molecular structure in the course of a chemical reaction, a phase transformation or a biological function has been a dream of scientists for decades. The development of time-resolved X-ray and electron-based methods is making this true. X-ray absorption spectroscopy is ideal for the study of structural dynamics in liquids, because it can be implemented in amorphous media. Furthermore, it is chemically selective. Using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) in laser pump/X-ray probe experiments allows the retrieval of the local geometric structure of the system under study, but also the underlying photoinduced electronic structure changes that drive the structural dynamics. Recent developments in picosecond and femtosecond X-ray absorption spectroscopy applied to molecular systems in solution are reviewed: examples on ultrafast photoinduced processes such as intramolecular electron transfer, low-to-high spin change, and bond formation are presented. [source]

Hydroxycinnamic Acids as DNA-Cleaving Agents in the Presence of CuII Ions: Mechanism, Structure,Activity Relationship, and Biological Implications

CHEMISTRY - A EUROPEAN JOURNAL, Issue 46 2009
Gui-Juan Fan
Abstract The effectiveness of hydroxycinnamic acids (HCAs), that is, caffeic acid (CaA), chlorogenic acid (ChA), sinapic acid (SA), ferulic acid (FA), 3-hydroxycinnamic acid (3-HCA), and 4-hydroxycinnamic acid (4-HCA), as pBR322 plasmid DNA-cleaving agents in the presence of CuII ions was investigated. Compounds bearing o -hydroxy or 3,5-dimethoxy groups on phenolic rings (CaA, SA, and ChA) were remarkably more effective at causing DNA damage than the compounds bearing no such groups; furthermore, CaA was the most active among the HCAs examined. The involvement of reactive oxygen species (ROS) and CuI ions in the DNA damage was affirmed by the inhibition of the DNA breakage by using specific scavengers of ROS and a CuI chelator. The interaction between CaA and CuII ions and the influence of ethylenediaminetetraacetic acid (EDTA), the solvent, and pH value on the interaction were also studied to help elucidate the detailed prooxidant mechanism by using UV/Vis spectroscopic analysis. On the basis of these observations, it is proposed that it is the CaA phenolate anion, instead of the parent molecule, that chelates with the CuII ion as a bidentate ligand, hence facilitating the intramolecular electron transfer to form the corresponding CaA semiquinone radical intermediate. The latter undergoes a second electron transfer with oxygen to form the corresponding o -quinone and a superoxide, which play a pivotal role in the DNA damage. The intermediacy of the semiquinone radical was supported by isolation of its dimer from the CuII -mediated oxidation products. Intriguingly, CaA was also the most cytotoxic compound among the HCAs toward human promyelocytic leukemia (HL-60) cell proliferation. Addition of exogenous CuII ions resulted in an effect dichotomy on cell viability depending on the concentration of CaA; that is, low concentrations of CaA enhanced the cell viability and, conversely, high concentrations of CaA almost completely inhibited the cell proliferation. On the other hand, when superoxide dismutase was added before, the two stimulation effects of exogenous CuII ions were significantly ameliorated, thus clearly indicating that the oxidative-stress level regulates cell proliferation and death. These findings provide direct evidence for the antioxidant/prooxidant mechanism of cancer chemoprevention. [source]

Synthesis and Photoinduced Electron-Transfer Properties of Phthalocyanine,[60]Fullerene Conjugates

CHEMISTRY - A EUROPEAN JOURNAL, Issue 12 2008
Maurizio Quintiliani Dr.
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). [source]