Modulation Of Electron Transfer Research Articles

Effects of Electronic Correlations for Adiabatic Electrochemical Reactions of Electron Transfer in Electrode Models with Nearly Empty and Nearly Filled Conduction Bands: General Relationships

Expressions for the calculation of the adiabatic free energy surfaces (AFES) and average number of electrons in the valence orbital of the reagent for adiabatic electrochemical reactions of electron transfer are obtained in terms of exactly solved models for a metal electrode with nearly empty and nearly filled conduction bands. The models are extreme cases of the Anderson model, which account exactly for the electron–electron correlation effects. In particular, the electrode model with a nearly filled conduction band can be applied to transition metals of Group VIII in the periodic table. Exact relationships connecting AFES and diagrams of kinetic modes (DKM) for electrodes with symmetric position of Fermi levels relative to the conduction band center are obtained. Two characteristic functions for analyzing the role of electron–electron correlation effects in the system under consideration are proposed and calculated. The results form a basis for calculating AFES and studying correlation effects in adiabatic electrochemical reactions of electron transfer and constructing DKM that would correspond to different electron transfer modes.

The reduction of tris-dithiolene complexes of molybdenum(VI) and tungsten(VI) by hydroxide ion: kinetics and mechanism.

The kinetic study of the spontaneous reduction of some neutral tris-dithiolene complexes [ML3] of molybdenum(VI) and tungsten(VI), (L = S2C6H4(2-), S2C6H3CH3(2-) and S2C2(CH3)2(2-); M = Mo or W) by tetrabutylammonium hydroxide in tetrahydrofuran-water solutions demonstrates that OH- is an effective reductant. Their reduction is fast, clean and quantitative. Depending upon both the molar ratio in which the reagents are mixed and the amount of water present, one- or two-electron reductions of these tris-dithiolene complexes were observed. If Bu4NOH is present in low concentration or/and at high concentrations of water, the total transformation of the neutral M(VI) complex into the monoanionic M(V) complex is the only observed process. Stopped-flow kinetic data for this reaction are consistent with the rate law: -d[ML3]/dt = d[ML3-]/dt = k[ML3][Bu4NOH]. The proposed mechanism involves nucleophilic attack of OH- to form a mono-anionic seven-coordinate intermediate [ML3OH]-, which interacts with another molecule of [ML3] to generate the monoanionic complex [ML3]- transfering the oxygen from coordinated OH- to water. Hydrogen peroxide was identified as the reaction product. The molybdenum complexes are more difficult to reduce than their corresponding tungsten complexes, and the values of k obtained for the molybdenum and tungsten series of complexes increase as the ene-1,2-dithiolate ligand becomes more electron-withdrawing (S2C6H4(2-) > S2C6H3CH3(2-) > S2C2(CH3)2(2-)). This investigation constitutes the only well-established interaction between hydroxide ion and a tris(dithiolene) complex, and supports a highly covalent bonding interaction between the metal and the hydroxide ion that modulates electron transfer reactions within these complexes.

Coupling a natural receptor protein with an artificial receptor to afford a semisynthetic fluorescent biosensor.

An artificial receptor and a signal transducer have been engineered on a lectin (saccharide-binding protein) surface by a post-photoaffinity labeling modification method. Saccharide binding can be directly and selectively read out by the fluorescence changes of the fluorophore via photoinduced electron transfer (PET) mode. Fluorescence titration with various saccharides reveals that molecular recognition by the artificial receptor is successfully coupled to the native binding site of the lectin, producing a novel fluorescent saccharide biosensor showing modulated specificity and enhanced affinity. Designed cooperativity between artificial and native molecular recognition modules was quantitatively demonstrated by the comparison of the binding affinities, and it represents a new strategy in molecular recognition. By using appropriate artificial receptors and various native lectins, this approach may provide many new semisynthetic biosensors for saccharide derivatives such as glycolipids and glycopeptides/proteins. An extended library of lectin-based biosensors is envisioned to be useful for glycome research, a newly emerging field of the post-genomic era.

An atypical haem in the cytochrome b(6)f complex.

Photosystems I and II (PSI and II) are reaction centres that capture light energy in order to drive oxygenic photosynthesis; however, they can only do so by interacting with the multisubunit cytochrome b(6)f complex. This complex receives electrons from PSII and passes them to PSI, pumping protons across the membrane and powering the Q-cycle. Unlike the mitochondrial and bacterial homologue cytochrome bc(1), cytochrome b(6)f can switch to a cyclic mode of electron transfer around PSI using an unknown pathway. Here we present the X-ray structure at 3.1 A of cytochrome b(6)f from the alga Chlamydomonas reinhardtii. The structure bears similarities to cytochrome bc(1) but also exhibits some unique features, such as binding chlorophyll, beta-carotene and an unexpected haem sharing a quinone site. This haem is atypical as it is covalently bound by one thioether linkage and has no axial amino acid ligand. This haem may be the missing link in oxygenic photosynthesis.

Vibrational coherence in electron transfer: an exactly solvable model

Abstract The exact solution for a transition probability in vibrationally modulated electron transfer is found by employing three independent methods based on: (a) a path integral technique, (b) direct diagonalization of the Hamiltonian by a unitary transformation, and (c) Keldysh–Green’s function techniques. We also consider a solution in the noninteracting blip approximation (NIBA). The comparative analysis reveals that the NIBA is valid at longer times, small reorganization energy, high temperatures, and for small transition matrix elements, Δ 0 , while at large Δ 0 the NIBA provides relaxation rates as much as twice greater. When Δ 0 =0, the NIBA transition probability difference oscillates in time with the frequency ∼ E r kT/ℏ 2 exhibiting incorrect behavior. At low temperatures, the exact solution yields power-law evolution. The NIBA solution exhibits incorrect behavior at small values of Δ 0 . At larger transition matrix elements, the NIBA practically yields a correct description, i.e. the decay in accordance with the exact solution while the coherent oscillations reveal some phase shift at longer times. An important case of several mode modulation is also studied. The Fourier analysis of the transition probability provides useful information of the vibrational couplings and frequencies. In general, the Fourier spectrum of the transition probability is the sum of the contributions from the harmonics with the frequencies |Δ 0 +∑ i=0 n (±)k i ω i | (k i =0,1,2,3,…) , where ω i is a modulating frequency. Thus, a generally used assumption about coincidence of coherence and vibrational frequencies appears to be incorrect.

Nitrogenase mechanism: Modulation of energy transduction and electron transfer within the Fe-protein-MoFe protein complex

Nitrogenase mechanism: Modulation of energy transduction and electron transfer within the Fe-protein-MoFe protein complex

Genetic modulation of metalloprotein electron transfer at bare gold.

Engineered metalloproteins and enzymes can be self assembled on pristine gold electrodes in robust, electrochemically-addressable, arrays.

Electron transfer studies of dithiolate complexes: effects of ligand variation and metal substitution

Solution redox potentials and heterogeneous electron transfer rate constants have been measured for mono-ene-1,2-dithiolate complexes of the type (Tp*)M(E)(S ∩S) [Tp*=hydrotris(3,5-dimethyl-1-pyrazolyl)borate; M=Mo, W; E=O, NO]. The dithiolate ligands (S ∩S) equatorially coordinated to the central metal include: 1,2-benzenedithiolate (bdt); 3,6-dichloro-1,2-benzenedithiolate (bdtCl 2); and 3,4-toluenedithiolate (tdt). Cyclic voltammograms reveal quasi-reversible one-electron reductions for all of the compounds; and, a one-electron quasi-reversible oxidation process is also observed for (Tp*)MoO(bdt), (Tp*)MoO(tdt), and (Tp*)WO(tdt). Electrochemical potentials are strongly dependent upon the nature of the substituents on the benzene ring of the dithiolate ligand. The nitrosyl–molybdenum complexes (E=NO) are more difficult to reduce than their corresponding oxo-molybdenum complexes. The tungsten compound, (Tp*)WO(tdt), has the most negative electrochemical potentials among the complexes investigated. Heterogeneous electron transfer rates are insensitive to the variations of the central metal and ligands. These results support a highly covalent bonding interaction between the metal and the dithiolate ligands that modulates electron transfer reactions within these compounds.

Modulation of charge transfer association and photo-induced electron transfer by steric encumbrance in Fréchet-type viologen dendrimer

The first and second generation of Fréchet-type dendrimer with a viologen core have been prepared; it has been observed that the association constant and the electron transfer quenching of these compounds with anthracene is modulated by the size of the polyether branches.

Dramatic modulation of electron transfer in protein complexes by crosslinking.

The transfer of electrons between proteins is an essential step in biological energy production. Two protein redox partners are often artificially crosslinked to investigate the poorly understood mechanism by which they interact. To better understand the effect of crosslinking on electron transfer rates, we have constructed dimers of azurin by crosslinking the monomers. The measured electron exchange rates, combined with crystal structures of the dimers, demonstrate that the length of the linker can have a dramatic effect on the structure of the dimer and the electron transfer rate. The presence of ordered water molecules in the protein-protein interface may considerably influence the electronic coupling between redox centers.

Coupled Electron/Proton Transfer in Complex Flavoproteins: SOLVENT KINETIC ISOTOPE EFFECT STUDIES OF ELECTRON TRANSFER IN XANTHINE OXIDASE AND TRIMETHYLAMINE DEHYDROGENASE

A solvent kinetic isotope effect study of electron transfer in two complex flavoproteins, xanthine oxidase and trimethylamine dehydrogenase, has been undertaken. With xanthine oxidase, electron transfer from the molybdenum center to the proximal iron-sulfur center of the enzyme occurs with a modest solvent kinetic isotope effect of 2.2, indicating that electron transfer out of the molybdenum center is at least partially coupled to deprotonation of the Mo(V) donor. A Marcus-type analysis yields a decay factor, beta, of 1.4 A(-1), indicating that, although the pyranopterin cofactor of the molybdenum center forms a nearly contiguous covalent bridge from the molybdenum atom to the proximal iron-sulfur center of the enzyme, it affords no exceptionally effective mode of electron transfer between the two centers. For trimethylamine dehydrogenase, rates of electron equilibration between the flavin and iron-sulfur center of the one-electron reduced enzyme have been determined, complementing previous studies of electron transfer in the two-electron reduced form. The results indicate a substantial solvent kinetic isotope effect of 10 +/- 4, consistent with a model for electron transfer that involves discrete protonation/deprotonation and electron transfer steps. This contrasts to the behavior seen with xanthine oxidase, and the basis for this difference is discussed in the context of the structures for the two proteins and the ionization properties of their flavin sites. With xanthine oxidase, a rationale is presented as to why it is desirable in certain cases that the physical layout of redox-active sites not be uniformly increasing in reduction potential in the direction of physiological electron transfer.

Control of Electron Transfer in Nitric-oxide Synthases: SWAPPING OF AUTOINHIBITORY ELEMENTS AMONG NITRIC-OXIDE SYNTHASE ISOFORMS

To clarify the role of the autoinhibitory insert in the endothelial (eNOS) and neuronal (nNOS) nitric-oxide synthases, the insert was excised from nNOS and chimeras with its reductase domain; the eNOS and nNOS inserts were swapped and put into the normally insertless inducible (iNOS) isoform and chimeras with the iNOS reductase domain; and an RRKRK sequence in the insert suggested by earlier peptide studies to be important (Salerno, J. C., Harris, D. E., Irizarry, K., Patel, B., Morales, A. J., Smith, S. M., Martasek, P., Roman, L. J., Masters, B. S., Jones, C. L., Weissman, B. A., Lane, P., Liu, Q., and Gross, S. S. (1997) J. Biol. Chem. 272, 29769-29777) was mutated. Insertless nNOS required calmodulin (CaM) for normal NOS activity, but the Ca(2+) requirement for this activity was relaxed. Furthermore, insert deletion enhanced CaM-free electron transfer within nNOS and chimeras with the nNOS reductase, emphasizing the involvement of the insert in modulating electron transfer. Swapping the nNOS and eNOS inserts gave proteins with normal NOS activities, and the nNOS insert acted normally in raising the Ca(2+) dependence when placed in eNOS. Insertion of the eNOS insert into iNOS and chimeras with the iNOS reductase domain significantly lowered NOS activity, consistent with inhibition of electron transfer by the insert. Mutation of the eNOS RRKRK to an AAAAA sequence did not alter the eNOS Ca(2+) dependence but marginally inhibited electron transfer. The salt dependence suggests that the insert modulates electron transfer within the reductase domain prior to the heme/reductase interface. The results clarify the role of the reductase insert in modulating the Ca(2+) requirement, electron transfer rate, and overall activity of nNOS and eNOS.

Electron transfer and vibrational modes in a finite molecular chain

A statistical quantum discrete model for a molecular bridge between metallic electrodes is proposed. A theory of conduction is developed on the assumption that the main contribution to the electron transfer is due to nonequilibrium-populated electron-affinity states of the molecule. It is shown that the field-induced modification of the states of the bridge molecule, together with the Coulomb blockade effect, leads to suppression of the electron transfer. Relations are obtained for the electron–vibron interaction, and the contribution of atomic vibrations to the conductance of the chain is discussed. The current–voltage characteristics of molecular bridges are calculated for different positions of the spectrum of the molecule with respect to the levels of the chemical potentials of the electrodes. Explanations are given for the stepped and asymmetric character of the current–voltage characteristic observed experimentally and for the fractional charging of the bridge molecule.

EPR investigation of Cu2+-substituted photosynthetic bacterial reaction centers: evidence for histidine ligation at the surface metal site.

The coordination environments of two distinct metal sites on the bacterial photosynthetic reaction center (RC) protein were probed with pulsed electron paramagnetic resonance (EPR) spectroscopy. For these studies, Cu2+ was bound specifically to a surface site on native Fe2+-containing RCs from Rhodobacter sphaeroides R-26 and to the native non-heme Fe site in biochemically Fe-removed RCs. The cw and pulsed EPR results clearly indicate two spectroscopically different Cu2+ environments. In the dark, the RCs with Cu2+ bound to the surface site exhibit an axially symmetric EPR spectrum with g(parallel) = 2.24, A(parallel) = 160 G, g(perpendicular) = 2.06, whereas the values g(parallel) = 2.31, A(parallel) = 143 G, and g(perpendicular) = 2.07 were observed when Cu(2+) was substituted in the Fe site. Examination of the light-induced spectral changes indicate that the surface Cu2+ is at least 23 A removed from the primary donor (P+) and reduced quinone acceptor (QA-). Electron spin-echo envelope modulation (ESEEM) spectra of these Cu-RC proteins have been obtained and provide the first direct solution structural information about the ligands in the surface metal site. From these pulsed EPR experiments, modulations were observed that are consistent with multiple weakly hyperfine coupled 14N nuclei in close proximity to Cu2+, indicating that two or more histidines ligate the Cu2+ at the surface site. Thus, metal and EPR analyses confirm that we have developed reliable methods for stoichiometrically and specifically binding Cu2+ to a surface site that is distinct from the well characterized Fe site and support the view that Cu2+ is bound at or near the Zn site that modulates electron transfer between the quinones QA and QB (QA-QB --> QAQB-) (Utschig, L. M., Ohigashi, Y., Thurnauer, M. C., and Tiede, D. M (1998) Biochemistry 37, 8278-8281) and proton uptake by QB- (Paddock, M. L., Graige, M. S., Feher, G., and Okamura, M. Y. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 6183-6188). Detailed EPR spectroscopic characterization of these Cu2+-RCs will provide a means to investigate the role of local protein environments in modulating electron and proton transfer.

Metal-assisted unusual hydroxylation at the carbon atom of the triazine ring in dinuclear ruthenium(II) and osmium(II) complexes bridged by 2,4,6-tris(2-pyridyl)-1,3,5-triazine: synthesis, structural characterization, stereochemistry, and electrochemical studies.

The reaction of cis-[M(bpy)2Cl2] (M = Ru(II), and Os(II) with 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) in refluxing ethanol-water resulted in the formation of dinuclear complexes of the composition [(M(bpy)2)2(tptz-OH)](PF6)3.nH2O (n = 1 for Ru and n = 0 for Os). In this reaction an unusual metal-induced hydroxylation at the carbon atom of the triazine ring of bridged tptz occurred. However, hydroxylation did not occur in the corresponding mononuclear complexes under similar reaction condition. A comparative study revealed that sufficient electrophilicity on the carbon atom and free movement of the attached pyridyl ring promoted the hydroxylation reaction. The hydroxylated dinuclear complexes exist in two stereoisomeric forms, a rac form (delta delta/lambda lambda) and a meso form (delta lambda/lambda delta). Both diastereoisomers have been isolated in pure form and characterized. The molecular structures of the rac form of Ru(II) complex (3-II) and meso form of the Os(II) complex (4-I) have been established by single-crystal X-ray studies. Crystal data: complex 3-II, monoclinic, C2/c, a = 24.584(7) A, b = 14.309(4) A, c = 41.044(13) A, beta = 92.84(2) degrees, V = 14420.0(7) A3, Z = 8, R = 0.179, wR2 = 0.479; complex 4-I, triclinic, P1, a = 13.444(7) A, b = 14.576(5) A, c = 19.641(7) A, alpha = 98.21(3) degrees, beta = 101.67(4) degrees, gamma = 105.80(4) degrees, V = 3546.0(3) A3, Z = 2, R = 0.093, wR2 = 0.279. The poor data quality of 3-II did not allow anisotropic refinement of non-hydrogen atoms except Ru and P. A PLUTO drawing of this compound is given only to support the molecular structure. 1H NMR data have been used to characterize the diastereoisomers. The dinuclear complexes exhibit unusual electrochemical behavior; cathodic shifts of the metal-centered oxidations and ligand-based first reduction compared to mononuclear complexes have been observed. There is a splitting in the metal-centered oxidation potentials, indicating strong electronic communication between the metal centers. Comproportionation constants (Kcom) of the mixed-valence species have been calculated; the values are in the range 6.03 x 10(4)-4.7 x 10(6). It appears that a metal-metal interaction occurred by an electron-transfer mode across the low-lying pi* orbital of the bridged tptz.

Six conserved cysteines of the membrane protein DsbD are required for the transfer of electrons from the cytoplasm to the periplasm of Escherichia coli.

The active-site cysteines of the Escherichia coli periplasmic protein disulfide bond isomerase (DsbC) are kept reduced by the cytoplasmic membrane protein, DsbD. DsbD, in turn, is reduced by cytoplasmic thioredoxin, indicating that DsbD transfers disulfidereducing potential from the cytoplasm to the periplasm. To understand the mechanism of this unusual mode of electron transfer, we have undertaken a genetic analysis of DsbD. In the process, we discovered that the previously suggested start site for the DsbD protein is incorrect. Our results permit the formulation of a model of DsbD membrane topology. Also, we show that six cysteines of DsbD conserved among DsbD homologs are essential for the reduction of DsbC, DsbG and for a reductive pathway leading to c-type cytochrome assembly in the periplasm. Our findings suggest a testable model for the DsbD-dependent transfer of electrons across the membrane, involving a cascade of disulfide bond reduction steps.

Kinetics and Mechanism of Ruthenium(iii) Chloride Catalyzed Oxidation of Propane-1,3-diol by Thallium(iii) in Acid Perchlorate Medium

The mode of electron transfer in ruthenium(III) chloride catalyzed oxidation of propane-1,3-diol by thallium(III) is explained by hydride ion abstraction.

Kinetics and Mechanism of Ruthenium(III) Chloride Catalyzed Oxidation of Propane-1,3-diol by Thallium(III) in Acid Perchlorate Medium

The mode of electron transfer in ruthenium(III) chloride catalyzed oxidation of propane-1,3-diol by thallium(III) is explained by hydride ion abstraction.

Electrostatic Effect on Electron Transfer at the Active Site of Heme Peroxidases: A Comparative Molecular Orbital Study on Cytochrome C Peroxidase and Ascorbate Peroxidase

Ab initio minimal basis set molecular orbital and discretized Poisson−Boltzmann electrostatic potential calculations were carried out on large models of the active site of heme peroxidases (cytochrome C peroxidase and cytosolic ascorbate peroxidase) to rationalize the location of the radical site in Compound I, an intermediate of the enzymatic reaction. Both molecular orbital and electrostatic potential calculations suggest that the spin distribution depends on the protonation state of the proximal His...Asp...Trp triad. If the transferable proton is shifted from the Trp side chain to Asp, the radical localizes on the indole group, while if it remains on the Trp the unpaired electron transfers to the heme group. Therefore, in cytochrome C peroxidase, Trp is deprotonated in Compound I, while it is protonated and neutral in cytosolic ascorbate peroxidase. Protonation state of the proximal residues is influenced by the electrostatic effect of the protein environment that differs in these enzymes, especially in the immediate vicinity of the Asp side chain. In contrast to earlier propositions we did not find evidence for the effect of the neighboring potassium-binding loop on the localization of the free radical. The heme peroxidases studied provide examples for the electrostatic, protonation-mediated modulation of electron transfer.

DNA-mediated electron transfer from a modified base to ethidium: π-stacking as a modulator of reactivity

Background: The DNA double helix is composed of an array of aromatic heterocyclic base pairs and, as a molecular π-stack, represents a novel system for studying long-range electron transfer. Because many base damage and repair processes result from electron-transfer reactions, the ability of DNA to mediate charge transport holds important biological implications. Seemingly contradictory conclusions have been drawn about electron transfer in DNA from the many different studies that have been carried out. These studies must be reconciled so that this phenomenon can be understood both at a fundamental level and in the context of biological systems. Results: The photoinduced oxidation of a modified base, 7-deazaguanine, has been examined as a function of distance, sequence, and base stacking in DNA duplexes covalently modified with ethidium. Over ethidium/deazaguanine separations of 6–27 Å, the photooxidation reaction proceeded on a subnanosecond time scale, and the quenching yield exhibited a shallow distance dependence. The efficiency of the reaction was highly sensitive to small changes in base composition. Moreover, the overall distance-dependence of the reaction is sensitive to sequence, despite the constancy of photoexcited ethidium as acceptor. Conclusions: The remarkable efficiency of deazaguanine photooxidation by intercalated ethidium over long distances provides new evidence for fast electron-transfer pathways through DNA. By varying sequence as well as reactant separation, this work provides the first experimental demonstration of the importance of reactant stacking in the modulation of long-range DNA-mediated electron transfer.