Electron Exchange Mechanism Research Articles

Electronic magnetic relaxation in manganese ferrites

Two electronic magnetic relaxation phenomena are reported for the manganese ferrite system, which are both explained by electron hopping processes between octahedral iron ions: the first one by hopping between Fe/sup 2+/ and Fe/sup 3+/ ions with a Mn/sup 2+/-ion as nearest neighbor and the second one by hopping between iron ions with only iron nearest neighbors. Electrical conductivity measurements performed on a single crystal, showed further that the composition dependence of activation energy for the conductivity behaves in a similar way as the magnetic activation energy which is a strong evidence for the proposed Fe/sup 2+/-Fe/sup 3+/ electron exchange mechanism. >

Product rotational alignment in the excitation transfer reaction Ar(3P2)+N2→Ar+N2(C 3Πu)

The velocity dependence of the N2(C) rotational alignment from the title excitation transfer reaction was studied over the collision energy range 42–460 meV, using a beam-gas time-of-flight method. The alignment was found to be positive, and to increase approximately linearly with energy: the extrapolated line has a large positive intercept, suggesting that the alignment persists down to zero energy. The present results, together with previous studies of this reaction, are consistent with a model in which the reaction occurs preferentially in the plane of rotation of the N2. It is suggested that this steric effect is due primarily to the orbital overlap requirements of the electron exchange mechanism. The polarization of the fluorescence from the dissociative excitation reactions of Ar*+CS2, Ar*+H2O, Kr*+CS2, and Xe*+BrCN was also measured and found to be zero at all velocities.

Memory of spin polarization in triplet-doublet systems

The interaction between triplet molecules and nitroxide radicals is studied in solution by the time-resolved ESR technique. Spin polarization induced in the radical reflects that of the triplet molecule which is an encounter partner. The spin-polarized ESR signals observed in nitroxide radicals are interpreted in terms of electron and/or spin exchange mechanisms.

Temperature dependence of the deexcitation of He(2 3S) by atoms and molecules as studied by pulse radiolysis method

The temperature dependence of the rate constants for deexcitation of He(2 3S) by Ar, Kr, N2, O2, CO, NO, CO2, and C2H4 has been measured in the temperature range from 133 to 300 K by a pulse radiolysis method, and thus the collisional energy dependence of the deexcitation cross sections is obtained. The results are interpreted in terms of an electron exchange mechanism. The value of α/β, the ratio of the exponential factor of the potential width Γ(R)=A exp(−αR) to that of the interaction potential V*(R)=B exp(−βR), is obtained for each target molecule. It is found that the value of α/β decreases with the increase of the deexcitation probability.

Rapid kinetic studies of the electron-exchange reaction between cytochrome c3 and ferredoxin from Desulfovibrio desulfuricans Norway strain and their individual reactions with dithionite

The electron-exchange reaction between Desulfovibrio desulfuricans Norway cytochrome c 3 and ferredoxin was investigated by rapid-mixing techniques under anaerobic conditions. Their reduction by dithionite is biphasic when several redox centres are present in the molecule. The observed first-order rate constant of the rapid and slow phases are both attributed to the reduction by the radical anion SO ⨪ 2. The reduction rate constants for cytochrome c 3 (10 7 M −1 · s −1) are higher than for ferredoxin (10 5 M −1 · s −1) at 5°C. These results are compared with data also obtained with Desulfovibrio vulgaris cytochrome c 3 and discussed in terms of kinetic models implying inter- and/or intramolecular electron exchange. The electron-transfer kinetics between reduced cytochrome c 3 and oxidized ferredoxin is monophasic with a rate-limit of 160 s −1 at 5°C. The amplitude detected corresponds to the oxidation of one heme per molecule. A reaction mechanism involving the formation of one intermediate complex, followed by an intramolecular electron exchange leading to the oxidation of the heme of lowest oxidation-reduction potential is proposed. The reverse reaction (i.e., mixing the oxidized cytochrome c 3 with reduced ferredoxin) is still monophasic and of an apparent second-order character with a rate constant of 7 · 10 7 M −1 · s −1 at 5°C. At extremely low ferredoxin concentrations the reaction is limited by the rate of the reverse reaction ( k − = 150 s −1). The reduction of four hemes is observed. This extent of reduction reveals that the apparent equilibrium constant for the electron-exchange reaction is greater than 1. Nevertheless, the data support the view that a complex is formed rapidly between the two proteins and is followed by a rapid intramolecular transfer of electrons in either direction. The rate constants for the reduction of cytochromes c 3 by ferredoxins, both from different species, are also reported. It is concluded that cytochrome c 3 is not equally efficient in both directions of the electron exchange. Depending on the relative oxidation-reduction states and concentrations, one or two hemes and four hemes might be involved in the electron exchange with ferredoxin. The influence of oxidation-reduction potentials, electrostatic interactions and charge distribution on the proteins are taken into account for the interpretation of the electron-exchange mechanism.

Enzyme electrodes based on organic metals

The parameters of enzyme electrodes based on organic metals are presented. Cytochrome b 2 (E.C. 1.1.2.3), glucose oxidase (E.C. 1.1.3.4), xanthine oxidase (E.C. 1.2.3.2) and peroxidase (E.C. 1.11.1.7) were used in electrodes sensitive to l-lactate, glucose, hypoxanthine and hydrogen peroxide. Electrocatalytic oxidation of NADH on organic metals and ethanol and acetaldehyde sensitive electrodes containing alcohol dehydrogenase (E.C. 1.1.1.1) were studied. Biocatalytic charge accumulation, the mechanism of electron exchange between the enzyme active centres and organic metals, and the future application of organic metals are discussed.

Dipole-Forbidden energy transfer process in solution

The transfer of electronic excitation energy to or from d-d excited states of transition metal compounds is formally dipole forbidden and is allowed by an electron exchange mechanism. In this limit, the quenching rate constant may be factored into contributions of an electronic term and a Franck-Condon, or nuclear term. A formalism based on non-radiative relaxation of electronic excited states is used to identify important features of the Franck-Condon term in the strong coupling and the weak coupling limits. Many of the systematic studies so far reported in fluid solution have involved the 4A 2 ↔ 2E electronic transition of Cr(III) in either the donor or acceptor and such system often correspond to the weak coupling limit. The behavior observed in this limit has been thought to deviate from expectation, but the proper accounting for the vibrational modes available to accommodate the excess energy in a weak coupling formalism accounts for the observed gap dependencies. When ( 2E)Cr(III) excited states are employed with ( 1A 1 → 3T 1)Co(III) acceptor, a strong coupling formalism may be employed to extract information about the electronic factor from variations in the quenching rates. The strong coupling and weak coupling formalisms, based on unimolecular radiationless transitions, have successfully described many aspects of bimolecular reactions, but several issues remain. Some of these issues will require observations on metal-to-metal energy transfer in multimetal molecules. A few pertinent studies are becoming available.

An electromotive force series for redox couples in a borosilicate melt: The basis for electron exchange interactions of the redox couples

Oxidation-reduction equilibria for Ag, Ce, Co, Cr, Cu, Eu, Fe, Mn, Mo, Ni, Sn, Ti, U, and V in a borosilicate liquid at 1150°C have been studied as a function of imposed oxygen fugacity. Redox couples for these multivalent elements could then be assigned a relative reduction potential and, thus, could be arranged in an electromotive force series in this glass-forming solvent. The electromotive force series in the borosilicate melt is comparable to that in other solvent systems and even to that in aqueous solution. All involve the redox ions associated with a coordination sphere of oxygens, but each possesses unique characteristics due to specific solvation interactions. The relative reduction potentials can be employed to predict the percentages of oxidized and reduced species of a multivalent element during the processing of borosilicate glass. For example, the difference in reduction potential of two interacting redox couples can predict to a first approximation the degree of mutual interaction by electron exchange. The mechanism for redox interaction is probably analogous to the inner-sphere redox mechanism for electron exchange between inorganic coordination complexes. The borosilicate oxygen ligands associated with the redox ions play an integral role in the electron transfer process. This concept provides credence to the clustering of redox ions in melts.

Mechanism of electron exchange between low energy He + and solid surfaces

A general picture on the mechanism of electron exchange between low energy He + ions and solid surfaces is proposed on the basis of experiments on three-dimensional angle resolving ion scattering spectroscopy in which not only He + but neutral He is used as a projectile.

Mechanism of electron exchange between low energy He + and solid surfaces

A general picture on the mechanism of electron exchange between low energy He + ions and solid surfaces is proposed on the basis of experiments on three-dimensional angle resolving ion scattering spectroscopy in which not only He + but neutral He is used as a projectile.

Electron transfer mecanism and interaction studies between cytochrome C 3 and ferredoxin

Ferredoxin, cytochrome c 3 and hydrogenase are specific partners of the sulfate reduction pathway of Desulfovibrio desulfuricans Norway and might be exemplary for electron exchange mechanism studies. Cytochrome c 3 contains four low redox potential haems for 13 000 molecular weight. Two ferredoxins isolated from the same bacteria are dimers of 6 000 molecular weight per subunit (Ferredoxin I : one (4 Fe-4S) cluster per subunit, ferredoxin II : two (4 Fe-4 S) clusters per subunit). The amino acid sequence of ferredoxin I is reported and compared to the ferredoxin II sequence. The structural characteristics of ferredoxins and cytochrome c 3 should allow a discussion on the nature of the interaction. 1H-NMR spectra of ferredoxin I and cytochrome c 3 in the absence and presence of ferredoxin are presented.

Photoinduced Charge Separation Process of Porphyrin in Liposomal Membrane

側鎖長の異なるメソポルフィリン誘導体meso-IX-Cn.およびZn-meso-K-Cnを合成し,それらをDPPCリボソーム膜に埋め込み,内液EDTA(電子供与体)から外液MV2+(電子受容体)への光電子移動効率におよぼす影響を,吸収,蛍光スペクトル,蛍光偏光解消,および温度変化などの測定により調つた。ポルフィリン色素の膜中における存在状態には,モノマー,オリゴマー凝集体,および固体状凝集体の3種の存在が示唆され,モノマーのみが効率よく光電子移動反応に寄与し,色素/脂質濃度比は1/100ぐらいが最適であることがわかった。ポルフィリン色素の可溶化により膜の流動性が変化し,流動性の増加により効率は増加し,C8が最適であった。ポルフィリン色素の膜中での存在位置も大きな影響を与え,親水領域に存在するものほど効率が大であった。バリノマィシンVal添加による膜電位効果は期待するほど現われなかったが,それはmeso-IX=Cn%がプロトン担体となっているためであることがわかった。膜内電子移動機構として,内側励起ポルフィリンからMV2+への酸化的消光によって生成した寿命の長いポルフィリンカチオンへの直接の電子移動(二光子励起型電子交換)を考えると,実験結果がもっともよく説明される。

In situ electron spin resonance spectroscopy studies of electrode processes

The usefulness of ESR spectroscopy for studying electrode processes is discussed and then illustrated by two selected examples. The first one deals with the identification of a radical intermediate in the electroreduction of some naphthazarin derivatives. The second example shows the possibility of elucidating the mechanism of electron exchange inside an electrode material, such as M 2 P 2 S 6 , during its electroreduction with simultaneous intercalation of lithium ions.

In situ electron spin resonance spectroscopy studies of electrode processes

The usefulness of ESR spectroscopy for studying electrode processes is discussed and then illustrated by two selected examples. The first one deals with the identification of a radical intermediate in the electroreduction of some naphthazarin derivatives. The second example shows the possibility of elucidating the mechanism of electron exchange inside an electrode material, such as M 2P 2S 6, during its electroreduction with simultaneous intercalation of lithium ions.

Electron transfer mechanism studies of cytochrome c 3: pH dependence of the redox equilibria

Cytochromes c 3 are tetrahaem electron transfer proteins isolated from Desulfovibrio spp. which play an important role in the metabolism of sulfur compounds and hydrogen [1]. They have a molecular weight of approx. 13,000 Daltons [1, 2]. The four haems, mesoporphyrins, are covalently bound to the polypeptide chain through thioether linkages provided by cysteinyl residues. The axial ligands are two hystidinyl residues [3, 4] and all the haems have negative, although different mid-point redox potentials [5–8]. Several physico-chemical techniques, mainly Mössbauer [9], EPR [4–7], NMR [3, 10–16], cyclic voltammetry, and different pulse polarography [6, 7], have been applied to elucidate the mechanism of electron transfer in cytochrome c 3 . However, the mechanism by which electrons are transferred between cytochrome c 3 and other electron carriers is only poorly understood. NMR has been shown to be a suitable technique to elucidate the electron exchange mechanism of cytochrome

Polymer films on electrodes. 8. Investigation of charge-transport mechanisms in Nafion polymer modified electrodes

The mechanism of charge transport through Nafion polymers coated on glassy carbon and containing Cp2FeTMA+, R~(bpy),~+, and O~(bpy),~+ (where Cp2FeTMA+ is ((trimethylammonio)methyl)ferrocene, bpy = 2,2'-bipyridine) is described. The apparent diffusion coefficients, Dam,, of modified electrodes, as measured by conventional electrochemical methods and which can include contributions from intermolecular electron transfer, are compared to the diffusion coefficients of actual mass transport of the electroactive species through the polymer, D,, as measured from the rate of permeation of the electroactive material. The results indicate that Dapp = D, for Cp2FeTMA+, Dapp 20, for Os(bpy),*+, and DaP4 = 15D, for Ru(bpy)t+. These results are discussed in terms of the Dahms-Ruff model, where the observed diffusional behavior is due to both physical diffusion of the electroactive species and an electron-transfer component that can contribute to the observed behavior. The contribution of the electron-exchange mechanism toward the value of Dapp decreases in the order of electroactive species, Ru(bpy),*+ >> O~(bpy),~+ 2 Cp2FeTMA+. A key question in the behavior of polymer electrodes (elec- trodes coated with a thin layer of polymer1-*) involves the mechanism of charge transport through the layers. The rate of charge transport frequently governs the rate of electrochemical and catalytic processes at such and has been the subject of numerous recent inve~tigations.~,~.~- Charge can be trans- ported through the layer by electron transfer between redox centers, counterion diffusion, and diffusion of the electroactive species; the relative contribution of these effects is probably different for different kinds of coatings.

Ion–ion–solvent interactions in solution. Part 6.—Aqueous solutions of metal nitrates having cations with incomplete valence shells

The Raman spectra of aqueous solutions of AgNO3, TlNO3, Zn(NO3)2, Cd(NO3)2, La(NO3)3 and Th(NO3)4 are analysed as a function of concentration. For all solutes, bands due to aquated-nitrate ions, solvent-separated ion pairs (outer-sphere complexes) and contact ion pairs (inner-sphere complexes) are reported. The nature of the inner-sphere complex leads to a distinctly different band from contact ion pairs for simple monovalent cations. This difference is interpreted in terms of an electron-exchange mechanism. The inner-sphere complexes are discernable at all concentrations studied. The association quotients compare favourably with values from other measurements for solutions of Zn(NO3)2 and Th(NO3)4.

Synthesis, photodegradation, and energy transfer in a series of poly(ethylene terephthalate–co–4,4′-biphenyldicarboxylate) copolymers

Poly(ethylene terephthalate) (PET) filament yarns were photostabilized by addition of 0.5–4.0 mole % dimethyl 4,4′-biphenyldicarboxylate (4,4′-BPDC) to the polymerization feed. The mechanism of photostabilization is proposed to be a triplet–triplet energy transfer from excited terephthalate units to ground-state biphenyldicarboxylate units. The mechanism of transfer is reported to be via an electron exchange mechanism, with the “quenching sphere” calculated to be 14.9 Å. Kinetic studies show the “pseudo” zero-order rate constant of initial photodegradation to decrease from 3.4 × 10−19 for the PET homopolymer to 2.0 × 10−19% breaking strength loss/quantum exposure/cm2 for the copolyester containing 4.0 mole % of the 4,4′-biphenyldicarboxyl moieties. The photophysical processes available to the dimethyl 4,4′-biphenyldicarboxylate monomer were characterized by absorption and luminescence studies. In solution, dimethyl 4,4′-biphenyldicarboxylate was shown to emit an intense fluorescence from a 2(π,π*) state and a weaker (∼10−2×) phosphorescence from a 1(π,π*) state derived from the 1A→supn1Lb absorption. The copolymer yarns were shown to exhibit both fluorescence and phosphorescence from the biphenyldicarboxylate units: the fluorescence from direct excitation, the phosphorescence by sensitized transfer.

The influence of non-radiative transitions on bombardment induced photon emission from Ti and Ti oxide

Photon yields from Ti atoms sputtered from Ti and Ti oxide due to 55 keV Ar + and O 2 + ion bombardment have been studied. Photon yields and line profiles of selected lines have also been studied as a function of background O 2 exposure. An attempt has been made to correlate changes in yields and line shapes with variations of the band structure in the solid. Resonance ionization processes are shown not to contribute to the observed yield, however Auger processes cannot be excluded. Lifetime effects have a strong influence on the observed results. It is shown that the distribution of excited states produced by the ion bombardment is Boltzmannlike. We note that it is possible to interpret these results in terms of a model which assumes that the probability of excitation or ionization is exponentially dependent on the excitation or ionization energy, without invoking electron exchange mechanisms.

Characteristics of azurin from pseudomonas aeruginosa via 270-MHz 1H nuclear magnetic resonance spectroscopy

Assignments of resonances in the 1H nmr spectra of Cu(I) azurin to proton groups in the protein are discussed in detail. Comparisons are drawn between Cu(I), Cu(II), apo, Hg(II), and Co(II) azurin samples. Redox titration of Cu(I) azurin with K 3Fe(CN) 6, is used to correlate Cu(I) and Cu(II) 1H nmr spectral features, and observed line broadenings deriving from Cu(II) paramagnetic effects are used to deduce the distances of assigned proton groups from the copper center. Histidine residues are characterized in terms of p K * values, rates of acid-base exchange near the p K *, and rates of C2H exchange with solvent deuterium. The possibility of histidine involvement in the azurincytochrome 551 electron exchange mechanism is discussed. A small number of NH protons observed to be distinctively inert to 2H exchange with solvent 2H 2O, in the Cu(I) protein, are found to show increased lability on removal of the metal.