Fast, long-range electron-transfer reactions of a "blue" copper protein coupled non-covalently to an electrode through a stilbenyl thiolate monolayer

A self-assembled monolayer (SAM), formed by the insitu saponification of a stilbenyl thioacetate on a gold electrode, yields fast electron transfer (ET)(the exchange rate at zero driving force exceeds 1600 s-1) with adsorbed molecules of the blue copper protein, azurin, over a distance exceeding 15 angstroms .     

Factors controlling the efficiencies of photoinduced electron-transfer reactions

The overall efficiencies of photoinduced electron transfer reactions in polar solvents are usually determined by the efficiency with which separated radical ions are formed from the initially formed geminate radical-ion pairs. These separation efficiencies are determined by the competition between retum electron transfer and separation within the geminate pairs. A method is described for determining whether variations in the quantum yields for formation of separated radical ions are due to changes in the reorganization parameters for the return electron transfer reactions, or to other factors. The use of the method is illustrated in studies of the effects of varying steric bulk and molecular size of the donors, and also in studies of the effect of using a charged sensitizer.     

Fast kinetics of the reactions of hydroxyl radicals with nitrone spin traps

The technique of spin trapping with nitrone spin traps nas gained wide acceptance as a method for estimating·OH yields in ESR studies. In our study, fast optical kinetic techniques applied to a series of these traps (PBN, 2-PyBN, 3-PyBN, 4-PyBN, 3-PyOBN and 4-PyOBN) reveal relaxation spectra that indicate two absorption maxima with different time constants, with all except 4-PyOBN showing second order behavior. These two spectral regions show different kinetics. Thus, two reaction sites are indicated, only one of which is necessarily a measure of initial · OH when ESR methods are used. One other trap (DMPO) after · OH reaction decays in one mode suggesting that its final product might be useful as a measure of initial · OH. Also, our ESR evidence shows that OH detection can be improved significantly by spin trapping -hydroxyalkyl radicals formed by · OH attack on alcohols.     

Theory of migration-controlled electron-transfer reactions in polar solvents

A general solution of the problem of finding the rate constant of electron-transfer reactions in polar solvents without the restrictions of the diffusional approximation has been obtained. Expressions for the reaction rate constant at the limit of the random-jump mechanism, as well as a convenient equation describing the transition between the random-jump and diffusional reaction mechanisms, have been found. A test for identifying a random-jump electron-transfer mechanism has been proposed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 1, pp. 27–33, January–February, 1985.     

Microthermogravimetric studies of the kinetics and thermodynamics of heterogeneous reactions in sulfur-containing atmospheres

A thermobalance for studies of the high-temperature sulfidation of metals and alloys is described. This apparatus permits the determination of mass changes in the sulfidized sample as functions of temperature and sulfur vapour pressure with an accuracy of 10^–6 g. Besides heterogeneous kinetics measurements, it has been shown that the chemical diffusion coefficients and deviations from stoichiometry of metal sulfides can also be studied as functions of temperature and sulfur pressure by means of equilibration measurements.

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Zusammenfassung Es wird eine Thermowaage zur Untersuchung der Hochtemperatur-Sulfidierung von Metallen und Legierungen beschrieben. Diese Apparatur ermöglicht die Bestimmung von MassenverÄnderungen der sulfidierten Proben in AbhÄngigkeit von der Temperatur und dem Schwefeldampfdruck mit einer Genauigkeit von 10^–6 g. Es wird gezeigt, da\ damit, sofern Gleichgewichtsmessungen ausgeführt werden, auch die chemischen Diffusionskoeffizienten und Abweichungen der Metallsulfide von der Stöchiometrie in AbhÄngigkeit von der Temperatur und dem Schwefeldampfdruck untersucht werden können.

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. 10^–6 . , , , .

     

Femtosecond electron-transfer reactions in mono- and polynucleotides and in DNA

Quenching of redox active, intercalating dyes by guanine bases in DNA can occur on a femtosecond time scale both in DNA and in nucleotide complexes. Notwithstanding the ultrafast rate coefficients, we find that a classical, nonadiabatic Marcus model for electron transfer explains the experimental observations, which allows us to estimate the electronic coupling (330 cm(-1)) and reorganization (8070 cm(-1)) energies involved for thionine-[poly(dG-dC)](2) complexes. Making the simplifying assumption that other charged, pi-stacked DNA intercalators also have approximately these same values, the electron-transfer rate coefficients as a function of the driving force, DeltaG, are derived for similar molecules. The rate of electron transfer is found to be independent of the speed of molecular reorientation. Electron transfer to the thionine singlet excited state from DNA obtained from calf thymus, salmon testes, and the bacterium, micrococcus luteus (lysodeikticus) containing different fractions of G-C pairs, has also been studied. Using a Monte Carlo model for electron transfer in DNA and allowing for reaction of the dye with the nearest 10 bases in the chain, the distance dependence scaling parameter, beta, is found to be 0.8 +/- 0.1 A(-1). The model also predicts the redox potential for guanine dimers, and we find this to be close to the value for isolated guanine bases. Additionally, we find that the pyrimidine bases are barriers to efficient electron transfer within the superexchange limit, and we also infer from this model that the electrons do not cross between strands on the picosecond time scale; that is, the electronic coupling occurs predominantly through the pi-stack and is not increased substantially by the presence of hydrogen bonding within the duplex. We conclude that long-range electron transfer in DNA is not exceptionally fast as would be expected if DNA behaved as a "molecular wire" but nor is it as slow as is seen in proteins, which do not benefit from pi-stacking.     

Solvent dynamics effect in condensed-phase electron-transfer reactions

Channel-based reaction-diffusion equations are solved analytically for two electron transfer (ET) models, where the fast inner-sphere coordinate leads to an ET reaction treated by Fermi's golden rule, and the slow solvent coordinate moves via diffusion. The analytic solution has let us derive an ET rate constant that modifies the Marcus-Jortner formula by adding a constant alpha which we call a dynamic correction factor. The dynamic correction factor measures the effect of solvent friction. When the relaxation of solvent dynamics is fast, the dynamic correction can be neglected and the ET rate constant reduces to the traditional Marcus-Jortner formula. If the solvent dynamic relaxation is slow, the dynamic correction can be very large and the ET rate can be reduced by orders of magnitude. Using a generalized Zusman-Sumi-Marcus model as a starting point, we introduce two variants, GZSM-A and GZSM-B, where in model A, only one quantum mode is considered for inner-sphere motion and in model B, a classical mode for inner-sphere motion is added. By comparing the two models with experimental data, it is shown that model B is better than model A. For the solvents that have a relaxation time ranging between 0 and 5 ps, our result agrees fairly well with experimental data; for the solvents that have a relaxation time ranging between 5 and 40 ps, our result deviates from the experimental values. After introducing an adjustable scaling index in the effective time correlation function of the reaction coordinate, good agreement is achieved between the experiment and the theory for model B for all of the solvents studied in this paper.     

LIGA-electrodes in voltammetric and spectroelectrochemical studies

The advantages of lithographic-galvanic (LIGA) fabricated microstructured honeycomb electrodes are demonstrated for spectroelectrochemical cells with respect to the response time (the time necessary to generate the product in a sufficient layer thickness close to the electrode to be detectable by UV-Vis-NIR spectroscopy) and to the conversion of the redox system in solution under thin-layer conditions. Transmission UV-Vis-NIR spectroscopy for several electrochemical applications can be performed in a special spectroelectrochemical cell based on the LIGA electrode and the two quartz rods, forming the walls of the cell and conducting the light beam through the cell. They are limiting the diffusion layer at the structured part of the working LIGA electrode. These microstructured LIGA-electrodes can be used as well defined models of porous electrodes at which redox processes occur under finite diffusion conditions. Such electrodes have been successfully used in the voltammetric and spectroelectrochemical study of various redox systems in both aqueous and non-aqueous solvents. The possibility to fabricate the well defined microstructures from various organic conducting polymers is demonstrated by the electrochemical deposition of polypyrrole in moulded LIGA-forms at high current densities in aqueous solutions.     

Rate of homogeneous electron-transfer in self-exchange reactions of methylviologen

The rate of the self-exchange reactions between 1,1′-dimethyl-4,4′-bipyridinium diperchlorate (methylviologen, MV²⁺) and its one-electron reduction product (MV⁺•) and between MV⁺• and the two-electron reduction product (MV) is measured from the broadening of the lines in the ESR-spectrum of MV⁺• in DMF. The bond and solvent reorganization energies λ(O) of these reactions are calculated.     

Structures and energetics of neutral and ionic silicon-germanium clusters: density functional theory and coupled cluster studies

We have calculated the structural and energetic properties of neutral and ionic (singly charged anionic and cationic) semiconductor binary silicon-germanium clusters Si(m)Ge(n) for s = m + n ≤ 12 using the density functional theory (DFT-B3LYP) and coupled cluster [CCSD(T)] methods with Pople's 6-311++G(3df, 3pd) basis set. Neutral and anionic clusters share similar ground state structures for s = 3-7, independent of the stoichiometry and atom locations, but start to deviate at s = 8. The relative energetic stability of the calculated ground state structures among possible isomers has been analyzed through a bond strength propensity model where the pair interactions of Si-Si, Si-Ge, and Ge-Ge are competing. Electron affinities, ionization potentials, energy gaps between the highest and lowest occupied molecular orbitals (HOMO-LUMO gaps), and cluster mixing energies were calculated and analyzed. Overall, for a fixed s, the vertical ionization potential increases as the number of silicon atoms m increases, while the vertical electron affinity shows a dip at m = 2. As s increases, the ionization potentials increase from s = 2 to s = 3 and then decrease slowly to s = 8. The mixing energies for neutral and ionic clusters are all negative, indicating that the binary clusters are more stable than pure elemental clusters. Except for s = 4 and 8, cationic clusters are more stable than anionic ones and, thus, are more likely to be observed in experiments.     

Fluorescence studies of the kinetics of binding and removal of metal ions in proteins

Fluorescence-quenching studies involving native protein fluorescence are used to monitor the rates of binding and removal of Hg(II), Cu(II), Ag(I), methylmercury(I), and p-chloro-mercuribenzoate in various protein systems (ovalbumin, bovine serum albumin, myoglobin, lysozyme, and insulin). In some cases, the fluorescence quenching as a function of time can be used to evaluate the rate constants for the binding of a particular metal ion to a protein. In many cases, multiple binding sites with different rate constants can be differentiated. The restoration of fluorescence vs. time on addition of various chelating agents (BAL, EDTA, cysteine and penicilamine) to the metal/protein system can be used to monitor metal ion removal. Multiple binding sites also can be differentiated kinetically in the removal experiments. In some cases, the appearance of multiple steps in the binding or removal or a metal or ion could be explained by small conformational changes. The rates of removal can help in estimating the effectiveness of various reagents as models for drugs in the treatment of heavy-metal poisoning.     

Solvent reorganization energy of electron-transfer reactions in polar solvents

A microscopic theory of solvent reorganization energy in polar molecular solvents is developed. The theory represents the solvent response as a combination of the density and polarization fluctuations of the solvent given in terms of the density and polarization structure factors. A fully analytical formulation of the theory is provided for a solute of arbitrary shape with an arbitrary distribution of charge. A good agreement between the analytical procedure and the results of Monte Carlo simulations of model systems is achieved. The reorganization energy splits into the contributions from density fluctuations and polarization fluctuations. The polarization part is dominated by longitudinal polarization response. The density part is inversely proportional to temperature. The dependence of the solvent reorganization energy on the solvent dipole moment and refractive index is discussed.     

Direct electron-transfer of native hemoglobin in blood: kinetics and catalysis

A novel approach that uses nature biological tissues, fish blood, for the study of the direct electron-transfer of hemoglobin and its catalytic activity for H(2)O(2) and NO(2)(-) is observed. The direct electron-transfer of hemoglobin in red blood cells in fish blood on glassy carbon electrode was observed for the first time. By simply casting fish blood on GC electrode surface and being air-dried, a pair of well-defined redox peaks for HbFe (III)/HbFe (II) appeared at about -0.36 V (vs SCE) at the fish blood film modified GCE in a pH 7.0 phosphate buffer solution. Ultraviolet visible (UV/VIS) characterization and the enhancement of the redox response of Hb by adding pure Hb in fish blood suggested that Hb preserved the native second structures in the fish blood film. Optical micrographs showed that the RBCs retained its integrity in blood. Hb in blood/GCE maintained its activity and could be used to electrocatalyze the reduction H(2)O(2) and NO(2)(-).     

Control of the stability, electron-transfer kinetics, and pH-dependent energetics of Si/H2O interfaces through methyl termination of Si(111) surfaces

Methyl-terminated, n-type, (111)-oriented Si surfaces were prepared via a two-step chlorination-alkylation method. This surface modification passivated the Si surface toward electrochemical oxidation and thereby allowed measurements of interfacial electron-transfer processes in contact with aqueous solutions. The resulting semiconductor/liquid junctions exhibited interfacial kinetics behavior in accord with the ideal model of a semiconductor/liquid junction. In contrast to the behavior of H-terminated Si(111) surfaces, current density vs. potential measurements of CH(3)-terminated Si(111) surfaces in contact with an electron acceptor having a pH-independent redox potential (methyl viologen(2+/+)) were used to verify that the band edges of the modified Si electrode were fixed with respect to changes in solution pH. The results provide strong evidence that the energetics of chemically modified Si interfaces can be fixed with respect to pH and show that the band-edge energies of Si can be tuned independently of pH-derived variations in the electrochemical potential of the solution redox species.