Electron Exchange Properties Research Articles

Insights into Interprotein Electron Transfer of Human Cytochrome c Variants Arranged in Multilayer Architectures by Means of an Artificial Silica Nanoparticle Matrix.

The redox behavior of proteins plays a crucial part in the design of bioelectronic systems. We have demonstrated several functional systems exploiting the electron exchange properties of the redox protein cytochrome c (cyt c) in combination with enzymes and photoactive proteins. The operation is based on an effective reaction at modified electrodes but also to a large extent on the capability of self-exchange between cyt c molecules in a surface-fixed state. In this context, different variants of human cyt c have been examined here with respect to an altered heterogeneous electron transfer (ET) rate in a monolayer on electrodes as well as an enhanced self-exchange rate while being incorporated in multilayer architectures. For this purpose, mutants of the wild-type (WT) protein have been prepared to change the chemical nature of the surface contact area near the heme edge. The structural integrity of the variants has been verified by NMR and UV–vis measurements. It is shown that the single-point mutations can significantly influence the heterogeneous ET rate at thiol-modified gold electrodes and that electroactive protein/silica nanoparticle multilayers can be constructed with all forms of human cyt c prepared. The kinetic behavior of electron exchange for the mutant proteins in comparison with that of the WT has been found altered in some multilayer arrangements. Higher self-exchange rates have been found for K79A. The results demonstrate that the position of the introduced change in the charge situation of cyt c has a profound influence on the exchange behavior. In addition, the behavior of the cyt c variants in assembled multilayers is found to be rather similar to the situation of cyt c self-exchange in solution verified by NMR.

Polyazetidine-Coated Microelectrodes: Electrochemical and Diffusion Characterization of Different Redox Substrates

The present paper reports on the diffusion characteristics and electron transfer properties of a membrane obtained from polyazetidine prepolymer (PAP) consisting of repeating units of 1-(aminomethyl)-1-{2-[(6-oxyhexane)amino]ethyl}-3-hydroxyazetidinium chloride studied in the presence of seven simple redox electroactive molecules: ABTS, catechol, dopamine, ferrocenecarboxylic acid, ferricyanide, ferrocyanide, and the osmium complex bis(2,2-bipyridyl)-4-aminomethylpyridine chloride hexafluorophosphate (Os[(bpy)(2) 4-AMP Cl](+)). Using water as medium, the apparent diffusion coefficients (D(app)), the concentrations of the compounds in the membrane, and the heterogeneous rate constants (k(s)) were calculated as a function of temperature, and the influence thereof on these parameters was evaluated. Even if D(app) and k(s) values in the presence of PAP are smaller than in solution, this decrease is small enough to indicate that the PAP membrane shows excellent diffusion and electron-exchange properties with respect to other commonly used membranes reported in the literature.

Study of the sorption-kinetic properties of carbon molecular sieves

To elucidate the role of the electron-exchange properties of arene molecules that form the pore walls of carbon molecular sieves in their interactions with sorbate molecules, the correlation between the electronexchange and sorption-kinetic properties was studied. The electron-exchange properties were estimated by using quantum-chemical modeling based on the energies of formation of singly charged anions and cations and neutral (both oxidized and reduced) molecules.

Effects of bias cooling on charge states in heterostructures embedding self-assembled quantum dots

Carrier transfer behavior has been investigated in selectively doped InGaAs/AlGaAs heterojunctions, in which a layer of self-assembled InAs quantum dots is embedded in close vicinity to a two-dimensional electron gas (2DEG). We applied the bias-cooling technique to derive information on the electron exchange properties from the bias dependence of the capacitance-voltage (CV) spectroscopy. Noise is observed in the CV results at large reverse biases after sufficiently negative bias cooling. The noise is temperature sensitive and can be eliminated when raising the temperature to 130 K. In combination with the experimental results in the control sample, we find that the noise induced by the bias cooling is most likely associated with the hot-electron trapping and detrapping in DX centers in the selectively doped AlGaAs layer. The effect of light illumination on the CV results supports our speculation on the noise origin. Furthermore, the variation in the 2DEG carrier concentration and mobility with bias-cooling processes has been discussed, which provides a direct insight into the vertical charge-transfer mechanism among the quantum dot related electronic states, DX centers, and 2DEG channel.

Effects of dimerization on protein electron transfer.

In order to investigate the relationship between the rate of protein-protein electron transfer and the structure of the association complex, a dimer of the blue copper protein azurin was constructed and its electron exchange properties were determined. For this purpose, a site for covalent cross-linking was engineered by replacing the surface-exposed asparagine 42 with a cysteine. This mutation enabled the formation of disulfide-linked homo-dimers of azurin. Based on NMR line-broadening experiments, the electron self-exchange (e.s.e.) rate constant for this dimer was determined to be 4.2(+/-0.7) x 10(5)M(-1)s(-1), which is a seven-fold decrease relative to wild-type azurin. This difference is ascribed to a less accessible hydrophobic patch in the dimer. To discriminate between intramolecular electron transfer within a dimer and intermolecular electron transfer between two dimers, the e.s.e. rate constant of (Cu-Cu)-N42C dimers was compared with that of (Zn-Cu)- and (Ag-Cu)-N42C dimers. As Zn and Ag are redox inactive, the intramolecular electron transfer reaction in these latter dimers can be eliminated. The e.s.e. rate constants of the three dimers are the same and an upper limit for the intramolecular electron transfer rate of 10 s(-1) could be determined. This rate is compatible with a Cu-Cu distance of 18 A or more, which is larger than the Cu - Cu distance of 15 A observed in the wild-type crystal structure that shows two monomers that face each other with opposing hydrophobic patches. Modelling of the dimer shows that the Cu-Cu distance should be in the range of 17 A < rCu-Cu < 28 A, which is in agreement with the experimental findings. For efficient electron transfer, it appears crucial that the two molecules interact in the proper orientation. Direct cross-linking may disturb the formation of such an optimal electron transfer complex.

Direct versus indirect bioleaching

The dissolution of metal sulfides is controlled by their solubility product and thus, the [H +] concentration of the solution, and further enhanced by several chemical mechanisms which lead to a disruption of sulfide chemical bonds. They include extraction of electrons and bond breaking by [Fe 3+], extraction of sulfur by polysulfide and iron complexes forming reactants [Y +] and electrochemical dissolution by polarization of the sulfide [high Fe 3+ concentration]. All these mechanisms have been exploited by sulfide and iron-oxidizing bacteria. Basically, the bacterial action is a catalytic one during which [H +], [Fe 3+] and [Y +] are breaking chemical bonds and are recycled by the bacterial metabolism. While the cyclic bacterial oxidative action via [H +] and [Fe 3+] can be called indirect, bacteria had difficulties harvesting chemical energy from an abundant sulfide such as FeS 2, the electron exchange properties of which are governed by coordination chemical mechanisms (extraction of electrons does not lead to a disruption of chemical bonds but to an increase of the oxidation state of interfacial iron). Here, bacteria have evolved alternative strategies which require an extracellular polymeric layer for appropriately conditioned contact with the sulfide. Thiobacillus ferrooxidans cycles [Y +] across such a layer to disrupt FeS 2 and Leptospirillum ferrooxidans accumulates [Fe 3+] in it to depolarize FeS 2 to a potential where electrochemical oxidation to sulfate occurs. Corrosion pits and high resolution electron microscopy leave no doubt that these mechanisms are strictly localized and depend on specific conditions which bacteria create. Nevertheless, they cannot be called ‘direct’ because the definition would require an enzymatic interaction between the bacterial membrane and the cell. Therefore, the term ‘contact’ leaching is proposed for this situation. In practice, multiple patterns of bacterial leaching coexist, including indirect leaching, contact leaching and a recently discovered cooperative (symbiotic) leaching where ‘contact’ leaching bacteria are feeding so wastefully that soluble and particulate sulfide species are supplied to bacteria in the surrounding electrolyte.

Effect of the type of Lewis acid on the structure, properties and yield of polycondensation products of p-benzoquinone

A study was made of the polycondensation of benzoquinone in the presence of ZnCl 2, AlCl 3 and BF 3·OEt 2, and the order of efficiency of these catalysts was found to be BF 3·OEt 2>AlCl 3>ZnCl 2. Boron trifluoride etherate produces higher yields and results in the purest polyhydroquinone; the polymers prepared in presence of this catalyst have higher heat stability. Structural analyses of the polymers by chemical and spectral methods showed that the latter possessed semi-conducting and electron exchange properties.