Decrease In Electron Transfer Rate Research Articles

Tuning the Photocatalytic Performance of Plasmonic Nanocomposites (ZnO/Aux) Driven in Visible Light

: In this article, we explored the possibility of controlling the reactivity of ZnO nanostructures by modifying its surface with gold nanoparticles (Au NPs). By varying the concentration of Au with different wt% (x = 0.01, 0.05, 0.08, 1 and 2), we have synthesized a series of (ZnO/Aux) nanocomposites (NCs). A thorough investigation of the photocatalytic performance of different wt% of Au NPs on ZnO nanosurface has been carried out. It was observed that ZnO/Au0.08 nanocomposite showed the highest photocatalytic activity among all concentrations of Au on the ZnO surface, which degrades the dye concentration within 2 minutes of visible light exposure. It was further revealed that with an increase in the size of plasmonic nanoparticles beyond 0.08%, the accessible surface area of the Au nanoparticle decreases. The photon absorption capacity of Au nanoparticle decreases beyond 0.08% resulting in a decrease in electron transfer rate from Au to ZnO and a decrease of photocatalytic activity. Background: Due to the industrialization process, most of the toxic materials go into the water bodies, affecting the water and our ecological system. The conventional techniques to remove dyes are expensive and inefficient. Recently, heterogeneous semiconductor materials like TiO2 and ZnO have been regarded as potential candidates for the removal of dye from the water system. Objective: To investigate the photocatalytic performance of different wt% of Au NPs on ZnO nanosurface and the effect of the size of Au NPs for photocatalytic performance in the degradation process. Methods: A facile microwave method has been adopted for the synthesis of ZnO nanostructure followed by a reduction of gold salt in the presence of ZnO nanostructure to form the composite. Results: ZnO/Au0.08 nanocomposite showed the highest photocatalytic activity which degrades the dye concentration within 2 minutes of visible light exposure. The schematic mechanism of electron transfer rate was discussed. Conclusion: Raspberry shaped ZnO nanoparticles modified with different percentages of Au NPs showed good photocatalytic behavior in the degradation of dye molecules. The synergetic effect of unique morphology of ZnO and well anchored Au nanostructures plays a crucial role.

Unresolved quenching mechanisms of chlorophyll fluorescence may invalidate MT saturating pulse analyses of photosynthetic electron transfer in microalgae.

Chlorophyll a fluorescence is a powerful tool for estimating photosynthetic efficiency, but there are still unanswered questions that hinder the use of its full potential. The present results describe a caveat in estimation of photosynthetic performance with so-called rapid light curves (RLCs) with pulse amplitude modulation fluorometers. RLCs of microalgae show a severe decrease in photosynthetic performance in high light, although a similar decrease cannot be seen with other methods. We show that this decrease cannot be assigned to energy-dependent non-photochemical quenching or photoinhibition or to the geometry of the algal sample. The measured decrease in electron transfer rate is small in the tested siphonaceuous algae and higher plants, but very notable in all planktonic species, exhibiting species-dependent variation in extent and reversibility. We performed in-depth analysis of the phenomenon in the diatom Phaeodactylum tricornutum, in which the decrease is the most pronounced and reversible among the tested organisms. The results suggest that quenching of fluorescence by oxidized plastoquinone alone cannot explain the phenomenon, and alternative quenching mechanisms within PSII need to be considered.

Role of Conductive Nanoparticles in the Direct Unmediated Bioelectrocatalysis of Immobilized Sulfite Oxidase

AbstractWe report efficient bioelectrocatalytic sulfite oxidation by human sulfite oxidase (hSO) immobilized on a gold nanoparticle (AuNP) modified gold electrode. The AuNP were synthesized in aqueous phase by using branched polyethyleneimine (PEI) as reducing as well as stabilizing agent. Gold electrodes were modified by a self assembled monolayer of dithio‐bis(N‐hydroxysuccinimidyl propionate) (DTSP) onto which the NP and hSO were immobilized. Cyclic voltammetry of the hSO modified electrode in the absence of substrate revealed a quasi‐reversible direct electrochemical reaction of the heme domain of hSO with fast electron transfer rate. The electron transfer rate constant of ks=32 s−1 and the formal potential E0′=−0.155 V vs. Ag/AgCl/1 M KCl were estimated. Comparative studies with nanoparticles of BaSO4 indicate the importance of the NP conductivity for charge transfer and enhancement of direct electron transfer communication. Sulfite addition resulted in a largely enhanced oxidation current at low overpotential. Use of longer thiols for assembly of the AuNP resulted in a decrease of electron transfer rate and increase of overpotential for sulfite bioelectrocatalysis. The electrode modified with DTSP and AuNP exhibited a very high steady state catalytic current density of 1 µA/cm2. The assembly can be applied for the amperometric biosensing of sulfite at a potential of 0 mV vs. Ag/AgCl/1 M KCl.

Barrier-Layer-Mediated Electron Transfer from Semiconductor Electrodes to Molecules in Solution: Sensitivity of Mechanism to Barrier-Layer Thickness

Electron transfer (ET) phenomena at and near semiconductor/molecule interfaces are of fundamental significance for applications involving liquid-junction photovoltaics, organic photovoltaics, and electrochemical heterogeneous catalysis. To probe mechanisms of electron delivery through barrier layers at such interfaces, we make use of atomic layer deposition to deposit ultrathin films of TiO2 conformally onto SnO2 electrodes. In the presence of TiO2 films (i.e., barrier layers) up to 10 Å thick, electrons are delivered from the electrode to molecules in solution by tunneling through the layers, as evidenced, in part, by an exponential decrease in ET rate with layer thickness. For films thicker than 10 Å, there is little change in ET rate as a function of TiO2 thickness. To our surprise, thermally annealing a 55 Å layer of TiO2 on SnO2 yielded a 10-fold decrease in ET rate compared to that imposed by the as-deposited layer. At applied potentials near the conduction-band edge of SnO2, and significantly below the band edge of TiO2, electrochemical impedance spectroscopy with nominally flat, as-deposited TiO2 indicates the presence of nearly twice the density of electronic states as found with air-annealed samples. These and related observations point to a barrier-layer-thickness-dependent change in the mechanism of electron delivery, from the underlying electrode to solution species, from one based on tunneling to one entailing trap-facilitated hopping. The findings have design implications for the application of interfacial barrier layers to electrochemical and photoelectrochemical problems.

Modulating Electron Transfer Dynamics at Dye–Semiconductor Interfaces via Self-Assembled Bilayers

Forward and back electron transfer at dye–semiconductor interfaces are pivotal events in dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells. Here we introduce self-assembled bilayers as a strategy for manipulating electron transfer dynamics at these interfaces. The bilayer films are achieved by stepwise layering of bridging molecules, linking ions, and dye molecules on the metal oxide surface. The formation of the proposed architecture is supported by ATR-IR and UV–vis spectroscopy. By using time-resolved emission and transient absorption, we establish that the films exhibit an exponential decrease in electron transfer rate with increasing bridge length. The findings indicate that self-assembled bilayers offer a simple, straightforward, and modular method for manipulating electron transfer dynamics at dye–semiconductor interfaces.

Influence of the Electron Transfer Rate on Electrochemically Controlled Hydrogen Bonding

Hydrogen bonding is an essential interaction in natural and artificial systems. Its strength can be modulated by employing process known as Electrochemically Controlled Hydrogen bonding (ECHB). Although these processes are assumed to operate under thermodynamic control no experimental evidence for kinetic control exists. In this work, ECHB processes where studied using electrogenerated radical anions from 5-nitroimidazole derivatives as receptor molecules and 1,3-diethylurea as hydrogen bond donor species. Results revealed that kinetic control occurs due to an increase in the internal reorganization energy of the receptor molecule, which cause a decrease in electron transfer rate. Electronic structure calculations and experimental Kb values suggested that kinetic limitations were the product of a competition between intra and intermolecular hydrogen bonding formation during the global process.

The Critical Role of Conserved Residues E90 and H212 in Subunit III of Rhodobacter sphaeroides Cytochrome C Oxidase on Enzyme Function

Cytochrome c oxidase (COX) is the terminal enzyme in the mitochondrial respiratory chain. Similar to the mammalian homolog, Rhodobacter sphaeroides (RBS) COX utilizes the reduction of oxygen to water to drive an electrochemical potential gradient across the membrane, which is ultimately used for the generation of ATP. Expression of COX by RBS yields a protein that retains the three catalytic core subunits of COX, which exhibit similar enzymatic activities as mitochondrial COX. Previous studies have shown that chemical modification of E90 of subunit III(SIII) using dicyclohexylcarbodiimide leads to a decrease in electron transfer rate and proton pumping in the mitochondrial enzyme. E90 is located within a nonpolar region of SIII and possibly interacts, via salt bridge or hydrogen bonds, with the conserved residue H212(SIII). Mutant RBS COX enzymes E90H(SIII) and E90H(III)/H212E(III) were constructed and isolated. Visible absorbance spectra of the E90H(III)/H212(III) mutant enzyme exhibited similar characteristics to those displayed by wild-type RBS COX; however, the α-region of the reduced spectrum for E90H(SIII) displayed a slight red shift of the absorbance maximum by 1 nm, resembling that seen for the SIII-depleted oxidase. E90H(III) and E90H/H212(III) mutant proteins were determined to contain stoichiometric amount of each subunit by SDS-PAGE gel, similar to wild-type COX. Electron transfer rates observed at pH 6.5, for E90H(III)/H212E(III) and E90H(SIII) COX, were decreased by 17 and 39% respectively, as compared to WT COX. The mutants exhibit little or no suicide inactivation when compared to the SIII-depleted RBS COX. pH dependence of electron transfer activity for E90H(III)/H212E(III) and E90H(III) were also not altered when compared to WT enzyme. Proton pumping experiments on reconstituted E90H(III) and E90H(III)/H212E(III) in liposomes will be discussed.

Surface modification imparts selectivity, facilitating redox catalytic studies: quinone mediated oxygen reduction

Modifying a gold electrode surface with hydroxy-alkyl-thiols significantly reduces the observed rate of electron transfer. This designed and controlled decrease in electron transfer rate readily allows reversible and irreversible electrode processes to be more clearly delineated. Separation of such voltammetric responses can facilitate the direct study of redox catalytic processes, which would, under other experimental conditions be obscured. The reduction of oxygen by anthraquinone hydroxy derivatives are used as a paradigmatic example, demonstrating the importance of the hydroxyl groups in the reduction process. Specifically, the reduced form of 1,8-dihydroxy-anthraquinone is shown to be significantly more reactive towards oxygen reduction than the 1,4-dihydroxyl analogue.

Chemical elimination of amorphous carbon on amorphous carbon nanotubes and its electrochemical performance

Chemical elimination of amorphous carbon on amorphous carbon nanotubes (ACNTs) was for the first time investigated by different treatment processes. Electrochemical performance of the modified ACNTs/carbon paste electrode (ACNTs/CPE) was measured by cyclic voltammetry. Field emission scanning and transmission electron microscope (STEM) observation reveals that the diameter of ACNTs is in the range of 60–100 nm. The amorphous nature of ACNTs was proved by the result of Raman analysis. FT-IR spectra showed that it might be one of the low-cost ways to eliminate amorphous carbon on the surface of ACNTs to treat ACNTs with HNO 3 in microwave oven. Further oxidation in air would lead to the decrease of electron transfer rate on the ACNTs/CPE because OH groups on the wall of ACNTs were partly eliminated by oxidation in air. The results of cyclic voltammetry showed that ACNTs/CPE treated with HNO 3 in microwave oven has optimal peak in relation to the highest redox peak current.

Electrostatic role of the non-heme iron complex in bacterial photosynthetic reaction center

To elucidate the role of the non-heme iron complex (Fe-complex) in the electron transfer (ET) events of bacterial photosynthetic reaction centers (bRC), we calculated redox potentials of primary/secondary quinones Q A/B ( E m(Q A/B)) in the Fe-depleted bRC. Removing the Fe-complex, the calculated E m(Q A/B) are downshifted by ∼220 mV/∼80 mV explaining both the 15-fold decrease in ET rate from bacteriopheophytin ( H A - ) to Q A and triplet state occurrence in Fe-depleted bRC. The larger downshift in E m(Q A) relative to E m(Q B) increases the driving-energy for ET from Q A to Q B by 140 meV, in agreement with ∼100 meV increase derived from kinetic studies.

Influence of pigment substitution on the electrochemical properties ofRhodobacter sphaeroides 601 reaction centers

With the help of pigment substitution, self-assembled monolayer film and square wave voltammetry, the influence of pigment substitution on the electrochemical properties of Rhodobacter sphaeroides 601 reaction centers was investigated. Results showed that the charge separation could also be driven by externally electric field, similar to the primary photochemical reaction in purple bacterial reaction center. On the surface of Au electrode, a self-assembled monolayer film (the RC-PDDA-DMSA film) was made up of 2,3-dimercaptosuccinic acid (DMSA), poly-dimethyldiallylammonium chloride (PDDA) and reaction center (RC). When square wave voltammetry was used to study the RC-PDDA-DMSA film, four redox pairs in the photochemical reaction of RC were observed by changing frequency. With nonlinear fitting, the standard potential of P/P(+) and the corresponding electrode reaction rate constant were determined to be 0.522 V and 13.04 S(-1), respectively. It was found that the redox peak at -0.02 V changed greatly when bacteriopheophytin was substituted by plant pheophytin in the reaction center. Further studies indicated that this change resulted from the decrease in electron transfer rate between Bphe(-)/Bphe (Phe(-)/Phe) and Q(A) (-)/Q(A) after pigment substitution. After investigations of spectra and electrochemical properties of different RCs and comparisons of different function groups of pigments, it was indicated that the phytyl tail, similar to other substituted groups of pheophytin, affected the efficiencies of pigment substitution.

Kinetics of Heterogeneous Electron Transfer at Liquid/Liquid Interfaces As Studied by SECM

Scanning electrochemical microscopy (SECM) was used to investigate the kinetics of heterogeneous electron transfer (ET) as a function of driving force at the interface between two immiscible electrolyte solutions. At high driving force, experimental rate constants decreased with increasing overpotential, deviating from predictions based on Butler−Volmer kinetics. This decrease in ET rate with increasing driving force is consistent with Marcus theory inverted region behavior. At low driving force, the potential dependence of the forward and reverse ET rate constants followed Butler−Volmer theory. SECM is also demonstrated to be a useful means of studying the effect of high ionic strength on the kinetics of heterogeneous ET.

The role of individual lysine residues in the basic patch on turnip cytochrome f for electrostatic interactions with plastocyanin in vitro.

The role of electrostatic interactions in determining the rate of electron transfer between cytochrome f and plastocyanin has been examined in vitro with mutants of turnip cytochrome f and mutants of pea and spinach plastocyanins. Mutation of lysine residues Lys58, Lys65 and Lys187 of cytochrome f to neutral or acidic residues resulted in decreased binding constants and decreased rates of electron transfer to wild-type pea plastocyanin. Interaction of the cytochrome f mutant K187E with the pea plastocyanin mutant D51K gave a further decrease in electron transfer rate, indicating that a complementary charge pair at these positions could not compensate for the decreased overall charge on the proteins. Similar results were obtained with the interaction of the cytochrome f mutant K187E with single, double and triple mutants of residues in the acidic patches of spinach plastocyanin. These results suggest that the lysine residues of the basic patch on cytochrome f are predominantly involved in long-range electrostatic interactions with plastocyanin. However, analysis of the data using thermodynamic cycles provided evidence for the interaction of Lys187 of cytochrome f with Asp51, Asp42 and Glu43 of plastocyanin in the complex, in agreement with a structural model of a cytochrome f-plastocyanin complex determined by NMR.

Tyr 30 of amicyanin is not critical for electron transfer to cytochrome c-551i: implications for predicting electron transfer pathways

A Pathways analysis of the methylamine dehydrogenase–amicyanin–cytochrome c-551i protein electron transfer (ET) complex predicts two sets of ET pathways of comparable efficiency from the type I copper of amicyanin to the heme of cytochrome c-551i. In one pathway, the electron exits copper via the Cys 92 copper ligand, and in the other, it exits via the Met 98 copper ligand. If the Pathways algorithm is modified to include contributions from the anisotropy of metal–ligand coupling, independent of differences in copper–ligand bond length, then the pathways via Cys 92 are predicted to be at least 100-fold more strongly coupled than the pathways via any of the other copper ligands. All of the favored pathways via Cys 92 include a through-space jump from Cys 92 to the side chain of Tyr 30. To determine whether or not the pathways via Cys 92 are preferentially used for ET, Tyr 30 was changed to other amino acid residues by site-directed mutagenesis. Some mutant proteins were very unstable suggesting a role for Tyr 30 in stabilizing the protein structure. Y30F and Y30I mutant amicyanins could be isolated and analyzed. For the Y30I mutant, the modified Pathways analysis which favors ET via Cys 92 predicts a decrease in ET rate of at least two orders of magnitude, whereas the standard Pathways analysis predicts no change in ET rate since ET via Met 98 is not affected. Experimentally, the ET rates of the Y30I and Y30F mutants were indistinguishable from that of wild-type amicyanin. Likely explanations for these observations are discussed as are their implications for predicting pathways for ET reactions of metalloproteins.

Scanning Electrochemical Microscopy Studies of Electron Transfer through Monolayers Containing Conjugated Species at the Liquid−Liquid Interface

The rates of electron transfer (ET) between two redox species through a monolayer of saturated dipalmytoyl phosphocholine and polyconjugated 2(3-(diphenylhexatrienyl)propanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine phospholipids adsorbed at the interface of two immiscible electrolyte solutions (ITIES) were measured by scanning electrochemical microscopy. Comparison of the ET rates shows that addition of phospholipids with conjugated hydrocarbon chains increases the ET rate by at least a factor of 2 compared to films with only saturated hydrocarbon chains. This difference was sufficiently high to obtain information about distribution of lipid molecules in monolayers formed by mixing two lipids. Lateral scanning of an ultramicroelectrode tip across the lipid monolayer comprised of two different phospholipids showed that each lipid forms microsize domains. The rate of ET across a monolayer of lipid adsorbed at the ITIES was also probed as a function of temperature. A sharp decrease in ET rate with tempe...

Alteration of the Midpoint Potential and Catalytic Activity of the Rieske Iron-Sulfur Protein by Changes of Amino Acids Forming Hydrogen Bonds to the Iron-Sulfur Cluster

The crystal structure of the bovine Rieske iron-sulfur protein indicates a sulfur atom (S-1) of the iron-sulfur cluster and the sulfur atom (Sgamma) of a cysteine residue that coordinates one of the iron atoms form hydrogen bonds with the hydroxyl groups of Ser-163 and Tyr-165, respectively. We have altered the equivalent Ser-183 and Tyr-185 in the Saccharomyces cerevisiae Rieske iron-sulfur protein by site-directed mutagenesis of the iron-sulfur protein gene to examine how these hydrogen bonds affect the midpoint potential of the iron-sulfur cluster and how changes in the midpoint potential affect the activity of the enzyme. Eliminating the hydrogen bond from the hydroxyl group of Ser-183 to S-1 of the cluster lowers the midpoint potential of the cluster by 130 mV, and eliminating the hydrogen bond from the hydroxyl group of Tyr-185 to Sgamma of Cys-159 lowers the midpoint potential by 65 mV. Eliminating both hydrogen bonds has an approximately additive effect, lowering the midpoint potential by 180 mV. Thus, these hydrogen bonds contribute significantly to the positive midpoint potential of the cluster but are not essential for its assembly. The activity of the bc1 complex decreases with the decrease in midpoint potential, confirming that oxidation of ubiquinol by the iron-sulfur protein is the rate-limiting partial reaction in the bc1 complex, and that the rate of this reaction is extensively influenced by the midpoint potential of the iron-sulfur cluster.

Photoinduced Electron Transfer in Ruthenium(II)−Rhodium(III) Terpyridine Dyads

A series of Ru(II)−Rh(III) dyads of general formula (ttpy)Ru−tpy−(ph)n−tpy−Rh(ttpy)5+ with n = 0, 1, 2 [ttpy = 4‘-p-tolyl-2,2‘:6,2‘‘-terpyridine; tpy−(ph)n−tpy = bridging ligand where two 2,2‘:6‘,2‘‘-terpyridine units are connected at the 4‘-position through a variable number of p-phenylene spacers] have been studied, in acetonitrile at room temperature, by picosecond and nanosecond time-resolved emission spectroscopy. When n = 1, excitation of the Ru(II)-based molecular component is followed by efficient intramolecular quenching by electron transfer to the Rh(III) center. The rate constant, k ≥ 3 × 109 s-1, is high despite the relatively small driving force of the process (ca. 0.1 eV). When n = 2, with the same driving force as above, no intramolecular electron transfer quenching is observed (upper limit for the rate constant of the electron transfer process: k < 5 × 108 s-1). The decrease in electron transfer rate obtained in going from n = 1 to n = 2 is in line with the behavior of other systems containing poly-p-phenylene spacers. The dyad with n = 0 is definitely not homogeneous with the other two: the intercomponent electronic coupling is much stronger, the Ru(II)-based excited state is lower in energy, and the electron transfer has a smaller driving force (ΔG ≈ 0). The lifetime of the Ru(II)-based emission is 17 ns. The lack of an obvious model compound makes it difficult to draw firm conclusions for this system.

Release of Cytochrome c from Liver Mitochondria during Permeability Transition

The mitochondrial permeability transition (PT) follows opening of megachannels in the inner membrane and may be part of a programmed death pathway. Recently a role for cytochrome c in programmed cell death has been proposed, although its relationship to PT has not been been determined. We studied the release of cytochrome c from liver mitochondria undergoing PT. Well-coupled mitochondria treated with 5 mM atractyloside (ATR) or 100 μM calcium chloride were found to undergo PT and release cytochrome c into the incubation buffer within 5 minutes. Control mitochondria and mitochondria treated with the uncoupler FCCP did not undergo PT or release cytochrome c at 5 or 15 minutes. PT induced by ATR could be prevented by pretreatment with 10 μM cyclosporin A. Mitochondria incubated with ATR or calcium caused a 20-30% decrease in electron transfer rate via cytochrome c and cytochrome c oxidase. We conclude that cytochrome c release is an early event during mitochondrial PT, and is sufficient to decrease electron transfer through the terminal electron transport components of the mitochondrion.