Short-range Electron Transfer Research Articles

Ultrafast Nonequilibrium Dynamics of Vibrationally Hot Electron Transfer in Flavodoxin.

The understanding of ultrafast short-range electron transfer (ET) in proteins remains challenging, and thorough studies on well-defined biological systems are demanding. Here, we utilized two types of flavodoxins and designed a series of mutants on two positions to systematically characterize the complete photoinduced redox cycles. We identified one position with a favorable orientation and distance for ultrafast ET in a few femtoseconds and the other position is relatively flexible with a longer ET time scale. We found that all forward and back ET dynamics are ultrafast nonequilibrium processes, occurring through highly vibronic states and ending in vibrationally hot ground states with subsequent cooling relaxation to efficiently dissipate photon energy into the protein environment.

Reconstructing bridging carbon atoms (from sp3-C to sp2-C) of pyrolytic N-doped carbon to enhance oxygen reduction reaction

The development of new regulation strategy for constructing functional carbon skeletons is a standing challenge for oxygen reduction reaction (ORR). Herein, the locally ordered graphene microcrystals with suitable N-sites derived from glucose are synthesized under the assistance of low melting point zinc molten salt. Utilizing the structural features of graphene microcrystals (“house of cards” model), the work function of carbon skeleton is reduced after inducing the conversion of bridging sp3-C to bridging sp2-C (ratio of sp2-C to sp3-C increases from 3.75 to 7.41). The lower work function of locally ordered microarchitectures decreases the long-range electron transfer barrier in outer-Helmholtz plane. And the synergy between pyridinic N and graphitic N is beneficial for enhancing short-range electron transfer process in inner-Helmholtz plane. After rearrangement of carbon skeletons, the half-wave potential can increase from 0.846 V to 0.874 V, and the HO2– by-product yield decreases from 31.5 % to 6.9 %. Furthermore, the discharge voltage of locally ordered microarchitecture (1.31 V) at 10 mA cm−2 can surpass the commercial Pt/C (1.25 V) in Zn-air battery devices. In brief, it provides a new direction of structure design for carbonaceous catalysts.

Binding Molybdenum selenide with dual conductive carbon as Self-Supporting anode for an efficient sodium storage

Compounding with conductive carbon is usually an effective strategy to improve the conductivity of TMDCs and alleviate volume expansion. However, single carbon matrix modification may lead to shedding of the electrode material. In this work, a self-supporting electrode structure is proposed that has MoSe2 as the TMDC sandwiched between the carbon cloth(CC) core and the outer carbon layer to form C-C/MoSe2/CC frame. The dual conductive carbon structure not only can suppress volume expansion, but also can form long-range conductive network. The outer carbon layer can provide short-range electron transfer paths, prevent the active materials from falling off, improve the cycling stability. The unique freestanding structure was synthesized via a multi-step solvothermal and calcination approach. With the above advantages, the C-C/MoSe2/CC heterostructure delivers remarkable cycling stability. In detail, a capacity of 355 mAh g−1 was obtained after 700 cycles at 1 A g−1 as the anode of SIBs. The Kinetic results show that most of the Na+ storage in C-C/MoSe2/CC is governed by a pseudocapacitive process.

Consecutive chemical bonds reconstructing surface structure of silicon anode for high-performance lithium-ion battery

The development of stable, high-energy electrode materials for lithium ion-batteries requires an elaborate effort to optimize the active materials as well as the chemical bonds and electron/ion transport in the electrode. However, hindered by the intrinsic structure and electrochemical degradation which is attributed to the volume expansion of materials, an increase in battery safety and reliability is concerned. Here, taking silicon as an example, we propose a strategy to stabilize this anode by successive chemical bonds reconstructing the surface. In this study, silicon nanoparticles are assembled in a carbon-copper framework via a facile and scalable pyrolysis process to provide a short-range electron transfer and pulverization suppression. Dissimilar to the current carbon coating methods, with the aid of Cu-O-C, Si-O-C, and Si-C chemical bonds, silicon hybridized reduced graphene oxide (rGO) and double-faced adhesive tape derived carbon composite (Si+rGO@DFAT-C) exhibits high structural integrity and immune to delamination. Hence, it demonstrates superior capacity (1536 mAh g−1 at 0.1 A g−1), high rate capability (1126 mAh g−1 at 2 A g−1), and stable electron stability (968.1 mAh g−1 after 200 cycles at 0.5 A g−1). This study emphasizes the crucial importance of well-tailor surface chemical bond reconstruction for the anode stabilization for high-performance LIBs.

Branched In2O3 Mesocrystal of Ordered Architecture Derived from the Oriented Alignment of a Metal–Organic Framework for Accelerated Hydrogen Evolution over In2O3–ZnIn2S4

It is fascinating yet challenging to assemble anisotropic nanowires into ordered architectures of high complexity and intriguing functions. We exploited a facile strategy involving oriented etching of a metal-organic fragment (MOF) to advance the rational design of highly ordered nanostructures. As a proof of concept, a microscale MIL-68(In) single crystal was etched with a K3[Co(CN)6] solution to give a microtube composed of aligned MIL-68(In) nanorods. Annealing such a MIL-68(In) microtube readily created an unprecedented branched In2O3 mesocrystal by assembly of In2O3 nanorods aligned in order. The derived ordered-In2O3-ZnIn2S4 is more efficient in catalyzing visible-light-driven H2 evolution (8753 μmol h-1 g-1) outperforming the disordered-In2O3-ZnIn2S4 counterpart (2700 μmol h-1 g-1) as well as many other state-of-the-art ZnIn2S4-based photocatalysts. The ordered architecture significantly boosts the short-range electron transfer in an In2O3-ZnIn2S4 heterojunction but has a negligible impact on the long-range electron transfer among In2O3 mesocrystals. The density functional theory (DFT) calculation reveals that the oriented etching is achieved by the selective binding of the [Co(CN)6]3- etchant on the (110) plane of MIL-68(In), which can drag the In atoms out of the framework in order. Our findings could broaden the technical sense toward advanced photocatalyst design and impose scientific impacts on unveiling how ordered photosystems operate.

Effects of biofilm transfer and electron mediators transfer on Klebsiella quasipneumoniae sp. 203 electricity generation performance in MFCs

BackgroundExtracellular electron transfer (EET) is essential in improving the power generation performance of electrochemically active bacteria (EAB) in microbial fuel cells (MFCs). Currently, the EET mechanisms of dissimilatory metal-reducing (DMR) model bacteria Shewanella oneidensis and Geobacter sulfurreducens have been thoroughly studied. Klebsiella has also been proved to be an EAB capable of EET, but the EET mechanism has not been perfected. This study investigated the effects of biofilm transfer and electron mediators transfer on Klebsiella quasipneumoniae sp. 203 electricity generation performance in MFCs.ResultsHerein, we covered the anode of MFC with a layer of microfiltration membrane to block the effect of the biofilm mechanism, and then explore the EET of the electron mediator mechanism of K. quasipneumoniae sp. 203 and electricity generation performance. In the absence of short-range electron transfer, we found that K. quasipneumoniae sp. 203 can still produce a certain power generation performance, and coated-MFC reached 40.26 mW/m2 at a current density of 770.9 mA/m2, whereas the uncoated-MFC reached 90.69 mW/m2 at a current density of 1224.49 mA/m2. The difference in the electricity generation performance between coated-MFC and uncoated-MFC was probably due to the microfiltration membrane covered in anode, which inhibited the growth of EAB on the anode. Therefore, we speculated that K. quasipneumoniae sp. 203 can also perform EET through the biofilm mechanism. The protein content, the integrity of biofilm and the biofilm activity all proved that the difference in the electricity generation performance between coated-MFC and uncoated-MFC was due to the extremely little biomass of the anode biofilm. To further verify the effect of electron mediators on electricity generation performance of MFCs, 10 µM 2,6-DTBBQ, 2,6-DTBHQ and DHNA were added to coated-MFC and uncoated-MFC. Combining the time–voltage curve and CV curve, we found that 2,6-DTBBQ and 2,6-DTBHQ had high electrocatalytic activity toward the redox reaction of K. quasipneumoniae sp. 203-inoculated MFCs. It was also speculated that K. quasipneumoniae sp. 203 produced 2,6-DTBHQ and 2,6-DTBBQ.ConclusionsTo the best of our knowledge, the three modes of EET did not exist separately. K. quasipneumoniae sp.203 will adopt the corresponding electron transfer mode or multiple ways to realize EET according to the living environment to improve electricity generation performance.

Understanding Short-Range Electron-Transfer Dynamics in Proteins.

Short-range electron-transfer (ET) reactions in biological systems are usually ultrafast, having transfer rates comparable to or even faster than corresponding environmental fluctuations, and often display nonexponential behaviors. To understand these nonequilibrium ET dynamics, we carried out detailed theoretical analyses based on the Sumi-Marcus model. It is shown that the ET dynamics is largely determined by the relative time scales of the ET reaction and its surrounding motions. Significantly, different environmental fluctuations can produce a variety of apparent ET dynamics even with the same driving force, Δ Go, and reorganization energy, λ. We applied our analyses to an ultrafast ET process in DNA repair by (6-4) photolyase and directly obtained the inner and outer reorganization energies (λi and λo) as well as the free energy Δ Go of various mutants, providing mechanical insight into ultrafast short-range ET reactions in proteins.

Short-Range Electron Transfer in Reduced Flavodoxin: Ultrafast Nonequilibrium Dynamics Coupled with Protein Fluctuations.

Short-range electron transfer (ET) in proteins is an ultrafast process on the similar time scales as local protein-solvent fluctuation, and thus the two dynamics are coupled. Here we use semiquinone flavodoxin and systematically characterized the photoinduced redox cycle with 11 mutations of different aromatic electron donors (tryptophan and tyrosine) and local residues to change redox properties. We observed the forward and backward ET dynamics in a few picoseconds, strongly following a stretched behavior resulting from a coupling between local environment relaxations and these ET processes. We further observed the hot vibrational-state formation through charge recombination and the subsequent cooling dynamics also in a few picoseconds. Combined with the ET studies in oxidized flavodoxin, these results coherently reveal the evolution of the ET dynamics from single to stretched exponential behaviors and thus elucidate critical time scales for the coupling. The observed hot vibration-state formation is robust and should be considered in all photoinduced back ET processes in flavoproteins.

A review on sediment microbial fuel cells as a new source of sustainable energy and heavy metal remediation: mechanisms and future prospective

Sediment microbial fuel cells (SMFCs) are different from microbial fuel cells because they are completely anoxic and lack a membrane. SMFCs are a novel technology for the simultaneous production of renewable energy and bioremediation of heavy metals. Recently, SMFCs have attracted the attention of many researchers because of their moderate functioning parameters and ability to use a range of biodegradable substrates like glucose, glutamic acid, river water, cysteine, acetate, and starch. The inocula used in SMFCs include river sediment, marine sediment, and wastewater. For power generation, many exoelectrogens in SMFCs have the ability to transfer electrons from electrodes by using natural electron shuttles. Exoelectrogens use four primary pathways to transfer electrons to the electrodes, including short-range electron transfer through redox-active proteins, soluble electron shuttling molecules, long-range electron transport by conductive pili, and direct interspecies electron transfer. The most dominant mechanism is long-range electron transfer via conductive pili because pili have metal-like conductivity. The powering by microbes is an emerging technique for the remediation of heavy metals from sediments. The pathways for transferring electrons in electrotrophs operate in the opposite direction from those in exoelectrogens. To further upgrade SMFC technology, this review targets the prototype, operating factors, working mechanisms, applications, and future perspectives of SMFCs. Copyright © 2017 John Wiley & Sons, Ltd.

Solvatomorphs of 25,26,27,28-tetrahydroxycalix[4]arene and 5,11,17,23-tetramino-25,26,27,28-tetrabutoxycalix[4]arene: quenching photoluminescence through switching the guest

Here, we have disclosed two solid state forms of 5,11,17,23-tetramino-25,26,27,28-tetrabutoxycalix[4]arene (1) and two solid state forms of the non-functionalized 25,26,27,28-tetrahydroxycalix[4]arene (2). This is the first structural knowledge of a tetra-amino functionalized calixarene derivative, even though this compound is well known and used as a precursor of several other functionalized calix[4]arenes. The two solid forms of 1 differ by the presence of either water or water/dimethylsulfoxide (DMSO) solvent molecules entrapped in the major calixarene cavity, even though the pinched conformation is adopted in both forms as a consequence of the contacts between solvent molecules and phenyl rings. Likewise, the switch from water to DMSO in the cone cavity of 1 has abrogated the photoluminescence (PL) found only in the dihydrate form. Frontier molecular orbital calculations at the B3LYP/6-31G* level of theory support a short-range electron transfer between guest (DMSO) and host (1) molecules quenching the solid state photoluminescence when DMSO is entrapped in the cone. This solvatomorphism approach for PL search in calixarenes opens a perspective on tuning and even increasing the performance of calixarenes through changing the guest solvent molecule. Similarly, the two crystal forms of 2 entrap either methyl alcohol or DMSO in their cones, which, as well as both crystal forms of 1, are packed into sheets through different fashions and contact patterns.

Gold nanoparticle-mediated electron transfer of cytochrome c on a self-assembled surface

Gold nanoparticles provide short-range electron transfer between cytochrome c and gold electrode.

Electron transfer with self-assembled copper ions at Au-deposited biomimetic films: mechanistic ‘anomalies’ disclosed by temperature- and pressure-assisted fast-scan voltammetry

It has been suggested that electron transfer (ET) processes occurring in complex environments capable of glass transitions, specifically in biomolecules, under certain conditions may experience the medium’s nonlinear response and nonergodic kinetic patterns. The interiors of self-assembled organic films (SAMs) deposited on solid conducting platforms (electrodes) are known to undergo glassy dynamics as well, hence they may also exhibit the abovementioned ‘irregularities’. We took advantage of Cu2+ ions as redox-active probes trapped in the Au-deposited −COOH-terminated SAMs, either L-cysteine, or 3-mercaptopropionic acid diluted by the inert 2-mercaptoethanol, to systematically study the impact of glassy dynamics on ET using the fast-scan voltammetry technique and its temperature and high-pressure extensions. We found that respective kinetic data can be rationalized within the extended Marcus theory, taking into account the frictionally controlled (adiabatic) mechanism for short-range ET, and complications due to the medium’s nonlinear response and broken ergodicity. This combination shows up in essential deviations from the conventional energy gap (overpotential) dependence and in essentially nonlinear temperature (Arrhenius) and high-pressure patterns, respectively. Biomimetic aspects for these systems are also discussed in the context of recently published results for interfacial ET involving self-assembled blue copper protein (azurin) placed in contact with a glassy environment.

Evidence of short-range electron transfer of a redox enzyme on graphene oxide electrodes

Direct electron transfer (DET) between redox enzymes and electrode surfaces is of growing interest and an important strategy in the development of biofuel cells and biosensors. Among the nanomaterials utilized at electrode/enzyme interfaces to enhance the electronic communication, graphene oxide (GO) has been identified as a highly promising candidate. It is postulated that GO layers decrease the distance between the flavin cofactor (FAD/FADH2) of the glucose oxidase enzyme (GOx) and the electrode surface, though experimental evidence concerning the distance dependence of the rate constant for heterogeneous electron-transfer (k(het)) has not yet been observed. In this work, we report the experimentally observed DET of the GOx enzyme adsorbed on flexible carbon fiber (FCF) electrodes modified with GO (FCF-GO), where the k(het) between GO and electroactive GOx has been measured at a structurally well-defined interface. The curves obtained from the Marcus theory were used to obtain k(het), by using the model proposed by Chidsey. In agreement with experimental data, this model proved to be useful to systematically probe the dependence of electron transfer rates on distance, in order to provide an empirical basis to understand the origin of interfacial DET between GO and GOx. We also demonstrate that the presence of GO at the enzyme/electrode interface diminishes the activation energy by decreasing the distance between the electrode surface and FAD/FADH2.

Formation of ionic complexes in cryogenic matrices: A case study using co-deposition of Cu− with rare gas cations in solid argon

Matrix isolation spectra have been obtained for ionic species formed from a beam of mass-selected ions, with a coincident beam of externally generated counter-ions used to provide charge balance. Infrared spectra were obtained for copper carbonyl complexes formed following deposition of Cu(-) ions with rare-gas counter-cations into CO-doped argon matrices. Both anionic and neutral copper carbonyl complexes Cu(CO)(n)(q) (n = 1-3; q = 0, -1) were observed in the spectra, with peak positions corresponding to previously reported assignments; new partially resolved bands appearing in the range 1830-1845 cm(-1) are assigned to larger [Cu(CO)3●(CO)n](-) aggregates, having additional CO ligands in the second solvation shell. The experimental geometry ensures that all Cu-centers initially arrive at the matrix as anions, so the relative abundance of anionic relative to neutral complexes is much higher than in previous studies employing alternative methods for ion deposition; this allows for monitoring of electron-transfer processes between anions and cations in the matrix. Comparison of time-dependent vs. temperature-dependent trends reveals that there are two distinct mechanisms by which the population of anionic complexes is converted into neutral complexes: short-range electron transfer between a cation-anion pair following diffusion, and long-range electron transfer involving photodetachment of an electron from the anion into the conduction band of solid argon, resulting in eventual recombination of the electron with a cation in a remote matrix site. The spectra also show a marked dependence on the deposition temperature and dopant concentration, in that 100-fold higher CO concentrations were required during deposition with the sample window at 10 K compared to that used at 20 K, in order to obtain a similar distribution of copper carbonyl complexes. Furthermore, although no carbonyl complexes are observed initially when low concentrations of CO are used at 10 K, upon warming the matrix to 15 K, the neutral di- and tricarbonyl peaks appear abruptly, which is attributed to fast diffusion of CO stimulated by the energy released upon short-range electron-transfer between Cu(-):counter-cation pairs.

Understanding the Mechanism of Short-Range Electron Transfer Using an Immobilized Cupredoxin

The hydrophobic patch of azurin (AZ) from Pseudomonas aeruginosa is an important recognition surface for electron transfer (ET) reactions. The influence of changing the size of this region, by mutating the C-terminal copper-binding loop, on the ET reactivity of AZ adsorbed on gold electrodes modified with alkanethiol self-assembled monolayers (SAMs) has been studied. The distance-dependence of ET kinetics measured by cyclic voltammetry using SAMs of variable chain length, demonstrates that the activation barrier for short-range ET is dominated by the dynamics of molecular rearrangements accompanying ET at the AZ-SAM interface. These include internal electric field-dependent low-amplitude protein motions and the reorganization of interfacial water molecules, but not protein reorientation. Interfacial molecular dynamics also control the kinetics of short-range ET for electrostatically and covalently immobilized cytochrome c. This mechanism therefore may be utilized for short-distance ET irrespective of the type of metal center, the surface electrostatic potential, and the nature of the protein-SAM interaction.

Photoinduced short-range electron transfer in DNA with fluorescent DNA bases: lessons from ethidium and thiazole orange as charge donors

Charge transfer processes through the double helix of DNA cover a broad range of mechanistic models ranging from superexchange to hopping mechanisms. Over the last decade, these processes were studied by our group in a photoinduced fashion since (i) the starting time for the charge transfer is clearly defined by the absorption of the photon and (ii) photoexcitation delivers the necessary driving force to the DNA system. It is a prerequisite to modify oligonucleotides synthetically with suitable organic fluorophores that serve as photoinducable charge donors. In the first part of this perspective article we summarize our recent advances in the area of DNA-mediated reductive electron transfer processes over short ranges using synthetic DNA-donor-acceptor systems. The second part of this article focuses on ethidium as the photoinducable charge donor. Ethidium-modified DNA can be used to compare oxidative hole transfer with reductive electron transfer since the type of charge transfer can be controlled by choosing the right charge acceptor. Recent results showed that an efficient charge transfer through DNA using covalently bound ethidium is strongly influenced mainly by DNA dynamics but also by several other parameters that affect the electronic coupling between charge donor and acceptor.

Unusual mechanism for the short-range electron transfer within gold–alkanethiol–ionic-liquid films of subnanometer thickness

Exploiting nanoscopically tunable composite gold-alkanethiol-ionic-liquid/ferrocene self-assembled systems with tunable electron transfer distance, we discovered in the case of thinner alkanethiol films a thermally activated electron transfer pattern totally controlled by the viscosity-related slow relaxation mode(s) of the ionic liquid acting as the reactant's fluctuating environment. This pattern manifested through the activation enthalpy and volume parameters that are identical to those for viscous flow was explained in terms of the extreme adiabatic mechanism with a vanishing Marcus barrier (via the exponential Franck-Condon-like term approaching unity).

Single‐Molecule Fluorescence Studies Reveal Long‐Range Electron‐Transfer Dynamics Through Double‐Stranded DNA

Complex dynamics of combined long-range fluorescence resonance energy transfer (FRET) and short-range electron transfer (ET) processes in a single double-stranded DNA (dsDNA) molecule (see figure) reveals that FRET remains almost unaltered in the presence of ET. Present systems also demonstrate the potential for long-range ET studies through the base stack in single dsDNA molecules.

Thiazole Orange and Cy3: Improvement of Fluorescent DNA Probes with Use of Short Range Electron Transfer

Thiazole orange was synthetically incorporated into oligonucleotides by using the corresponding phosphoramidite as the building block for automated DNA synthesis. Due to the covalent fixation of the TO dye as a DNA base surrogate, the TO-modified oligonucleotides do not exhibit a significant increase of fluorescence upon hybridization with the counterstrand. However, if 5-nitroindole (NI) is present as a second artificial DNA base (two base pairs away from the TO dye) a fluorescence increase upon DNA hybridization can be observed. That suggests that a short-range photoinduced electron transfer causes the fluorescence quenching in the single strand. The latter result represents a concept that can be transferred to the commercially available Cy3 label. It enables the Cy3 fluorophore to display the DNA hybridization by a fluorescence increase that is normally not observed with this dye.

Optimized Spin Crossings and Transition States for Short-range Electron Transfer in Transition Metal Dimers

Electron-transfer reactions in eight mixed-valence manganese dimers are studied using B3LYP. One of the dimers is a model of the active site of manganese catalase, while another represents a basic building block of the oxygen-evolving complex in photosystem II. The adiabatic reactions are characterized by fully optimized transition states where the single imaginary frequency represents the electron-transfer coordinate. When there is antiferromagnetic coupling between different high-spin centers, electron transfer must be accompanied by a spin transition. Spin transitions are characterized by minimum-energy crossing points between spin surfaces. Three reaction mechanisms have been investigated. First, a single-step reaction where spin flip is concerted with electron transfer. Second, an initial transition to a center with intermediate spin that can be followed by electron transfer. Third, an initial transition to a ferromagnetic state from which the electron can be transferred adiabatically. The complexes prefer the third route with rate-determining barriers ranging from 5.7 kcal/mol to 17.2 kcal/mol for different complexes. The origins of these differences are discussed in terms of oxidation states and ligand environments. Many DFT functionals overestimate charge-transfer interactions, but for the present complexes, the error should be limited because of short Mn-Mn distances.