Distance Dependence Of Electron Transfer Research Articles

Electron-transfer kinetics through nucleic acids untangled by single-molecular fluorescence blinking

•Kinetics of distance-dependent electron transfer (ET) in single molecules •ET rate shows large heterogeneity •Blinking allows site-specific single-molecule measurement on cell blocks •Measurement of single mRNAs can identify point mutations on cell blocks Ensemble-averaged measurements have revealed the physicochemical basis of electron transfer (ET) through biomolecules, which is foundational to the development of bioelectronic devices for analysis and diagnosis. However, such measurements can obscure single-molecule kinetics that are pertinent to the underlying science and practical applications. We used DNA to study the distance dependence of the intramolecular ET kinetics by measuring fluorescence blinking. The systematic measurement of the distance dependence of the ET rate up to ∼27 Å clearly demonstrated the single-molecule measurement of ET kinetics and highlighted the large heterogeneity in the ET rate. Each nucleic acid has a specific ET rate, and thus one can obtain sequence information by measuring the ET rate through nucleic acids. We applied single-molecule ET measurement to detect isocitrate dehydrogenase (IDH) mRNA point mutations, which are correlated with treatment efficacy in adult gliomas. We identified the mRNA point mutation in pathological specimens prepared from human glioma model cultured cells. Ensemble-averaged measurements have revealed the physicochemical basis of electron transfer (ET) through biomolecules, which is foundational to the development of bioelectronic devices for analysis and diagnosis. However, such measurements can obscure single-molecule kinetics that are pertinent to the underlying science and practical applications. We used DNA to study the distance dependence of the intramolecular ET kinetics by measuring fluorescence blinking. The systematic measurement of the distance dependence of the ET rate up to ∼27 Å clearly demonstrated the single-molecule measurement of ET kinetics and highlighted the large heterogeneity in the ET rate. Each nucleic acid has a specific ET rate, and thus one can obtain sequence information by measuring the ET rate through nucleic acids. We applied single-molecule ET measurement to detect isocitrate dehydrogenase (IDH) mRNA point mutations, which are correlated with treatment efficacy in adult gliomas. We identified the mRNA point mutation in pathological specimens prepared from human glioma model cultured cells.

Distance Dependence of Electron Transfer through Conformationally Constrained Peptides

Understanding and the ability of peptides to mediate long-range electron tunneling in proteins is an evergreen topic. In this context, the study of electron transfer (ET) through model peptides has been particularly useful, especially to assess the occurrence (or competition) of the superexchange and sequential hopping mechanisms as a function of the structure and length of the peptide bridge. In previous studies [1,2], we stressed the importance of dealing with peptides with a well-defined and rigid structure to properly correlate kinetic and structural data. This structural control was made possible by using model peptide bridges based on oligomers of the α-aminoisobutyric acid (Aib) residue. The Aib unit is characterized by a marked steric hindrance at the α-carbon, and this allows the preparation of very rigid peptides. Rigidity is ensured by a framework of intramolecular C=O···H-N hydrogen bonds. Consequently, these peptides possess a strong dipole moment along the main axis [3]. In previous studies, we used cyclic voltammetry to study the dissociative ETs in two series of Donor − Aib-peptide − Acceptor systems in which we changed the donor group, whereas the acceptor was kept constant (a dialkyl peroxide moiety, resulting in the cleavage of the O-O peroxide bond [4]). In this communication, we describe the outcome of dissociative ETs carried out using a new series of Donor − Aib-peptide − Acceptor system in which the acceptor was a perbenzoate moiety [5]. This change also affects the dissociative ET driving force. The same peptide-length dependent dissociative ETs were tested for a possible solvent effect.

Insights into the Distance Dependence of Electron Transfer through Conformationally Constrained Peptides

AbstractUnderstanding the distance dependence of electron transfer (ET) through peptides is a topic of continuous interest. We studied a series of donor‐bridge‐acceptor (D‐B‐A) systems. The peptide bridges (B) were oligomers of the strongly helicogenic α‐aminoisobutyric acid, and were prepared in different lengths to form 0, 1, 2, 3, 4, or 5 intramolecular H‐bonds. D was a phthalimide group and A was a perester. The study was carried out in DMF, acetonitrile, and dichloromethane. Cyclic voltammetry (CV) and convolution analysis of the model donors and acceptor yielded the relevant thermodynamic information, whereas homogenous redox catalysis provided information on the competitive intermolecular ET. CV analysis of the D‐B‐A systems showed that, independently of the solvent used, the intramolecular ET occurs by a superexchange mechanism strongly affected by intramolecular H‐bonds and the extent of conjugation of the peptide bridge with D and A.

Electron Transfer around a Molecular Corner.

The distance dependence of electron transfer (ET) is commonly investigated in linear rigid rod-like compounds, but studies of molecular wires with integrated corners imposing 90° angles are very rare. By using spirobifluorene as a key bridging element and by substituting it at different positions, two isomeric series of donor-bridge-acceptor compounds with either nearly linear or angled geometries were obtained. Photoinduced ET in both series is dominated by rapid through-bond hole hopping across oligofluorene bridges over distances of up to 70 Å. Despite considerable conformational flexibility, direct through-space and through-solvent ET is negligible even in the angled series. The independence of the ET rate constant on the total number of fluorene units in the angled series is attributed to a rate-limiting tunneling step through the spirobifluorene corner. This finding is relevant for multidimensional ET systems and grids in which individual molecular wires are interlinked at 90° angles.

Increased Charge Separation Rates with Increasing Donor–Acceptor Distance in Molecular Triads: The Effect of Solvent Polarity

Distance-dependent electron transfer in donor–spacer–acceptor systems is accepted to occur via two distinct mechanisms, that is, by coherent superexchange or incoherent hopping. In general, the rate of electron transfer (kET) decreases with increasing donor–acceptor distances, irrespective of the actual mechanism being responsible for the process. However, recently Wenger and his group showed that in the frame of the superexchange mechanism electron-transfer rates can pass a maximum when increasing the transfer distance. This manuscript presents an investigation of the forward electron transfer in a series of donor (N-methylphenothiazine)–photocenter (Ru(II) bis(terpyridine) complex)–acceptor (N-methylfulleropyrrolidine) triads that reveals the control of the electron-transfer rates by solvent variation to an extent that in acetonitrile an increasing electron-transfer rate is observed with increasing donor–acceptor distance, while in dichloromethane an increase in the separation causes the electron transf...

Power Conversion Efficiency of Arylamine Organic Dyes for Dye-Sensitized Solar Cells (DSSCs) Explicit to Cobalt Electrolyte: Understanding the Structural Attributes Using a Direct QSPR Approach

Post silicon solar cell era involves light-absorbing dyes for dye-sensitized solar systems (DSSCs). Therefore, there is great interest in the design of competent organic dyes for DSSCs with high power conversion efficiency (PCE) to bypass some of the disadvantages of silicon-based solar cell technologies, such as high cost, heavy weight, limited silicon resources, and production methods that lead to high environmental pollution. The DSSC has the unique feature of a distance-dependent electron transfer step. This depends on the relative position of the sensitized organic dye in the metal oxide composite system. In the present work, we developed quantitative structure-property relationship (QSPR) models to set up the quantitative relationship between the overall PCE and quantum chemical molecular descriptors. They were calculated from density functional theory (DFT) and time-dependent DFT (TD-DFT) methods as well as from DRAGON software. This allows for understanding the basic electron transfer mechanism along with the structural attributes of arylamine-organic dye sensitizers for the DSSCs explicit to cobalt electrolyte. The identified properties and structural fragments are particularly valuable for guiding time-saving synthetic efforts for development of efficient arylamine organic dyes with improved power conversion efficiency.

Distance-Dependent Electron Transfer in Chemically Engineered Carbon Dots

The highly complex nanostructured framework of carbon dots (CDs) elevates scientific debates regarding their molecular and electronic structures. Here, we are able to highlight the free electron distribution on surface of newly synthesized Ru(III) and amine (ethylenediamine) doped carbon nano dots, Ru:CNDEDAs by identifying CD+• radical ions produced through photoinduced electron transfer to a model quinone drug, menadione (MQ) using laser flash photolysis corroborated with magnetic field of the order of 0.08 T. Amine passivation renders Ru:CNDEDAs surface to be a potential electron donor, which exhibits distance-dependent rapid electron transfer with MQ by chemical engineering with homocystein thiolactone (HCTL) in physiological condition. This modification is basically executed to mimic the post-translational modification (homocysteinylation) of protein–lysine residues by HCTL. Moreover, the high values of hyperfine splitting constants of CDs calculated from the magnetic field effect indicate the presen...

Molecular tilt-dependent and tyrosine-enhanced electron transfer across ITO/SAM/[DPPC–Au NP–Tyrosine] Janus nanoparticle junction

Enhanced interfacial electron transfer (ET) across the otherwise insulating indium tin oxide/alkanethiol self-assembled monolayer (SAM)/redox molecule junction was accomplished when a Janus gold nanoparticle (JNP) protected by bioinspired phosphatidylcholine (DPPC) lipid and tyrosine amino acid ligands was anchored on it. In addition to the most theoretical and experimental investigations on the distance-dependent ET across Metal–Organic SAM–Nanoparticle (NP) architectures, the current results succinctly illustrate molecular tilt angle of the SAM and the characteristic of JNP as key factors in expediting the ET rate via electron tunneling. In the absence of JNP, electron tunneling with a tunneling factor β = 1.1 A−1 across the SAM was the rate-limiting step, evidenced from electrochemical impedance spectroscopy (EIS). The apparent electron transfer rate constant (k app 0 ) for anchored SAM was enhanced by at least one order of magnitude than the DPPC-only protected nanoparticle, suggesting the potential role of tyrosine towards the enhanced ET. The asymmetric and biogenic nature of the construct sheds light on a potential bioelectronic device for novel electronic attributes. Entry of TOC .

Distance-Dependent Electron Transfer At Passivated Electrodes Decorated By Gold Nanoparticles

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Distance Dependent Electron Transfer at TiO2 Interfaces Sensitized with Phenylene Ethynylene Bridged RuII–Isothiocyanate Compounds

Excess electrons present in semiconductor nanocrystallites generate a significant electric field, yet the role this field plays in molecular charge transfer processes remains poorly understood. Three ruthenium bipyridyl cis-Ru(bpy)(LL)(NCS)2 compounds, where LL is a 4-substituted bpy, with zero, one, or two phenylene ethynylene bridge units, were anchored to mesoporous nanocrystalline TiO2 thin films to specifically quantify interfacial charge transfer with chromophores designed to be set at variable distances from the surface. Injection of electrons into TiO2 resulted in a blue shift of the metal-to-ligand charge transfer absorption consistent with an underlying Stark effect. The electroabsorption data were used to quantify the electric field experienced by the compounds that decreased from 0.85 to 0.22 MV/cm as the number of OPE spacers increased from 0 to 2. Charge recombination on the 10(-8)-10(-5) s time scale correlated with the magnitude of the electric field with an apparent attenuation factor β = 0.12 Å(-1). Slow components to charge recombination observed on the 10(-4)-10(-1) s time scale that were unaffected by temperature, irradiance, or the bridge units present on the molecular sensitizer were attributed to electron tunneling between TiO2 acceptor states. The photocurrent efficiencies of solar cells based on these compounds decreased markedly when the bridge units were present on the sensitizer. Iodine was found to form adducts with all three compounds, K = 1.8 ± 0.2 × 10(4) M(-1), but only significantly lowered the excited state injection yield for those that possessed the bridge units.

A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photogeneration and Interconversion through Electron Transfer

Bond cleavage and bond formation are central to organic chemistry. Carbocations play a key role in our understanding of nucleophilic substitution reactions that involve both processes. The precise understanding of the mechanism and dynamics of the photogeneration of carbocations and carbon radicals is therefore an important quest. In particular, the role of electron transfer for the generation of carbocations from the radical pair is still unclear. A quantitative femtosecond absorption study is presented, with ultrabroad probing on selected donor and acceptor substituted benzhydryl chlorides irradiated with 270 nm (35 fs) pulses. The ultrafast bond cleavage within 300 fs is almost exclusively homolytic, thus leading to a radical pair. The carbocations observable in the nanosecond regime are generated from these radicals by electron transfer from the benzhydryl to the chlorine radical within the first tens of picoseconds. Their concentration is reduced by geminate recombination within hundreds of picoseconds. In moderately polar solvents this depletion almost extinguishes the cation population; in highly polar solvents free ions are still observable on the nanosecond timescale. The explanation of the experimental findings requires the microscopic realm of the intermediates to be accounted for, including their spatial and environmental distributions. The distance dependent electron transfer described by Marcus theory is combined with Smoluchowski diffusion. The depletion of the radical pair distribution at small distances causes a temporal increase of the mean distance and the observed stretched exponential electron transfer. A close accord with experiment can only be reached for a broad distribution of the nascent radical pairs. The increase in the inter-radical and inter-ion pair distance is measured directly as a shift of the UV/Vis absorption of the products. The results demonstrate that, at least for aprotic solvents, traditional descriptions of reaction mechanisms based on the concept of contact and solvent-separated pairs have to be reassessed.

Distance-Dependent Electron Transfer at Passivated Electrodes Decorated by Gold Nanoparticles

The phenomenon of nanoparticles attached to an electrode passivated by an organic layer allowing efficient electron transfer between redox species in solution and the underlying electrode to be restored has resulted in Chazalviel and Allongue proposing a theory [Chazalviel, J.-N.; Allongue, P. J. Am. Chem. Soc.2011, 133, 762-764] to explain this phenomenon. The theory suggests that with electrode-organic layer-nanoparticle constructs, high exchange current densities, compared with when the nanoparticles are absent, results in the rate of electron transfer being independent of the thickness of the organic layer until a threshold thickness is exceeded. Thereafter, the thicker the organic layer, the slower the rate of electron transfer. Herein we provide the first experimental data to support this theory using a single experimental system that can show the transition from thickness independent electron transfer kinetics to distant dependent kinetics. This was achieved using ethylenediamine electrodeposited on a glassy carbon electrode. Different numbers of deposition cycles were applied in order to fabricate different thicknesses of the organic film. The deposited films showed progressively greater blocking abilities toward ruthenium hexamine, as a redox active probe in solution, as the films got thicker. Electron transfer kinetics of nanoparticle-decorated surfaces showed a change from thickness independent to thickness dependent as the organic layer exceeded an average thickness of 20 Å. Electrochemical impedance spectroscopy, cyclic voltammetry, scanning electron microscopy, ellipsometry, and atomic force microscopy were used to characterize the fabricated surfaces.

Influence of the diffuse double layer on steady-state voltammetry

The influence of the diffuse double layer on the passage of Faradaic current is investigated for steady-state voltammetry. Both mathematical analysis and numerical solution with the Nernst–Planck–Poisson equations are employed. We report a comprehensive study of the effects of reactant charge, electrode charge, electrode size and quantity of supporting electrolyte. Both infinite and finite electrode kinetics are investigated, as well as distance-dependent electron transfer (tunnelling) and activity effects. Certain combinations of reactant and electrode charge are shown to profoundly alter the predicted current by exclusion of the reactant (Levich effect) or deceleration of apparent kinetics (Frumkin effect), although tunnelling can overcome both effects by moving the plane of electron transfer. The structurally altered double layer at nanoelectrodes is shown to either increase or decrease the predicted current depending on the electrode charge, due to an unscreened electric field.

Structure and Photoinduced Electron Transfer Dynamics of a Series of Hydrogen-Bonded Supramolecular Complexes Composed of Electron Donors and a Saddle-Distorted Diprotonated Porphyrin

The excited-state photodynamics of intrasupramolecular photoinduced electron transfer was investigated in a series of hydrogen-bonded supramolecular complexes composed of diprotonated 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (H(4)DPP(2+)) and electron donors bearing a carboxylate group. The formation of supramolecular complexes was examined by spectroscopic measurements. The binding constants obtained by spectroscopic titration indicate the strong binding (10(8)-10(10) M(-2)) even in a polar and coordinating solvent, benzonitrile (PhCN). The crystal structure of the supramolecular assembly using ferrocenecarboxylate (FcCOO(-)) was determined to reveal a new structural motif involving two-point and single-point hydrogen bonding among saddle-distorted H(4)DPP(2+) dication and two FcCOO(-) anions. Femtosecond laser flash photolysis was applied to investigate the photodynamics in the hydrogen-bonded supramolecular complexes. Rate constants obtained were evaluated in light of the Marcus theory of electron transfer, allowing us to determine the reorganization energy and the electronic coupling matrix constant of photoinduced electron transfer and back electron transfer to be 0.68 eV and 43 cm(-1), respectively. The distance dependence of electron transfer was also examined by using a series of ferrocenecarboxylate derivatives connected by linear phenylene linkers, and the distance dependence of the rate constant of electron transfer (k(ET)) was determined to be k(ET) = k(0) exp(-beta r), in which beta = 0.64 A(-1).

Distance and orientation dependence of photoinduced electron transfer through twisted, bent and helical bridges: a Karplus relation for charge transfer interaction

A simple theoretical model is presented that describes orientation effects in electron transfer. In this model the twist and dihedral angles between electron donor (D) and acceptor (A) play a role in the distance dependence of charge transfer (CT), next to the conventional distance dependence of electron transfer that is not influenced by the angles between D and A. This can be applied to, for example, helical bridges, when twist and dihedral angles can modulate the rate. For this aim, parts are taken from the work of J. W. Verhoeven in the period 1970-1975 on dihedral angular dependence of CT interaction. Furthermore, recent work of Harriman on the effects of the twist angle on CT is used. The combination leads to a new equation that is dubbed: Karplus Relation for Charge Transfer Interaction. It can be used to describe orientation effects and non-linear plots of ln(k(cs)) vs. distance. This new equation is applied to two test-cases.

A simple way to calculate the intermolecular electron transfer integral - the isolated orbital method

Under the two-state model approximation,we estimated the intermolecular electron transfer integrals of the face-to-face benzene dimers with different methods.Among these methods,the isolated orbital method based on the self-consistent field (SCF) single electron Hamiltonian (Fock matrix) can readily do the Lwdin's symmetric transformation to get reasonable coupling values.The basis set and distance dependent electron transfer integrals were systemically investigated by isolated orbital method.Compared with the accurate values from the searching minimum splitting between the two state based along the reaction coordinates,the isolated orbital method with D95V basis set show very good performance in distance dependence.

The voltammetric responses of nanometer-sized electrodes in weakly supported electrolyte: A theoretical study

The effect of the supporting electrolyte concentration on the interfacial profiles and voltammetric responses of nanometer-sized disk electrodes have been investigated theoretically by combining the Poisson–Nernst–Planck (PNP) theory and Butler–Volmer (BV) equation. The PNP-theory is used to treat the nonlinear couplings of electric field, concentration field and dielectric field at electrochemical interface without the electroneutrality assumption that has been long adopted in various voltammetric theories for macro/microelectrodes. The BV equation is modified by using the Frumkin correction to account for the effect of the diffuse double layer potential on interfacial electron-transfer (ET) rate and by including a distance-dependent ET probability in the expression of rate constant to describe the radial heterogeneity of the ET rate constant at nanometer-sized disk electrodes. The computed voltammetric responses for disk electrodes larger than 200 nm in radii in the absence of the excess of the supporting electrolyte using the present theoretical scheme show reasonable agreements with the predications of the conventional microelectrode voltammetric theory which uses the combined Nernst–Planck equation and electroneutrality equation to describe the mixed electromigration-diffusion mass transport without including the possible effects of the diffuse double layer (Amatore et al. [25]). For electrodes smaller than 200 nm, however, the voltammetric responses predicated by the present theory exhibit significant deviation from the microelectrode theory. It is shown that the deviations are mainly resulted from the overlap between the diffuse double layer and the concentration depletion layer (CDL) at nanoscale electrochemical interfaces in weakly supported media, which will result in the invalidation of the electroneutrality condition in CDL, and from the radial inhomogeneity of ET probability at nanometer-sized disk electrodes.

Distance-dependent electron transfer rate of immobilized redox proteins: A statistical physics approach

The electron transfer kinetics of redox proteins adsorbed on metal electrodes coated with self-assembled monolayers (SAMs) of mercaptanes shows an unusual distance-dependence. For thick SAMs, the experimentally measured electron transfer rate constant k{exp} obeys the behavior predicted by Marcus theory [R. A. Marcus and N. Sutin, Biochim. Biophys. Acta 811, 265 (1985)], whereas for thin SAMs, k{exp} remains virtually constant [Z. Q. Feng, J. Chem. Soc., Faraday Trans. 93, 1367 (1997)]. In this work, we present a simple theoretical model system for the redox protein cytochrome c electrostatically bound to a SAM-coated electrode. A statistical average of the electron tunneling rate is calculated by accounting for all possible orientations of the model protein. This approach, which takes into account the electric field dependent orientational distribution, allows for a satisfactory description of the "saturation" regime in the high electric field limit. It further predicts a nonexponential behavior of the average electron transfer processes that may be experimentally checked by extending kinetic experiments to shorter sampling times, i.e., 1/k{exp}. For a comprehensive description of the overall kinetics in the saturation regime at sampling times of the order of <<1/k{exp}, it is essential to consider the dynamics of protein reorientation, which is not implemented in the present model.

Distance Dependence of Electron Transfer Kinetics for Azurin Protein Adsorbed to Monolayer Protected Nanoparticle Film Assemblies

The distance dependence and kinetics of the heterogeneous electron transfer (ET) reaction for the redox protein azurin adsorbed to an electrode modified with a gold nanoparticle film are investigated using cyclic voltammetry. The nanoparticle films are comprised of nonaqueous nanoparticles, known as monolayer-protected clusters (MPCs), which are covalently networked with dithiol linkers. The MPC film assembly serves as an alternative adsorption platform to the traditional alkanethiolate self-assembled monolayer (SAM) modified electrodes that are commonly employed to study the ET kinetics of immobilized redox proteins, a strategy known as protein monolayer electrochemistry. Voltammetric analysis of the ET kinetics for azurin adsorbed to SAMs of increasing chain length results in quasi-reversible voltammetry with significant peak splitting. We observed rate constants (k degrees (ET)) of 12-20 s(-1) for the protein at SAMs of shorter alkanethiolates that decays exponentially (beta = 0.9/CH(2) or 0.8/A) at SAMs of longer alkanethiolates (9-11 methylene units) or an estimated distance of 1.23 nm and is representative of classical electronic tunneling behavior over increasing distance. Azurin adsorbed to the MPC film platforms of increasing thickness results in reversible voltammetry with very little voltammetric peaks splitting and nearly negligible decay of the ET rate over significant distances up to 20 nm. The apparent lack of distance dependence for heterogeneous ET reactions at MPC film assemblies is attributed to a two-step mechanism involving extremely fast electronic hopping through the MPC film architecture. These results suggest that MPC platforms may be used in protein monolayer electrochemistry to create adsorption platforms of higher architecture that can accommodate greater than monolayer protein coverage and increase the Faradaic signal, a finding with significant implications for amperometric biosensor design and development.

Long-Range Electron Transfer across Cytochrome−Hematite (α-Fe2O3) Interfaces

Electrochemical scanning tunneling microscopy was used to assess the distance dependence of electron transfer facilitated by a bacterial multiheme cytochrome to a single crystal iron oxide surface. We measured tunneling current-distance (I-s) profiles across the nanoscale space between Au STM tips and the basal (001) surface of a hematite (alpha-Fe2O3) crystal and compared them to the case in which an intervening small tetraheme cytochrome (STC) from Shewanella oneidensis was covalently linked to the end of the Au tip. Tunneling profiles were collected at constant surface potentials in solutions having a range of ionic strengths. For the case without intervening cytochrome, at short tip-sample separation, the distance dependence of the tunneling current shows a quasi-linear behavior, whereas at longer distances, near-exponential decay is observed. The different regions can be understood first in terms of reduction of interfacial water and ion layers in the electrical double layer associated with the hematite surface, followed by electron tunneling through bulk water. The effective tunneling range and the transition between the two conduction mechanisms are substantially increased when STC is present in the tunneling junction, suggesting that cytochrome molecules provide enhanced tunneling pathways and stronger electronic coupling to the hematite surface. On the basis of these results, cytochrome-mediated electron transfer during bacterial metal reduction may be possible at distances farther than originally speculated. In addition, as multiheme cytochromes and other similar molecules gain attention for their promising role in fuel cells and molecular electronics, we demonstrate that the solution conditions and surface properties of the substrate must be carefully considered.