Kinetics Of Long-range Electron Transfer Research Articles

Double-Layer Inhibition of Peroxydisulfate Reduction at Mercury Ultramicroelectrodes. A Quantitative Analysis of the Frumkin Effect Including Molecular Transport and Long-Range Electron Transfer

The electrical double layer (EDL) can lead to unexpected voltammetric phenomena. A notable example is the Frumkin effect, observed for the reduction of peroxydisulfate (S2O82–). In the absence of excess supporting electrolyte, the rate of S2O82– reduction decreases as the cathodic overpotential is increased, contrary to expectations for faradaic electron-transfer processes. Here, we report a demonstration of the Frumkin effect for S2O82– reduction at Hg ultramicroelectrodes, revealing steady-state voltammetry behavior consistent with prior observations at rotating disk electrodes. A finite element model is used to simulate the effect of the EDL on S2O82– reduction in the presence and absence of excess supporting electrolyte (K2SO4). Semiquantitative agreement between experimental and simulated voltammograms is obtained, including capturing the decrease in current for S2O82– reduction with increasing overpotential. We show that S2O82– migration within the EDL, at electrode potentials negative of the potential of zero charge, is the primary process responsible for the observed decrease in current with increasing driving force. These combined experiments and finite element simulations represent the first quantitative description of the Frumkin effect with rigorous inclusion of the EDL, migrational and diffusional mass-transport, and long-range electron transfer kinetics.

Dipole-induced effects on charge transfer and charge transport. Why do molecular electrets matter?

Charge transfer (CT) and charge transport (CTr) are at the core of life-sustaining biological processes and of processes that govern the performance of electronic and energy-conversion devices. Electric fields are invaluable for guiding charge movement. Therefore, as electrostatic analogues of magnets, electrets have unexplored potential for generating local electric fields for accelerating desired CT processes and suppressing undesired ones. The notion about dipole-generated local fields affecting CT has evolved since the middle of the 20th century. In the 1990s, the first reports demonstrating the dipole effects on the kinetics of long-range electron transfer appeared. Concurrently, the development of molecular-level designs of electric junctions has led the exploration of dipole effects on CTr. Biomimetic molecular electrets such as polypeptide helices are often the dipole sources in CT systems. Conversely, surface-charge electrets and self-assembled monolayers of small polar conjugates are the preferred sources for modifying interfacial electric fields for controlling CTr. The multifaceted complexity of such effects on CT and CTr testifies for the challenges and the wealth of this field that still remains largely unexplored. This review outlines the basic concepts about dipole effects on CT and CTr, discusses their evolution, and provides accounts for their future developments and impacts.

Fluctuating hydrogen-bond networks govern anomalous electron transfer kinetics in a blue copper protein

We combine experimental and computational methods to address the anomalous kinetics of long-range electron transfer (ET) in mutants of Pseudomonas aeruginosa azurin. ET rates and driving forces for wild type (WT) and three N47X mutants (X = L, S, and D) of Ru(2,2'-bipyridine)2 (imidazole)(His83) azurin are reported. An enhanced ET rate for the N47L mutant suggests either an increase of the donor-acceptor (DA) electronic coupling or a decrease in the reorganization energy for the reaction. The underlying atomistic features are investigated using a recently developed nonadiabatic molecular dynamics method to simulate ET in each of the azurin mutants, revealing unexpected aspects of DA electronic coupling. In particular, WT azurin and all studied mutants exhibit more DA compression during ET (>2 Å) than previously recognized. Moreover, it is found that DA compression involves an extended network of hydrogen bonds, the fluctuations of which gate the ET reaction, such that DA compression is facilitated by transiently rupturing hydrogen bonds. It is found that the N47L mutant intrinsically disrupts this hydrogen-bond network, enabling particularly facile DA compression. This work, which reveals the surprisingly fluctional nature of ET in azurin, suggests that hydrogen-bond networks can modulate the efficiency of long-range biological ET.

Interpretation of fluorescence decay kinetics in 3-methylbenzimidazolyl(5′-5′)guanosine dinucleotides: exponential dependence on the number of phosphates in the polyphosphate bridge

The number of phosphate groups in the 5',5'-polyphosphate bridge of mRNA-cap dinucleotide analogues affects kinetics of long-range electron transfer (ET) responsible for 3-methylbenzimidazole (m(3)B) fluorescence quenching in model dinucleotides. For instance, 3-methylbenzimidazolyl(5'-5')guanosine dinucleotides (m(3)Bp( n )G, n = 2, 3, 4) having m(3)B donor, 5'-5' polyphosphate bridge, and guanine (G) acceptor, exhibit exponential dependence of the ET rate on the number of phosphates, i.e. donor-acceptor distance. Involvement of the 5'-5' polyphosphate bridge in the ET is strongly indicated by lack of m(3)B-G stacking effect on the exponential factor, which is the same at 20 degrees C, where m(3)B-G intramolecular stacking dominates, as that at 75 degrees C where stacking-unstacking equilibrium is shifted in favour of the unstacked structure.

Unusual Electron Tunneling Constant for Long Range Electron Transfer in Hydroquinone (H2Q)-terminated Self-Assembled Monolayers on Gold in Alkaline Solution

In order to study the interfacial electron transfer, selfassembled monolayers (SAMs) on electrode surface have been attractive as a model system because they provide a stable and structurally well-defined monolayer with an adjustable thickness and desirable function. This characteristic of SAMs affords an opportunity to study fundamental issues such as the effects of distance and interfacial structure on the long-range electron transfer kinetics between a redox active species and an electrode. Weaver and Li obtained the first evidence of the distance-dependence of heterogeneous electron transfer rate constant for reduction of pentaaminecobalt(III) complexes anchored to gold and mercury surface. After Chidsey and co-workers reported heterogeneous electron transfer rate and electron tunneling constant (β) for the ferrocene-terminated alkanethiol monolayers with different alkyl chain length, a number of groups have reported β values for SAMs containing redox couples such as pentaamine (pyridine) Ru(II) complex, Os(II) bipyridyl complex, viologen, naphtoquinone, azobenzene, and hydroquinone. All of these reports demonstrated that the logarithmic heterogeneous apparent rate constant (kapp) linearly decreases as the distance between the electroactive center and the electrode surface increases as expected by Marcus theory. Those investigations on the distance dependence of electron transfer gave β values which are roughly ranging from 0.7 to 1.3 per methylene unit in the alkyl chain spacer. It is interesting to note that these β values are quite similar each other in their magnitude even though the electroactive SAMs contain different redox molecules and they follow significantly different electron transfer mechanism each other. For example, the electron transfer of ruthenium complex containing pentaamine and pyridine tether is highly reversible due to one-electron outer-sphere redox couple. However, redox center such as azobenzene undergoes substantially slow heterogeneous electron transfer because of its protonation reaction and structural change during its 2e−, 2H redox process. Among these redox centers studied, the electron transfer kinetics of a hydroquinone (H2Q) is also quite complicated due to its 2e −, 2H transfer reaction though Laviron presented a theoretical treatment of proton-coupled electron transfer reaction based on the nine-member square scheme. The electrochemical properties of hydroquinone/benzoquinone derivatives have been extensively studied in solutions because of their important biological activities. Especially, Hubbard and Soriaga have studied on the orientation of various quinones and mercaptohydroquinone derivatives adsorbed on metal surfaces using thin-layer electrochemistry. And Uosaki and coworkers have investigated the pH dependent redox behaviors of mercaptohydroquinone adsorbed on gold surface. We have reported the distance dependence of heterogeneous proton-coupled electron transfer rate constant of H2Q redox center in ω-mercaptoalkylhydroquinone SAMs on gold in 0.1 M HClO4 solution a few years ago. The β value reported for the H2Q-SAMs was 1.04 ± 0.06 and it was in good agreement with the values for the electroactive SAM systems reported up to now. However, the electron transfer kinetics of the H2Q-SAMs on gold shows remarkably different kinetic behavior in 0.1 M NaOH solution. In this note, we report an unusual observation for long range electron transfer in the ω-mercaptoalkylhydroquinone SAMs on gold electrode in a strong basic media. At this moment, the distance dependence of electron transfer of H2Q group is not observed in basic solution unlike that in acidic condition. Almost zero value of β determined from this phenomenon, to the best of our knowledge, is the first one in investigation of electron tunneling constant for long range electron transfer in the electroactive SAMs so far. The present work provides some understandings on the controlling factors related to proton-coupled electron transfer reaction nature.

Electron Transfer at Liquid/Liquid Interfaces. The Effects of Ionic Adsorption, Electrolyte Concentration, and Spacer Length on the Reaction Rate

Several factors affecting the rate of heterogeneous electron transfer (ET) reactions at the nonpolarized interface between two immiscible electrolyte solutions (ITIES) were investigated by scanning electrochemical microscopy (SECM). The reactions between zinc porphyrin in organic phase and aqueous Ru(CN)63/4- or Fe(CN)63/4- redox species, which have previously served as model systems for probing the potential dependence of the ET rate constant, were used to investigate the effects of adsorption of a reactant and a potential-determining ion on ET rate at the interfaces between water and different organic solvents. It was demonstrated that the rate constant of an interfacial ET can be either potential-independent or potential-dependent under different experimental conditions. The same model experimental systems were used to probe the kinetics of long-range ET across a monolayer of phospholipid adsorbed at the ITIES. The results obtained with a family of phosphatidylserine lipids are compared to those previously obtained with synthetic phosphatidylcholine lipids. The extent of blocking effect of adsorbed lipids on interfacial ET was investigated at different pHs and also in the presence of calcium ions in the aqueous phase. The micrometer-sized domain formation in the monolayer was observed in the presence of divalent cations.

Kinetics of long-range electron transfer through alkanethiolate monolayers containing amide bonds

Non-electroactive alkanethiolate monolayers containing internal amide bonds were used as model systems for the studies of the effect of structure of the intervening medium on long-range electron transfer. The blocking properties and the kinetics of electron transfer across the monolayers immobilized on gold were studied by voltammetry with the hexachloroiridate(IV) ion as the redox probe in the solution. The electron transfer efficiency was measured over a large potential window. The three types of monolayers studied were simple octadecanethiol and two amide-containing systems with one or two amide moieties in place of selected methylene groups in the main alkyl chain. Enhanced electronic coupling between the redox probe and the metal of the electrode was found for the monolayers with internal amide bonds. We ascribed it to the contribution of a hydrogen bonded network to electron tunneling through the monolayer. In the case of monolayers formed by molecules containing two secondary amide groups, the location of amide moieties inside the monolayer was shown to play an important role in the electron transfer efficiency. The second amide moiety placed in the alkyl chain in the odd position relative to the first amide did not increase electronic coupling in the monolayer. This behavior can be explained as due to larger distances between the amide groups in the external plane of the monolayer leading to difficulty in the formation of the hydrogen bond network. The position of the amide group relative to the electrode surface may be also considered as an important factor determining the efficiency of electron tunneling through the monolayer.

Theory for electron transfer from a mixed-valence dimer with paramagnetic sites to a mononuclear acceptor

Polynuclear transition-metal complexes, such as Fe–S clusters, are the prosthetic groups in a large number of metalloproteins and serve as temporary electron storage units in a number of important redox-based biological processes. Polynuclearity distinguishes clusters from mononuclear centers and confers upon them unique properties, such as spin ordering and the presence of thermally accessible excited spin states in clusters with paramagnetic sites, and fractional valencies in clusters of the mixed-valence type. In an earlier study we presented an effective-mode (EM) analysis of electron transfer from a binuclear mixed-valence donor with paramagnetic sites to a mononuclear acceptor which revealed that the cluster-specific attributes have an important impact on the kinetics of long-range electron transfer. In the present study, the validity of these results is tested in the framework of more detailed theories which we have termed the multimode semiclassical (SC) model and the quantum-mechanical (QM) model. It is found that the qualitative trends in the rate constant are the same in all treatments and that the semiclassical models provide a good approximation of the more rigorous quantum-mechanical description of electron transfer under physiologically relevant conditions. In particular, the present results corroborate the importance of electron transfer via excited spin states in reactions with a low driving force and justify the use of semiclassical theory in cases in which the QM model is computationally too demanding. We consider cases in which either one or two donor sites of a dimer are electronically coupled to the acceptor. In the case of multiconnectivity, the rate constant for electron transfer from a valence-delocalized (class-III) donor is nonadditive with respect to transfer from individual metal sites of the donor and undergoes an order-of-magnitude change by reversing the sign of the intradimer metal–metal resonance parameter (β). In the case of single connectivity, the rate constant for electron transfer from a valence-localized (class-II) donor can readily be tuned over several orders of magnitude by introducing differences in the electronic potentials at the two metal sites of the donor. These results indicate that theories of cluster-based electron transfer, in order to be realistic, need to consider both intrinsic electronic structure and extrinsic interactions of the cluster with the protein environment.

Constructing different `bridges' for interfacial electron transfer in azobenzene LB/SAM composite bilayers

Abstract Two methods, Langmuir–Blodgett (LB) and self-assembly (SA), were used in the present work to fabricate certain interfacial structures for the study of long-range electron transfer. We have successfully constructed two different `bridges' for the interfacial electron transfer in azobenzene LB/SAM composite bilayer systems. One has ionic bonding character, while the other has face-to-face hydrogen bonding at the interface of an ABD (4-octyl-4′-(3-carboxy-trimethylene-oxy)-azobenzene) LB monolayer and the underlying aminothiol SAM. These structures have been confirmed by using RA-FTIR and contact angle titration. Furthermore, the apparent electron transfer rate across the ionic bonding `bridge' is much more rapid than in the hydrogen-bonded case, indicating that the interfacial structure plays a crucial role in the long-range electron transfer kinetics.

Yeast cytochrome c with phenylalanine or tyrosine at position 87 transfers electrons to (zinc cytochrome c peroxidase)+ at a rate ten thousand times that of the serine-87 or glycine-87 variants.

Of the many factors known to influence the rate of electron transfer between two metalloproteins, it is particularly difficult to assess the role of the polypeptide matrix intervening between the donor and acceptor sites. To determine whether the phylogenetically conserved Phe-87 of yeast iso-1-cytochrome c helps to mediate electron transfer between cytochrome c and cytochrome c peroxidase, we have constructed mutants of cytochrome c that are altered at this position and now have studied the kinetics of long-range electron transfer within their complexes with zinc-substituted cytochrome c peroxidase. We find that the rate of electron transfer from reduced cytochrome c to the zinc cytochrome c peroxidase pi-cation radical is four orders of magnitude greater when phenylalanine or tyrosine is present at position 87 than when serine or glycine is present.