ET Rate Constant Research Articles

Dynamical effects in metalloprotein heterogeneous electron transfer

In this work we assess the influence of physiological viscosities on metalloprotein electron transfer reactions. To that end we investigated the direct electrochemistry of the copper proteins azurin and CuA adsorbed on SAM-coated electrodes. The experimental results show a change of ET regime from non-adiabatic to friction control upon shortening tunneling distances, which is paralleled by a sigmoidal increase of the apparent ET reorganization energies. These data could be accurately described using novel Matyushov’s model, thereby validating this recent theoretical development, which however did not describe viscosity effects in its original formulation. We demonstrate that ET rate constants and effective relaxation times vary with viscosity following power laws. Moreover, the crossover parameter g that determines the ET regime could be redefined in terms of the exponents of these power laws. The magnitude of g, i.e. the extent of frictional control, was found to be protein specific as it is determined by the dynamical features of the protein milieu. Interestingly, Stokes shift and diffusional relaxation times were found to be of similar magnitude, thus resulting in non-negligible frictional control even at tunneling distances as long as 19 Å, and this effect is amplified by physiologically high viscosities, thus highlighting the influence of intracellular macromolecular crowding in modulating protein ET reactions.

Backbone-Constrained Peptides: Temperature and Secondary Structure Affect Solid-State Electron Transport.

The primary sequence and secondary structure of a peptide are crucial to charge migration, not only in solution (electron transfer, ET), but also in the solid-state (electron transport, ETp). Hence, understanding the charge migration mechanisms is fundamental to the development of biomolecular devices and sensors. We report studies on four Aib-containing helical peptide analogues: two acyclic linear peptides with one and two electron-rich alkene-based side chains, respectively, and two peptides that are further rigidified into a macrocycle by a side bridge constraint, containing one or no alkene. ETp was investigated across Au/peptide/Au junctions, between 80 and 340 K in combination with the molecular dynamic (MD) simulations. The results reveal that the helical structure of the peptide and electron-rich side chain both facilitate the ETp. As temperature increases, the loss of helical structure, change of monolayer tilt angle, and increase of thermally activated fluctuations affect the conductance of peptides. Specifically, room temperature conductance across the peptide monolayers correlates well with previously observed ET rate constants, where an interplay between backbone rigidity and electron-rich side chains was revealed. Our findings provide new means to manipulate electronic transport across solid-state peptide junctions.

Functionalized carbon nanotube adsorption interfaces for electron transfer studies of galactose oxidase

Modified electrodes featuring specific adsorption platforms able to access the electrochemistry of the copper containing enzyme galactose oxidase (GaOx) were explored, including interfaces featuring nanomaterials such as nanoparticles and carbon nanotubes (CNTs). Electrodes modified with various self-assembled monolayers (SAMs) including those with attached nanoparticles or amide-coupled functionalized CNTs were examined for their ability to effectively immobilize GaOx and study the redox activity related to its copper core. While stable GaOx electrochemistry has been notoriously difficult to achieve at modified electrodes, strategically designed functionalized CNT-based interfaces, cysteamine SAM-modified electrode subsequently amide-coupled to carboxylic acid functionalized single wall CNTs, were significantly more effective with high GaOx surface adsorption along with well-defined, more reversible, stable (≥ 8 days) voltammetry and an average ET rate constant of 0.74 s−1 in spite of increased ET distance - a result attributed to effective electronic coupling at the GaOx active site. Both amperometric and fluorescence assay results suggest embedded GaOx remains active. Fundamental ET properties of GaOx may be relevant to biosensor development targeting galactosemia while the use functionalized CNT platforms for adsorption/electrochemistry of electroactive enzymes/proteins may present an approach for fundamental protein electrochemistry and their future use in both direct and indirect biosensor schemes.

Effect of viscosity on photoinduced electron transfer reaction: An observation of the Marcus inverted region in homogeneous solvents

The viscosity effect of homogeneous solvents on the dynamics of photoinduced electron transfer (PET) reaction among the coumarins and N,N-dimethylaniline (DMA) is investigated using steady-state and time-resolved fluorescence spectroscopy. A bell shape Marcus inversion in the ET rates has been detected in the plot of ET rate constant (kq) with free energy change (ΔG0) in viscous solvents decanol and EG, but it is not observed in DMSO like low viscous solvent. We have also reported that there is no complex formation between the coumarin dye and DMA molecule by using fluorescence correlation spectroscopy.

Extension of Hopfield's Electron Transfer Model To Accommodate Site-Site Correlation.

Extension of the Förster analogue for the ET rate constant (based on virtual intermediate electron detachment or attachment states) with inclusion of site-site correlation due to coulomb terms associated with solvent reorganization energy and the driving force, has been developed and illustrated for a simple three-state, two-mode model. The model is applicable to charge separation (CS), recombination (CR), and shift (CSh) ET processes, with or without an intervening bridge. The model provides a unified perspective on the role of virtual intermediate states in accounting for the thermal Franck-Condon weighted density of states (FCWD), the gaps controlling superexchange coupling, and mean absolute redox potentials, with full accommodation of site-site coulomb interactions. Two types of correlation have been analyzed: aside from the site-site correlation due to coulomb interactions, we have emphasized the intrinsic "nonorthogonality" which generally pertains to reaction coordinates (RCs) for different ET processes involving multiple electronic states, as may be expressed by suitably defined direction cosines (cos(θ)). A pair of RCs may be nonorthogonal even when the site-site coulomb correlations are absent. While different RCs are linearly independent in the mathematical sense for all θ ≠ 0°, they are independent in the sense of being "uncorrelated" only in the limit of orthogonality (θ = 90°). Application to more than two coordinates is straightforward and may include both discrete and continuum contributions.

Direct electrochemistry of recombinant tobacco peroxidase on gold

Direct electron transfer (DET) reactions of recombinant tobacco peroxidase (rTOP), namely direct electroreduction of Compound I/Compound II and heme Fe 3+/2+ conversion, were studied on gold electrodes. rTOP of wild type, non-glycosylated, was produced using an Escherichia coli expression system. At pH 5.0, the redox potential for direct electrochemical transformation of the Fe 3+/2+ of the peroxidase heme was −143 mV vs. Ag∣AgCl, and 0.26 ± 0.07 pmol of the adsorbed rTOP were in DET contact with the gold electrode. The total amount of the adsorbed rTOP estimated from QCM data was 53 ± 5 pmol/cm 2 or 1.67 pmol when referred to the surface area of the electrodes used for electrochemical measurements. Of 1.67 pmol of adsorbed rTOP, only 0.76 pmol were catalytically active. DET between Au and the enzyme was also studied in the reaction of the bioelectrocatalytic reduction of H 2O 2 by cyclic voltammetry and amperometric detection of H 2O 2 at +50 mV with rTOP-modified Au electrodes placed in a wall-jet flow-through electrochemical cell. Maximal bioelectrocatalytic current response of the rTOP-modified gold electrodes to H 2O 2 was observed at pH 5.0 and stemmed from its bioelectrocatalytic reduction based on DET between Au and the active site of rTOP. Kinetic analysis of the DET reactions gave 52% of the adsorbed rTOP molecules active in DET reactions (0.4 pmol of adsorbed catalytically active rTOP, correspondingly), which correlated well with the non-catalytic-voltammetry data. DET was characterised by a heterogeneous ET rate constant of 13.2 s −1, if one takes into account the QCM data, and 19.6 s −1, if the amount of rTOP estimated from the data on DET transformation of Fe 3+/2+ couple of rTOP is considered. The sensitivity for H 2O 2 obtained for the rTOP-modified Au electrodes was 0.7 ± 0.1 A M −1 cm −2. These are the first ever-reported data on DET reactions of anionic plant peroxidases on bare gold electrodes.

Direct electron transfer in the system gold electrode–recombinant horseradish peroxidases

The kinetics of the bioelectrocatalytic reduction of hydrogen peroxide has been studied at gold electrodes modified with different forms of horseradish peroxidase (HRP). Native HRP, wild type recombinant HRP (rec-HRP) and its two mutant forms containing a six-histidine tag at the C- or N-terminus, C Hisrec-HRP and N Hisrec-HRP, respectively, have been used for an adsorptive modification of the gold electrodes. The histidine sequences, i.e. histidine tags, were introduced into the peroxidase structure by genetic engineering of non-glycosylated rec-HRP using an Escherichia coli expression system. Experiments with a gold rotating disc electrode demonstrated that electrodes with the adsorbed rec-HRP forms exhibited high and stable current response to H 2O 2 due to its bioelectrocatalytic reduction based on direct (mediatorless) ET between gold and the active site of HRP. The heterogeneous ET rate constants were evaluated to be in the order of 20 or 33 s −1 between rec-HRP or its histidine mutants and gold, respectively, in 0.01 M phosphate buffer (pH 7.4) containing 0.15 M NaCl. The increase in the heterogeneous ET rate found for C Hisrec-HRP and N Hisrec-HRP is probably due to the interaction of the histidine tag with the electrode surface. The kinetic data demonstrate that new possibilities for enhancing the catalytic activity of the enzyme at the electrode ∣ solution interface can be achieved by genetic engineering design of the enzyme molecules.

Stereoselective Electron-Transfer Reactions of Myoglobin and Cytochrome c with Chiral Viologen-Radical Cations

Abstract Electron-transfer (ET) reactions of metmyoglobin (metMb) and the oxidized form of cytochrome c (cyt c(III)) with chiral viologen-radical cations containing (1-phenyl- and 1-cyclohexylethyl)carbamoylmethyl groups were examined at pH 7.0 (10 mM phosphate buffer), ionic strengths (I) of 0.040–0.50 M, and temperatures of 10–35 °C. In the metMb system, the pseudo first-order rate constant of the decay of the viologen-radical cation was saturated with increasing concentrations of metMb, indicating that metMb interacts with the viologen-radical cation to form a complex followed by an intramolecular ET reaction. The stereoselectivity was observed in both the complex formation and the intramolecular ET processes for the (S,S)-isomers of chiral viologens. The ratios of the association constants and the intramolecular ET rate constants of the (S,S)-isomer with those of the (R,R)-isomer were 1.1–1.3 and 1.1–1.5, respectively. The saturation kinetics was not observed for the ET reaction between metMb and the achiral benzylviologen-radical cation, which has no CONH bond, suggesting that the hydrogen bonding of the CONH group of the chiral viologen with the side chain of the polypeptide in metMb plays an important role in the stereoselectivity. No appreciable complex formation was observed in the ET reaction between cyt c(III) and viologen-radical cations, although the stereoselectivity was observed. The ratios of the second-order rate constants of the (S,S)-isomer with those of the (R,R)-isomer were 1.1–1.4. The stereoselectivity is discussed based on the activation parameters.

Reorganization Parameters of Electronic Transitions in Electronically Delocalized Systems. 1. Charge Transfer Reactions

This paper addresses the problem of calculating the free energy surfaces of charge transfer (CT) reactions in electronically delocalized systems involving direct electronic overlap of the donor and acceptor units. The model includes two electronic states of the solute linearly coupled to a linearly responding solvent in both the diagonal and off-diagonal terms of the Hamiltonian matrix. The exact solution for the adiabatic free energy surfaces along the CT reaction coordinate is derived as a function of the reorganization parameters invariant to the extent of electron delocalization and the parameter of electron delocalization. For dipolar solutes, the latter is defined through adiabatic differential and transition dipoles measured by optical spectroscopy. The invariant reorganization energy is the real, observable reorganization energy determining the vertical transition energy, bandwidth, and the activation energy of self-exchange transitions. The Golden Rule expression for the ET rate constant is obtai...

Diffusionless electron transfer of microperoxidase-11 on gold electrodes

Abstract Microperoxidase-11, MP-11, is made by proteolytic digestion of cytochrome c, cyt. c. It consists of a polypeptide of 11 amino residues attached covalently to the heme. Given that MP-11 has a more exposed heme than the complete protein, it would seem that electron transfer, ET, between immobilized MP-11 and electrodes would be at least as fast as for intact cyt. c. However, while the maximal heterogeneous ET rate for immobilized cyt. c is around 1000 s−1, that reported previously for immobilized MP-11 does not exceed 20 s−1. This work attempts to understand this difference in measured ET rates. The MP-11 was immobilized on gold electrodes using several protocols: (electrode A) the immobilization was done following a previously published carbodiimide based recipe yielding ET rates of the order of 20 s−1; (B) MP-11 was bound to gold electrodes by Lomant’s reagent and gave an ET rate close to 4000 s−1; (C) physisorbed MP-11 on gold electrodes with a self assembled monolayer, SAM, of alkane thiols gave an ET rate approaching 2000 s−1 for the shortest length alkane thiol. Inspection of the immobilization chemistries suggests that the procedure employed in producing electrodes B and C are likely to lead to a monolayer or less of immobilized MP-11 while the procedure employed for electrode A may lead to a film comprised of a multilayer of MP-11. The presence of such a film on electrode A complicates the ET process since the MP-11 in the layer adjacent to the electrode could have fast ET rates while the MP-11 in the outer layers may have significantly slower ET rates. The net result would be an apparent ET rate constant which is much smaller than the value for the first layer. The measurements and calculations are presented in support of such an interpretation.

Generation and Mesolysis of PhSeSiR3]•-: Mechanistic Studies by Laser Flash Photolysis and Application for Bimolecular Group Transfer Radical Reactions

The investigation presented in this paper explores the mechanistic aspects and synthetic potentials of PET promoted reductive activation of selenosilane 1a to its radical anion 1a•-. PET activation of 1a is achieved through a photosystem comprising a light-absorbing electron-rich aromatic (ERA), such as DMN or DMA, as an electron donor and ascorbic acid as a co-oxidant. The evidence for the ET from excited singlet states of DMN as well as DMA to 1a is suggested by estimating negative ΔGet (−51 and −43.46 kcal mol-1, respectively) values and nearly diffusion-controlled fluorescence quenching rate constants (kqTR) 0.36 × 1010 M-1 s-1 and 0.28 × 1010 M-1 s-1, respectively, from time-resolved fluorescence quenching study. The transient absorption spectra of DMN•+, DMA•+, and 1a•- are obtained initially by pulse radiolysis in order to correlate the time-resolved absorption spectral data. Laser flash photolysis studies in the nanosecond time domain have confirmed the generation of 1a•-, DMN•+, and DMA•+, supporting the participation of the triplet state of DMN or DMA in the ET reaction. Mesolytic cleavage of 1a•- produced a silyl radical and a phenyl selenide anion. The preparative PET activation of 1a in acetonitrile in the presence of DMN or DMA leads to the formation of 5 and 6, confirming the fragmentation pattern of 1a•-. The overall ET rate constants (kr(DMN) = 0.99 × 1010 M-1 s-1 and kr(DMA) = 1.62 × 1010 M-1 s-1) and limiting quantum yields (φlim(DMN) = 0.034 and φlim(DMA) = 0.12) are estimated from the inverse plot (1/[1a] vs 1/φdis) obtained by measuring the dependence of photodissociation quantum yields of 1a at its maximum concentration in the presence of DMN or DMA. Silicon-centered radical species generated from the mesolysis of 1a•- are utilized for initiating a radical reaction by the abstraction of halogen atom from −C−X (X = Cl, Br) bonds, while PhSe- terminates the radical sequences via PhSeSePh. This concept is successfully applied for the bimolecular group transfer (BMGT) radical reactions and intermolecular radical chain addition reactions.

Distance dependence of electron transfer rates in bilayers of a ferrocene Langmuir-Blodgett monolayer and a self-assembled monolayer on gold

Bilayers consisting of a self-assembled alkanethiol monolayer of variable chain length and Langmuir-Blodgett monolayers of 16-ferrocenylhexadecanoic acid were fabricated on Au electrodes. The hydrophobic ferrocene groups reside at the monolayer-monolayer interface and are variably spaced from the underlying electrode. Interfacial electron transfer rates k” for ferrocene electron transfer across the alkanethiol spacer monolayer were measured cyclic voltammetrically. The rates were found to fall off exponentially with increasing alkanethiol chain length, giving a decay constant of -0.96 per methylene. The electron transfer rate with a dodecanethiol spacer layer (k” = 12 s-l), which involves a non-bonded donor-acceptor couple, is almost 2 orders of magnitude lower than those for the analogous ferrocenes chemically bound on Au through alkane thiols of similar bond length. The higher rate in the latter may be attributed to a through-bond mechanism, as opposed to the former in which there exists a single “break” in the chemical bond network. The results have their implications on the electron transfer studies of biological systems. We have sought to probe long-distance interfacial electron transfer’ (ET) by using designed electrochemical interfaces based on self-assembled monolayer2 (SAM) and LangmuirBlodgett (LB) films.3 In particular, we have fabricated on gold surfaces bilayer structures composed of an inner spacer monolayer of long-chain alkanethiols (CnH2,+1SH, n 2 10) and an outer electroactive LB monolayer, with electrochemically reactive head groups (ferrocene) positioned at the monolayermonolayer interface (Scheme la). The distance dependence of the heterogeneous ET rate constants can be investigated by varying the alkanethiol chain length. Furthermore, a comparison of these rates with those obtained at gold electrodes modified with ferrocene-terminated alkanethiol monolayers2c.f (Scheme lb) can address the question of the relative efficiency of “through-bond” versus “through-space’’ ET pathways. This is one of the key issues currently under investigation4 in the field of long range electron transfer, a reaction that is ubiquitous in biological systems and is involved in such important processes as oxidative phosphorylation and photosynthesis. In a biological electron transfer reaction, the electron donor and acceptor are separated by polypeptides organized into defined threedimensional structures mostly through noncovalent interactions such as hydrogen bonds and van der Waals contacts. The modulation of these noncovalent interactions on the overall electron tunneling efficiency through the polypeptide matrix is therefore well worth investigating. Experimental measurements5 as well as theoretical calculations6 have been carried out using metal-derivatized proteins and physiological protein-protein complexes to examine the effect of biological medium on electron transfer kinetics, although the ability to control the structural details of these systems has not been achieved yet. With the rapid advancement in the field of molecular selfassembly, we are now in a position to fabricate highly ordered monomolecular and multilayer films on solid surfaces with molecular level control over the structures and employ them in

Weak Temperature Dependence of Electron Transfer Rates in Fixed-Distance Porphyrin-Quinone Model Systems

Electron transfer rate constants of several derivatives of [5-(4'-(4-(2',5'-benzoquinonyl)bicyclo[2.2.2]octyl) phenyl)-2,3,7,8,12,13,17,18-octamethylporphyrinato]zinc(II) have been measured as a function of temperature in 2-methyltetrahydrofuran, toluene, and toluene-d_g. The observed temperature dependencies of the electron transfer rate constants are relatively weak in both the polar and nonpolar solvents. Nonexponential ET dynamics are observed at low temperatures and described in terms of an initial (k_(ET)^0) and an average ET rate constant (k_(av)). The k_(ET)^0 values for the molecules with different driving forces, spanning a range of 0.2 eV, show parallel trends over the range of temperatures studied. The trends in k_(ET)^0 are described in terms of the effects of temperature-dependent changes in solvent dielectric constants on the barrier height. Good agreement is observed for the case of toluene solvent, using a semiclassical model, but poorer quantitative agreement is found for the 2-methyltetrahydrofuran data. The temperature dependence of k_(av) is described using a model incorporating an angle-dependent electronic coupling and interconversion of rotational conformers. A temperature-dependent solvent isotope effect is observed on going from toluene to toluene-d_8, with k_(ET)^0( toluene)/k_(ET)^0( toluene-d_8) being as large at 1.5 over the range of temperatures studied.

Electron transfer in ruthenium-modified cytochromes c. .sigma.-tunneling pathways through aromatic residues

The rates of intramolecular electron-transfer (ET) reactions from the ferroheme to bis(2,2'-bipyridine)(imidazole)-ruthenium(II1) complexes bound to genetically engineered histidines (58 and 66) on the surface of yeast iso-1-cytochrome c (cyt c) have been measured by using a laser flashquench technique. The crystal structure of the wild-type protein indicates that the ET pathways involve aromatic side chains: Ru(His58)cyt c includes a bridging tryptophan at position 59, and Ru(His66)cyt c has a tyrosine at 67. A variant in which the bridging Tyr67 in the His66 mutant had been replaced with a phenylalanine also was examined. The Fe^2+ → Ru^(3+)ET rate constants (25 °C, pH 7.0) are as follows: 5.2(5) × l0^4 (ΔE° = 0.69(5)), Ru(His58)cyt c; 1.0(1) × 10^6 (ΔE° = 0.72(5)), Ru(His66)cyt c; and 3.1(3) × l0^6 s^-1 (ΔE° = 0.77(5) eV), Ru(His66Phe67)cyt c. The experimentally derived electronic coupling constants [H_AB(His66)= 0.014; H_AB(HiS66)= 0.060 cm-1 are in closer agreement with the lengths of a-tunneling pathways than with the direct donor-acceptor distances, and there is no indication that the u orbitals of intervening groups enhance any of these couplings. Maximum ET rates in the modified cytochromes drop by 2 orders of magnitude for every 6.3-A increase in the a-tunneling length. Analysis of the results also suggests that an internal water molecule in Ru(His66Phe67)cyt c plays a role in linking the Ru(His66) group to the heme.

Self-exchange electron transfer rate constants and reorganization energies for some aromatic compounds in N,N-dimethylformamide determined by elect

The rate constants for homogeneous self-exchange electron transfer reactions of nine aromatic and heteroaromatic compounds in N,N-dimethylformamide have been measured by electron spin resonance line-broadening experiments. The aromatic anion radicals were generated electrochemically in a flow cell. The measured rate constants are compared with literature values when possible and self-exchange reorganization energies λ have been calculated. It is found that for all the aromatic and heteroaromatic compounds, except benzophenone, λ 11 ± 1.5 kcal mol −1. For benzophenone a significantly higher value for λ is found (14.7 kcal mol −1. An electrochemical flow cell connected with the cavity of an ESR spectrometer has been used to measure self-exchange ET rate constants and self-exchange reorganization energies for a number of aromatic and heteroaromatic compounds. The obtained values agree with some of the literature values, but with others the differences observed are unsatisfactory and inexplicable. It seems a general problem with such measurements that although accurate measurements (± 10%) can be made in several laboratories, the results obtained in different laboratories may deviate by more than 100%. When comparing reorganization energies for different compounds it seems safest to compare values obtained in the same investigation.