An Electrochemical Approach to Investigate Gated Electron Transfer Using a Physiological Model System: Cytochrome c Immobilized on Carboxylic Acid-Terminated Alkanethiol Self-Assembled Monolayers on Gold Electrodes

Electron Conduction across Electrode-Immobilized Neutravidin Bound with Biotin-Labeled Ruthenium Pentaamine

Voltammetry is reported here of a self-assembled redox-protein conjugate consisting of neutravidin conjugated with a biotin derivative redox probe, Ru(NH3)5(N-[(N-[(4-pyridyl)methyl]biotinamide], immobilized on gold electrodes modified by self-assembled monolayers of mercaptoundecanoic acid. This voltammetry indicates that self-assembly of the conjugate/electrode electronic interface, driven by electrostatic binding between the monolayer and a single redox probe, favors orientation of the conjugate, resulting in electronic accessibility of the remaining three redox probes.

Direct electron transfer of heme- and molybdopterin cofactor-containing chicken liver sulfite oxidase on alkanethiol-modified gold electrodes

Direct heterogeneous electron transfer (ET) of sulfite oxidase (SOx), a heme- and molybdopterin cofactor-containing intermembrane enzyme, was studied on alkanethiol-modified Au electrodes both with SOx entrapped between the modified Au electrode and a permselective membrane and with SOx adsorbed at the electrode surface, in the absence of any membrane. SOx in direct electronic communication with the electrode surface gave a quasi-reversible electrochemical signal with a midpoint potential of--120 mV vs Ag/AgCl corresponding to the redox transformations of the heme domain of SOx and with a heterogeneous ET constant in the order of 15 s(-1). The efficiency of the bioelectrocatalytic 2e- oxidation of sulfite catalyzed by SOx in direct ET exchange with the electrode was shown to depend essentially on the nature of the alkanethiol layer. Adsorption and orientation of SOx on an 11-mercapto-1-undecanol (MuD-OH) self-assembled monolayer, i.e., terminally functionalized with OH groups, provided efficient catalytic oxidation of sulfite, contrary to nonfunctionalized alkanethiols, e.g., 1-decanethiol, or alkanethiol layers terminally functionalized with NH2 groups. Comparative studies with short-chain alkanethiols, e.g., cysteamine and 2-mercaptoethanol, revealed an evidently different mode of adsorption of SOx on these layers, onto which SOx was not catalytically active. Coadsorption of MuD-OH and 11-mercapto-1-undecanamine improved the surface properties of the SAM, resulting in a higher surface coverage with bioelectrocatalytically active SOx but not in an increased apparent catalytic rate constant, kcat, ranging in the order of 18-24 s(-1) at pH 7.4. The achieved efficiency of SOx bioelectrocatalysis in direct ET reaction between the modified electrode and the enzyme approached the rates characteristic for the catalysis mediated by cytochrome c, the natural redox partner of SOx, thus implying the retention of the biological function of SOx under the heterogeneous electrode reaction conditions. Results obtained enable the development of a third-generation biosensor for sulfite monitoring.

Fast, long-range electron-transfer reactions of a "blue" copper protein coupled non-covalently to an electrode through a stilbenyl thiolate monolayer

A self-assembled monolayer (SAM), formed by the insitu saponification of a stilbenyl thioacetate on a gold electrode, yields fast electron transfer (ET)(the exchange rate at zero driving force exceeds 1600 s-1) with adsorbed molecules of the blue copper protein, azurin, over a distance exceeding 15 angstroms .

Redox and conformational equilibria and dynamics of cytochrome c at high electric fields

Cytochrome c (Cyt-c) adsorbed in the electrical double layer of the Ag electrode/electrolyte interface has been studied by stationary and time-resolved surface-enhanced resonance Raman spectroscopy to analyse the effect of strong electric fields on structure and reaction equilibria and dynamics of the protein. In the potential range between +0.1 and -0.55 V (versus saturated calomel electrode), the adsorbed Cyt-c forms a potential-dependent reversible equilibrium between the native state B1 and a conformational state B2. The redox potentials of the bis-histidine-coordinated six-coordinated low-spin and five-coordinated high-spin substates of B2 were determined to be -0.425 and -0.385 V, respectively, whereas the additional six-coordinated aquo-histidine-coordinated high-spin substate was found to be redox-inactive. The redox potential for the conformational state B1 was found to be the same as in solution in agreement with the structural identity of the adsorbed B1 and the native Cyt-c. For all three redox-active species, the formal heterogeneous electron transfer rate constants are small and of the same order of magnitude (3-13 s-1), which implies that the rate-limiting step is largely independent of the redox-site structure. These findings, as well as the slow and potential-dependent transitions between the various conformational (sub-)states, can be rationalized in terms of an electric field-induced increase of the activation energy for proton-transfer steps linked to protein structural reorganisation. Further increasing the electric field strength by shifting the electrode potential above +0.1 V leads to irreversible structural changes that are attributed to an unfolding of the polypeptide chain.

Charge-transfer mechanism for cytochrome c adsorbed on nanometer thick films. Distinguishing frictional control from conformational gating

Using nanometer thick tunneling barriers with specifically attached cytochrome c, the electron-transfer rate constant was studied as a function of the SAM composition (alkane versus terthiophene), the omega-terminating group type (pyridine, imidazole, nitrile), and the solution viscosity. At large electrode-reactant separations, the pyridine terminated alkanethiols exhibit an exponential decline of the rate constant with increasing electron-transfer distance. At short separations, a plateau behavior, analogous to systems involving -COOH terminal groups to which cytochrome c can be attached electrostatically, is observed. The dependence of the rate constant in the plateau region on system properties is investigated. The rate constant is insensitive to the mode of attachment to the surface but displays a significant viscosity dependence, change with spacer composition (alkane versus terthiophene), and nature of the solvent (H(2)O versus D(2)O). Based on these findings and others, the conclusion is drawn that the charge-transfer rate constant at short distance is determined by polarization relaxation processes in the structure, rather than the electron tunneling probability or large-amplitude conformational rearrangement (gating). The transition in reaction mechanism with distance reflects a gradual transition between the tunneling and frictional mechanisms. This conclusion is consistent with data from a number of other sources as well.

Conformational reorganisation in interfacial protein electron transfer

Protein-protein electron transfer (ET) plays an essential role in all redox chains. Earlier studies which used cross-linking and increased solution viscosity indicated that the rate of many ET reactions is limited (i.e., gated) by conformational reorientations at the surface interface. These results are later supported by structural studies using NMR and molecular modelling. New insights into conformational gating have also come from electrochemical experiments in which proteins are noncovalently adsorbed on the electrode surface. These systems have the advantage that it is relatively easy to vary systematically the driving force and electronic coupling. In this review we summarize the current knowledge obtained from these electrochemical experiments and compare it with some of the results obtained for protein-protein ET.

Heterogeneous electron-transfer kinetics for ruthenium and ferrocene redox moieties through alkanethiol monolayers on gold

The standard heterogeneous electron-transfer rate constants between substrate gold electrodes and either ferrocene or pentaaminepyridine ruthenium redox couples attached to the electrode surface by various lengths of an alkanethiol bridge as a constituent of a mixed self-assembled monolayer were measured as a function of temperature. The ferrocene was either directly attached to the alkanethiol bridge or attached through an ester (CO(2)) linkage. For long bridge lengths (containing more than 11 methylene groups) the rate constants were measured using either chronoamperometry or cyclic voltammetry; for the shorter bridges, the indirect laser induced temperature jump technique was employed to measure the rate constants. Analysis of the distance (bridge length) dependence of the preexponential factors obtained from an Arrhenius analysis of the rate constant versus temperature data demonstrates a clear limiting behavior at a surprisingly small value of this preexponential factor (much lower than would be expected on the basis of aqueous solvent dynamics). This limit is independent of both the identity of the redox couple and the nature of the linkage of the couple to the bridge, and it is definitely different (smaller) from the limit derived from an equivalent analysis of the rate constant (versus temperature) data for the interfacial electron-transfer reaction through oligophenylenevinylene bridges between gold electrodes and ferrocene. There are a number of possible explanations for this behavior including, for example, the possible effects of bridge conformational flexibility upon the electron-transfer kinetics. Nevertheless, conventional ideas regarding electronic coupling through alkane bridges and solvent dynamics are insufficient to explain the results reported here.

Nonideal electrochemical behavior of biomimetic iron porphyrins: interfacial potential distribution across multilayer films

The electrochemical behavior of multilayer films formed by iron porphyrins deposited on an edge plane graphite electrode has been examined under anaerobic conditions. In the scan rate interval (1-250 mV/s) where the electrode reaction is reversible, CV diagrams of these films demonstrate substantial deviations from ideality in broadening and separation of the peaks. A model that describes the observed behavior is proposed by taking into account the potential distribution at the electrode/film interface and the concentration dependence of surface activity coefficients. The peak separation is described in terms of the electric double layer that affects the potential difference driving the electrode reaction. The effective potential difference deviates from the applied value due to the potential distribution across the film. The interfacial potential distribution depends on the ionic concentration inside the film. When different ionic concentrations are assumed for oxidation and reduction, different shifts from the applied potential lead to a hysteresis of the peaks. The peak broadening is modeled by using the lattice theory expression for the surface activity coefficients. The model shows that the midpoint potentials of the redox centers depend on the ionic concentration inside the film. At low ionic concentrations, they are remarkably close to the midpoints of the cytochrome c oxidase heme a3/CuB site.

Electron-transfer mechanisms through biological redox chains in multicenter enzymes

A new approach for studying intramolecular electron transfer in multicenter enzymes is described. Two fumarate reductases, adsorbed on an electrode in a fully active state, have been studied using square-wave voltammetry as a kinetic method to probe the mechanism of the long-range electron transfer to and from the buried active site. Flavocytochrome c(3) (Fcc(3)), the globular fumarate reductase from Shewanella frigidimarina, and the soluble subcomplex of the membrane-bound fumarate reductase of Escherichia coli (FrdAB) each contain an active site FAD that is redox-connected to the surface by a chain of hemes or Fe-S clusters, respectively. Using square-wave voltammetry with large amplitudes, we have measured the electron-transfer kinetics of the FAD cofactor as a function of overpotential. The results were modeled in terms of the FAD group receiving or donating electrons either via a direct mechanism or one involving hopping via the redox chain. The FrdAB kinetics could be described by both models, while the Fcc(3) data could only be fit on the basis of a direct electron-transfer mechanism. This raises the likelihood that electron transfer can occur via a superexchange mechanism utilizing the heme groups to enhance electronic coupling. Finally, the FrdAB data show, in contrast to Fcc(3), that the maximum ET rate at high overpotential is related to the turnover number for FrdAB measured previously so that electron transfer is the limiting step during catalysis.

Monolayers of a de novo designed 4-alpha-helix bundle carboprotein and partial structures on Au(111)-surfaces

Mapping of structure and function of proteins adsorbed on solid surfaces is important in many contexts. Electrochemical techniques based on single-crystal metal surfaces and in situ scanning probe microscopies (SPM) have recently opened new perspectives for mapping at the single-molecule level. De novo design of model proteins has evolved in parallel and holds promise for test and control of protein folding and for new tailored protein structural motifs. These two strategies are combined in the present report. We present a synthetic scheme for a new 4-alpha-helix bundle carboprotein built on a galactopyranoside derivative with a thiol anchor aglycon suitable for surface immobilization on gold. The galactopyranoside with thiol anchor and the thiol anchor alone were prepared for comparison. Voltammetry of the three molecules on Au(111) showed reductive desorption peaks caused by monolayer adsorption via thiolate-Au bonding. In situ STM of the thiol anchor disclosed an ordered adlayer with clear domains and molecular features. This holds promise, broadly for single-molecule voltammetry and the SPM and scanning tunnelling microscopy (STM) of natural and synthetic proteins.

Intermolecular biological electron transfer: an electrochemical approach

We investigated the electron transfer (ET) rates between a well-defined gold electrode and cytochrome c immobilized at the carboxylic acid terminus of alkanethiol self-assembled monolayers (SAMs) by using the potential modulated electroreflectance technique. A logarithmic plot of ET rates against the chain length of the alkanethiol is linear with long chain alkanethiols. The ET rates become independent of the chain length with short alkanethiols. It is proposed that the rate-limiting ET step through short alkyl chains results from a configurational rearrangement process preceding the ET event. This "gating" process arises from a rearrangement of the cytochrome c from a thermodynamically stable binding form on the carboxylic acid terminus to a configuration, which facilitates the most efficient ET pathways (surface diffusion process). We propose that the lysine-13 of mammalian cytochrome c facilitates the most efficient ET pathway to the carboxylate terminus and this proposal is supported by the ET reaction rate of a rat cytochrome c mutant (RC9-K13A) [Elektrokhimiya (2001) in press], in which lysine-13 is replaced by alanine. The ET rate of K13A is more than six orders of magnitude smaller than that of the native protein.

Proton-coupled electron transfer of cytochrome c

Cytochrome c (Cyt-c) was electrostatically bound to self-assembled monolayers (SAM) on an Ag electrode, which are formed by omega-carboxyl alkanethiols of different chain lengths (C(x)). The dynamics of the electron-transfer (ET) reaction of the adsorbed heme protein, initiated by a rapid potential jump to the redox potential, was monitored by time-resolved surface enhanced resonance Raman (SERR) spectroscopy. Under conditions of the present experiments, only the reduced and oxidized forms of the native protein state contribute to the SERR spectra. Thus, the data obtained from the spectra were described by a one-step relaxation process yielding the rate constants of the ET between the adsorbed Cyt-c and the electrode for a driving force of zero electronvolts. For C(16)- and C(11)-SAMs, the respective rate constants of 0.073 and 43 s(-1) correspond to an exponential distance dependence of the ET (beta = 1.28 A(-1)), very similar to that observed for long-range intramolecular ET of redox proteins. Upon further decreasing the chain length, the rate constant only slightly increases to 134 s(-1) at C(6)- and remains essentially unchanged at C(3)- and C(2)-SAMs. The onset of the nonexponential distance dependence is paralleled by a kinetic H/D effect that increases from 1.2 at C(6)- to 4.0 at C(2)-coatings, indicating a coupling of the redox reaction with proton-transfer (PT) steps. These PT processes are attributed to the rearrangement of the hydrogen-bonding network of the protein associated with the transition between the oxidized and reduced state of Cyt-c. Since this unusual kinetic behavior has not been observed for electron-transferring proteins in solution, it is concluded that at the Ag/SAM interface the energy barrier for the PT processes of the adsorbed Cyt-c is raised by the electric field. This effect increases upon reducing the distance to the electrode, until nuclear tunneling becomes the rate-limiting step of the redox process. The electric field dependence of the proton-coupled ET may represent a possible mechanism for controlling biological redox reactions via changes of the transmembrane potential.