Mimicking Protein−Protein Electron Transfer: Voltammetry of Pseudomonas aeruginosa Azurin and the Thermus thermophilus CuA Domain at ω-Derivatized Self-Assembled-Monolayer Gold Electrodes

Fundamental signatures of short- and long-range electron transfer for the blue copper protein azurin at Au/SAM junctions

The blue copper protein from Pseudomonas aeruginosa, azurin, immobilized at gold electrodes through hydrophobic interaction with alkanethiol self-assembled monolayers (SAMs) of the general type [-S-(CH(2))(n)-CH(3)] (n = 4, 10, and 15) was employed to gain detailed insight into the physical mechanisms of short- and long-range biomolecular electron transfer (ET). Fast scan cyclic voltammetry and a Marcus equation analysis were used to determine unimolecular standard rate constants and reorganization free energies for variable n, temperature (2-55 degrees C), and pressure (5-150 MPa) conditions. A novel global fitting procedure was found to account for the reduced ET rate constant over almost five orders of magnitude (covering different n, temperature, and pressure) and revealed that electron exchange is a direct ET process and not conformationally gated. All the ET data, addressing SAMs with thickness variable over ca. 12 A, could be described by using a single reorganization energy (0.3 eV), however, the values for the enthalpies and volumes of activation were found to vary with n. These data and their comparison with theory show how to discriminate between the fundamental signatures of short- and long-range biomolecular ET that are theoretically anticipated for the adiabatic and nonadiabatic ET mechanisms, respectively.

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.

Fluctuations in biological and bioinspired electron-transfer reactions

Central to theories of electron transfer (ET) is the idea that nuclear motion generates a transition state that enables electron flow to proceed, but nuclear motion also induces fluctuations in the donor-acceptor (DA) electronic coupling that is the rate-limiting parameter for nonadiabatic ET. The interplay between the DA energy gap and DA coupling fluctuations is particularly noteworthy in biological ET, where flexible protein and mobile water bridges take center stage. Here, we discuss the critical timescales at play for ET reactions in fluctuating media, highlighting issues of the Condon approximation, average medium versus fluctuation-controlled electron tunneling, gated and solvent relaxation controlled electron transfer, and the influence of inelastic tunneling on electronic coupling pathway interferences. Taken together, one may use this framework to establish principles to describe how macromolecular structure and structural fluctuations influence ET reactions. This framework deepens our understanding of ET chemistry in fluctuating media. Moreover, it provides a unifying perspective for biophysical charge-transfer processes and helps to frame new questions associated with energy harvesting and transduction in fluctuating media.

Characterization of the electron transfer of a ferrocene redox probe and a histidine-tagged hemoprotein specifically bound to a nitrilotriacetic-terminated self-assembled monolayer

We report the selective, controlled binding of a model redox probe, 1,1'-bis(N-imidazolylmethyl)ferrocene (Fc-Im2), and a small redox hemoprotein, histidine-tagged recombinant human neuroglobin (hNb), at the surface of metal electrodes (gold and SER-active silver) modified by a self-assembled monolayer (SAM) of a nitrilotriacetic (NTA)-terminated thiol. The resulting SAMs were characterized by cyclic voltammetry and surface-enhanced resonance Raman (SERR) spectroscopy coupled to electrochemistry. Once specifically bounded to the Ni(II)-NTA-modified gold electrode, nearly ideal cyclic voltammetric behavior with relatively fast electron-transfer (ET) communication through the SAM was determined for the Fc-Im2 redox probe. However, no direct electron transfer could be evidenced for the hNb redox protein under the same conditions. This outcome was different from the result obtained during SERR experiments coupled to electrochemistry in which a direct electrochemical conversion of hNb immobilized on a Ni(II)-NTA-modified SER-active Ag electrode was observed. The SERR spectra of the immobilized hNb was the same as the resonance Raman spectra of the protein in homogeneous solution, allowing us to conclude that the native structure of hNb was retained upon immobilization and that the direct ET was not the result of some partial or complete protein denaturation. The long-range ET rate constant (kET) through the SAM was determined by time-resolved SERR spectroscopy. A value of kET=0.12 s(-1) was obtained, which is within the predicted range of a fully nonadiabatic ET through a SAM thickness of approximately 26 A and close to the values previously determined for analogous small redox proteins at similar long-range ET distances. A SERR spectroelectrochemical titration of the immobilized hNb was also carried out, showing both an apparent standard potential (E0') negatively shifted by 100 mV compared with hNb in solution and a gentle slope in the titration curve. These results suggest a range of chemical environments in the surroundings of the redox protein and a variety of interactions with the NTA-terminated SAM. The influence of protein immobilization on E0' is discussed together with the long-range ET rate constant and molecular orientation of the surface-immobilized hNb.

Electron transfer in peptides and proteins

Proteins and peptides use their amino acids as medium for electron-transfer reactions that occur either in single-step superexchange or in multistep hopping processes. Whereas the rate of the single-step electron transfer dramatically decreases with the distance, a hopping process is less distance dependent. Electron hopping is possible if amino acids carry oxidizable side chains, like the phenol group in tyrosine. These side chains become intermediate charge carriers. Because of the weak distance dependency of hopping processes, fast electron transfer over very long distances occurs in multistep reactions, as in the enzyme ribonucleotide reductase.

Electron Tunneling through Pseudomonas aeruginosa Azurins on SAM Gold Electrodes

Robust voltammetric responses were obtained for wild-type and Y72F/H83Q/Q107H/Y108F azurins adsorbed on CH(3)(CH(2))(n)SH:HO(CH(2))(m)SH (n=m=4,6,8,11; n=13,15 m=11) self-assembled monolayer (SAM) gold electrodes in acidic solution (pH 4.6) at high ionic strengths. Electron-transfer (ET) rates do not vary substantially with ionic strength, suggesting that the SAM methyl headgroup binds to azurin by hydrophobic interactions. The voltammetric responses for both proteins at higher pH values (>4.6 to 11) also were strong. A binding model in which the SAM hydroxyl headgroup interacts with the Asn47 carboxamide accounts for the relatively strong coupling to the copper center that can be inferred from the ET rates. Of particular interest is the finding that rate constants for electron tunneling through n = 8, 13 SAMs are higher at pH 11 than those at pH 4.6, possibly owing to enhanced coupling of the SAM to Asn 47 caused by deprotonation of nearby surface residues.

DNA-Directed Immobilization of Horseradish Peroxidase–DNA Conjugates on Microelectrode Arrays: Towards Electrochemical Screening of Enzyme Libraries

This work is aimed towards the generation of enzyme arrays on electrochemically active surfaces by taking advantage of the DNA-directed immobilization (DDI) technique. To this end, two different types of horseradish peroxidase (HRP)-DNA conjugates were prepared, either by covalent coupling with a bifunctional cross-linker or by the reconstitution of apo-HRP, that is, HRP lacking its prosthetic heme (protoporphyrin IX) group, with a covalently DNA-modified heme cofactor. Both conjugates were characterized in bulk and also subsequent to their immobilization on gold electrodes through specific DNA hybridization. Electrochemical measurements by using the phenolic mediator ortho-phenylendiamine indicated that, due to the high degree of conformational orientation, the apparent Michaelis-Menten constants of the reconstituted HRP conjugate were lower than those of the covalent conjugate. Due to the reversible nature of DDI, both conjugates could be readily removed from the electrode surface by simple washing and, subsequently, the electrodes could be reloaded with fresh enzymes, thereby restoring the initial amperometric-response activity. Moreover, the specific DNA hybridization allowed us to direct the two conjugates to distinct sites on a microelectrode array. Therefore, the self-assembly and regeneration capabilities of this approach should open the door to the generation of arrays of redox-enzyme devices for the screening of enzymes and their effectors.

Modulation of the electrochemical behavior of tyrosyl radicals by the electrode surface

The ability to adsorb proteins and enzymes on electrode surfaces enhances opportunities for studying enzyme activity and redox-based catalysis. Proteins may be bound in a chosen orientation on surfaces so that specific sites within them may be preferentially studied, but to date no systematic study of a redox moiety from solvent to electrode surface to the protein milieu has been performed. We report the redox and ionization behavior of tyrosine-cysteine, using the cysteine residue to form covalent linkages with Au and self-assembled-monolayer (SAM)-modified Au surfaces and using the tyrosine for redox activity. In addition, the same redox fragment incorporated into a protein bound to a SAM is examined. We find that directly binding the dipeptide to Au causes the greatest change in properties, while binding it to the SAM causes a slight perturbation in redox potential and a significant perturbation in pK(a). When azurin with a surface-exposed tyrosine is bound to a SAM-modified electrode, the redox potential and pK(a) of the tyrosine are nearly unperturbed from the values found for the dipeptide free in solution. Finally, quantification of the voltammetric signal indicates that tyrosine oxidation in the protein triggers the additional oxidation of another nearby amino acid.

Elucidating the mechanisms of coupled electron transfer and catalytic reactions by protein film voltammetry

Protein film voltammetry, the direct electrochemistry of redox enzymes and proteins, provides precise and comprehensive information on complicated reaction mechanisms. By controlling the driving force for a reaction (using the applied potential) and monitoring the reaction in real time (using the current), it allows thermodynamic and kinetic information to be determined simultaneously. Two challenges are inherent to protein film voltammetry: (i) to adsorb the protein or enzyme in a native and active configuration on the electrode surface, and (ii) to understand and interpret voltammetric results on both a qualitative and quantitative level, allowing mechanistic models to be proposed and rigorous experiments to test these models to be devised. This review focuses on the second of these two challenges. It describes how to use protein film voltammetry to derive mechanistic and biochemically relevant information about redox proteins and enzymes, and how to evaluate and interpret voltammetric results. Selected key studies are described in detail, to illustrate their underlying principles, strategies and physical interpretations.

Long-range interfacial electron transfer of metalloproteins based on molecular wiring assemblies

We address some physical features associated with long-range interfacial electron transfer (ET) of metalloproteins in both electrochemical and electrochemical scanning tunneling microscopy (ECSTM) configurations, which offer a brief foundation for understanding of the ET mechanisms. These features are illustrated experimentally by new developments of two systems with the blue copper protein azurin and enzyme nitrite reductase as model metalloproteins. Azurin and nitrite reductase were assembled on Au(111) surfaces by molecular wiring to establish effective electronic coupling between the redox centers in the proteins and the electrode surface for ET and biological electrocatalysis. With such assemblies, interfacial ET proceeds through chemically defined and well oriented sites and parallels biological ET. In the case of azurin, the ET properties can be characterized comprehensively and even down to the single-molecule level with direct observation of redox-gated electron tunnelling resonance. Molecular wiring using a pi-conjugated thiol is suitable for assembling monolayers of the enzyme with catalytic activity well-retained. The catalytic mechanism involves multiple-ET steps including both intramolecular and interfacial processes. Interestingly, ET appears to exhibit a substrate-gated pattern observed preliminarily in both voltammetry and ECSTM.

Metalloprotein tunnel junctions: compressional modulation of barrier height and transport mechanism

Though the incorporation of sensory or potentially-switchable biological entities into electronic devices brings with it a number of complicating issues associated with hydration, structural complexity/delicacy, and low conductance, the possibility of resolving properties of fundamental importance (such as the influence of protein fold on conductance) at a molecularly-resolved level, are exciting. Our ability to analyse charge transport through a biological macromolecule remains, though, a significant practical and theoretical challenge. Though much information can be gained by carrying out such examinations at a molecular level, there exist few methods where such controlled analyses are, in fact, feasible. Here we report on the electron transport characteristics of a blue copper metalloprotein as characterized by conductive-probe atomic force microscopy. At very low imposed force, contact resistance is high, electrical contact unstable, and the junction undergoes dielectric breakdown at 1.1-1.5 GV m(-1). At increased applied force, the current-voltage characteristics are entirely reproducible and well-described by a Simmons (non-resonant) tunnelling model. Though highly resistive, observations demonstrate the ability of the protein matrix to mediate appreciable tunnelling current. Non-resonant behaviour is consistent with observations of bias-independent tunnelling imaging. In fitting observed transport characteristics to this model, it is possible to deconvolute barrier height and length at specific experimental conditions and, specifically, to monitor the modulation of these parameters by imposed compressional force. At higher field spectroscopic features assignable to metal based density of states are reproducibly observed. These vanish in a force regime where the tunnel barrier to direct tip-sample communication decreases.

Long-range protein electron transfer observed at the single-molecule level: In situ mapping of redox-gated tunneling resonance

A biomimetic long-range electron transfer (ET) system consisting of the blue copper protein azurin, a tunneling barrier bridge, and a gold single-crystal electrode was designed on the basis of molecular wiring self-assembly principles. This system is sufficiently stable and sensitive in a quasi-biological environment, suitable for detailed observations of long-range protein interfacial ET at the nanoscale and single-molecule levels. Because azurin is located at clearly identifiable fixed sites in well controlled orientation, the ET configuration parallels biological ET. The ET is nonadiabatic, and the rate constants display tunneling features with distance-decay factors of 0.83 and 0.91 A(-1) in H(2)O and D(2)O, respectively. Redox-gated tunneling resonance is observed in situ at the single-molecule level by using electrochemical scanning tunneling microscopy, exhibiting an asymmetric dependence on the redox potential. Maximum resonance appears around the equilibrium redox potential of azurin with an on/off current ratio of approximately 9. Simulation analyses, based on a two-step interfacial ET model for the scanning tunneling microscopy redox process, were performed and provide quantitative information for rational understanding of the ET mechanism.

Self-assembled monolayers of alkanethiols on Au(111): surface structures, defects and dynamics

The surface structures, defects and dynamics of self-assembled monolayers (SAMs) on Au(111) are reviewed. In the case of the well-known c(4 x 2) and radical 3 x radical 3 R30 degrees surface structures, the present discussion is centered on the determination of the adsorption sites. A more complex scenario emerges for the striped phases, where a variety of surface structures that depends on surface coverage are described. Recently reported surface structures at non-saturation coverage show the richness of the self-assembly process. The study of surface dynamics sheds light on the relative stability of some of these surface structures. Typical defects at the alkanethiol monolayer are shown and discussed in relation to SAMs applications.