Electron Transport via Cytochrome C on Si–H Surfaces: Roles of Fe and Heme

Deciphering the Roles of Interfacial Amino Acids in Inter-Protein Charge Transport

Elucidating charge transport (CT) through proteins is critical for gaining insights into ubiquitous CT chain reactions in biological systems and developing high-performance bioelectronic devices. While intra-protein CT has been extensively studied, crucial knowledge about inter-protein CT via interfacial amino acids is still absent due to the structural complexity. Herein, by loading cytochrome c (Cyt c) on well-defined peptide self-assembled monolayers to mimic the protein-protein interface, we provide a precisely controlled platform for identifying the roles of interfacial amino acids in solid-state CT via peptide-Cyt c junctions. The terminal amino acid of peptides serves as a fine-tuning factor for both the interfacial interaction between peptides and Cyt c and the immobilized Cyt c orientation, resulting in a nearly 10-fold difference in current through peptide-Cyt c junctions with varied asymmetry. This work provides a valuable platform for studying CT across proteins and contributes to the understanding of fundamental principles governing inter-protein CT.

Ionogel-Electrode for the Study of Protein Tunnel Junctions under Physiologically Relevant Conditions

The study of charge transport through proteins is essential for understanding complicated electrochemical processes in biological activities while the reasons for the coexistence of tunneling and hopping phenomena in protein junctions still remain unclear. In this work, a flexible and conductive ionogel electrode is synthesized and is used as a top contact to form highly reproducible protein junctions. The junctions of proteins, including human serum albumin, cytochrome C and hemoglobin, show temperature-independent electron tunneling characteristics when the junctions are in solid states while with a different mechanism of temperature-dependent electron hopping when junctions are hydrated under physiologically relevant conditions. It is demonstrated that the solvent reorganization energy plays an important role in the electron-hopping process and experimentally shown that it requires ≈100 meV for electron hopping through one heme group inside a hydrated protein molecule connected between two electrodes.

Living electronics: A catalogue of engineered living electronic components

Biology leverages a range of electrical phenomena to extract and store energy, control molecular reactions and enable multicellular communication. Microbes, in particular, have evolved genetically encoded machinery enabling them to utilize the abundant redox-active molecules and minerals available on Earth, which in turn drive global-scale biogeochemical cycles. Recently, the microbial machinery enabling these redox reactions have been leveraged for interfacing cells and biomolecules with electrical circuits for biotechnological applications. Synthetic biology is allowing for the use of these machinery as components of engineered living materials with tuneable electrical properties. Herein, we review the state of such living electronic components including wires, capacitors, transistors, diodes, optoelectronic components, spin filters, sensors, logic processors, bioactuators, information storage media and methods for assembling these components into living electronic circuits.

Biotin Binding Hardly Affects Electron Transport Efficiency across Streptavidin Solid-State Junctions

The electron transport (ETp) efficiency of solid-state protein-mediated junctions is highly influenced by the presence of electron-rich organic cofactors or transition metal ions. Hence, we chose to investigate an interesting cofactor-free non-redox protein, streptavidin (STV), which has unmatched strong binding affinity for an organic small-molecule ligand, biotin, which lacks any electron-rich features. We describe for the first time meso-scale ETp via electrical junctions of STV monolayers and focus on the question of whether the rate of ETp across both native and thiolated STV monolayers is influenced by ligand binding, a process that we show to cause some structural conformation changes in the STV monolayers. Au nanowire-electrode-protein monolayer-microelectrode junctions, fabricated by modifying an earlier procedure to improve the yields of usable junctions, were employed for ETp measurements. Our results on compactly integrated, dense, uniform, ∼3 nm thick STV monolayers indicate that, notwithstanding the slight structural changes in the STV monolayers upon biotin binding, there is no statistically significant conductance change between the free STV and that bound to biotin. The ETp temperature (T) dependence over the 80-300 K range is very small but with an unusual, slightly negative (metallic-like) dependence toward room temperature. Such dependence can be accounted for by the reversible structural shrinkage of the STV at temperatures below 160 K.

Assessing Thermal Response of Redox Conduction for Anti-Arrhenius Kinetics in a Microbial Cytochrome Nanowire

A micrometers-long helical homopolymer of the outer-membrane cytochrome type S (OmcS) from Geobacter sulfurreducens is proposed to transport electrons to extracellular acceptors in an ancient respiratory strategy of biogeochemical and technological significance. OmcS surprisingly exhibits higher conductivity upon cooling (anti-Arrhenius kinetics), an effect previously attributed to H-bond restructuring and heme redox potential shifts. Herein, the temperature sensitivity of redox conductivity is more thoroughly examined with conventional and constant-redox and -pH molecular dynamics and quantum mechanics/molecular mechanics. A 30 K drop in temperature constituted a weak perturbation to electron transfer energetics, changing electronic couplings (⟨H_mn⟩), reaction free energies (ΔG_mn), reorganization energies (λ_mn), and activation energies (E_a) by at most |0.002|, |0.050|, |0.120|, and |0.045| eV, respectively. Changes in ΔG_mn reflected -0.07 ± 0.03 V shifts in redox potentials that were caused in roughly equal measure by altered electrostatic interactions with the solvent and protein. Changes in intraprotein H-bonding reproduced the earlier observations. Single-particle diffusion and multiparticle steady-state flux models, parametrized with Marcus theory rates, showed that biologically relevant incoherent hopping cannot qualitatively or quantitatively describe electrical conductivity measured by atomic force microscopy in filamentous OmcS. The discrepancy is attributed to differences between solution-phase simulations and solid-state measurements and the need to model intra- and intermolecular vibrations explicitly.

The Role of Structural Order in the Mechanism of Charge Transport across Tunnel Junctions with Various Iron-Storing Proteins

In biomolecular electronics, the role of structural order in charge transport (CT) is poorly understood. It has been reported that the metal oxide cores of protein cages (e.g., iron oxide and ferrihydrite nanoparticles (NPs) present in ferritin and E2-LFtn, which is E2 protein engineered with an iron-binding sequence) play an important role in the mechanism of CT. At the same time, the NP core also plays a major role in the structural integrity of the proteins. This paper describes the role of structural order in CT across tunnel junctions by comparing three iron-storing proteins. They are (1) DNA binding protein from starved cells (Dps, diameter (∅) = 9 nm); (2) engineered archaeal ferritin (AfFtn-AA, ∅ = 12 nm); and (3) engineered E2 of pyruvate dehydrogenase enzyme complex (E2-LFtn, ∅ = 25 nm). Both holo-Dps and apo-Dps proteins undergo CT by coherent tunneling because their globular architecture and relative structural stability provide a coherent conduction pathway. In contrast, apo-AfFtn-AA forms a disordered structure across which charges have to tunnel incoherently, but holo-AfFtn-AA retains its globular structure and supports coherent tunneling. The large E2-LFtn always forms disordered structures across which charges incoherently tunnel regardless of the presence of the NP core. These findings highlight the importance of structural order in the mechanism of CT across biomolecular tunnel junctions.

Conformation- and phosphorylation-dependent electron tunnelling across self-assembled monolayers of tau peptides

We report on charge transport across self-assembled monolayers (SAMs) of short tau peptides by probing the electron tunneling rates and quantum mechanical simulation. We measured the electron tunneling rates across SAMs of carboxyl-terminated linker molecules (C₆H₁₂O₂S) and short cis-tau (CT) and trans-tau (TT) peptides, supported on template-stripped gold (Au^TS) bottom electrode, with Eutectic Gallium-Indium (EGaIn)(EGaIn) top electrode. Measurements of the current density across thousands of Au^TS/linker/tau//Ga₂O₃/EGaIn single-molecule junctions show that the tunneling current across CT peptide is one order of magnitude lower than that of TT peptide. Quantum mechanical simulation demonstrated a wider energy bandgap of the CT peptide, as compared to the TT peptide, which causes a reduction in its electron tunneling current. Our findings also revealed the critical role of phosphorylation in altering the charge transport characteristics of short peptides; more specifically, we found that the presence of phosphate groups can reduce the energy band gap in tau peptides and alter their electrical properties. Our results suggest that conformational and phosphorylation of short peptides (e.g., tau) can significantly change their charge transport characteristics and energy levels.

Electrochemical Potential-Driven High-Throughput Molecular Electronic and Spintronic Devices: From Molecules to Applications

Molecules are fascinating candidates for constructing tunable and electrically conducting devices by the assembly of either a single molecule or an ensemble of molecules between two electrical contacts followed by current-voltage (I-V) analysis, which is often termed "molecular electronics". Recently, there has been also an upsurge of interest in spin-based electronics or spintronics across the molecules, which offer additional scope to create ultrafast responsive devices with less power consumption and lower heat generation using the intrinsic spin property rather than electronic charge. Researchers have been exploring this idea of utilizing organic molecules, organometallics, coordination complexes, polymers, and biomolecules (proteins, enzymes, oligopeptides, DNA) in integrating molecular electronics and spintronics devices. Although several methods exist to prepare molecular thin-films on suitable electrodes, the electrochemical potential-driven technique has emerged as highly efficient. In this Review we describe recent advances in the electrochemical potential driven growth of nanometric various molecular films on technologically relevant substrates, including non-magnetic and magnetic electrodes to investigate the stimuli-responsive charge and spin transport phenomena.

The porphyrin ring rather than the metal ion dictates long-range electron transport across proteins suggesting coherence-assisted mechanism

The fundamental biological process of electron transfer (ET) takes place across proteins with common ET pathways of several nanometers. Recent discoveries push this limit and show long-range extracellular ET over several micrometers. Here, we aim in deciphering how protein-bound intramolecular cofactors can facilitate such long-range ET. In contrast to natural systems, our protein-based platform enables us to modulate important factors associated with ET in a facile manner, such as the type of the cofactor and its quantity within the protein. We choose here the biologically relevant protoporphyrin molecule as the electron mediator. Unlike natural systems having only Fe-containing protoporphyrins, i.e., heme, as electron mediators, we use here porphyrins with different metal centers, or lacking a metal center. We show that the metal redox center has no role in ET and that ET is mediated solely by the conjugated backbone of the molecule. We further discuss several ET mechanisms, accounting to our observations with possible contribution of coherent processes. Our findings contribute to our understanding of the participation of heme molecules in long-range biological ET.

Unidirectional electron injection and accelerated proton transport in bacteriorhodopsin based Bio-p-n junctions

Hampered by the absence of evidence and theoretical model of biological semiconductors, the unidirectional electron transport via the p-n junction between functional proteins and abiotic materials remains a challenge for bioelectronics. Bacteriorhodopsin (bR), a representative transmembrane protein, has demonstrated exceptional optoelectronic effects in bR/semiconductor hybrid materials and offers a possible pathway for addressing this challenge. In the present work, for the first time, bR is proved to be an n-type semiconductor with an indirect electron transition. Through the photo-electrochemical method used for studying the p-n junction effect in the bR and p-type semiconductor combined electrodes, we reached several important conclusions: The self-corrosion of bR integrated Cu2O electrodes is delayed for about 36 times; The photocurrent of bR integrated CuSCN electrodes is enhanced by about 400%, which is attributed to the directional migration of electrons via the p-n junction. Furthermore, the ultrafast kinetics we have explored, shows that the injection of electrons shortens the lifetime of the intermediate state O640 from 37.3 μs to 20.1 μs, what means that the protons transport rate accompanying the bR photocycle process is accelerated. Therefore, we believe that the concept of the bio-p-n junction and the mechanism of electron coupled proton transport, which are discussed herein, will promote useful research on bioelectronic applications for bR and its homologs.

Dynamic relaying properties of a β-turn peptide in long-range electron transfer

The relay stations play a significant role in long-range charge hopping transfer in proteins. Although studies have clarified that many more protein structural motifs can function as relays in charge hopping transfers by acting as intermediate charge carriers, the relaying properties are still poorly understood. In this work, taking a β-turn oligopeptide as an example, we report a dynamic character of a relay with tunable relaying properties using the density functional theory calculations. Our main finding is that a β-turn peptide can serve as an effective electron relay in facilitating long-range electron migration and its relay properties is vibration-tunable. The vibration-induced structural transient distortions remarkably affect the lowest occupied molecular orbital (LUMO) energy, vertical electron affinity and electron-binding mode of the β-turn oligopeptide and the singly occupied molecular orbital (SOMO) energy of the corresponding electron adduct and thus the relaying properties. Different vibration modes lead to different structural distortions and thus have different effects on the relaying properties and ability of the β-turn peptide. For the relaying properties, there approximately is a linear negative correlation of electron affinity with the LUMO energy of the β-turn or the SOMO energy of its electron adduct. Besides, such relaying properties also vary in the vibration evolution process, and the electron-binding modes may be tunable. As an important addition to the known static charge relaying properties occurring in various protein structural motifs, this work reports the dynamic electron-relaying characteristics of a β-turn oligopeptide with variable relaying properties governed by molecular vibrations which can be applied to different proteins in mediating long-range charge transfers. Clearly, this work reveals molecular vibration effects on the electron relaying properties of protein structural motifs and provides new insights into the dynamics of long-range charge transfers in proteins. © 2018 Wiley Periodicals, Inc.

Tunneling explains efficient electron transport via protein junctions

Metalloproteins, proteins containing a transition metal ion cofactor, are electron transfer agents that perform key functions in cells. Inspired by this fact, electron transport across these proteins has been widely studied in solid-state settings, triggering the interest in examining potential use of proteins as building blocks in bioelectronic devices. Here, we report results of low-temperature (10 K) electron transport measurements via monolayer junctions based on the blue copper protein azurin (Az), which strongly suggest quantum tunneling of electrons as the dominant charge transport mechanism. Specifically, we show that, weakening the protein-electrode coupling by introducing a spacer, one can switch the electron transport from off-resonant to resonant tunneling. This is a consequence of reducing the electrode's perturbation of the Cu(II)-localized electronic state, a pattern that has not been observed before in protein-based junctions. Moreover, we identify vibronic features of the Cu(II) coordination sphere in transport characteristics that show directly the active role of the metal ion in resonance tunneling. Our results illustrate how quantum mechanical effects may dominate electron transport via protein-based junctions.

Protein bioelectronics: a review of what we do and do not know

We review the status of protein-based molecular electronics. First, we define and discuss fundamental concepts of electron transfer and transport in and across proteins and proposed mechanisms for these processes. We then describe the immobilization of proteins to solid-state surfaces in both nanoscale and macroscopic approaches, and highlight how different methodologies can alter protein electronic properties. Because immobilizing proteins while retaining biological activity is crucial to the successful development of bioelectronic devices, we discuss this process at length. We briefly discuss computational predictions and their connection to experimental results. We then summarize how the biological activity of immobilized proteins is beneficial for bioelectronic devices, and how conductance measurements can shed light on protein properties. Finally, we consider how the research to date could influence the development of future bioelectronic devices.

Solid-state electrical applications of protein and peptide based nanomaterials

This review summarizes recent advancements in electrical properties and applications of natural proteins and mutated variants, synthetic oligopeptides and peptide–π conjugates.

Photoswitchable Sn-Cyt c Solid-State Devices

Electron transfer across proteins plays an important role in many biological processes, including those relevant for the conversion of solar photons to chemical energy. Previous studies demonstrated the generation of photocurrents upon light irradiation in a number of photoactive proteins, such as photosystem I or bacteriorhodopsin. Here, it is shown that Sn-cytochrome c layers act as reversible and efficient photoelectrochemical switches upon integration into large-area solid-state junctions. Photocurrents are observed both in the Soret band (λ = 405 nm) and in the Q band (λ = 535 nm), with current on/off ratios reaching values of up to 25. The underlying modulation in charge-transfer rate is attributed to a hole-transport channel created by the photoexcitation of the Sn-porphyrin.

Biofunctionalisation of p-doped silicon with cytochrome c553minimises charge recombination and enhances photovoltaic performance of the all-solid-state photosystem I-based biophotoelectrode

Passivation of p-doped silicon substrate was achieved by its biofunctionalisation with hexahistidine-tagged cytochrome c₅₅₃, a soluble electroactive photosynthetic protein responsible for electron donation to photooxidised photosystem I.

Effects of Film Morphology and Surface Chemistry on the Direct Electrochemistry of Cytochrome c at Boron-Doped Diamond Electrodes

The effects of film morphology and surface termination on the direct electron transfer of horse heart cytochrome c on boron-doped ultrananocrystalline (B-UNCD) and microcrystalline (B-MCD) diamond thin-film electrodes were investigated. Quasi-reversible, diffusion-controlled cyclic voltammetric responses were observed on oxygen-terminated (atomic O/C ~0.015), but not hydrogen-terminated (atomic O/C ~0.02) diamond thin films. The effect of the surface termination was the same for both the nanostructured B-UNCD film with sp2-bonded carbon atoms in the grain boundaries and the well faceted B-MCD film with micron-sized grains and largely devoid of sp2 carbon. Stable cyclic voltammetric i-E curves were recorded with cycling for both oxygen-terminated films indicating the absence of protein denaturation and electrode fouling. The peak currents increased linearly with the square root of the scan rate and the protein concentration; both indicative of a reaction rate limited by semi-infinite linear diffusion of the protein. Similar heterogeneous electron-transfer rate constants were observed for oxygen-terminated B-UNCD (3.48 (± 1.25) × 10-3 cm/s) and B-MCD films (2.38 (± 0.72) × 10-3 cm/s). The results clearly reveal that the oxygen-terminated surface is more active for electron-transfer with this soluble redox protein than is the hydrogen-terminated surface. The film morphology does not influence the diffusion-controlled response of the redox protein.

Towards nanometer-spaced silicon contacts to proteins

A vertical nanogap device (VND) structure comprising all-silicon contacts as electrodes for the investigation of electronic transport processes in bioelectronic systems is reported. Devices were fabricated from silicon-on-insulator substrates whose buried oxide (SiO2) layer of a few nanometers in thickness is embedded within two highly doped single crystalline silicon layers. Individual VNDs were fabricated by standard photolithography and a combination of anisotropic and selective wet etching techniques, resulting in p(+) silicon contacts, vertically separated by 4 or 8 nm, depending on the chosen buried oxide thickness. The buried oxide was selectively recess-etched with buffered hydrofluoric acid, exposing a nanogap. For verification of the devices' electrical functionality, gold nanoparticles were successfully trapped onto the nanogap electrodes' edges using AC dielectrophoresis. Subsequently, the suitability of the VND structures for transport measurements on proteins was investigated by functionalizing the devices with cytochrome c protein from solution, thereby providing non-destructive, permanent semiconducting contacts to the proteins. Current-voltage measurements performed after protein deposition exhibited an increase in the junctions' conductance of up to several orders of magnitude relative to that measured prior to cytochrome c immobilization. This increase in conductance was lost upon heating the functionalized device to above the protein's denaturation temperature (80 °C). Thus, the VND junctions allow conductance measurements which reflect the averaged electronic transport through a large number of protein molecules, contacted in parallel with permanent contacts and, for the first time, in a symmetrical Si-protein-Si configuration.

Long-Range Tunneling Processes across Ferritin-Based Junctions

The mechanism of long-range charge transport across tunneling junctions with monolayers of ferritin is investigated. It is shown that the mechanism can be switched between coherent tunneling, sequential tunneling, and hopping by changing the iron content inside the ferritin. This study shows that ferritins are an interesting class of biomolecules to control charge transport.

Porphyrins at interfaces

Porphyrins and other tetrapyrrole macrocycles possess an impressive variety of functional properties that have been exploited in natural and artificial systems. Different metal centres incorporated within the tetradentate ligand are key for achieving and regulating vital processes, including reversible axial ligation of adducts, electron transfer, light-harvesting and catalytic transformations. Tailored substituents optimize their performance, dictating their arrangement in specific environments and mediating the assembly of molecular nanoarchitectures. Here we review the current understanding of these species at well-defined interfaces, disclosing exquisite insights into their structural and chemical properties, and also discussing methods by which to manipulate their intramolecular and organizational features. The distinct characteristics arising from the interfacial confinement offer intriguing prospects for molecular science and advanced materials. We assess the role of surface interactions with respect to electronic and physicochemical characteristics, and describe in situ metallation pathways, molecular magnetism, rotation and switching. The engineering of nanostructures, organized layers, interfacial hybrid and bio-inspired systems is also addressed.

Electron Transfer Proteins as Electronic Conductors: Significance of the Metal and Its Binding Site in the Blue Cu Protein, Azurin

Electron transfer (ET) proteins are biomolecules with specific functions, selected by evolution. As such they are attractive candidates for use in potential bioelectronic devices. The blue copper protein azurin (Az) is one of the most-studied ET proteins. Traditional spectroscopic, electrochemical, and kinetic methods employed for studying ET to/from the protein's Cu ion have been complemented more recently by studies of electrical conduction through a monolayer of Az in the solid-state, sandwiched between electrodes. As the latter type of measurement does not require involvement of a redox process, it also allows monitoring electronic transport (ETp) via redox-inactive Az-derivatives. Here, results of macroscopic ETp via redox-active and -inactive Az derivatives, i.e., Cu(II) and Cu(I)-Az, apo-Az, Co(II)-Az, Ni(II)-Az, and Zn(II)-Az are reported and compared. It is found that earlier reported temperature independence of ETp via Cu(II)-Az (from 20 K until denaturation) is unique, as ETp via all other derivatives is thermally activated at temperatures >≈200 K. Conduction via Cu(I)-Az shows unexpected temperature dependence >≈200 K, with currents decreasing at positive and increasing at negative bias. Taking all the data together we find a clear compensation effect of Az conduction around the Az denaturation temperature. This compensation can be understood by viewing the Az binding site as an electron trap, unless occupied by Cu(II), as in the native protein, with conduction of the native protein setting the upper transport efficiency limit.

Electronic transport via proteins

A central vision in molecular electronics is the creation of devices with functional molecular components that may provide unique properties. Proteins are attractive candidates for this purpose, as they have specific physical (optical, electrical) and chemical (selective binding, self-assembly) functions and offer a myriad of possibilities for (bio-)chemical modification. This Progress Report focuses on proteins as potential building components for future bioelectronic devices as they are quite efficient electronic conductors, compared with saturated organic molecules. The report addresses several questions: how general is this behavior; how does protein conduction compare with that of saturated and conjugated molecules; and what mechanisms enable efficient conduction across these large molecules? To answer these questions results of nanometer-scale and macroscopic electronic transport measurements across a range of organic molecules and proteins are compiled and analyzed, from single/few molecules to large molecular ensembles, and the influence of measurement methods on the results is considered. Generalizing, it is found that proteins conduct better than saturated molecules, and somewhat poorer than conjugated molecules. Significantly, the presence of cofactors (redox-active or conjugated) in the protein enhances their conduction, but without an obvious advantage for natural electron transfer proteins. Most likely, the conduction mechanisms are hopping (at higher temperatures) and tunneling (below ca. 150-200 K).

Solid-state electron transport via cytochrome c depends on electronic coupling to electrodes and across the protein

Electronic coupling to electrodes, Γ, as well as that across the examined molecules, H, is critical for solid-state electron transport (ETp) across proteins. Assessing the importance of each of these couplings helps to understand the mechanism of electron flow across molecules. We provide here experimental evidence for the importance of both couplings for solid-state ETp across the electron-mediating protein cytochrome c (CytC), measured in a monolayer configuration. Currents via CytC are temperature-independent between 30 and ∼130 K, consistent with tunneling by superexchange, and thermally activated at higher temperatures, ascribed to steady-state hopping. Covalent protein-electrode binding significantly increases Γ, as currents across CytC mutants, bound covalently to the electrode via a cysteine thiolate, are higher than those through electrostatically adsorbed CytC. Covalent binding also reduces the thermal activation energy, Ea, of the ETp by more than a factor of two. The importance of H was examined by using a series of seven CytC mutants with cysteine residues at different surface positions, yielding distinct electrode-protein(-heme) orientations and separation distances. We find that, in general, mutants with electrode-proximal heme have lower Ea values (from high-temperature data) and higher conductance at low temperatures (in the temperature-independent regime) than those with a distal heme. We conclude that ETp across these mutants depends on the distance between the heme group and the top or bottom electrode, rather than on the total separation distance between electrodes (protein width).