Electronic Conductance Resonance in Non-Redox-Active Proteins

Single-Molecule Electrical Profiling of Peptides and Proteins

In recent decades, there has been a significant increase in the application of single-molecule electrical analysis platforms in studying proteins and peptides. These advanced analysis methods have the potential for deep investigation of enzymatic working mechanisms and accurate monitoring of dynamic changes in protein configurations, which are often challenging to achieve in ensemble measurements. In this work, the prominent research progress in peptide and protein-related studies are surveyed using electronic devices with single-molecule/single-event sensitivity, including single-molecule junctions, single-molecule field-effect transistors, and nanopores. In particular, the successful commercial application of nanopores in DNA sequencing has made it one of the most promising techniques in protein sequencing at the single-molecule level. From single peptides to protein complexes, the correlation between their electrical characteristics, structures, and biological functions is gradually being established. This enables to distinguish different molecular configurations of these biomacromolecules through real-time electrical monitoring of their life activities, significantly improving the understanding of the mechanisms underlying various life processes.

Hierarchical consciousness: the Nested Observer Windows model

Foremost in our experience is the intuition that we possess a unified conscious experience. However, many observations run counter to this intuition: we experience paralyzing indecision when faced with two appealing behavioral choices, we simultaneously hold contradictory beliefs, and the content of our thought is often characterized by an internal debate. Here, we propose the Nested Observer Windows (NOW) Model, a framework for hierarchical consciousness wherein information processed across many spatiotemporal scales of the brain feeds into subjective experience. The model likens the mind to a hierarchy of nested mosaic tiles-where an image is composed of mosaic tiles, and each of these tiles is itself an image composed of mosaic tiles. Unitary consciousness exists at the apex of this nested hierarchy where perceptual constructs become fully integrated and complex behaviors are initiated via abstract commands. We define an observer window as a spatially and temporally constrained system within which information is integrated, e.g. in functional brain regions and neurons. Three principles from the signal analysis of electrical activity describe the nested hierarchy and generate testable predictions. First, nested observer windows disseminate information across spatiotemporal scales with cross-frequency coupling. Second, observer windows are characterized by a high degree of internal synchrony (with zero phase lag). Third, observer windows at the same spatiotemporal level share information with each other through coherence (with non-zero phase lag). The theoretical framework of the NOW Model accounts for a wide range of subjective experiences and a novel approach for integrating prominent theories of consciousness.

Dynamical control of nanoscale light-matter interactions in low-dimensional quantum materials

Tip-enhanced nano-spectroscopy and -imaging have significantly advanced our understanding of low-dimensional quantum materials and their interactions with light, providing a rich insight into the underlying physics at their natural length scale. Recently, various functionalities of the plasmonic tip expand the capabilities of the nanoscopy, enabling dynamic manipulation of light-matter interactions at the nanoscale. In this review, we focus on a new paradigm of the nanoscopy, shifting from the conventional role of imaging and spectroscopy to the dynamical control approach of the tip-induced light-matter interactions. We present three different approaches of tip-induced control of light-matter interactions, such as cavity-gap control, pressure control, and near-field polarization control. Specifically, we discuss the nanoscale modifications of radiative emissions for various emitters from weak to strong coupling regime, achieved by the precise engineering of the cavity-gap. Furthermore, we introduce recent works on light-matter interactions controlled by tip-pressure and near-field polarization, especially tunability of the bandgap, crystal structure, photoluminescence quantum yield, exciton density, and energy transfer in a wide range of quantum materials. We envision that this comprehensive review not only contributes to a deeper understanding of the physics of nanoscale light-matter interactions but also offers a valuable resource to nanophotonics, plasmonics, and materials science for future technological advancements.

The Protein Matrix of Plastocyanin Supports Long-Distance Charge Transport with Photosystem I and the Copper Ion Regulates Its Spatial Span and Conductance

Charge exchange is the fundamental process that sustains cellular respiration and photosynthesis by shuttling electrons in a cascade of electron transfer (ET) steps between redox cofactors. While intraprotein charge exchange is well characterized in protein complexes bearing multiple redox sites, interprotein processes are less understood due to the lack of suitable experimental approaches and the dynamic nature of the interactions. Proteins constrained between electrodes are known to support electron transport (ETp) through the protein matrix even without redox cofactors, as the charges housed by the redox sites in ET are furnished by the electrodes. However, it is unknown whether protein ETp mechanisms apply to the interprotein medium present under physiological conditions. We study interprotein charge exchange between plant photosystem I (PSI) and its soluble redox partner plastocyanin (Pc) and address the role of the Pc copper center. Using electrochemical scanning tunneling spectroscopy (ECSTS) current-distance and blinking measurements, we quantify the spatial span of charge exchange between individual Pc/PSI pairs and ETp through transient Pc/PSI complexes. Pc devoid of the redox center (Pcapo) can exchange charge with PSI at longer distances than with the copper ion (Pcholo). Conductance bursts associated with Pcapo/PSI complex formation are higher than in Pcholo/PSI. Thus, copper ions are not required for long-distance Pc/PSI ETp but regulate its spatial span and conductance. Our results suggest that the redox center that carries the charge in Pc is not necessary to exchange it in interprotein ET through the aqueous solution and question the canonical view of tight complex binding between redox protein partners.

Electrochemistry in sensing of molecular interactions of proteins and their behavior in an electric field

Electrochemical methods can be used not only for the sensitive analysis of proteins but also for deeper research into their structure, transport functions (transfer of electrons and protons), and sensing their interactions with soft and solid surfaces. Last but not least, electrochemical tools are useful for investigating the effect of an electric field on protein structure, the direct application of electrochemical methods for controlling protein function, or the micromanipulation of supramolecular protein structures. There are many experimental arrangements (modalities), from the classic configuration that works with an electrochemical cell to miniaturized electrochemical sensors and microchip platforms. The support of computational chemistry methods which appropriately complement the interpretation framework of experimental results is also important. This text describes recent directions in electrochemical methods for the determination of proteins and briefly summarizes available methodologies for the selective labeling of proteins using redox-active probes. Attention is also paid to the theoretical aspects of electron transport and the effect of an external electric field on the structure of selected proteins. Instead of providing a comprehensive overview, we aim to highlight areas of interest that have not been summarized recently, but, at the same time, represent current trends in the field.

Heavy Water Reduces the Electronic Conductance of Protein Wires via Deuteron Interactions with Aromatic Residues

Proteins are versatile, self-assembling nanoelectronic components, but their hopping conductivity is expected to be influenced by solvent fluctuations. The role of the solvent was investigated by measuring the single molecule conductance of several proteins in both H2O and D2O. The conductance of a homologous series of protein wires decreases more rapidly with length in D2O, indicating a 6-fold decrease in carrier diffusion constant relative to the same protein in H2O. The effect was found to depend on the specific aromatic amino acid composition. A tryptophan zipper protein showed a decrease in conductance similar to that of the protein wires, whereas a phenylalanine zipper protein was insensitive to solvent changes. Tryptophan contains an indole amine, whereas the phenylalanine aromatic ring has no exchangeable protons, so the effect of heavy water on conductance is a consequence of specific D- or H-interactions with the aromatic residues.

Long-Range Conductivity in Proteins Mediated by Aromatic Residues

Single-molecule measurements show that many proteins, lacking any redox cofactors, nonetheless exhibit electrical conductance on the order of a nanosiemen over 10 nm distances, implying that electrons can transit an entire protein in less than a nanosecond when subject to a potential difference of less than 1 V. This is puzzling because, for fast transport (i.e., a free energy barrier of zero), the hopping rate is determined by the reorganization energy of approximately 0.8 eV, and this sets the time scale of a single hop to at least 1 μs. Furthermore, the Fermi energies of typical metal electrodes are far removed from the energies required for sequential oxidation and reduction of the aromatic residues of the protein, which should further reduce the hopping current. Here, we combine all-atom molecular dynamics (MD) simulations of non-redox-active proteins (consensus tetratricopeptide repeats) with an electron transfer theory to demonstrate a molecular mechanism that can account for the unexpectedly fast electron transport. According to our MD simulations, the reorganization energy produced by the energy shift on charging (the Stokes shift) is close to the conventional value of 0.8 eV. However, the non-ergodic sampling of molecular configurations by the protein results in reaction-reorganization energies, extracted directly from the distribution of the electrostatic energy fluctuations, that are only ∼0.2 eV, which is small enough to enable long-range conductivity, without invoking quantum coherent transport. Using the MD values of the reorganization energies, we calculate a current decay with distance that is in agreement with experiment.

Control of charge transport in electronically active systems towards integrated biomolecular circuits (IbC)

The miniaturization of traditional silicon-based electronics will soon reach its limitation as quantum tunneling and heat become serious problems at the several-nanometer scale. Crafting integrated circuits via self-assembly of electronically active molecules using a "bottom-up" paradigm provides a potential solution to these technological challenges. In particular, integrated biomolecular circuits (IbC) offer promising advantages to achieve this goal, as nature offers countless examples of functionalities entailed by self-assembly and examples of controlling charge transport at the molecular level within the self-assembled structures. To this end, the review summarizes the progress in understanding how charge transport is regulated in biosystems and the key redox-active amino acids that enable the charge transport. In addition, charge transport mechanisms at different length scales are also reviewed, offering key insights for controlling charge transport in IbC in the future.

Single-molecular protein-based bioelectronics via electronic transport: fundamentals, devices and applications

Biomolecular electronics is a rapidly growing multidisciplinary field that combines biology, nanoscience, and engineering to bridge the two important fields of life sciences and molecular electronics. Proteins are remarkable for their ability to recognize molecules and transport electrons, making the integration of proteins into electronic devices a long sought-after goal and leading to the emergence of the field of protein-based bioelectronics, also known as proteotronics. This field seeks to design and create new biomolecular electronic platforms that allow for the understanding and manipulation of protein-mediated electronic charge transport and related functional applications. In recent decades, there have been numerous reports on protein-based bioelectronics using a variety of nano-gapped electrical devices and techniques at the single molecular level, which are not achievable with conventional ensemble approaches. This review focuses on recent advances in physical electron transport mechanisms, device fabrication methodologies, and various applications in protein-based bioelectronics. We discuss the most recent progress of the single or few protein-bridged electrical junction fabrication strategies, summarise the work on fundamental and functional applications of protein bioelectronics that enable high and dynamic electron transport, and highlight future perspectives and challenges that still need to be addressed. We believe that this specific review will stimulate the interdisciplinary research of topics related to protein-related bioelectronics, and open up new possibilities for single-molecule biophysics and biomedicine.

Reorganization energy of electron transfer

The theory of electron transfer reactions establishes the conceptual foundation for redox solution chemistry, electrochemistry, and bioenergetics. Electron and proton transfer across the cellular membrane provide all energy of life gained through natural photosynthesis and mitochondrial respiration. Rates of biological charge transfer set kinetic bottlenecks for biological energy storage. The main system-specific parameter determining the activation barrier for a single electron-transfer hop is the reorganization energy of the medium. Both harvesting of light energy in natural and artificial photosynthesis and efficient electron transport in biological energy chains require reduction of the reorganization energy to allow fast transitions. This review article discusses mechanisms by which small values of the reorganization energy are achieved in protein electron transfer and how similar mechanisms can operate in other media, such as nonpolar and ionic liquids. One of the major mechanisms of reorganization energy reduction is through non-Gibbsian (nonergodic) sampling of the medium configurations on the reaction time. A number of alternative mechanisms, such as electrowetting of active sites of proteins, give rise to non-parabolic free energy surfaces of electron transfer. These mechanisms, and nonequilibrium population of donor-acceptor vibrations, lead to a universal phenomenology of separation between the Stokes shift and variance reorganization energies of electron transfer.

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.

Multifrequency dielectric mapping of fixed mice colon tissues in cell culture media via scanning electrochemical microscopy

Alternating current scanning electrochemical microscopy (AC-SECM) is a powerful tool for characterizing the electrochemical reactivity of surfaces. Here, perturbation in the sample is induced by the alternating current and altered local potential is measured by the SECM probe. This technique has been used to investigate many exotic a range of biological interfaces including live cells and tissues, as well as the corrosive degradation of various metallic surfaces, etc. In principle, AC-SECM imaging is derived from electrochemical impedance spectroscopy (EIS) which has been used for a century to describe interfacial and diffusive behaviour of molecules in solution or on a surface. Increasingly bioimpedance centric medical devices have become an important tool to detect evolution of tissue biochemistry. Predictive implications of measuring electrochemical changes within a tissue is one of the core concepts in developing minimally invasive and smart medical devices. In this study, cross sections of mice colon tissue were used for AC-SECM imaging. A 10 micron sized platinum probe was used for two-dimensional (2D) tan δ mapping of histological sections at a frequency of 10 kHz, Thereafter, multifrequency scans were performed at 100 Hz, 10 kHz, 300 kHz, and 900 kHz. Loss tangent (tan δ) mapping of mice colon revealed microscale regions within a tissue possessing a discrete tan δ signature. This tan δ map may be an immediate measure of physiological conditions in biological tissues. Multifrequency scans highlight subtle changes in protein or lipid composition as a function of frequency which was recorded as loss tangent maps. Impedance profile at different frequencies could also be used to identify optimal contrast for imaging and extracting the electrochemical signature specific for a tissue and its electrolyte.

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.

Electron transport via tyrosine-doped oligo-alanine peptide junctions: role of charges and hydrogen bonding

A way of modulating the solid-state electron transport (ETp) properties of oligopeptide junctions is presented by charges and internal hydrogen bonding, which affect this process markedly. The ETp properties of a series of tyrosine (Tyr)-containing hexa-alanine peptides, self-assembled in monolayers and sandwiched between gold electrodes, are investigated in response to their protonation state. Inserting a Tyr residue into these peptides enhances the ETp carried via their junctions. Deprotonation of the Tyr-containing peptides causes a further increase of ETp efficiency that depends on this residue's position. Combined results of molecular dynamics simulations and spectroscopic experiments suggest that the increased conductance upon deprotonation is mainly a result of enhanced coupling between the charged C-terminus carboxylate group and the adjacent Au electrode. Moreover, intra-peptide hydrogen bonding of the Tyr hydroxyl to the C-terminus carboxylate reduces this coupling. Hence, the extent of such a conductance change depends on the Tyr-carboxylate distance in the peptide's sequence.

A Scalable CMOS Molecular Electronics Chip for Single-Molecule Biosensing

This work reports the first CMOS molecular electronics chip. It is configured as a biosensor, where the primary sensing element is a single molecule "molecular wire" consisting of a ∼100 GΩ, 25 nm long alpha-helical peptide integrated into a current monitoring circuit. The engineered peptide contains a central conjugation site for attachment of various probe molecules, such as DNA, proteins, enzymes, or antibodies, which program the biosensor to detect interactions with a specific target molecule. The current through the molecular wire under a dc applied voltage is monitored with millisecond temporal resolution. The detected signals are millisecond-scale, picoampere current pulses generated by each transient probe-target molecular interaction. Implemented in a 0.18 μm CMOS technology, 16k sensors are arrayed with a 20 μm pitch and read out at a 1 kHz frame rate. The resulting biosensor chip provides direct, real-time observation of the single-molecule interaction kinetics, unlike classical biosensors that measure ensemble averages of such events. This molecular electronics chip provides a platform for putting molecular biosensing "on-chip" to bring the power of semiconductor chips to diverse applications in biological research, diagnostics, sequencing, proteomics, drug discovery, and environmental monitoring.

Measuring conductance switching in single proteins using quantum tunneling

Interpreting the electrical signatures of single proteins in electronic junctions has facilitated a better understanding of the intrinsic properties of proteins that are fundamental to chemical and biological processes. Often, this information is not accessible using ensemble and even single-molecule approaches. In addition, the fabrication of nanoscale single-protein junctions remains challenging as they often require sophisticated methods. We report on the fabrication of tunneling probes, direct measurement, and active control (switching) of single-protein conductance with an external field in solution. The probes allowed us to bridge a single streptavidin molecule to two independently addressable, biotin-terminated electrodes and measure single-protein tunneling response over long periods. We show that charge transport through the protein has multiple conductive pathways that depend on the magnitude of the applied bias. These findings open the door for the reliable fabrication of protein-based junctions and can enable their use in future protein-embedded bioelectronics applications.

A Chirality-Based Quantum Leap

There is increasing interest in the study of chiral degrees of freedom occurring in matter and in electromagnetic fields. Opportunities in quantum sciences will likely exploit two main areas that are the focus of this Review: (1) recent observations of the chiral-induced spin selectivity (CISS) effect in chiral molecules and engineered nanomaterials and (2) rapidly evolving nanophotonic strategies designed to amplify chiral light-matter interactions. On the one hand, the CISS effect underpins the observation that charge transport through nanoscopic chiral structures favors a particular electronic spin orientation, resulting in large room-temperature spin polarizations. Observations of the CISS effect suggest opportunities for spin control and for the design and fabrication of room-temperature quantum devices from the bottom up, with atomic-scale precision and molecular modularity. On the other hand, chiral-optical effects that depend on both spin- and orbital-angular momentum of photons could offer key advantages in all-optical and quantum information technologies. In particular, amplification of these chiral light-matter interactions using rationally designed plasmonic and dielectric nanomaterials provide approaches to manipulate light intensity, polarization, and phase in confined nanoscale geometries. Any technology that relies on optimal charge transport, or optical control and readout, including quantum devices for logic, sensing, and storage, may benefit from chiral quantum properties. These properties can be theoretically and experimentally investigated from a quantum information perspective, which has not yet been fully developed. There are uncharted implications for the quantum sciences once chiral couplings can be engineered to control the storage, transduction, and manipulation of quantum information. This forward-looking Review provides a survey of the experimental and theoretical fundamentals of chiral-influenced quantum effects and presents a vision for their possible future roles in enabling room-temperature quantum technologies.

Protein nanowires with tunable functionality and programmable self-assembly using sequence-controlled synthesis

Advances in synthetic biology permit the genetic encoding of synthetic chemistries at monomeric precision, enabling the synthesis of programmable proteins with tunable properties. Bacterial pili serve as an attractive biomaterial for the development of engineered protein materials due to their ability to self-assemble into mechanically robust filaments. However, most biomaterials lack electronic functionality and atomic structures of putative conductive proteins are not known. Here, we engineer high electronic conductivity in pili produced by a genomically-recoded E. coli strain. Incorporation of tryptophan into pili increased conductivity of individual filaments >80-fold. Computationally-guided ordering of the pili into nanostructures increased conductivity 5-fold compared to unordered pili networks. Site-specific conjugation of pili with gold nanoparticles, facilitated by incorporating the nonstandard amino acid propargyloxy-phenylalanine, increased filament conductivity ~170-fold. This work demonstrates the sequence-defined production of highly-conductive protein nanowires and hybrid organic-inorganic biomaterials with genetically-programmable electronic functionalities not accessible in nature or through chemical-based synthesis.

Bacterial hairs called pili become highly-conductive electric wires upon addition of both natural and synthetic amino acids conjugated with gold nanoparticles. Here the authors use computationally-guided ordering further increasing their conductivity, thus yielding genetically-programmable materials.

Molecular electronics sensors on a scalable semiconductor chip: A platform for single-molecule measurement of binding kinetics and enzyme activity

Detection of molecular interactions is the foundation for many important biotechnology applications in society and industry, such as drug discovery, diagnostics, and DNA sequencing. This report describes a broadly applicable platform for detecting molecular interactions at the single-molecule scale, in real-time, label-free, and potentially highly multiplexable fashion, using single-molecule sensors on a highly scalable semiconductor sensor array chip. Such chips are both practically manufacturable in the near term, and have a durable long-term scaling roadmap, thus providing an ideal way to bring the power of modern chip technology to the broad area of biosensing. This work also realizes a 50-year-old scientific vision of integrating single molecules into electronic chips to achieve the ultimate miniaturization of electronics.

For nearly 50 years, the vision of using single molecules in circuits has been seen as providing the ultimate miniaturization of electronic chips. An advanced example of such a molecular electronics chip is presented here, with the important distinction that the molecular circuit elements play the role of general-purpose single-molecule sensors. The device consists of a semiconductor chip with a scalable array architecture. Each array element contains a synthetic molecular wire assembled to span nanoelectrodes in a current monitoring circuit. A central conjugation site is used to attach a single probe molecule that defines the target of the sensor. The chip digitizes the resulting picoamp-scale current-versus-time readout from each sensor element of the array at a rate of 1,000 frames per second. This provides detailed electrical signatures of the single-molecule interactions between the probe and targets present in a solution-phase test sample. This platform is used to measure the interaction kinetics of single molecules, without the use of labels, in a massively parallel fashion. To demonstrate broad applicability, examples are shown for probe molecule binding, including DNA oligos, aptamers, antibodies, and antigens, and the activity of enzymes relevant to diagnostics and sequencing, including a CRISPR/Cas enzyme binding a target DNA, and a DNA polymerase enzyme incorporating nucleotides as it copies a DNA template. All of these applications are accomplished with high sensitivity and resolution, on a manufacturable, scalable, all-electronic semiconductor chip device, thereby bringing the power of modern chips to these diverse areas of biosensing.

Distance and Potential Dependence of Charge Transport Through the Reaction Center of Individual Photosynthetic Complexes

Charge separation and transport through the reaction center of photosystem I (PSI) is an essential part of the photosynthetic electron transport chain. A strategy is developed to immobilize and orient PSI complexes on gold electrodes allowing to probe the complex's electron acceptor side, the chlorophyll special pair P700. Electrochemical scanning tunneling microscopy (ECSTM) imaging and current-distance spectroscopy of single protein complex shows lateral size in agreement with its known dimensions, and a PSI apparent height that depends on the probe potential revealing a gating effect in protein conductance. In current-distance spectroscopy, it is observed that the distance-decay constant of the current between PSI and the ECSTM probe depends on the sample and probe electrode potentials. The longest charge exchange distance (lowest distance-decay constant β) is observed at sample potential 0 mV/SSC (SSC: reference electrode silver/silver chloride) and probe potential 400 mV/SSC. These potentials correspond to hole injection into an electronic state that is available in the absence of illumination. It is proposed that a pair of tryptophan residues located at the interface between P700 and the solution and known to support the hydrophobic recognition of the PSI redox partner plastocyanin, may have an additional role as hole exchange mediator in charge transport through PSI.

Electronic Transport in Molecular Wires of Precisely Controlled Length Built from Modular Proteins

DNA molecular wires have been studied extensively because of the ease with which molecules of controlled length and composition can be synthesized. The same has not been true for proteins. Here, we have synthesized and studied a series of consensus tetratricopeptide repeat (CTPR) proteins, spanning 4 to 20 nm in length, in increments of 4 nm. For lengths in excess of 6 nm, their conductance exceeds that of the canonical molecular wire, oligo(phenylene-ethylenene), because of the more gradual decay of conductance with length in the protein. We show that, while the conductance decay fits an exponential (characteristic of quantum tunneling) and not a linear increase of resistance with length (characteristic of hopping transport), it is also accounted for by a square-law dependence on length (characteristic of weakly driven hopping). Measurements of the energy dependence of the decay length rule out the quantum tunneling case. A resonance in the carrier injection energy shows that allowed states in the protein align with the Fermi energy of the electrodes. Both the energy of these states and the long-range of hopping suggest that the reorganization induced by hole formation is greatly reduced inside the protein. We outline a model for calculating the molecular-electronic properties of proteins.

Plugging into bacterial nanowires: a comparison of model electrogenic organisms

Extracellular electron transport (EET) is an important metabolic process used by many bacteria to remove excess electrons generated through cellular metabolism. However, there is still limited understanding about how the molecular mechanisms used to export electrons impact cellular metabolism. Here the EET pathways of two of the best-studied electrogenic organisms, Shewanella oneidensis and Geobacter sulferreducens, are described. Both organisms have superficially similar overall EET routes, but differ in the mechanisms used to oxidise menaquinol, transfer electrons across the outer membrane and reduce extracellular substrates. These mechanistic differences substantially impact both substrate choice and bacterial lifestyle.

Probing Bioelectronic Connections Using Streptavidin Molecules with Modified Valency

As molecular electronic components, proteins are distinguished by a remarkably long electronic decay length (∼10 nm) together with high contact resistance and extreme sensitivity to the chemical details of the contact. As a consequence, the conductance of even a large bioelectronic assembly is largely controlled by the conductance of the contacts. Streptavidin is a versatile linker protein that can tether together biotinylated electrodes and biotinylated proteins but with an ambiguity about the contact geometry that arises from its four possible binding sites for biotin. Here, we use engineered streptavidin tetramers, selected to contain a defined ratio of active monomers to "dead" monomers so as to define the biotin binding sites. We find a strong dependence of conductance on the separation of the biotin molecules, consistent with a short-range tunneling interaction within the streptavidin and in contrast to the long-range transport observed inside larger proteins. Hexaglutamate tails label the active monomers, and the additional negative charge enhances conductance significantly. This effect is quantitatively accounted for by an electronic resonance in the protein conductance.

RNA BioMolecular Electronics: towards new tools for biophysics and biomedicine

The last half-century has witnessed the birth and development of a new multidisciplinary field at the edge between materials science, nanoscience, engineering, and chemistry known as Molecular Electronics. This field deals with the electronic properties of individual molecules and their integration as active components in electronic circuits and has also been applied to biomolecules, leading to BioMolecular Electronics and opening new perspectives for single-molecule biophysics and biomedicine. Herein, we provide a brief introduction and overview of the BioMolecular electronics field, focusing on nucleic acids and potential applications for these measurements. In particular, we review the recent demonstration of the first single-molecule electrical detection of a biologically-relevant nucleic acid. We also show how this could be used to study biomolecular interactions and applications in liquid biopsy for early cancer detection, among others. Finally, we discuss future perspectives and challenges in the applications of this fascinating research field.

Intrinsic electronic conductivity of individual atomically resolved amyloid crystals reveals micrometer-long hole hopping via tyrosines

Proteins are commonly known to transfer electrons over distances limited to a few nanometers. However, many biological processes require electron transport over far longer distances. For example, soil and sediment bacteria transport electrons, over hundreds of micrometers to even centimeters, via putative filamentous proteins rich in aromatic residues. However, measurements of true protein conductivity have been hampered by artifacts due to large contact resistances between proteins and electrodes. Using individual amyloid protein crystals with atomic-resolution structures as a model system, we perform contact-free measurements of intrinsic electronic conductivity using a four-electrode approach. We find hole transport through micrometer-long stacked tyrosines at physiologically relevant potentials. Notably, the transport rate through tyrosines (10⁵ s^-1) is comparable to cytochromes. Our studies therefore show that amyloid proteins can efficiently transport charges, under ordinary thermal conditions, without any need for redox-active metal cofactors, large driving force, or photosensitizers to generate a high oxidation state for charge injection. By measuring conductivity as a function of molecular length, voltage, and temperature, while eliminating the dominant contribution of contact resistances, we show that a multistep hopping mechanism (composed of multiple tunneling steps), not single-step tunneling, explains the measured conductivity. Combined experimental and computational studies reveal that proton-coupled electron transfer confers conductivity; both the energetics of the proton acceptor, a neighboring glutamine, and its proximity to tyrosine influence the hole transport rate through a proton rocking mechanism. Surprisingly, conductivity increases 200-fold upon cooling due to higher availability of the proton acceptor by increased hydrogen bonding.

Long-Range Charge Transport in Diazonium-Based Single-Molecule Junctions

Thin layers of cobalt and ruthenium polypyridyl-oligomers with thicknesses between 2 and 8 nm were deposited on gold by electrochemical reduction of diazonium salts. A scanning tunneling microscope was used to create single-molecule junctions (SMJs). The charge transport properties of the Au-[Co(tpy)₂]_n-Au (n = 1-4) SMJs do not depend markedly on the oligomer length, have an extremely low attenuation factor (β ∼ 0.19 nm^-1), and do not show a thickness-dependent transition between two mechanisms. Resonant charge transport is proposed as the main transport mechanism. The SMJ conductance decreases by 1 order of magnitude upon changing the metal from Co to Ru. In Au-[Ru(tpy)₂]_n-Au and Au-[Ru(bpy)₃]_n-Au SMJs, a charge transport transition from direct tunneling to hopping is evidenced by a break in the length-dependent β-plot. The three different mechanisms observed are a clear molecular signature on transport in SMJs. Most importantly, these results are in good agreement with those obtained on large-area molecular junctions.

Electric field stimulates production of highly conductive microbial OmcZ nanowires

Multifunctional living materials are attractive due to their powerful ability to self-repair and replicate. However, most natural materials lack electronic functionality. Here we show that an electric field, applied to electricity-producing Geobacter sulfurreducens biofilms, stimulates production of previously unknown cytochrome OmcZ nanowires with 1,000-fold higher conductivity (30 S/cm), and 3-fold higher stiffness (1.5 GPa), than the cytochrome OmcS nanowires that are important in natural environments. Using chemical imaging-based multimodal nanospectroscopy, we correlate protein structure with function, and observe pH-induced conformational switching to β-sheets in individual nanowires, which increases their stiffness and conductivity by 100-fold due to enhanced π-stacking of heme groups; this was further confirmed by computational modelling and bulk spectroscopic studies. These nanowires can transduce mechanical and chemical stimuli into electrical signals to perform sensing, synthesis and energy production. These findings of biologically-produced, highly-conductive protein nanowires may help to guide the development of seamless, bidirectional interfaces between biological and electronic systems.

Why are Vibrational Lines Narrow in Proteins?

The vibrational Stark effect in proteins yields line shifts indicative of strong internal electric fields up to a few volts per angstrom. These values are supported by numerical simulations of proteins. The simulations also show a significant breadth of field fluctuations translating to inhomogeneous broadening of vibrational lines. According to fluctuation-dissipation arguments, strong internal fields should lead to broad lines. Experimentally reported vibrational lines in proteins are, however, very narrow. This disconnect is explained here in terms of the insufficient (nonergodic) sampling of the protein's configurations on the lifetime of the vibrational probe. The slow component of the electric field fluctuations in proteins relaxes on the time scale of tens of nanoseconds and is dynamically frozen on the vibrational lifetime.

Ubiquitous Electron Transport in Non-Electron Transfer Proteins

Many proteins that have no known role in electron transfer processes are excellent electronic conductors. This surprising characteristic is not generally evident in bulk aggregates or crystals, or in isolated, solvated peptides, because the outer hydrophilic shell of the protein presents a barrier to charge injection. Ligands that penetrate this barrier make excellent electrical contacts, yielding conductivities on the order of a S/m. The Fermi Energy of metal electrodes is aligned with the energy of internal electronic states of the protein, as evidenced by resonant transmission peaks at about 0.3V on the Normal Hydrogen Electrode scale. This energy is about 0.7 V less than the oxidation potential of aromatic amino acids, indicating a large reduction in electrostatic reorganization energy losses in the interior of the proteins. Consistent with a possible biological role for this conductance, there is a strong dependence on protein conformation. Thus, direct measurement of conductance is a powerful new way to read out protein conformation in real time, opening the way to new types of single molecule sensors and sequencing devices.