Electron Transfer across Helical Peptides

Secondary structure determines electron transport in peptides

Proteins play a key role in biological electron transport, but the structure-function relationships governing the electronic properties of peptides are not fully understood. Despite recent progress, understanding the link between peptide conformational flexibility, hierarchical structures, and electron transport pathways has been challenging. Here, we use single-molecule experiments, molecular dynamics (MD) simulations, nonequilibrium Green's function-density functional theory (NEGF-DFT), and unsupervised machine learning to understand the role of secondary structure on electron transport in peptides. Our results reveal a two-state molecular conductance behavior for peptides across several different amino acid sequences. MD simulations and Gaussian mixture modeling are used to show that this two-state molecular conductance behavior arises due to the conformational flexibility of peptide backbones, with a high-conductance state arising due to a more defined secondary structure (beta turn or 310 helices) and a low-conductance state occurring for extended peptide structures. These results highlight the importance of helical conformations on electron transport in peptides. Conformer selection for the peptide structures is rationalized using principal component analysis of intramolecular hydrogen bonding distances along peptide backbones. Molecular conformations from MD simulations are used to model charge transport in NEGF-DFT calculations, and the results are in reasonable qualitative agreement with experiments. Projected density of states calculations and molecular orbital visualizations are further used to understand the role of amino acid side chains on transport. Overall, our results show that secondary structure plays a key role in electron transport in peptides, which provides broad avenues for understanding the electronic properties of proteins.

Exposure of polyethylene microplastics affects sulfur migration and transformation in anaerobic system

Polyethylene (PE) microplastic, which is detected in various environmental media worldwide, also inevitably enters wastewater treatment plants, which may have an impact on anaerobic processes in wastewater treatment. In this work, the effect of PE microplastics on anaerobic sulfur transformation was explored. Experimental results showed that PE microplastics addition at 0.1%- 0.5% w/w promoted H2S production by 14.8%-27.4%. PE microplastics enhanced the release of soluble organic sulfur and inorganic sulfate, and promoted the bioprocesses of organosulfur compounds hydrolysis and sulfate reduction. Mechanism analysis showed that PE microplastics increased the content of electroactive components (e.g., protein and humic acids) contained in extracellular polymeric substances (EPS). In particular, PE microplastics increased the proportion and the dipole moment of α-helix, an important component involved in electron transfer contained in extracelluar protein, which provided more electron transfer sites and promoted the α-helix mediated electron transfer. These enhanced the direct electron transfer ability of EPSs, which might explain why PE microplastics facilitated the bioprocesses of organosulfur compounds hydrolysis and sulfate reduction. Correspondingly, metagenomic analysis revealed that PE microplastics increased the relative abundance of S2- producers (e.g., Desulfobacula and Desulfonema) and the relative abundance of functional genes involved in anaerobic sulfur transformation (e.g., PepD and cysD), which were beneficial to H2S production in anaerobic system.

Clicked BODIPY-Fullerene-Peptide Assemblies: Studies of Electron Transfer Processes in Self-Assembled Monolayers on Gold Surfaces

Two BODIPY-C60-peptide assemblies were synthesized by CuAAC reactions of BODIPY-C60 dyads and a helical peptide functionalized with a terminal alkyne group and an azide group, respectively. The helical peptide within these assemblies was functionalized at its other end by a disulfide group, allowing formation of self-assembled monolayers (SAMs) on gold surfaces. Characterizations of these SAMs, as well as those of reference molecules (BODIPY-C60-alkyl, C60-peptide and BODIPY-peptide), were carried out by PM-IRRAS and cyclic voltammetry. BODIPY-C60-peptide SAMs are more densely packed than BODIPY-C60-alkyl and BODIPY-peptide based SAMs. These findings were attributed to the rigid peptide helical conformation along with peptide-peptide and C60-C60 interactions within the monolayers. However, less dense monolayers were obtained with the target assemblies compared to the C60-peptide, as the BODIPY entity likely disrupts organization within the monolayers. Finally, electron transfer kinetics measurements by ultra-fast electrochemistry experiments demonstrated that the helical peptide is a better electron mediator in comparison to alkyl chains. This property was exploited along with those of the BODIPY-C60 dyads in a photo-current generation experiment by converting the resulting excited and/or charge separated states from photo-illumination of the dyad into electrical energy.

Effect of Ba2+ on the biomineralization of Ca2+ and Mg2+ ions induced by Bacillus licheniformis

The effect of barium ions on the biomineralization of calcium and magnesium ions is often overlooked when utilizing microbial-induced carbonate precipitation technology for removing barium, calcium, and magnesium ions from oilfield wastewater. In this study, Bacillus licheniformis was used to bio-precipitate calcium, magnesium, and barium ions. The effects of barium ions on the physiological and biochemical characteristics of bacteria, as well as the components of extracellular polymers and mineral characteristics, were also studied in systems containing coexisting barium, calcium, and magnesium ions. The results show that the increasing concentrations of barium ions decreased pH, carbonic anhydrase activity, and concentrations of bicarbonate and carbonate ions, while it increased the contents of humic acids, proteins, polysaccharides, and DNA in extracellular polymers in the systems containing all three types of ions. With increasing concentrations of barium ions, the content of magnesium within magnesium-rich calcite and the size of minerals precipitated decreased, while the full width at half maximum of magnesium-rich calcite, the content of O-C=O and N-C=O, and the diversity of protein secondary structures in the minerals increased in systems containing all three coexisting ions. Barium ions does inhibit the precipitation of calcium and magnesium ions, but the immobilized bacteria can mitigate the inhibitory effect. The precipitation ratios of calcium, magnesium, and barium ions reached 81-94%, 68-82%, and 90-97%. This research provides insights into the formation of barium-enriched carbonate minerals and offers improvements for treating oilfield wastewater.

Peptide-based self-assembled monolayers (SAMs): what peptides can do for SAMs and vice versa

Self-assembled monolayers (SAMs) represent highly ordered molecular materials with versatile biochemical features and multidisciplinary applications. Research on SAMs has made much progress since the early begginings of Au substrates and alkanethiols, and numerous examples of peptide-displaying SAMs can be found in the literature. Peptides, presenting increasing structural complexity, stimuli-responsiveness, and biological relevance, represent versatile functional components in SAMs-based platforms. This review examines the major findings and progress made on the use of peptide building blocks displayed as part of SAMs with specific functions, such as selective cell adhesion, migration and differentiation, biomolecular binding, advanced biosensing, molecular electronics, antimicrobial, osteointegrative and antifouling surfaces, among others. Peptide selection and design, functionalisation strategies, as well as structural and functional characteristics from selected examples are discussed. Additionally, advanced fabrication methods for dynamic peptide spatiotemporal presentation are presented, as well as a number of characterisation techniques. All together, these features and approaches enable the preparation and use of increasingly complex peptide-based SAMs to mimic and study biological processes, and provide convergent platforms for high throughput screening discovery and validation of promising therapeutics and technologies.

Reason for the increased electroactivity of extracellular polymeric substances with electrical stimulation: Structural change of α-helix peptide of protein

Electroactivity is an important parameter to assess the ability of the extracellular polymeric substance (EPS) of microorganisms to participate in extracellular respiration. Many reports have found that the electroactivity of microbial sludge could be enhanced with electrical stimulation, but the reason remains unclear. The results of this study showed that the current generation of the three microbial electrolysis cells increased by 1.27-1.76 times during 49 days of electrical stimulation, but the typical electroactive microorganisms were not enriched. Meanwhile, the capacitance and conductivity of EPS of sludge after the electrical stimulation increased by 1.32-1.83 times and 1.27-1.32 times, respectively. In-situ FTIR analysis indicated that the electrical stimulation could lead to the polarization of amide groups in the protein, likely affecting the protein structure related to the electroactivity. Accordingly, the dipole moment of the α-helix peptide of protein of sludge increased from 220 D to 280 D after the electrical stimulation, which was conducive to electron transfer in the α-helix peptide. Moreover, the vertical ionization potential and E_LUMO-E_HOMO energy gap of the C-terminal in the α-helix peptide decreased from 4.43 eV to 4.10 eV and 0.41 eV to 0.24 eV, respectively, which indicated that the α-helix was easier to serve as the electron transfer site of electron hopping. These results meant that the enhancement of the dipole moment of the α-helix peptide unchoked the electron transfer chain of the protein, which was the main reason for the increased electroactivity of EPS protein.

Non-Conventional Peptide Self-Assembly into a Conductive Supramolecular Rope

Structures composed of alternating α and β amino acids can give rise to peculiar secondary structural motifs, which could self-assemble into complex structures of controlled geometries. This work describes the self-assembly properties of an α,β-peptide, containing three units of syn H2-(2-F-Phe)-h-PheGly-OH, able to self-organize on surfaces into a fascinating supramolecular rope. This material was characterized by AFM, electronic conduction and fluorescence measurements. Molecular dynamics simulations showed that this hexapeptide can self-assemble into an antiparallel β-sheet layer, stabilized by intermolecular H-bonds, which, in turn, can self-assemble into many side-by-side layers, due to π-π interactions. As a matter of fact, we demonstrated that in this system, the presence of aromatic residues at the intramolecular interface promoted by the alternation of α,β-amino-acids in the primary sequence, endorses the formation of a super-secondary structure where the aromatic groups are close to each other, conferring to the system good electron conduction properties. This work demonstrates the capability and future potential of designing and fabricating distinctive nanostructures and efficient bioelectronic interfaces based on an α,β-peptide, by controlling structure and interaction processes beyond those obtained with α- or β-peptides alone.

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.

Helical versus Flat Bis-Ferrocenyl End-Capped Peptides: The Influence of the Molecular Skeleton on Redox Properties

Despite the fact that peptide conjugates with a pendant ferrocenyl (Fc) have been widely investigated, bis-ferrocenyl end-capped peptides are rarely synthetized. In this paper, in addition to the full characterization of the Fc-CO-[L-Dap(Boc)]n-NH-Fc series, we report a comparison of the three series of bis-ferrocenyl homopeptides synthesized to date, to gain insights into the influence of α-amino isobutyric (Aib), 2,3-diamino propionic (Dap) and Cα,β-didehydroalanine (ΔAla) amino acids on the peptide secondary structure and on the ferrocene redox properties. The results obtained by 2D NMR analysis and X-ray crystal structures, and further supported by electrochemical data, evidence different behaviors depending on the nature of the amino acid; that is, the formation of 310-helices or fully extended (2.05-helix) structures. In these foldamers, the orientation of the carbonyl groups in the peptide helix yields a macrodipole with the positive pole on the N-terminal amino acid and the negative pole on the C-terminal amino acid, so that oxidation of the Fc moieties takes place more or less easily depending on the orientation of the macrodipole moment as the peptide chain grows. Conversely, the fully extended conformation adopted by ΔAla flat peptides neither generates a macrodipole nor affects Fc oxidation. The utilization as electrochemical and optical (Circular Dichroism) probes of the two terminal Fc groups, bound to the same peptide chain, makes it possible to study the end-to-end effects of the positive charges produced by single and double oxidations, and to evidence the presence "exciton-coupled" CD among the two intramolecularly interacting Fc groups of the L-Dap(Boc) series.

α-Helix in Cystathionine β-Synthase Enzyme Acts as an Electron Reservoir

The modulation of electron density at the Pyridoxal 5'-phosphate (PLP) catalytic center, because of charge transfer across the α-helix/PLP interface, is the determining factor for the enzymatic activities in the human Cystathionine β-Synthase (hCBS) enzyme. Applying density functional theory calculations, in conjunction with the real space density analysis, we investigated the charge density delocalization across the entire heme-α-helix-PLP electron communication channels. The electron delocalization due to hydrogen bonds at the heme/α-helix and α-helix/PLP interfaces are found to be extended over a very long range, as a result of redistribution of electron densities of the cofactors. Moreover, the internal hydrogen bonds of α-helix that are crucial for its secondary structure also participate in the electron redistribution through the structured hydrogen-bond network. α-Helix is found to accumulate the electron density at the ground state from both of the cofactors and behaves as an electron reservoir for catalytic reaction at the electrophilic center of PLP.

An interesting possibility of forming special hole stepping stones with high-stacking aromatic rings in proteins: three-π five-electron and four-π seven-electron resonance bindings

Long-range hole transfer of proteins plays an important role in many biological processes of living organisms. Therefore, it is highly useful to examine the possible hole stepping stones, which can facilitate hole transfer in proteins. However, the structures of stepping stones are diverse because of the complexity of the protein structures. In the present work, we proposed a series of special stepping stones, which are instantaneously formed by three and four packing aromatic side chains of amino acids to capture a hole, corresponding to three-π five-electron (π:π∴π↔π∴π:π) and four-π seven-electron (π:π∴π:π↔π:π:π∴π) resonance bindings with appropriate binding energies. The aromatic amino acids include histidine (His), phenylalanine (Phe), tyrosine (Tyr) and tryptophan (Trp). The formations of these special stepping stones can effectively reduce the local ionization potential of the high π-stacking region to efficiently capture the migration hole. The quick formations and separations of them promote the efficient hole transfer in proteins. More interestingly, we revealed that a hole cannot delocalize over infinite aromatic rings along the high π-π packing structure at the same time and the micro-surroundings of proteins can modulate the formations of π:π∴π↔π∴π:π and π:π∴π:π↔π:π:π∴π bindings. These results may contribute a new avenue to better understand the potential hole transfer pathway in proteins.

Flat, Cα,β -Didehydroalanine Foldamers with Ferrocene Pendants: Assessing the Role of α-Peptide Dipolar Moments

The foldamer field is continuously expanding as it allows to produce molecules endowed with 3D-structures and functions never observed in nature. We synthesized flat foldamers based on the natural, but non-coded, Cα,β -didehydroalanine α-amino acid, and covalently linked to them two ferrocene (Fc) moieties, as redox probes. These conjugates retain the flat and extended conformation of the 2.05 -helix, both in solution and in the crystal state (X-ray diffraction). Cyclic voltammetry measurements agree with the adoption of the 2.05 -helix, characterized by a negligible dipole moment. Thus, elongated α-peptide stretches of this type are insulators rather than charge conductors, the latter being constituted by peptide α-helices. Also, our homo-tetrapeptide has a N-to-C length of about 18.2 Å, almost double than that (9.7 Å) of an α-helical α-tetrapeptide.

Conformation-dependent charge transport through short peptides

We report on charge transport across single short peptides using the Mechanically Controlled Break Junction (MCBJ) method. We record thousands of electron transport events across single-molecule junctions and with an unsupervised machine learning algorithm, we identify several classes of traces with multifarious conductance values that may correspond to different peptide conformations. Data analysis shows that very short peptides, which are more rigid, show conductance plateaus at low conductance values of about 10^-3G₀ and below, with G₀ being the conductance quantum, whereas slightly longer, more flexible peptides also show plateaus at higher values. Fully stretched peptide chains exhibit conductance values that are of the same order as that of alkane chains of similar length. The measurements show that in the case of short peptides, different compositions and molecular lengths offer a wide range of junction conformations. Such information is crucial to understand mechanism(s) of charge transport in and across peptide-based biomolecules.

Electric field controlled uphill electron migration along α-helical oligopeptides

A systematic study on applied electric field effects (Eapp) on electron transfer along the peptides is very important for the regulation of electron transfer behaviors so as to realize the functions of proteins. In this work, we computationally investigated the uphill migration behaviors of excess electrons along the peptide chains under Eapp using the density functional theory method. We examined the electronic property changes of the model α-helical oligopeptides, the dynamics behavior of an excess electron along the peptide chains under Eapp opposite to the internal dipole field of peptides. We found that Eapp of different intensities can effectively modulate the electron-binding abilities, Frontier molecular orbital (FMO) energies and distributions, dipole moments and other corresponding properties with different degrees. The electron-binding abilities of α-helical oligopeptides revealed by vertical electron affinity and FMO energies decrease in weak Eapp and then increase greatly in high Eapp, while the dipole moments change mildly in weak Eapp and increase significantly until a threshold and then become gentle in high Eapp. Analysis of FMO and electron distributions indicates that an excess electron can migrate uphill from the N-terminus to the C-terminus of the α-helical peptides in an irregular jump mode as Eapp linearly increases. Another interesting finding is that α-helical peptides with diverse chain lengths have different sensitivities to Eapp. The longer the peptide is, the more obvious the effects of Eapp are. Additionally, compared to the Eapp effect on linear oligopeptides, we summarized the systematic rule about the Eapp effect on excess electron migration uphill along the peptide chains. Clearly, this work not only enriches the information of the Eapp effect on electronic properties and electron transfers in the helical peptides, but also provides a new perspective for modulating electron migration behaviors in protein electronics engineering.

Features of the crystallization of multicomponent solutions: a dipeptide, its salt and potassium carbonate

Various stages of crystallization of the dipeptide potassium salt on graphite and gold. Possible molecular structures of the dipeptide (a) and its potassium salt (b).

Electronics of peptide- and protein-based biomaterials

Biologically inspired peptide- and protein-based materials are at the forefront of organic bioelectronics research due to their inherent conduction properties and excellent biocompatibility. Peptides have the advantages of structural simplicity and ease of synthesis providing credible prospects for mass production, whereas naturally expressed proteins offer inspiration with many examples of high performance evolutionary optimised bioelectronics properties. We review recent advances in the fundamental conduction mechanisms, experimental techniques and exemplar applications for the bioelectronics of self-assembling peptides and proteins. Diverse charge transfer processes, such as tunnelling, hopping and coupled transfer, are found in naturally occurring biological systems with peptides and proteins as the predominant building blocks to enable conduction in biology. Both theory and experiments allow detailed investigation of bioelectronic properties in order to design functionalized peptide- and protein-based biomaterials, e.g. to create biocompatible aqueous electrodes. We also highlight the design of bioelectronics devices based on peptides/proteins including field-effect transistors, piezoelectric energy harvesters and optoelectronics.

Photoinduced Electron Transfer and Energy Transfer Processes in a Flexible BODIPY-C60 Dyad

A new donor-acceptor dyad composed of a BODIPY (4,4'-difluoro-4-bora-3a,4a-diaza-s-indacene) donor and a fullerene C₆₀ acceptor has been synthesized and characterized. This derivative has been prepared using a clickable fullerene building block that bears an alkyne moiety and a maleimide unit. The post-functionalization of the maleimide group by a BODIPY thiol leads to a BODIPY-C₆₀ dyad, leaving the alkyne moiety for further functional arrangement. On the basis of the combination of semi-empirical and density functional theory (DFT) calculations, spectroelectrochemical experiments, and steady-state and time-resolved spectroscopies, the photophysical properties of this new BODIPY-C₆₀ dyad were thoroughly studied. By using semi-empirical calculations, the equilibrium of three conformations of the BODIPY-C₆₀ dyad has been deduced, and their molecular orbital structures have been analyzed using DFT calculations. Two short fluorescence lifetimes were attributed to two extended conformers displaying variable donor-acceptor distances (17.5 and 20.0 Å). Additionally, the driving force for photoinduced electron transfer from the singlet excited state of BODIPY to the C₆₀ moiety was calculated using redox potentials determined with electrochemical studies. Spectroelectrochemical measurements were also carried out to investigate the absorption profiles of radicals in the BODIPY-C₆₀ dyad in order to assign the transient species in pump-probe experiments. Under selective photoexcitation of the BODIPY moiety, occurrences of both energy and electron transfers were demonstrated for the dyad by femtosecond and nanosecond transient absorption spectroscopies. Photoinduced electron transfer occurs in the folded conformer, while energy transfer is observed in extended conformers.

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

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

Tunable oligo-histidine self-assembled monolayer junction and charge transport by a pH modulated assembly

Histidine works as an important mediator in the charge transport process through proteins via its conjugate side group. It can also stabilize a peptide's secondary structure through hydrogen bonding of the imidazole group. In this study, the conformation of the self-assembled monolayer (SAM) and the charge transport of the tailor-made oligopeptide hepta-histidine derivative (7-His) were modulated through the pH control of the assembly environment. Histidine is found to be an efficient tunneling mediator in monolayer junctions with an attenuation factor of β = ∼0.5 Å-1. Successful theoretical model fitting indicates a linear increase in the number of tunneling sites as the 7-His SAM thickness increases, following the deprotonation of histidine. Combined with the ultraviolet photoelectron spectroscopy (UPS) measurements, a modulable charge transport pathway through 7-His with imidazole groups of histidine as tunneling foot stones is revealed. Histidine therefore possesses a large potential for modulable functional (bio)electronic devices.

Enhanced Copper-Temozolomide Interactions by Protein for Chemotherapy against Glioblastoma Multiforme

Current treatment of recurrent glioblastoma multiforme (GBM) demands dose-intense temozolomide (TMZ), a prodrug of 5-(3-methyltriazen-1-yl) imidazole-4-carboxamide (MTIC), based on the spontaneous hydrolysis of TMZ at basic pH. However, how to control the activity of MTIC remains unknown, which poses a particular challenge to search a reliable MTIC receptor. We reported that copper, for the first time, is found to recognize and bind MTIC in the process of TMZ degradation, which means copper can play an important role in enhancing the bioavailability of MTIC derived from TMZ. Using apoferritin as a model copper-bound protein, we studied the copper-TMZ interaction in protein and observed efficient MTIC immobilization with high binding efficiency (up to 92.9% based on original TMZ) and capacity (up to 185 MTIC moieties per protein). The system was stable against both alkaline and acidic pH and could be activated by glutathione to liberate MTIC, which paves a way to deliver a DNA-alkylating agent for both TMZ-sensitive and TMZ-resistant GBM chemotherapy. Our study provides a new insight for understanding the potential relationship between the special GBM microenvironment (specific copper accumulation) and the therapeutic effect of TMZ.

Electron transfer through protein-bound water and its bioelectronic application

This article reports that a metastructure of polypeptides with the bound water can have high and stable electron conductivity without classic electron-conducting components. We used gelatin as the model protein since the peptide chains contain numerous sites capable of forming hydrogen bonds with water molecules. The lack of redox sites and the trace amounts of aromatic amino acids also eliminate the possibility that the electron transfer is due to redox reactions or pi-stacking. Our Raman spectroscopy results show that the high electron-conductive metastructure is composed of bound water and unwound gelatin polypeptides. Further removal of bound water from the metastructure dramatically decreases the electron-conductivity, indicating that bound water is crucial to connect the polypeptide chains for the electron-conductivity. In addition, the ability to switch between the low-electron-conductive typical hydrogel state and the high-electron-conductive metastructure state of the gelatin hydrogel allows the gelatin hydrogel to exhibit rewritable nonvolatile resistive memory features. The high ON/OFF current ratio of 10⁵ at a low reading voltage of 0.09 V is superior to that of conventional nonvolatile resistive memories by one order of magnitude. The discovered phenomenon of using bound water and flexible polypeptide structure for long-distance electron transfer could provide a new direction for designing highly biocompatible conducting materials or functional devices in bioelectronics.

Diiron Dithiolate Complex Induced Helical Structure of Histone and Application in Photochemical Hydrogen Generation

Very-lysine-rich calf thymus histone proteins form disordered structure and hydrophobic interaction-driven aggregates in weakly acidic solution. We reported that the conjugation of diiron dithiolate complex to the lysine residues induced formation of helical conformation and condensed nanoassemblies with a high loading capacity up to 18.7 wt %. The incorporated diiron dithiolate complex showed photocatalytic activity for hydrogen evolution in aqueous solutions, with a turnover number (based on [FeFe] catalyst moiety) up to 359 that was more than 6 times that of the free catalyst. The increase of helical conformation in proteins was well correlated to the increasing enhancement of photocatalytic activity. We demonstrated that the [FeFe]-hydrogenase-mimic biohybrid system based on the photocatalyst-induced protein conformational conversion and reassembly is efficient for hydrogen generation regardless of the relatively large size.

Doping hepta-alanine with tryptophan: A theoretical study of its effect on the electrical conductance of peptide-based single-molecule junctions

Motivated by a recent experiment [C. Guo et al., Proc. Natl. Acad. Sci. U. S. A. 113, 10785 (2016)], we carry out a theoretical study of electron transport through peptide-based single-molecule junctions. We analyze the pristine hepta-alanine and its functionalizations with a single tryptophan unit, which is placed in three different locations along the backbone. Contrary to expectations from the experiment on self-assembled monolayers, we find that insertion of tryptophan does not raise the electrical conductance and that the resulting peptides instead remain insulating in the framework of a coherent transport picture. The poor performance of these molecules as conductors can be ascribed to the strongly off-resonant transport and low electrode-molecule coupling of the frontier orbitals. Although the introduction of tryptophan increases the energy of the highest occupied molecular orbital (HOMO) of the peptides in the gas phase, the new HOMO states are localized on the tryptophan unit and therefore essentially do not contribute to coherent charge transport.

Building Supramolecular DNA-Inspired Nanowires on Gold Surfaces: From 2D to 3D

Three building blocks have been designed to chemically link to a gold surface and vertically self-assemble through thymine-adenine hydrogen bonds. Starting from these building blocks, two different films were engineered on gold surface. Film 1 consists of adenine linked to lipoic acid (Lipo-A) to covalently bind to the gold surface, and ZnTPP linked to a thymine (T-ZnTPP). Film 2 has an additional noncovalently linked layer: a helical undecapeptide analogue of the trichogin GA IV peptide, in which four glycines were replaced by four lysines to favor a helical conformation and reduce flexibility and the two extremities were functionalized with thymine and adenine to enable Lipo-A and T-ZnTPP binding, respectively. These films were characterized by electrochemical and spectroscopic techniques, and were very stable over time and when in contact with solution. Under illumination, they could generate current with higher efficiency than similar previously described systems.

Exploring the thermal reversibility and tunability of a low molecular weight gelator using vibrational and electronic spectroscopy and rheology

Cationic glycylalanylglycine (GAG) self-assembles into a gel in a 55 mol% ethanol/45 mol% water mixture. The gel exhibits a network of crystalline fibrils grown to lengths on a 10-4-10-5 m scale (Farrel et al., Soft Matter, 2016, 12, 6096-6110). Rheological data are indicative of a rather strong gel with storage moduli in the 10 kPa regime. Spectroscopic data revealed the existence of two gel phases; one forms below T = 15 °C (phase I) while the other one forms in a temperature range between 15 °C and the melting temperature of ca. 35 °C (phase II). We explored the reformation of the cationic GAG gel in 55 mol% ethanol/45 mol% water after thermal annealing by spectroscopic and rheological means. Our data reveal that even a short residence time of 5 minutes in the sol phase at 50 °C produced a delay of the gelation process and a gel of lesser strength. These observations suggest that the residence time at the annealing temperature can be used to adjust the strength of both gel phases. Our spectroscopic data show that the annealing process does not change the chirality of peptide fibrils in the two gel phases and that the initial aggregation state of the reformation process is by far more ordered for phase I than it is for phase II. In the gel phases of GAG/ethanol/water mixtures, ethanol seems to function as a sort of catalyst that enables the self-assembly of the peptide in spite of its low intrinsic propensity for aggregation.

Iron sulfur clusters in protein nanocages for photocatalytic hydrogen generation in acidic aqueous solutions

We took advantage of the iron binding affinity of apoferritin to immobilize iron-sulfur clusters into apoferritin up to 312 moieties per protein, with a loading rate as high as 25 wt%. The photocatalytic hydrogen generation activity in acidic aqueous solutions was achieved with TONs up to 31 (based on a single catalyst moiety) or 8.3 × 103 (based on a single protein) upon 3 h of visible light irradiation. The present study provides a versatile strategy to construct uniform protein/photocatalyst supramolecular systems with FeFe-H2ase activity.

Thermoelectric properties of oligoglycine molecular wires

We have investigated the electrical and thermoelectrical properties of glycine chains with and without cysteine terminal groups. The electrical conductance of (Gly)n, (Gly)nCys and Cys(Gly)nCys molecules (where Gly, Cys represent glycine and cysteine and n = 1-3) was found to decay exponentially with length l as e-βl. Our results show that connecting the molecules to gold electrodes via the sulphur atom of the cysteine moiety leads to higher β factors of 1.57 Å-1 and 1.22 Å-1 for (Gly)nCys and Cys(Gly)nCys respectively, while β = 0.92 Å-1 for (Gly)n. We also find that replacing the peptide bond with a methylene group (-CH2-) increases the conductance of (Gly)3Cys. Furthermore, we find the (Gly)1Cys and Cys(Gly)1Cys systems show good thermoelectrical performance, because of their high Seebeck coefficients (∼0.2 mV K-1) induced by the sulphur of the cysteine(s). With the contributions of both electrons and phonons taken into consideration, a high figure of merit ZT = 0.8 is obtained for (Gly)1Cys at room temperature, which increases further with increasing temperature, suggesting that peptide-based SAM junctions are promising candidates for thermoelectric energy harvesting.

Orientation of Oligopeptides in Self-Assembled Monolayers Inferred from Infrared Reflection-Absorption Spectroscopy

A series of a single tryptophan containing oligo-alanine peptides were recently characterized as conductive molecules that enable electron transport between electrodes. IR reflection-absorption of self-assembled monolayers of such peptides on gold surfaces revealed that the relative intensities of amide I and II bands in the respective spectra depend on the tryptophan residue position in the oligopeptide sequence. This indicates different average peptide orientations with respect to the normal onto the carrying gold surface. We developed a model which calculates the polarized reflectivities of the amide I and II bands as function of the angle of the incident light, the average peptide orientation and the relative orientations of peptide group at the N-terminal. The orientation and strength of vibrational transition dipole moments were calculated by employing an excitonic coupling approach which considers probable conformational distributions of the disordered peptides. Our results revealed that the position of the tryptophan can affect the effective tilt angle of the peptide as well as the orientation of transition dipole moments with respect to the reflection plane. We have also calculated the average end to end distances of the examined peptides and found them to be in reasonable agreement with experimental values determined by ellipsometry. Some evidence is obtained for the notion that increasing the tilt angle of the investigated peptides reduces their conductivity.

Macroscale Biomolecular Electronics and Ionics

The conduction of ions and electrons over multiple length scales is central to the processes that drive the biological world. The multidisciplinary attempts to elucidate the physics and chemistry of electron, proton, and ion transfer in biological charge transfer have focused primarily on the nano- and microscales. However, recently significant progress has been made on biomolecular materials that can support ion and electron currents over millimeters if not centimeters. Likewise, similar transport phenomena in organic semiconductors and ionics have led to new innovations in a wide variety of applications from energy generation and storage to displays and bioelectronics. Here, the underlying principles of conduction on the macroscale in biomolecular materials are discussed, highlighting recent examples, and particularly the establishment of accurate structure-property relationships to guide rationale material and device design. The technological viability of biomolecular electronics and ionics is also discussed.

Self-assembly of Functional Nanostructures by Short Helical Peptide Building Blocks

The self-assembly of short peptide building blocks into well-ordered nanostructures is a key direction in bionanotechnology. The formation of β -sheet organizations by short peptides is well explored, leading to the development of a wide range of functional assemblies. Likewise, many natural proteinaceous materials, such as silk and amyloid fibrils, are based on β-sheet structures. In contrast, collagen, the most abundant protein in mammals, is based on helical arrangement. Similar to β-sheet structures, short helical peptides have been recently discovered to possess a diverse set of functionalities with the potential to fabricate artificial self-assembling materials. Here, we outline the functional roles of self-assembled nanostructures formed by short helical peptides and their potential as artificial materials. We focus on the association between self-assembled mesoscale structures and their material function and demonstrate the way by which this class of building blocks bears the potential for diverse applications, such as the future fabrication of smart devices.

Peptides as Bio-Inspired Electronic Materials: An Electrochemical and First-Principles Perspective

Molecular electronics is at the forefront of interdisciplinary research, offering a significant extension of the capabilities of conventional silicon-based technology as well as providing a possible stand-alone alternative. Bio-inspired molecular electronics is a particularly intriguing paradigm, as charge transfer in proteins/peptides, for example, plays a critical role in the energy storage and conversion processes for all living organisms. However, the structure and conformation of even the simplest protein is extremely complex, and therefore, synthetic model peptides comprising well-defined geometry and predetermined functionality are ideal platforms to mimic nature for the elucidation of fundamental biological processes while also enhancing the design and development of single-peptide electronic components. In this Account, we first present intramolecular electron transfer within two synthetic peptides, one with a well-defined helical conformation and the other with a random geometry, using electrochemical techniques and computational simulations. This study reveals two definitive electron transfer pathways (mechanisms), the natures of which are dependent on secondary structure. Following on from this, electron transfer within a series of well-defined helical peptides, constrained by either Huisgen cycloaddition, ring-closing metathesis, or a lactam bridge, was determined. The electrochemical results indicate that each constrained peptide, in contrast to a linear counterpart, exhibits a remarkable shift of the formal potential to the positive (>460 mV) and a significant reduction of the electron transfer rate constant (up to 15-fold), which represent two distinct electronic "on/off" states. High-level calculations demonstrate that the additional backbone rigidity provided by the side-bridge constraints leads to an increased reorganization energy barrier, which impedes the vibrational fluctuations necessary for efficient intramolecular electron transfer through the peptide backbone. Further calculations reveal a clear mechanistic transition from hopping to superexchange (tunneling) stemming from side-bridge gating. We then extended our research to fine-tuning of the electronic properties of peptides through both structural and chemical manipulation, to reveal an interplay between electron-rich side chains and backbone rigidity on electron transfer. Further to this, we explored the possibility that the side-bridge constraints present in our synthetic peptides provide an additional electronic transport pathway, which led to the discovery of two distinct forms of quantum interferometer. The effects of destructive quantum interference appear essentially through both the backbone and an alternative tunneling pathway provided by the side bridge in the constrained β-strand peptide, as evidenced by a correlation between electrochemical measurements and conductance simulations for both linear and constrained β-strand peptides. In contrast, an interplay between quantum interference effects and vibrational fluctuations is revealed in the linear and constrained 3₁₀-helical peptides. Collectively, these exciting findings augment our fundamental knowledge of charge transfer dynamics and kinetics in peptides and also open up new avenues to design and develop functional bio-inspired electronic devices, such as on/off switches and quantum interferometers, for practical applications in molecular electronics.

Electron Transport Through Homopeptides: Are They Really Good Conductors?

Motivated by recent experiments, we performed a theoretical study of electron transport through single-molecule junctions incorporating four kinds of homopeptides (based on alanine, glutamic acid, lysine, and tryptophan). Our results suggest that these molecules are rather insulating and operate in off-resonance tunneling as their main transport mechanism. We ascribe their poor performance as conductors to the high localization of their frontier orbitals. We found that binding scenarios in which side chains lie on the side of gold protuberances could give rise to an increase in conductance with respect to end-to-end binding configurations. These findings provide an insight into the conductance mechanism of the building blocks of proteins and identify key issues that need to be further investigated.

Microbial nanowires - Electron transport and the role of synthetic analogues

Electron transfer is central to cellular life, from photosynthesis to respiration. In the case of anaerobic respiration, some microbes have extracellular appendages that can be utilised to transport electrons over great distances. Two model organisms heavily studied in this arena are Shewanella oneidensis and Geobacter sulfurreducens. There is some debate over how, in particular, the Geobacter sulfurreducens nanowires (formed from pilin nanofilaments) are capable of achieving the impressive feats of natural conductivity that they display. In this article, we outline the mechanisms of electron transfer through delocalised electron transport, quantum tunnelling, and hopping as they pertain to biomaterials. These are described along with existing examples of the different types of conductivity observed in natural systems such as DNA and proteins in order to provide context for understanding the complexities involved in studying the electron transport properties of these unique nanowires. We then introduce some synthetic analogues, made using peptides, which may assist in resolving this debate. Microbial nanowires and the synthetic analogues thereof are of particular interest, not just for biogeochemistry, but also for the exciting potential bioelectronic and clinical applications as covered in the final section of the review.

STATEMENT OF SIGNIFICANCE

Some microbes have extracellular appendages that transport electrons over vast distances in order to respire, such as the dissimilatory metal-reducing bacteria Geobacter sulfurreducens. There is significant debate over how G. sulfurreducens nanowires are capable of achieving the impressive feats of natural conductivity that they display: This mechanism is a fundamental scientific challenge, with important environmental and technological implications. Through outlining the techniques and outcomes of investigations into the mechanisms of such protein-based nanofibrils, we provide a platform for the general study of the electronic properties of biomaterials. The implications are broad-reaching, with fundamental investigations into electron transfer processes in natural and biomimetic materials underway. From these studies, applications in the medical, energy, and IT industries can be developed utilising bioelectronics.

Probing Charge Transport through Peptide Bonds

We measure the conductance of unmodified peptides at the single-molecule level using the scanning tunneling microscope-based break-junction method, utilizing the N-terminal amine group and the C-terminal carboxyl group as gold metal-binding linkers. Our conductance measurements of oligoglycine and oligoalanine backbones do not rely on peptide side-chain linkers. We compare our results with alkanes terminated asymmetrically with an amine group on one end and a carboxyl group on the other to show that peptide bonds decrease the conductance of an otherwise saturated carbon chain. Using a newly developed first-principles approach, we attribute the decrease in conductance to charge localization at the peptide bond, which reduces the energy of the frontier orbitals relative to the Fermi energy and the electronic coupling to the leads, lowering the tunneling probability. Crucially, this manifests as an increase in conductance decay of peptide backbones with increasing length when compared with alkanes.

The electron's spin and molecular chirality - how are they related and how do they affect life processes?

The recently discovered chiral induced spin selectivity (CISS) effect gives rise to a spin selective electron transmission through biomolecules. Here we review the mechanism behind the CISS effect and its implication for processes in Biology. Specifically, three processes are discussed: long-range electron transfer, spin effects on the oxidation of water, and enantioselectivity in bio-recognition events. These phenomena imply that chirality and spin may play several important roles in biology, which have not been considered so far.

Electronic structure of dipeptides in the gas-phase and as an adsorbed monolayer

UPS and DFT reveal how frontier energy levels and molecular orbitals of peptides are modified upon peptide binding to a gold substrate.

Electron Hopping Across Hemin-Doped Serum Albumin Mats on Centimeter-Length Scales

Exploring long-range electron transport across protein assemblies is a central interest in both the fundamental research of biological processes and the emerging field of bioelectronics. This work examines the use of serum-albumin-based freestanding mats as macroscopic electron mediators in bioelectronic devices. In particular, this study focuses on how doping the protein mat with hemin improves charge-transport. It is demonstrated that doping can increase conductivity 40-fold via electron hopping between adjacent hemin molecules, resulting in the highest measured conductance for a protein-based material yet reported, and transport over centimeter length scales. The use of distance-dependent AC impedance and DC current-voltage measurements allows the contribution from electron hopping between adjacent hemin molecules to be isolated. Because the hemin-doped serum albumin mats have both biocompatibility and fabrication simplicity, they should be applicable to a range of bioelectronic devices of varying sizes, configurations, and applications.

Geobacter sulfurreducenspili support ohmic electronic conduction in aqueous solution

Solid-state and electrochemical observations of ohmic conductivity in purifiedGeobacter sulfurreducenspili.

Molecular conductance of double-stranded DNA evaluated by electrochemical capacitance spectroscopy

Conductance was measured in two different double stranded DNA (both with 20 bases), the more conducting poly(dG)-poly(dC) (ds-DNAc) and the less conducting poly(dA)-poly(dT) (ds-DNAi), by means of Electrochemical Capacitance Spectroscopy (ECS). The use of the ECS approach, exemplified herein with DNA nanowires, is equally a suitable and time-dependent advantageous alternative for conductance measurement of molecular systems, additionally allowing better understanding of the alignment existing between molecular scale conductance and electron transfer rate.