Effects of Dipole Moment, Linkers, and Chromophores at Side Chains on Long-Range Electron Transfer through Helical Peptides

Dimer Is Not Double: The Unexpected Behavior of Two-Floor Peptide Nanosponge

Using the framework of an investigation of the stimuli-responsive behavior of peptide assembly on a solid surface, this study on the behavior of a chemisorbed peptide on a gold surface was performed. The studied peptide is a dimeric form of the antimicrobial peptide Trichogin GAIV, which was also modified by substituting the glycine with lysine residues, while the N-terminus octanoyl group was replaced by a lipoic one that was able to bind to the gold surface. In this way, a chemically linked peptide assembly that is pH-responsive was obtained because of the protonation/deprotonation of the sidechains of the Lys residues. Information about the effect of protonation/deprotonation equilibria switching the pH from acid (pH = 3) to basic (pH = 11) conditions was obtained macroscopically by performing Quartz crystal microbalance with dissipation monitoring (QCM-D), Surface Plasmon Resonance (SPR), Nanoplasmonic Sensing (NPS), and FTIR techniques. Using molecular dynamics (MD) simulations, it is possible to explain, at the molecular level, our main experimental results: (1) pH changes induce a squeezing behavior in the system, consisting in thickness and mass variations in the peptide layer, which are mainly due to the pH-driven hydrophilic/hydrophobic character of the lysine residues, and (2) the observed hysteresis is due to small conformational rearrangements from helix to beta sheets occurring mainly on the first half of the peptide, closer to the surface, while the second half remains almost unaffected. The latter result, together with the evidence that the layer thickness is not simply double the assembly of the monomeric analog, indicates that the dimeric peptide does not behave as a double monomer, but assumes very peculiar features.

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.

Peptide Materials in Dye Sensitized Solar Cells

In September 2015, the ONU approved the Global Agenda for Sustainable Development, by which all countries of the world are mobilized to adopt a set of goals to be achieved by 2030. Within these goals, the aim of having a responsible production and consumption, as well as taking climate action, made is necessary to design new eco-friendly materials. Another important UN goal is the possibility for all the countries in the world to access affordable energy. The most promising and renewable energy source is solar energy. Current solar cells use non-biodegradable substrates, which generally contribute to environmental pollution at the end of their life cycles. Therefore, the production of green and biodegradable electronic devices is a great challenge, prompted by the need to find sustainable alternatives to the current materials, particularly in the field of dye-sensitized solar cells. Within the green alternatives, biopolymers extracted from biomass, such as polysaccharides and proteins, represent the most promising materials in view of a circular economy perspective. In particular, peptides, due to their stability, good self-assembly properties, and ease of functionalization, may be good candidates for the creation of dye sensitized solar cell (DSSC) technology. This work shows an overview of the use of peptides in DSSC. Peptides, due to their unique self-assembling properties, have been used both as dyes (mimicking natural photosynthesis) and as templating materials for TiO2 morphology. We are just at the beginning of the exploitation of these promising biomolecules, and a great deal of work remains to be done.

A pH-Induced Reversible Conformational Switch Able to Control the Photocurrent Efficiency in a Peptide Supramolecular System

External stimuli are potent tools that Nature uses to control protein function and activity. For instance, during viral entry and exit, pH variations are known to trigger large protein conformational changes. In Nature, also the electron transfer (ET) properties of ET proteins are influenced by pH-induced conformational changes. In this work, a pH-controlled, reversible 3₁₀ -helix to α-helix conversion (from acidic to highly basic pH values and vice versa) of a peptide supramolecular system built on a gold surface is described. The effect of pH on the ability of the peptide SAM to generate a photocurrent was investigated, with particular focus on the effect of the pH-induced conformational change on photocurrent efficiency. The films were characterized by electrochemical and spectroscopic techniques, and were found to be very stable over time, also in contact with a solution. They were also able to generate current under illumination, with an efficiency that is the highest recorded so far with biomolecular systems.

Helical Peptides Design for Molecular Dipoles Functionalization of Wide Band Gap Oxides

The use of helical hexapeptides to establish a surface dipole layer on a TiO₂ substrate, with the goal of influencing the energy levels of a coadsorbed chromophore, is explored. Two helical hexapeptides, synthesized from 2-amino isobutyric acid (Aib) residues, were protected at the N-terminus with a carboxybenzyl group (Z) and at the C-terminus carried either a carboxylic acid or an isophthalic acid (Ipa) anchor group to form Z-(Aib)₆-COOH or Z-(Aib)₆-Ipa, respectively. Using a combination of vibrational and photoemission spectroscopies, bonding of the two peptides to TiO₂ surfaces (either nanostructured or single-crystal TiO₂(110)) was found to be highly dependent on the anchor group, with Ipa establishing a monolayer much more efficiently than COOH. Furthermore, a monolayer of Z-(Aib)₆-Ipa on TiO₂(110) was exposed for different binding times to a solution of a zinc tetraphenylporphyrin (ZnTPP) derivative terminated with an Ipa anchor group (ZnTPP-P-Ipa). Photoemission spectroscopy revealed that ZnTPP-P-Ipa partly displaced Z-(Aib)₆-Ipa, forming a coadsorbed monolayer on the oxide surface. The presence of the peptide molecular dipole shifted the HOMO levels of the ZnTPP group to lower energy by ∼300 meV, in accordance with a simple parallel plate capacitor model. These results suggest that a mixed-layer approach, involving coadsorption of a strong molecular dipole compound with a chromophore, is a versatile method to shift the energy levels of such chromophores with respect to the band edges of the substrate.

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.

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.

Nonlinear Migration Dynamics of Excess Electrons along Linear Oligopeptides Controlled by an Applied Electric Field

Migration of an excess electron along linear oligopeptides governed by the external electric field (E_ex ) which is against the inner dipole electric field is theoretically investigated, including the effects of E_ex on the structural and electronic properties of electron migration. Two structural properties including electron-binding ability and the dipole moment of linear oligopeptides are sensitive to the E_ex values and can be largely modulated by E_ex due to the competition of E_ex and the inner electric field and electron transfer caused by E_ex . In the case of low E_ex values, two structural properties decrease slightly_, while for high E_ex values, the electron-binding ability continually increases strongly, with dipole moments firstly increasing significantly and then increasing more slowly at higher E_ex . Additionally, linear oligopeptides of different chain lengths influence the modulation extent of E_ex and the longer the chain length is, the more sensitive modulation of E_ex is. In addition, electronic properties represented by electron spin densities and singly occupied molecular orbital distributions vary with E_ex intensities, leading to an unusual electron migration behavior. As E_ex increases, an excess electron transfers from the N-terminus to the C-terminus and jumps over a neighboring dipole unit of two termini to other units, respectively, instead of transferring by means of a one-by-one dipole unit hopping mechanism. These findings not only promote a deeper understanding of the connection between E_ex and structural and electronic properties of electron transfer behavior in peptides, but also provide a new insight into the modulation of electron migration along the oligopeptides.

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.

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

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

Tailor-Made Functional Peptide Self-Assembling Nanostructures

Noncovalent interactions are the main driving force in the folding of proteins into a 3D functional structure. Motivated by the wish to reveal the mechanisms of the associated self-assembly processes, scientists are focusing on studying self-assembly processes of short protein segments (peptides). While this research has led to major advances in the understanding of biological and pathological process, only in recent years has the applicative potential of the resulting self-assembled peptide assemblies started to be explored. Here, major advances in the development of biomimetic supramolecular peptide assemblies as coatings, gels, and as electroactive materials, are highlighted. The guiding lines for the design of helical peptides, β strand peptides, as well as surface binding monolayer-forming peptides that can be utilized for a specific function are highlighted. Examples of their applications in diverse immerging applications in, e.g., ecology, biomedicine, and electronics, are described. Taking into account that, in addition to extraordinary design flexibility, these materials are naturally biocompatible and ecologically friendly, and their production is cost effective, the emergence of devices incorporating these biomimetic materials in the market is envisioned in the near future.

Chirality Dependent Charge Transfer Rate in Oligopeptides

It is shown that "spontaneous magnetization" occurs when chiral oligopeptides are attached to ferrocene and are self-assembled on a gold substrate. As a result, the electron transfer, measured by electrochemistry, shows asymmetry in the reduction and oxidation rate constants; this asymmetry is reversed between the two enantiomers. The results can be explained by the chiral induced spin selectivity of the electron transfer. The measured magnetization shows high anisotropy and the "easy axis" of magnetization is along the molecular axis.

Peptides as Bio-inspired Molecular Electronic Materials

Understanding the electronic properties of single peptides is not only of fundamental importance to biology, but it is also pivotal to the realization of bio-inspired molecular electronic materials. Natural proteins have evolved to promote electron transfer in many crucial biological processes. However, their complex conformational nature inhibits a thorough investigation, so in order to study electron transfer in proteins, simple peptide models containing redox active moieties present as ideal candidates. Here we highlight the importance of secondary structure characteristic to proteins/peptides, and its relevance to electron transfer. The proposed mechanisms responsible for such transfer are discussed, as are details of the electrochemical techniques used to investigate their electronic properties. Several factors that have been shown to influence electron transfer in peptides are also considered. Finally, a comprehensive experimental and theoretical study demonstrates that the electron transfer kinetics of peptides can be successfully fine tuned through manipulation of chemical composition and backbone rigidity. The methods used to characterize the conformation of all peptides synthesized throughout the study are outlined, along with the various approaches used to further constrain the peptides into their geometric conformations. The aforementioned sheds light on the potential of peptides to one day play an important role in the fledgling field of molecular electronics.

A controllable mechanistic transition of charge transfer in helical peptides: from hopping to superexchange

A controllable mechanistic transition of charge transfer in helical peptides is demonstrated as a direct result of side-bridge gating.

Electron transfer in peptides

In this review, we discuss the factors that influence electron transfer in peptides. We summarize experimental results from solution and surface studies and highlight the ongoing debate on the mechanistic aspects of this fundamental reaction. Here, we provide a balanced approach that remains unbiased and does not favor one mechanistic view over another. Support for a putative hopping mechanism in which an electron transfers in a stepwise manner is contrasted with experimental results that support electron tunneling or even some form of ballistic transfer or a pathway transfer for an electron between donor and acceptor sites. In some cases, experimental evidence suggests that a change in the electron transfer mechanism occurs as a result of donor-acceptor separation. However, this common understanding of the switch between tunneling and hopping as a function of chain length is not sufficient for explaining electron transfer in peptides. Apart from chain length, several other factors such as the extent of the secondary structure, backbone conformation, dipole orientation, the presence of special amino acids, hydrogen bonding, and the dynamic properties of a peptide also influence the rate and mode of electron transfer in peptides. Electron transfer plays a key role in physical, chemical and biological systems, so its control is a fundamental task in bioelectrochemical systems, the design of peptide based sensors and molecular junctions. Therefore, this topic is at the heart of a number of biological and technological processes and thus remains of vital interest.

Peptide flatlandia: a new-concept peptide for positioning of electroactive probes in proximity to a metal surface

A helical hexapeptide was designed to link in a rigid parallel orientation to a gold surface. The peptide sequence of the newly synthesized compound is characterized by the presence of two 4-amino-1,2-dithiolane-4-carboxylic acid (Adt) residues (positions 1 and 4) to promote a bidentate interaction with the gold surface, two L-Ala residues (positions 2 and 5) and two-aminoisobutyric acid (Aib) residues (positions 3 and 6) to favor a high population of the 310-helix conformation. Furthermore, a ferrocenoyl (Fc) probe was inserted at the N-terminus to investigate the electronic conduction properties of the peptide. X-Ray photoelectron spectroscopy and scanning tunneling microscopy techniques were used to characterize the binding of the peptide to the gold surface and the morphology of the peptide layer, respectively. Several electrochemical (cyclic voltammetry, chronoamperometry, square wave voltammetry) techniques were applied to analyze the electrochemical activity of the Fc probe, along with the influence of the peptide 3D-structure and the peptide layer morphology on electron transfer processes.

Computational insights into the charge relaying properties of β-turn peptides in protein charge transfers

Density functional theory calculations suggest that β-turn peptide segments can act as a novel dual-relay elements to facilitate long-range charge hopping transport in proteins, with the N terminus relaying electron hopping transfer and the C terminus relaying hole hopping migration. The electron- or hole-binding ability of such a β-turn is subject to the conformations of oligopeptides and lengths of its linking strands. On the one hand, strand extension at the C-terminal end of a β-turn considerably enhances the electron-binding of the β-turn N terminus, due to its unique electropositivity in the macro-dipole, but does not enhance hole-forming of the β-turn C terminus because of competition from other sites within the β-strand. On the other hand, strand extension at the N terminal end of the β-turn greatly enhances hole-binding of the β-turn C terminus, due to its distinct electronegativity in the macro-dipole, but does not considerably enhance electron-binding ability of the N terminus because of the shared responsibility of other sites in the β-strand. Thus, in the β-hairpin structures, electron- or hole-binding abilities of both termini of the β-turn motif degenerate compared with those of the two hook structures, due to the decreased macro-dipole polarity caused by the extending the two terminal strands. In general, the high polarity of a macro-dipole always plays a principal role in determining charge-relay properties through modifying the components and energies of the highest occupied and lowest unoccupied molecular orbitals of the β-turn motif, whereas local dipoles with low polarity only play a cooperative assisting role. Further exploration is needed to identify other factors that influence relay properties in these protein motifs.

Unraveling the interplay of backbone rigidity and electron rich side-chains on electron transfer in peptides: the realization of tunable molecular wires

Electrochemical studies are reported on a series of peptides constrained into either a 310-helix (1-6) or β-strand (7-9) conformation, with variable numbers of electron rich alkene containing side chains. Peptides (1 and 2) and (7 and 8) are further constrained into these geometries with a suitable side chain tether introduced by ring closing metathesis (RCM). Peptides 1, 4 and 5, each containing a single alkene side chain reveal a direct link between backbone rigidity and electron transfer, in isolation from any effects due to the electronic properties of the electron rich side-chains. Further studies on the linear peptides 3-6 confirm the ability of the alkene to facilitate electron transfer through the peptide. A comparison of the electrochemical data for the unsaturated tethered peptides (1 and 7) and saturated tethered peptides (2 and 8) reveals an interplay between backbone rigidity and effects arising from the electron rich alkene side-chains on electron transfer. Theoretical calculations on β-strand models analogous to 7, 8 and 9 provide further insights into the relative roles of backbone rigidity and electron rich side-chains on intramolecular electron transfer. Furthermore, electron population analysis confirms the role of the alkene as a "stepping stone" for electron transfer. These findings provide a new approach for fine-tuning the electronic properties of peptides by controlling backbone rigidity, and through the inclusion of electron rich side-chains. This allows for manipulation of energy barriers and hence conductance in peptides, a crucial step in the design and fabrication of molecular-based electronic devices.

Electron Transfer Mechanism in Helical Peptides

Electrochemical studies of a set of ferrocene-labeled helical peptides of increasing length were carried out by forming self-assembled monolayers (SAMs) on gold electrodes. Electron transfer (ET) rates showed a very weakly distance dependent nature that has been interpreted as a result of a dynamically controlled tunneling mechanism. Specifically, the slow equilibrium between the α- and the 310 helical conformers in a SAM has been invoked, and the rate of formation of the more conductive 310 conformer has been proposed to be related to the ET rates observed.

Ultra-long-range electron transfer through a self-assembled monolayer on gold composed of 120-Å-long α-helices

Electron transfer through α-helices has attracted much attention from the viewpoints of their contributions to efficient long-range electron transfer occurring in biological systems and their utility as molecular-electronics elements. In this study, we synthesized a long 80mer helical peptide carrying a redox-active ferrocene unit at the terminal and immobilized the helical peptide on a gold surface. The molecular length is calculated to be 134 Å, in which the helix accounts for 120 Å. The preparation conditions of the self-assembled monolayers were intentionally changed to obtain monolayers with different physical states to study the correlation between molecular motions and electron transfer. Ellipsometry and infrared spectroscopy showed that the helical peptide forms a self-assembled monolayer with vertical orientation. Electrochemical measurements revealed that an electron is transferred from the ferrocene unit to gold through the monolayer composed of this long helical peptide, and the experimental data are well explained by theoretical results calculated under the assumption that electron transfer occurs by a unique hopping mechanism with the amide groups as hopping sites. Furthermore, we have observed a unique dependence of electron transfer on the monolayer packing, suggesting the importance of structural fluctuations of peptides on the electron transfer controlled by the hopping mechanism.

Electrochemistry of redox-active self-assembled monolayers

Redox-active self-assembled monolayers (SAMs) provide an excellent platform for investigating electron transfer kinetics. Using a well-defined bridge, a redox center can be positioned at a fixed distance from the electrode and electron transfer kinetics probed using a variety of electrochemical techniques. Cyclic voltammetry, AC voltammetry, electrochemical impedance spectroscopy, and chronoamperometry are most commonly used to determine the rate of electron transfer of redox-activated SAMs. A variety of redox species have been attached to SAMs, and include transition metal complexes (e.g., ferrocene, ruthenium pentaammine, osmium bisbipyridine, metal clusters) and organic molecules (e.g., galvinol, C(60)). SAMs offer an ideal environment to study the outer-sphere interactions of redox species. The composition and integrity of the monolayer and the electrode material influence the electron transfer kinetics and can be investigated using electrochemical methods. Theoretical models have been developed for investigating SAM structure. This review discusses methods and monolayer compositions for electrochemical measurements of redox-active SAMs.

Effect of a strong interfacial electric field on the orientation of the dipole moment of thiolated aib-oligopeptides tethered to mercury on either the N- or C-terminus

Four oligopeptides consisting of a sequence of alpha-aminoisobutyric acid (Aib) residues, thiolated at either the N- or C-terminus by means of a -(CH(2))(2)-SH anchor, were self-assembled on mercury, which is a substrate known to impart a high fluidity to self-assembled monolayers (SAMs). The surface dipole potential of these peptide SAMs was estimated in 0.1 M KCl aqueous solution at a negatively charged electrode, where the interfacial electric field is directed toward the metal. To the best of our knowledge, this is the first estimate of the surface dipole potential of peptide SAMs in aqueous solution. The procedure adopted consisted in measuring the charge involved in the gradual expansion of a peptide-coated mercury drop and then combining the resulting information with an estimate of the charge density experienced by diffuse layer ions. The dipole moment of the tethered thiolated peptides was found to be directed toward the metal, independent of whether they were thiolated at the C- or N-terminus. This result was confirmed by the effect of these SAMs on the kinetics and thermodynamics of the Eu(III)/Eu(II) redox couple. The combined outcome of these studies indicates that a strong interfacial electric field orients the dipole moment of peptide SAMs tethered to mercury, even against their "natural" dipole moment.

Electrochemistry of ferrocenoyl beta-peptide monolayers on gold

The electrochemistry of self-assembled monolayers (SAMs) on gold containing a lipoic acid linker, the beta-peptide sequence (beta(3)Val-beta(3)Ala-beta(3)Leu)(n) for n = 1, 2, and a terminal ferrocenyl group has been described for the first time. Circular dichroism (CD), NMR, and molecular modeling were used to evaluate the beta-peptide structure in solution, while the monolayer film organization and electron-transfer kinetics were evaluated by cyclic voltammetry, chronoamperometry (CA), and ellipsometry. The peptides were assembled from trifluoroethanol solutions, where they are linear (n = 1) or helical (n = 2) based on CD, NMR, ellipsometry, and modeling evidence. The structure of the SAMs is less well understood. There is evidence for noncompact layers that allow electrolyte ions to approach the interface. Electron-transfer rates for n = 1, 2 were found to be 2500 and 1200 s(-1), respectively, and CA evidence indicated that the transfer is based on the hopping mechanism.

Electron transfer through a self-assembled monolayer of a double-helix peptide with linking the terminals by ferrocene

A unique molecular structure, a double-helix peptide, was self-assembled on gold, and the electron transfer through the monolayer was studied. The double-helix peptide consists of two 9mer 3(10)-helical peptide chains having a disulfide group at each N terminal and being linked by a ferrocene dicarboxylic acid between the C terminals. Each helical peptide chain has three naphthyl groups in a linear arrangement along the helix. The monolayer properties and the electron transfer from the ferrocene unit to gold were studied with reference peptides with a similar double helix but without naphthyl groups, a single helix with a dicarboxylic ferrocene unit, and a single helix with a monocarboxylic ferrocene unit. It was demonstrated that the naphthyl groups on the side chains had no effect on electron transfer, and the electron-transfer rate in the double-helix monolayer was not promoted, despite the two electron pathways in the molecule. We propose that in the double-helix monolayer, molecular motions are suppressed, possibly by its rigid structure tethered by the two linkers on gold to cancel out acceleration effects of the 2-fold electron pathways and the ferrocene substitution number. The factors that affect the electron-transfer reaction across the helical peptide SAMs are discussed in depth.

Electron transfer in peptides and proteins

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

Electroconductive and photocurrent generation properties of self-assembled monolayers formed by functionalized, conformationally-constrained peptides on gold electrodes

The electroconductive properties and photocurrent generation capabilities of self-assembled monolayers formed by conformationally-constrained hexapeptides were studied by cyclic voltammetry, chronoamperometry, and photocurrent generation experiments. Lipoic acid was covalently linked to the N-terminus of the peptides investigated to exploit the high affinity of the disulfide group to the gold substrates. Smart functionalization of the peptide scaffold with a redox-active (TOAC) or a photosensitizer (Trp) amino acid allowed us to study the efficiency of peptide-based self-assembled monolayers to mediate electron transfer and photoinduced electron transfer processes on gold substrates. Interdigitated microelectrodes have shown higher film stability under photoexcitation, lower dark currents, and higher sensitivity with respect to standard gold electrodes.

The Influence of Molecular Dipole Moment on the Redox-Induced Reorganization of α-Helical Peptide Self-Assembled Monolayers: An Electrochemical SPR Investigation

Self-assembled monolayers (SAMs) of ferrocene-labeled α-helical peptides were prepared on gold surfaces and studied using electrochemical surface plasmon resonance (EC-SPR). The leucine-rich peptides were synthesized with a cysteine sulfhydryl group either at the C- or N-terminus, enabling their immobilization onto gold surfaces with control of the direction of the molecular dipole moment. Two electroactive SAMs were studied, one in which all of the peptide dipole moments are oriented in the same direction (SAM1), and the other in which the peptide dipole moment of one peptide is aligned in the opposite direction to that of its surrounding peptide molecules (SAM2). Cyclic voltammetry combined with SPR measurements revealed that SAM reorientations concomitant with the oxidation of the ferrocene label were more significant in SAM2 than in SAM1. The substantially greater change in the peptide film thickness in the case of SAM2 is attributed to the electrostatic repulsion between the electrogenerated ferrocinium moiety and the positively charged gold surface. The greater permeability of SAM1 to electrolyte anions, on the other hand, appears to effectively neutralize this electrostatic repulsion. The film thickness change in SAM2 was estimated to be 0.25 ± 0.05 nm using numerical simulation. The timescale of the redox-induced SPR changes was established by chronoamperometry and time-resolved SPR measurements, followed by fitting of the SPR response to a stretched exponential function. The time constants measured for the anodic process were 16 and 6 ms for SAM1 and SAM2 respectively, indicating that the SAM thickness changes are notably fast.

A novel polypseudorotaxane composed of cyclic β-peptide as bead component

A polypseudorotaxane composed of cyclic hexamers of beta-glucosamino acid and poly(ethylene glycol) (PEG) was constructed, where the beads were stacked unidirectionally together via hydrogen bonds to form a peptide nanotube.

Conductance of α-Helical Peptides Trapped within Molecular Junctions

Self-assembled monolayers of alpha-helical peptides on a gold surface were employed as model systems for the investigation of mediated electron transfer. The peptides contained 14, 15, 16, and 17 amino acid residues. The measurements of electron transmission through single molecules of helical peptides were performed using scanning tunneling spectroscopy (STS). The molecules were trapped between the gold tip and the substrate. Electrical contact between the molecule and the gold probe was achieved by the use of peptides containing thiol groups present at each end of the helix. The conductance behavior of the peptides was examined as a function of tip-substrate distance at fixed bias voltage. Measurements performed with peptides containing different numbers of amino acid residues indicate that the distance dependence of electron transmission through an alpha-helix is weaker than that through simple n-alkyl bridges.

Parallel assembly of dipolar columns composed of a stacked cyclic tri-beta-peptide

A novel cyclic trimer of a beta-amino acid, trans-2-aminocyclohexylcarboxylic acid, was synthesized and its conformation and ability to form assemblies investigated. FT-IR and NMR measurements and computational calculations showed that this cyclic tri-beta-peptide has a C3-symmetric conformation with trans amide groups. A notable feature of the conformation is a vertical and parallel orientation of the three amide groups to the cyclic skeleton. The cyclic tri-beta-peptide was crystallized from a solution in trifluoroacetic acid-methanol (or trifluoroacetic acid-water) to yield a rod-shaped molecular assembly, as observed by TEM. The electron crystallography of the rod-shaped assembly both in suspension and in ultrathin cross-section revealed that the cyclic tri-beta-peptides were stacked up to form molecular columns, and that a two-fold screw symmetry operation along the column direction was present in the unit cell, which contained two cyclic tri-beta-peptides. This indicates that all the amide groups are oriented in the same direction. Since any two molecular columns are staggered by a quarter of a c-axis length and aligned parallel to each other, the dipole moments of the columns are aligned to enhance the strength additively in the whole assembly.