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

Ultrasensitive Detection of Hydrogen Peroxide Using Methylene Blue Grafted on Molecular Wires as Nanozyme with Catalase-like Activity

In this study, we present the development of an electrochemical sensor designed for ultrasensitive detection of endogenous H2O2. This sensor relies on signal amplification achieved through nanozyme activity exhibited by methylene blue (MB) grafted onto a peptide support. The sensor exhibited excellent selectivity and sensitivity, with a limit of detection of 18 nM and a linear detection range of 20-200 nM. Thus, we have validated the concept of the MB-peptide system, serving as both an electroactive label and a catalyst for H2O2 decomposition under electrochemical conditions. The implemented signal amplification system enables the rapid detection of H2O2, with an overall assay time of 1-2 min, a significant improvement compared to amperometric detection using surface-immobilized enzymes.

The Renaissance of Ferrocene-Based Electrocatalysts: Properties, Synthesis Strategies, and Applications

The fascinating electrochemical properties of the redox-active compound ferrocene have inspired researchers across the globe to develop ferrocene-based electrocatalysts for a wide variety of applications. Advantages including excellent chemical and thermal stability, solubility in organic solvents, a pair of stable redox states, rapid electron transfer, and nontoxic nature improve its utility in various electrochemical applications. The use of ferrocene-based electrocatalysts enables control over the intrinsic properties and electroactive sites at the surface of the electrode to achieve specific electrochemical activities. Ferrocene and its derivatives can function as a potential redox medium that promotes electron transfer rates, thereby enhancing the reaction kinetics and electrochemical responses of the device. The outstanding electrocatalytic activity of ferrocene-based compounds at lower operating potentials enhances the specificity and sensitivity of reactions and also amplifies the response signals. Owing to their versatile redox chemistry and catalytic activities, ferrocene-based electrocatalysts are widely employed in various energy-related systems, molecular machines, and agricultural, biological, medicinal, and sensing applications. This review highlights the importance of ferrocene-based electrocatalysts, with emphasis on their properties, synthesis strategies for obtaining different ferrocene-based compounds, and their electrochemical applications.

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.

Multifaceted aspects of charge transfer

Charge transfer and charge transport are by far among the most important processes for sustaining life on Earth and for making our modern ways of living possible. Involving multiple electron-transfer steps, photosynthesis and cellular respiration have been principally responsible for managing the energy flow in the biosphere of our planet since the Great Oxygen Event. It is impossible to imagine living organisms without charge transport mediated by ion channels, or electron and proton transfer mediated by redox enzymes. Concurrently, transfer and transport of electrons and holes drive the functionalities of electronic and photonic devices that are intricate for our lives. While fueling advances in engineering, charge-transfer science has established itself as an important independent field, originating from physical chemistry and chemical physics, focusing on paradigms from biology, and gaining momentum from solar-energy research. Here, we review the fundamental concepts of charge transfer, and outline its core role in a broad range of unrelated fields, such as medicine, environmental science, catalysis, electronics and photonics. The ubiquitous nature of dipoles, for example, sets demands on deepening the understanding of how localized electric fields affect charge transfer. Charge-transfer electrets, thus, prove important for advancing the field and for interfacing fundamental science with engineering. Synergy between the vastly different aspects of charge-transfer science sets the stage for the broad global impacts that the advances in this field have.

Biomimetic and bioinspired molecular electrets. How to make them and why does the established peptide chemistry not always work?

“Biomimetic” and “bioinspired” define different aspects of the impacts that biology exerts on science and engineering. Biomimicking improves the understanding of how living systems work, and builds tools for bioinspired endeavors. Biological inspiration takes ideas from biology and implements them in unorthodox manners, exceeding what nature offers. Molecular electrets, i.e. systems with ordered electric dipoles, are key for advancing charge-transfer (CT) science and engineering. Protein helices and their biomimetic analogues, based on synthetic polypeptides, are the best-known molecular electrets. The inability of native polypeptide backbones to efficiently mediate long-range CT, however, limits their utility. Bioinspired molecular electrets based on anthranilamides can overcome the limitations of their biological and biomimetic counterparts. Polypeptide helices are easy to synthesize using established automated protocols. These protocols, however, fail to produce even short anthranilamide oligomers. For making anthranilamides, the residues are introduced as their nitrobenzoic-acid derivatives, and the oligomers are built from their C- to their N-termini via amide-coupling and nitro-reduction steps. The stringent requirements for these reduction and coupling steps pose non-trivial challenges, such as high selectivity, quantitative yields, and fast completion under mild conditions. Addressing these challenges will provide access to bioinspired molecular electrets essential for organic electronics and energy conversion.

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.

Dipole-induced effects on charge transfer and charge transport. Why do molecular electrets matter?

Charge transfer (CT) and charge transport (CTr) are at the core of life-sustaining biological processes and of processes that govern the performance of electronic and energy-conversion devices. Electric fields are invaluable for guiding charge movement. Therefore, as electrostatic analogues of magnets, electrets have unexplored potential for generating local electric fields for accelerating desired CT processes and suppressing undesired ones. The notion about dipole-generated local fields affecting CT has evolved since the middle of the 20th century. In the 1990s, the first reports demonstrating the dipole effects on the kinetics of long-range electron transfer appeared. Concurrently, the development of molecular-level designs of electric junctions has led the exploration of dipole effects on CTr. Biomimetic molecular electrets such as polypeptide helices are often the dipole sources in CT systems. Conversely, surface-charge electrets and self-assembled monolayers of small polar conjugates are the preferred sources for modifying interfacial electric fields for controlling CTr. The multifaceted complexity of such effects on CT and CTr testifies for the challenges and the wealth of this field that still remains largely unexplored. This review outlines the basic concepts about dipole effects on CT and CTr, discusses their evolution, and provides accounts for their future developments and impacts.

Balancing hydrophobicity and sequence pattern to influence self-assembly of amphipathic peptides

Amphipathic peptides with alternating polar and nonpolar amino acid sequences efficiently self-assemble into functional β-sheet fibrils as long as the nonpolar residues have sufficient hydrophobicity. For example, the Ac-(FKFE)2 -NH2 peptide rapidly self-assembles into β-sheet bilayer nanoribbons, while Ac-(AKAE)2 -NH2 fails to self-assemble under similar conditions due to the significantly reduced hydrophobicity and β-sheet propensity of Ala relative to Phe. Herein, we systematically explore the effect of substituting only two of the four Ala residues at various positions in the Ac-(AKAE)2 -NH2 peptide with amino acids of increasing hydrophobicity, β-sheet potential, and surface area (including Phe, 1-naphthylalanine (1-Nal), 2-naphthylalanine (2-Nal), cyclohexylalanine (Cha), and pentafluorophenylalanine (F5 -Phe)) on the self-assembly propensity of the resulting sequences. It was found that double Phe variants, regardless of the position of substitution, failed to self-assemble under the conditions used in this study. In contrast, all double 1-Nal and 2-Nal variants readily self-assembled, albeit at differing rates depending on the substitution patterns. To determine whether this was due to hydrophobicity or side chain surface area, we also prepared double Cha and F5 -Phe variant peptides (both side chain groups are more hydrophobic than Phe). Each of these variants also underwent effective self-assembly, with the aromatic F5 -Phe peptides doing so with greater efficiency. These findings provide insight into the role of amino acid hydrophobicity and sequence pattern on self-assembly proclivity of amphipathic peptides and on how targeted substitutions of nonpolar residues in these sequences can be exploited to tune the characteristics of the resulting self-assembled materials.

Peptide-based biosensors: From self-assembled interfaces to molecular probes in electrochemical assays

Redox-tagged peptides have emerged as functional materials with multiple applications in the area of sensing and biosensing applications due to their high stability, excellent redox properties and versatility of biomolecular interactions. They allow direct observation of molecular interactions in a wide range of affinity and enzymatic assays and act as electron mediators. Short helical peptides possess the ability to self-assemble in specific configurations with the possibility to develop in highly-ordered, stable 1D, 2D and 3D architectures in a hierarchical controlled manner. We provide here a brief overview of the electrochemical techniques available to study the electron transfer in peptide films with particular interest in developing biosensors with immobilized peptide motifs, for biological and clinical applications.

Synthesis, Crystal Structures and Properties of Ferrocenyl Bis-Amide Derivatives Yielded via the Ugi Four-Component Reaction

Ten ferrocenyl bis-amide derivatives were successfully synthesized via the Ugi four-component reaction by treating ferrocenecarboxylic acid with diverse aldehydes, amines, and isocyanides in methanol solution. Their chemical structures were fully characterized by IR, NMR, HR-MS, and X-ray diffraction analyses. They feature unique molecular morphologies and create a 14-membered ring motif in the centro-symmetric dimers generated in the solid state. Moreover, the electrochemical behavior of these ferrocenyl bis-amides was assessed by cyclic voltammetry.

Rapid Electron Transport Phenomenon in the Bis(terpyridine) Metal Complex Wire: Marcus Theory and Electrochemical Impedance Spectroscopy Study

The authors reported previously that bis(terpyiridne)iron(II) complex oligomer wires possess outstanding long-range intrawire electron transport ability. Here, molecular arrays of gold-electrode-bis(terpyridine)iron(II)-ferrocene are constructed by stepwise coordination as simple models of the oligomer wire system. The fast electron transfer between the terminal ferrocene and the gold electrode through the bis(terpyiridne)iron(II) complex unit is studied by potential step chronoamperometry (PSCA) and electrochemical impedance spectroscopy (EIS). Tafel plots derived from PSCA are analyzed based on Marcus theory. The plots reveal greater first-order electron transfer rate constant, weaker electronic coupling between the terminal ferrocene and the gold electrode, and smaller reorganization energy than shown by a conventional ferrocenylalkanethiol self-assembled monolayer. The electron transfer rate constants estimated by EIS agree with the PSCA results.

Two novel ferrocenyl dipeptide-like compounds generated via the Ugi four-component reaction

The Ugi four-component reaction, a powerful method for the synthesis of diverse dipeptide-like derivatives in combinatorial chemistry, was used to synthesize (S)-1'-{N-[1-(anthracen-9-yl)-2-(tert-butylamino)-2-oxoethyl]-N-(4-methoxyphenyl)carbamoyl}ferrocene-1-carboxylic acid dichloromethane disolvate, [Fe(C6H5O2)(C33H31N2O3)]·2CH2Cl2, (I), and (S)-2-(anthracen-9-yl)-N-tert-butyl-2-[N-(4-methylphenyl)ferrocenylformamido]acetamide, [Fe(C5H5)(C33H31N2O2)], (II). They adopt broadly similar molecular conformations, with near-eclipsed cyclopentadienyl rings and near-perpendicular amide planes in their dipeptide-like chains, one of which is almost coplanar with its attached cyclopentadienyl ring but perpendicular to the aromatic ring bound to the N atom. In the supramolecular structure of (I), a two-dimensional network is constructed based on molecular dimers and a combination of intermolecular O-H···O, N-H···O and C-H···O hydrogen bonds, forming R2(2)(11), R2(2)(16), R2(2)(22) and C(9) motifs. These two-dimensional networks are connected by C-H···O and C-H···Cl contacts to create a three-dimensional framework, where one dichloromethane solvent molecule acts as a bridge between two neighbouring networks. In the packing of (II), classical hydrogen bonds are absent and an infinite one-dimensional chain is generated via a combination of C-H···O hydrogen bonds and C-H···π interactions, producing a C(7) motif. This work describes a simple synthesis and the supramolecuar structures of ferrocenyl dipeptide-like compounds and is significant in the development of redox-active receptors.

A modular electrochemical peptide-based sensor for antibody detection

We report a modular electrochemical peptide-based sensor targeting the anti-deamidated gliadin peptide (DGP) antibody. A recognition peptide, here DGP, is grafted onto a supporting peptide bearing a redox label. The fabricated peptide-based sensor supports the detection of the target antibody (anti-DGP antibody) in the nanomolar range.

A dielectric model of self-assembled monolayer interfaces by capacitive spectroscopy

The presence of self-assembled monolayers at an electrode introduces capacitance and resistance contributions that can profoundly affect subsequently observed electronic characteristics. Despite the impact of this on any voltammetry, these contributions are not directly resolvable with any clarity by standard electrochemical means. A capacitive analysis of such interfaces (by capacitance spectroscopy), introduced here, enables a clean mapping of these features and additionally presents a means of studying layer polarizability and Cole-Cole relaxation effects. The resolved resistive term contributes directly to an intrinsic monolayer uncompensated resistance that has a linear dependence on the layer thickness. The dielectric model proposed is fully aligned with the classic Helmholtz plate capacitor model and additionally explains the inherently associated resistive features of molecular films.

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.

The critical main-chain length for helix formation in water: determined in a peptide series with alternating Aib and Ala residues exclusively and detected with ECD spectroscopy

Critical main-chain length for peptide helix formation in the crystal (solid) state and in organic solvents has been already reported. In this short communication, we describe our results aiming at assessing the aforementioned parameter in water solution. To this goal, we synthesized step-by-step by solution procedures a complete series of N-terminally acetylated, C-terminally methoxylated oligopeptides, characterized only by alternating Aib and Ala residues, from the dimer to the nonamer level. All these compounds were investigated by electronic circular dichroism in the far-UV region in water solution as a function of chemical structure, namely presence/absence of an ester moiety or a negative charge at the C-terminus, and temperature. We find that the critical main-chain lengths for 3(10)- and α-helices, although still formed to a limited extent, in aqueous solution are six and eight residues, respectively.

Playing with peptides: how to build a supramolecular peptide nanostructure by exploiting helix···helix macrodipole interactions

A novel method to build bicomponent peptide self-assembled monolayers (SAMs) has been developed, by exploiting helix···helix macrodipole interactions. In this work, a peptide-based self-assembled monolayer composed of two helical peptides was immobilized on a gold surface. Specifically, a pyrene-containing octapeptide, devoid of any sulfur atom (A8Pyr), and a hexapeptide, functionalized at the N-terminus with (S,R) lipoic acid, for binding to gold substrates (SSA4WA) via a Au-S linkage, have been employed. Both peptides investigated attain a helical structure, because they are almost exclusively formed by strongly folding inducer C(α)-tetrasubstituted α-amino acids. We demonstrate that the two peptides generate a stable supramolecular nanostructure (a densely packed bicomponent peptide monolayer), where A8Pyr is incorporated into the SSA4WA palisade by exploiting helix···helix macrodipole interactions. The presence of both peptides on the gold surface was investigated by spectroscopic and electrochemical techniques, while the morphology of the monolayer was analyzed by ultra high-vacuum scanning tunnelling microscopy. The composition of the bicomponent SAM on the surface was studied by a combination of electrochemical and spectroscopic techniques. In particular, the amount of Au-S linkages from the sulfur-containing peptides was quantified from reductive desorption of the peptide-based SAM, while the amount of A8Pyr was estimated by fluorescence spectroscopy. The antiparallel orientation of the A8Pyr and SSA4WA peptide chains minimizes the interaction energy between the helix dipoles, suggesting that this kind of electrostatic phenomenon is the driving force that stabilizes the bicomponent SAM.

Formation process and solvent-dependent structure of a polyproline self-assembled monolayer on a gold surface

The formation process and structure of a self-assembled monolayer (SAM) of lipoic-acid-terminated polyproline on a gold surface in aqueous solution were investigated by several techniques. The amount of polyproline molecules on the gold surface was determined from the area of the reductive desorption peak, and orientation and thickness of the polyproline SAM were determined in situ by attenuated total reflection infrared (ATR-IR) spectroscopy and ellipsometry. The kinetics of the polyproline SAM formation process were discussed on the basis of these results. The in situ IR study confirmed that the conformation of the polyproline SAM was changed by changing the solvent from water to methanol and methanol to water, as is the case for polyproline dissolved in solution.

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.

De novo designed coiled-coil proteins with variable conformations as components of molecular electronic devices

Conformational changes of proteins are widely used in nature for controlling cellular functions, including ligand binding, oligomerization, and catalysis. Despite the fact that different proteins and artificial peptides have been utilized as electron-transfer mediators in electronic devices, the unique propensity of proteins to switch between different conformations has not been used as a mechanism to control device properties and performance. Toward this aim, we have designed and prepared new dimeric coiled-coil proteins that adopt different conformations due to parallel or antiparallel relative orientations of their monomers. We show here that controlling the conformation of these proteins attached as monolayers to gold, which dictates the direction and magnitude of the molecular dipole relative to the surface, results in quantitative modulation of the gold work function. Furthermore, charge transport through the proteins as molecular bridges is controlled by the different protein conformations, producing either rectifying or ohmic-like behavior.

EC-STM study of potential-controlled adsorption of substituted pyrimidinethiol on Au(111)

The ability of the pyrimidine derivatives to form numerous complexes and supramolecular assemblies makes them suitable for the construction of new functional surfaces. Therefore, in this paper, the adsorption behavior of 4-hydroxy-6-(trifluoromethyl)pyrimidine-2-thiol (HTPT) on a Au(111) surface has been investigated using electrochemical scanning tunneling microscopy (EC-STM). High-resolution imaging revealed that the HTPT molecules organize on a gold surface producing a highly ordered monolayer consistent with a (4 x radical3)R-30(0) superstructure. It has been observed that the arrangement of the molecules, as well as their orientation with respect to the substrate, remains stable over a relatively broad potential range from -0.40 to 0.55 V. It has been demonstrated that the presence of the functional groups attached to the aromatic ring affects the final structure of the HTPT adlayer, giving rise to the formation of the assembly with a uniform orientation of the molecules on the substrate.