Charge Transfer in Polypeptides: Effect of Secondary Structures on Charge-Transfer Integral and Site Energies

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.

Bidirectional Electron-Transfer in Polypeptides with Various Secondary Structures

The protein-mediated bidirectional electron transfer (ET) is the foundation of protein molecular wire, and plays an important role in the rapid detection of oxo-guanine-adenine DNA mismatches by MutY glycosylase. However, the influences of structural transitions on bidirectional ET are still not clear. In this work, the modified through-bond coupling (MTBC) model was further refined to correlate the structural transition and ET rate more quantitatively. With this model, various polyglycine structures (3₁₀-helix, α-helix, β-sheets, linear, polyproline helical I and II) were studied to explore the influences of structural transitions on bidirectional ET. It was found that the HOMO-LUMO gaps (ΔE) in CN (from the carboxyl to amino terminus) direction are much lower than that in opposite direction, except for polypro I. However, with the equal tunneling energy, the differences between bidirectional ET rates are slight for all structures. In structural transitions, we found that the ET rates are not only affected by the Ramachandran angles, but also correlated to the alignment of C = O vectors, the alignment of peptide planes and the rearrangement of other structure factors. The detailed information can be used to rationalize the inhomogeneous ET across different protein structures and design more efficient protein molecular wires.

Computational Studies on Structural, Excitation, and Charge-Transfer Properties of Ureidopeptidomimetics

Peptides with ureido group enclosing backbones are considered peptidomimetics and are known for their higher stabilities, biocompatibilities, antibiotic, inhibitor, and charge-transduction activities. These peptidomimetics have some unique applications, which are quite different from those of natural peptides. Hence, it is imperative to appreciate their properties at a microscopic level. In this regard, this work outlines, in detail, the charge transfer (CT) properties, hole-migration dynamics, and electronic structures of various experimentally comprehended ureidopeptidomimetic models using density functional theory (DFT). Time-dependent DFT and complete active space self-consistent field computations on basic models provide the necessary evidence for the viability of CT from the end enfolding the ureido group to the other end with a carboxylate entity. This donor-to-acceptor CT has been reflected in excitation studies, in which the higher intensity band corresponds to CT from the π orbital of the ureido group to the π* orbital of the carboxylate entity. Further, hole-migration studies have shown that charge can evolve from the ureido end, whereas the hole generated at the carboxylate end does not migrate. However, hole migration has been reported to occur from both ends (amino and carboxylate ends) in glycine oligopeptides, and our studies show that the ability to transfer and migrate charge can be tuned by modifying the donor and acceptor functional groups in both the neutral and cationic charge states. We have analyzed the possibility of hole migration following ionization using DFT-based wave-packet propagation and found its occurrence on a ∼2-5 fs time scale, which reflects the charge-transduction ability of peptidomimetics.

Design of single peptides for self-assembled conduction channels

Self-assembly of peptides provides the possibility of achieving relatively long range order on surfaces. These ordered peptides can also form channels that can be used as conduction channels. In the past, studies were focused on electron conduction through the secondary structure and amine bond of peptides and these restrict conduction of electrons over a short range (a few nanometers). In this work, we demonstrate the realization of electron conduction over a longer range of a few hundred nanometers via π-π stacking of the phenyl groups in the tyrosine residue of a single peptide. The peptide used in this work was designed with a phenyl ring for π-π stacking at one end and a carboxylic group at the other end for binding to aminopropyltriethoxysilane (APTES) treated silicon wafer. The distance between the peptides is controlled by a disulfide bond formed between neighboring cysteine residue and also by the amine groups of aminopropyltriethoxysilane. We demonstrate that the self-assembled peptide is conducting in the dry state over hundreds of nanometers, realizing the possibility of using peptide as a molecular wire.

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.