Tuning electronic transport via hepta-alanine peptides junction by tryptophan doping

Abstract

Charge migration for electron transfer via the polypeptide matrix of proteins is a key process in biological energy conversion and signaling systems. It is sensitive to the sequence of amino acids composing the protein and, therefore, offers a tool for chemical control of charge transport across biomaterial-based devices. We designed a series of linear oligoalanine peptides with a single tryptophan substitution that acts as a "dopant," introducing an energy level closer to the electrodes' Fermi level than that of the alanine homopeptide. We investigated the solid-state electron transport (ETp) across a selfassembled monolayer of these peptides between gold contacts. The single tryptophan "doping" markedly increased the conductance of the peptide chain, especially when its location in the sequence is close to the electrodes. Combining inelastic tunneling spectroscopy, UV photoelectron spectroscopy, electronic structure calculations by advanced density-functional theory, and dc current-voltage analysis, the role of tryptophan in ETp is rationalized by charge tunneling across a heterogeneous energy barrier, via electronic states of alanine and tryptophan, and by relatively efficient direct coupling of tryptophan to a Au electrode. These results reveal a controlled way of modulating the electrical properties of molecular junctions by tailormade "building block" peptides. oligopeptide | electron transport | self-assembled monolayer | inelastic electron tunneling spectroscopy | doping E lectron transfer (ET) processes taking place between prosthetic groups across the polypeptide matrices of proteins (1-3) have been extensively explored by, e.g., time-resolved photolysis (4, 5), pulse radiolysis (6, 7), scanning tunneling microscopy (8, 9), electrochemical methods (10-12), and by theoretical studies (13-16). The surprisingly fast and efficient ET over considerable distances (up to ∼25 Å) (17) via peptide matrices in proteins suggests possible development of peptide-and protein-based bioelectronics with diverse functionality and relative ease of modification (18,19). Studying electron transport (ETp) via solid-state molecular junctions represents an approach, bridging the study of basic ET-related phenomena and electronic devices, where measuring ET rates as a function of an electrochemical driving force is replaced by measuring current across molecular junctions as a function of an applied electrical voltage (20). Combining the concepts and methods of molecular electronics such as currentvoltage (I-V) line-shape analysis (21, 22), temperature-dependent measurements (23, 24), and electronic spectroscopy techniques (25-27) may yield new insights into the mechanism of ETp across the peptide matrix of proteins (28-30) and pave the road to peptide-and protein-based electronic devices for switching, rectification, and memory (18, 31).ETp via homopeptides has been investigated, probing the roles of specific amino acid residues, their protonation, and secondary structure (29,32). Synthetic heteropeptides with defined compositi...

Protein bioelectronics: a review of what we do and do not know

“…Nevertheless, ETp comparisons showed that conduction via the helices was substantially higher than via the extended peptides. 155,410 In further work, coupling to the electrode contact was found to be critical for ETp efficiency across Au/7-peptide/Au junctions, as shown by comparing ETp between peptides with tryptophan (an amino acid with an aromatic indole side chain) in different positions along the chain. 403 In most biological ET processes, the cofactor of the protein is thought to enhance the electron tunneling probability with respect to vacuum tunneling.…”

Impact of Hydrogen Bonding on the Susceptibility of Peptides to Oxidation

“…Due to the ubiquity of hydrogen bonds in peptides, inter alia as a prime determinant of the three‐dimensional structure of proteins, this phenomenon is potentially of great importance. In the current article we examine it in detail, and explore its relevance to peptide function, for example, in electron transport and enzyme catalysis, post‐translational metabolism, and susceptibility and resistance to degradation in oxidative environments.…”

“…While this qualitative change is note ntirely unexpected, it is also of interestt oe xamine itsb iological impact. [7] Due to the ubiquity of hydrogen bonds in peptides, inter alia as ap rime determinant of the three-dimensional structure of proteins, [8] this phenomenon is potentially of great importance.I nt he current article we examinei ti nd etail,a nd explore its relevance to peptidef unction, fore xample, in electron transport [9, 10] and enzyme catalysis, [11, 12] post-translational metabolism, [13] and susceptibility and resistance [14] to degradation in oxidativee nvironments.Am ost characteristic feature of all peptides is the amide bond. We use as mall model system for this peptide backbone linkage,n amely, N-methylacetamide (Figure 1), to initially examine effects on IEs.…”

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

“…31 This work is motivated by the experimental results of Ref. 34. In that paper, the authors reported that doping a 7-alanine (7A) homopeptide with a tryptophan unit can significantly enhance the conductance of junctions formed by contacting self-assembled monolayers (SAMs) to gold electrodes.…”