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

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.

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

We review the status of protein-based molecular electronics. First, we define and discuss fundamental concepts of electron transfer and transport in and across proteins and proposed mechanisms for these processes. We then describe the immobilization of proteins to solid-state surfaces in both nanoscale and macroscopic approaches, and highlight how different methodologies can alter protein electronic properties. Because immobilizing proteins while retaining biological activity is crucial to the successful development of bioelectronic devices, we discuss this process at length. We briefly discuss computational predictions and their connection to experimental results. We then summarize how the biological activity of immobilized proteins is beneficial for bioelectronic devices, and how conductance measurements can shed light on protein properties. Finally, we consider how the research to date could influence the development of future bioelectronic devices.

Work function and temperature dependence of electron tunneling through an N-type perylene diimide molecular junction with isocyanide surface linkers

Conducting probe atomic force microscopy (CP-AFM) was employed to examine electron tunneling in self-assembled monolayer (SAM) junctions. A 2.3 nm long perylene tetracarboxylic acid diimide (PDI) acceptor molecule equipped with isocyanide linker groups was synthesized, adsorbed onto Ag, Au and Pt substrates, and the current-voltage (I-V) properties were measured by CP-AFM. The dependence of the low-bias resistance (R) on contact work function indicates that transport is LUMO-assisted ('n-type behavior'). A single-level tunneling model combined with transition voltage spectroscopy (TVS) was employed to analyze the experimental I-V curves and to extract the effective LUMO position ε_l = E_LUMO - E_F and the effective electronic coupling (Γ) between the PDI redox core and the contacts. This analysis revealed a strong Fermi level (E_F) pinning effect in all the junctions, likely due to interface dipoles that significantly increased with increasing contact work function, as revealed by scanning Kelvin probe microscopy (SKPM). Furthermore, the temperature (T) dependence of R was found to be substantial. For Pt/Pt junctions, R varied more than two orders of magnitude in the range 248 K < T < 338 K. Importantly, the R(T) data are consistent with a single step electron tunneling mechanism and allow independent determination of ε_l, giving values compatible with estimates of ε_l based on analysis of the full I-V data. Theoretical analysis revealed a general criterion to unambiguously rule out a two-step transport mechanism: namely, if measured resistance data exhibit a pronounced Arrhenius-type temperature dependence, a two-step electron transfer scenario should be excluded in cases where the activation energy depends on contact metallurgy. Overall, our results indicate (1) the generality of the Fermi level pinning phenomenon in molecular junctions, (2) the utility of employing the single level tunneling model for determining essential electronic structure parameters (ε_l and Γ), and (3) the importance of changing the nature of the contacts to verify transport mechanisms.

Bicomponent β-sheet assembly of dipeptide fluorophores of opposite polarity and sensitive detection of nitro-explosives

Fluorescent hydrogels formed by the bicomponent β-sheet co-assembly of dipeptide–pyrene amphiphiles of opposite polarity provide a 3D microenvironment to detect toxic nitro-explosives.

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.

Impact of Hydrogen Bonding on the Susceptibility of Peptides to Oxidation

The tendency of peptides to be oxidized is intimately connected with their function and even their ability to exist in an oxidative environment. Here we report high-level theoretical studies that show that hydrogen bonding can alter the susceptibility of peptides to oxidation, with complexation to a hydrogen-bond acceptor facilitating oxidation, and vice versa, impacting the feasibility of a diverse range of biological processes. It can even provide an energetically viable mechanistic alternative to direct hydrogen-atom abstraction. We find that hydrogen bonding to representative reactive groups leads to a broad (≈400 kJ mol^-1 ) spectrum of ionization energies in the case of model amide, thiol and phenol systems. While some of the oxidative processes at the extreme ends of the spectrum are energetically prohibitive, subtle environmental and solvent effects could potentially mitigate the situation, leading to a balance between hydrogen bonding and oxidative susceptibility.

Effect of Heteroatom Substitution on Transport in Alkanedithiol-Based Molecular Tunnel Junctions: Evidence for Universal Behavior

The transport properties of molecular junctions based on alkanedithiols with three different methylene chain lengths were compared with junctions based on similar chains wherein every third -CH₂- was replaced with O or S, that is, following the general formula HS(CH₂CH₂X)_nCH₂CH₂SH, where X = CH₂, O, or S and n = 1, 2, or 3. Conducting probe atomic force microscopy revealed that the low bias resistance of the chains increased upon substitution in the order CH₂ < O < S. This change in resistance is ascribed to the observed identical trend in contact resistance, R_c, whereas the exponential prefactor β (length sensitivity) was essentially the same for all chains. Using an established, analytical single-level model, we computed the effective energy offset ε_h (i.e., Fermi level relative to the effective HOMO level) and the electronic coupling strength Γ from the current-voltage (I-V) data. The ε_h values were only weakly affected by heteroatom substitution, whereas the interface coupling strength Γ varied by over an order of magnitude. Consequently, we ascribe the strong variation in R_c to the systematic change in Γ. Quantum chemical calculations reveal that the HOMO density shifts from the terminal SH groups for the alkanedithiols to the heteroatoms in the substituted chains, which provides a plausible explanation for the marked decrease in Γ for the dithiols with electron-rich heteroatoms. The results indicate that the electronic coupling and thus the resistance of alkanedithiols can be tuned by substitution of even a single atom in the middle of the molecule. Importantly, when appropriately normalized, the experimental I-V curves were accurately simulated over the full bias range (±1.5 V) using the single-level model with no adjustable parameters. The data could be collapsed to a single universal curve predicted by the model, providing clear evidence that the essential physics is captured by this analytical approach and supporting its utility for molecular electronics.

Genetically encoded conductive protein nanofibers secreted by engineered cells

Bacterial biofilms are promising tools for functional applications as bionanomaterials.

Electronic characterization of Geobacter sulfurreducens pilins in self-assembled monolayers unmasks tunnelling and hopping conduction pathways

The peptide subunit of Geobacter nanowires (pili) metal-reducing bacterium Geobacter sulfurreducens was self-assembled as a conductive monolayer. Its electronic characterized revealed tunneling and hopping regimes.