Electronic Transport in Molecular Wires of Precisely Controlled Length Built from Modular Proteins

Abstract

DNA molecular wires have been studied extensively because of the ease with which molecules of controlled length and composition can be synthesized. The same has not been true for proteins. Here, we have synthesized and studied a series of consensus tetratricopeptide repeat (CTPR) proteins, spanning 4 to 20 nm in length, in increments of 4 nm. For lengths in excess of 6 nm, their conductance exceeds that of the canonical molecular wire, oligo­(phenylene-ethylenene), because of the more gradual decay of conductance with length in the protein. We show that, while the conductance decay fits an exponential (characteristic of quantum tunneling) and not a linear increase of resistance with length (characteristic of hopping transport), it is also accounted for by a square-law dependence on length (characteristic of weakly driven hopping). Measurements of the energy dependence of the decay length rule out the quantum tunneling case. A resonance in the carrier injection energy shows that allowed states in the protein align with the Fermi energy of the electrodes. Both the energy of these states and the long-range of hopping suggest that the reorganization induced by hole formation is greatly reduced inside the protein. We outline a model for calculating the molecular-electronic properties of proteins.

A 300-fold conductivity increase in microbial cytochrome nanowires due to temperature-induced restructuring of hydrogen bonding networks

“…Combining theory with measurements of intrinsic, contact-free, conductivity measurements of individual nanowires as a function of nanowire length, voltage, and temperature, we find that a hopping mechanism explains the unexpected length and temperature dependence of nanowire conductivity. This was previously reported only in synthetic nanowires and nonredox proteins where it was limited to distances less than 20 nm ( 2 , 16 ).…”

“…This finding contrasts with most proteins that can transfer electrons only up to 1 to 2 nm, due to the exponential decay of current with distance, characteristic of the tunneling mechanism ( 1 ). Conductance in noncytochrome proteins has been limited to 20 nm ( 2 ). In contrast, conductance of OmcS nanowires is more comparable to synthetic molecular systems ( 13 – 18 ), for which the transport mechanism has been observed to transition from tunneling to hopping for molecular lengths greater than 4 nm ( 15 ).…”

“…The increase in H-bonding frequency upon cooling that we observed in our model is typical of secondary structures such as α helices and β sheets ( 38 ). However, OmcS has no β sheets and only 13% of helices ( 2 ). Therefore, the restructuring of the H-bonding network and its role in nanowire conductivity may be unique to OmcS.…”

“…The protein architecture between cofactors has long been considered a passive tunneling bridge resulting in transport that decays exponentially with distance, thus limiting transport to 1 to 2 nm ( 1 ). Some proteins can conduct over distances up to 20 nm ( 2 ). Long-distance (>20 nm) charge transport is usually achieved via a redox potential gradient generated by distinct redox enzymes, which provides an energetic driving force for the transport of charge toward the terminal acceptor ( 3 ).…”

Theory of Protein Charge Transfer: Electron Transfer between Tryptophan Residue and Active Site of Azurin

“…Proteins are increasingly viewed as charge conducting media. [1][2][3] The reason for conductivity is sought in chains of aromatic residues that can be oxidized and serve as relay elements to direct oxidizing electron holes to the protein surface to avoid oxidative damage of active sites of enzymes. [4][5][6] Each hop in this sequence is viewed as a single electron-transfer step 7 as traditionally described by the Marcus theory of electron transfer reactions.…”