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

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.

Automated summary

DNA molecular wires have been extensively studied, while the same is not true for proteins. In this study, a series of consensus tetratricopeptide repeat (CTPR) proteins were synthesized and studied, and it was found that their conductance exceeded that of the canonical molecular wire oligo(phenylene-ethylenene). The decay of conductance with length in the protein followed an exponential pattern, characteristic of quantum tunneling, and a square-law dependence on length, characteristic of weakly driven hopping. Measurements of the energy dependence of the decay length ruled out the quantum tunneling case, and a resonance in the carrier injection energy showed alignment with the Fermi energy of the electrodes.