Can Electron Transport through a Blue-Copper Azurin Be Coherent? An Ab Initio Study

Multiple experiments on the electron transport through solid-state junctions based on different proteins have suggested that the dominant transport mechanism is quantum tunneling (or coherent transport). This is extremely surprising given the length of these molecules (2–7 nm) and their electronic structure (mainly comprising very localized molecular orbitals). Overall, this is probably the single most important puzzle in the field of biomolecular electronics and calls for rigorous calculations of the tunneling probability in protein-based junctions. Motivated by these experiments, we tackle here this problem and report a comprehensive theoretical study of the coherent electron transport in metal–protein–metal junctions based on the blue-copper azurin from Pseudomonas aeruginosa, which is the workhorse in protein electronics. More precisely, we focus on single-molecule junctions realized in STM-based experiments and analyze a wide variety of contact scenarios. Our calculations are based on a combination of molecular dynamics simulations and ab initio transport calculations. Our results unambiguously show that when azurin is not deformed and retains its pristine structure, the end-to-end tunneling probability is exceedingly small and does not give rise to any measurable electrical current. On the other hand, we find that much higher tunneling probabilities are possible when either the STM tip (indented from the top) substantially compresses the protein or the protein is contacted sideways, significantly reducing the effective junction length. We also show that in certain configurations, the presence of surrounding water can also increase the conductance but it cannot explain the high conductance values reported experimentally. In all cases, the current is found to flow through the Cu atom of this metalloprotein, although the role of several other levels close to the Fermi energy cannot be ruled out. We remark that we only evaluate the efficiency of coherent transport and the analysis of the relevance of other potential charge-transport mechanisms is out of the scope of our work.