Structural,\nSolvent, and Temperature Effects on Protein\nJunction Conductance

Cytochrome <i>b</i><sub>562</sub> is a small\nredox-active\nheme protein that has served as an important model system for understanding\nbiological electron transfer processes. Here, we present a comprehensive\ntheoretical study of electron transport mechanisms in protein–metal\njunctions incorporating cytochrome <i>b</i><sub>562</sub> using a multi-scale computational approach. Employing molecular\ndynamics (MD) simulations, we generated junction geometries for both\nvacuum-dried and solvated conditions, with the protein covalently\nbound to gold contacts in various configurations. Coherent tunneling,\ndescribed by the Landauer–Buttiker formalism within the density\nfunctional theory (DFT) framework, is compared to the incoherent hopping\ncharge transport mechanism captured by the semi-classical Marcus theory.\nThe tunneling was identified as the dominant mechanism explaining\nthe experimental data measured on the cytochrome <i>b</i><sub>562</sub> junctions, exhibiting exponential yet very shallow\ndistance dependence. While the structural orientations and protein\ncontacts with the electrodes influence the junction conductance significantly,\nthe solvation effects are relatively small, affecting the electronic\nproperties mostly via the adsorption arrangement. On the other hand,\nthe considerable temperature dependence of the conductance was found\nstrong only for hopping, while the tunneling current magnitudes remain\npractically unaffected and are a good indicator of the coherent mechanism\nin this case.