Electric-Field Effects on Structure and Conductance in a Cytochrome b₅₆₂ Junction

Electric fields present under experimental bias can significantly affect the structure and conductance of biomolecular junctions, especially in large, charged proteins. Here, we develop a multiscale computational framework to study these effects in an extended junction based on the redox protein cytochrome b₅₆₂, a system too large for straightforward application of non-equilibrium Green's function (NEGF) methods, which are typically used for molecular-junction calculations. The approach combines non-equilibrium classical molecular dynamics (MD), density functional theory (DFT) calculations with external electric fields, and approximate transport calculations based on the projection operator diabatization (POD) method. This enables us to disentangle electronic polarization from field-induced structural changes and to assess their combined influence on tunneling conductance in vacuum and aqueous solution. We show that electric fields induce asymmetries in conductance and current responses with respect to applied bias. The structural field response is partly compensated by electronic polarization in both vacuum and aqueous solution. Whereas the protein model is neutral in vacuum and the field response is dominated by dipole interactions, charged (de)protonated groups in solution are screened by polar water molecules and ions. Therefore, the total field effects are relatively weak and represent only a small perturbation to the conductance.