Modeling electronic coupling between Heme groups in extracellular cytochrome polymers with first-principles density functional theory

OmcS is an example of a micro-meter extracellular cytochrome peptide wire that transfers electrons; vital to the metabolism of Geobacter sulfurreducens, the bacteria that excretes it. Cryo-EM experiments (PDB: 6ef8) show a single peptide monomer with 6 heme groups stacking to form the polymer wire, creating a linear chain of heme groups. For OmcS and other similar peptide wires, these heme groups are identified in the literature as the charge carrier sites for an excess electron under aerobic conditions; orthogonalized heme charge carrier sites are used for modeling charge carrier evolution. To better understand the charge carrier dynamics in these peptide wires, we present canonical Kohn-Sham orbitals of isolated single and double heme group subsystems from the OmcS structure, manipulating the inter-heme geometry in order to tune the extracted charge carrier site energies and couplings. DFT simulations thus show the relationship between charge carrier binding energy, coupling, and dimer structure. Our results indicate that modifying the inter-heme geometry can result in desired charge carrier properties.