Probing the Role of Molecular Orientation on Spin-Selective Electron Transport through the Bacterial Multiheme Cytochrome MtrA

Extracellular electron transfer allows microbes to gain energy by transferring electrons between the cellular interior and external minerals that serve as electron acceptors or donors for respiration. Multiheme cytochromes, including the periplasmic decaheme MtrA, are key molecular conduits that mediate this biotic-abiotic electron exchange. Previous studies of electron transport through multiheme cytochromes adsorbed on ferromagnetic substrates demonstrated that these molecules exhibit the chirality induced spin selectivity (CISS) effect. This property is linked to efficient electron transfer by enhancing the transmission probability of one preferred electron spin, dependent on the chirality of the molecule. However, key questions remain about the factors that control the magnitude and sign of spin polarization in these molecules. While the effects of molecular size and secondary structure were previously analyzed, the roles of the molecule-substrate interface and orientation of the cytochromes were not investigated. Here, using conductive probe atomic force measurements of MtrA monolayers adsorbed onto ferromagnetic substrates, we demonstrate that both orientations of molecular adsorption on the magnetic substrate show higher conduction for the up-magnetization direction, indicating that the protein's preferred spin direction is independent of its orientation in the junction, unlike contrasting results previously reported for smaller chiral molecules. The maintained spin preference for opposite molecular orientations on the substrate indicates that the spin selectivity is controlled primarily by the inherent cytochrome chirality and structure rather than the precise coupling to the substrate.