Within biology, long-range electron transport can be achieved through multiple protein assemblies, including small tetraheme cytochrome (STC) proteins. STC proteins can form nanocrystals, where close contacts between hemes allows the excitations to hop rapidly between them, allowing for long-range electron transfer to take place. The STC nanocrystals therefore can transport electrons over long distances up to several micrometers, creating potential applications as biological nanowires or sensors in the medical, industrial or environmental sector. Recent crystallographic structures revealed heme organization within STCs, with fixed distances between hemes within the crystal. Thermalization at room temperature expands the crystal, which subtly increases these distances. Molecular dynamics simulations of water solvated STC crystal chunks allows the quantification of temperature dependent distance changes. After obtaining ensemble averages for distance distributions from simulation trajectories, we calculate electron transfer rates within simplified STC nanocrystal models using graph theory tools. We find that the wire dimensions significantly influence the electron transfer rate. Narrow nanowires feature lower potential currents as their length increases due to natural variation in inter-heme distances within a thermalized crystal. This is remedied by increasing the diameter for the nanocrystal, which provides more pathways to avoid a local bottleneck within the electron transfer network. Our hope is to contribute with this research to the development of bio-compatible wires and sensors.

Electron transport rates depend on wire dimensions in small tetraheme cytochrome nanowires

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