Nonuniform Distance Decay of Conductance in a Series of Multiheme Cytochrome Bioelectronic Junctions.

Multiheme cytochromes have emerged as a promising new class of bioelectronic materials for potential use in soft and biocompatible electronics. To aid these developments it is essential to understand how conductance decays with increasing size of multiheme proteins and which parts of the proteins govern their high conductance. Here we present a systematic investigation of six native multiheme cytochromes forming electronic junctions of increasing size. We find that the computed conductances are best described by a protein-specific intercept model with a single exponential distance decay constant of β = 1.8 nm-1 shared across all six proteins and an increasing intercept with protein size. Moreover, our calculations suggest that conductance for larger multiheme cytochromes tends to be less sensitive to thermal protein fluctuations than for smaller proteins. Both observations can be explained by the higher density of protein electronic states with increasing protein size. Fe ions appear to be unimportant for electronic conduction and the contribution of heme-cofactors is relatively small compared to their essential role for electron transfer in native biological environment. Our calculations suggest that it is mainly the protein frame and the contacts between amino acids and electrodes that govern electronic conductance in the series of multiheme cytochromes investigated.