Biological films containing the microbe Geobacter sulfurreducens, as well as protein layers composed of pili filaments, have been recently reported to exhibit electrical conductivities similar to those of doped organic semiconductors. The temperature dependence of these conductivities has led to lively debate on the most appropriate transport mechanisms: “electron hopping” or “metallic conduction”. One line of thinking proposes that charge equivalents are conducted on pili by a succession of electron transfer reactions among redox proteins. An opposing view provides the idea that the pili have metallic conductivity and invokes π–π interchain stacking between aromatic amino acid residues as the critical electron-coupling unit for charge transport. Difficulties in purifying pili crystals and identifying the distances between extracellular cytochromes on pili prevent the correlation of the electrical properties to an assembled electroactive unit; clear structure-to-property relationships at the molecular level thus remain lacking.
In this study, we approached the problem from the perspective of what is known on organic semiconducting materials and the exploration on the conventional charge transport between aromatic units within the protein framework(s). We constructed two protein models of GS pili based on its protein sequence, a recently reported NMR monomer structure, and a related type IVa pili protein model from Neisseria gonorrhoeae as both share high sequence conservation regions in subunits responsible for superstructure assembly. Resulting inter-aromatic distances extracted from their spatial location within GS pili superstructure suggest that aromatic amino acids are not packed sufficiently tight for π–π interactions. Analysis combining the aromatic units with quantum mechanical electronic calculations leads to low electronic transport properties and charge-carrier mobilities that are insufficient to account for experimentally determined conductivities. These results based on such geometry are in contrast with highly cited mechanistic proposals for those biological films, which invoke inter-aromatic contacts as being critical to the conductivity measurements of biological films, thus paralleling the conductivity mechanisms of biological films with that in conducting conjugated materials. Therefore, a different electron transport mechanism for GS pili needs to be sought.