Scientists have found that the hair-like nanoscale filaments on some bacteria have electrical conductivity comparable to that of copper, an advance that may lead to low-cost, non-toxic, biological components for light-weight electronics.
Although proteins are usually electrically insulating, hair-like nanoscale filaments (called pili) on the surface of Geobacter bacteria exhibit metallic-like conductivity.
To understand why pili are conductive, scientists from the University of Massachusetts and Brookhaven National Laboratory in the US used X-ray diffraction to analyse the structure of the filaments.
They found that the electronic arrangement and the small molecular separation distances (about 0.3 nanometres) give the pili an electrical conductivity comparable to that of copper.
Enhance pili’s electrical conductivity through genetic engineering could be used to construct low-cost, non-toxic, nanoscale, biological sources of electricity for light-weight electronics and for bioremediation, researchers said.
Direct measurement of multiple physical properties of Geobacter sulfurreducens pili have demonstrated that they possess metallic-like conductivity, but several studies have suggested that this conductivity is unlikely based on the structures of the G sulfurreducens pilus predicted from homology models.
Researchers examined the pili with synchrotron X-ray microdiffraction and rocking-curve X-ray diffraction.
Both techniques showed a periodic 0.32-nanometre spacing in G sulfurreducens pili. This was missing in the pili of strain Aro5, which lack key aromatic acids required for conductivity.
The intensity of the 0.32-nanometre peak increased 100-fold when the pH was shifted from 10.5 to 2, corresponding with a previously reported 100-fold increase in pilus conductivity with this pH change.
These results suggest a clear structure-function correlation for metallic-like conductivity that can be attributed to overlapping orbitals of aromatic amino acids.
A homology model of the G sulfurreducens pilus predicted that aromatic amino acids in it are packed within 0.3 to 0.4 nanometre, consistent with the experimental results.
Thus, the predictions of homology modelling are highly sensitive to assumptions inherent in the model construction.
The experimental results further support the concept that the pili of G sulfurreducens represent a novel class of electronically functional proteins in which aromatic amino acids promote long-distance electron transport.
The mechanism for long-range electron transport along the conductive pili of G sulfurreducens is of interest because these “microbial nanowires” are important in biogeochemical cycling as well as applications in bioenergy and bioelectronics.
The findings are expected to be useful in the design of novel bioelectronic materials, researchers said.