MD simulations of WT and Y27A pili. (A,B) Snapshots of the geometry-optimized WT (A) and Y27A (B) pilus model (black, Y27; orange, other aromatic residues) and detail of aromatic clusters with inter-aromatic distances. (C,D) 3D-projection of aromatic density (C), aromatic contacts (D), and electrostatic surface map (E) of WT and Y27A pili.
A microbial protein fiber discovered by a Michigan State University scientist transports charges at rates high enough to be applied in humanmade nanotechnologies. The discovery describes the high-speed protein fiber produced by uranium-reducing Geobacter bacteria. The fibers are hair-like protein filaments called “pili” that have the unique property of transporting charges at speeds of 1 billion electrons per second.
“This microbial nanowire is made of but a single peptide subunit,” said Gemma Reguera. “Being made of protein, these organic nanowires are biodegradable and biocompatible. This discovery thus opens many applications in nanoelectronics such as the development of medical sensors and electronic devices that can be interfaced with human tissues.”
Since existing nanotechnologies incorporate exotic metals into their designs, the cost of organic nanowires is much more cost effective as well, she added. How the nanowires function in nature is comparable to breathing. Bacterial cells, like humans, have to breathe. The process of respiration involves moving electrons out of an organism. Geobacter bacteria use protein nanowires to bind and breathe metal-containing minerals such as iron oxides and soluble toxic metals such as uranium. The toxins are mineralized on the nanowires’ surface, preventing metals from permeating the cell.
Reguera’s team purified their protein fibers, which are about 2 nanometers in diameter. Using the same toolset of nanotechnologists, the scientists were able to measure the high velocities at which the proteins were passing electrons. “They are like power lines at the nanoscale,” Reguera said. “This also is the first study to show the ability of electrons to travel such long distances –more than a 1,000 times what’s been previously proven — along proteins.”
They also identified metal traps on the surface of the protein nanowires that bind uranium with great affinity and could trap other metals. These findings could provide the basis for systems that integrate protein nanowires to mine gold and other precious metals, scrubbers that can be deployed to immobilize uranium at remediation sites and more. Reguera’s nanowires also can be modified to seek out other materials in which to help them breathe.
“The Geobacter cells are making these protein fibers naturally to breathe certain metals. We can use genetic engineering to tune the electronic and biochemical properties of the nanowires and enable new functionalities. We also can mimic the natural manufacturing process in the lab to mass-produce them in inexpensive and environmentally friendly processes,” Reguera said. “This contrasts dramatically with the manufacturing of humanmade inorganic nanowires, which involve high temperatures, toxic solvents, vacuums and specialized equipment.”
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