In natural form, the protein resembles the misshapen compounds riddling the brains of mad cow disease and Alzheimer's patients. Scientists have made fibers from the protein, called NM, that have proven unusually durable against extremes of acid, heat and cold -- the same harsh physical conditions that might be encountered during factory manufacturing.
"It's exciting because it really suggests we can build devices harnessing the extraordinarily diverse and wonderful properties of proteins that nature has provided us after millions of years of evolution," researcher Susan Lindquist, director of the Whitehead Institute for Biomedical Research, told United Press International. "It's sort of a dream world of science fiction in its baby steps, but I think it's not too far off from what we can achieve very soon."
For years, scientists have tinkered with creating wires only nanometers -- billionths of a meter -- wide. A nanometer is to an inch what a small grape is to the whole planet.
Instead of building such wires by pouring metal down long tubes, some investigators instead are experimenting with biological materials that can arrange themselves into strings spontaneously -- DNA and bacteria-killing viruses, for examples. These strands would then serve as the backbone for metal to attach onto, making wires.
The problem with these schemes is that biological components are fragile and not highly electrically conductive. In findings appearing online on March 31 from Proceedings of the National Academy of Sciences, Lindquist and her team reveal they might circumvent this problem and develop nanowires for electronics by using genetically engineered yeast amyloids.
Amyloids are misfolded proteins found plaguing the victims of cystic fibrosis, Alzheimer's disease and many other ailments. The researchers used a kind of yeast protein called a prion. Amyloid prions are believed to be the cause of mad cow disease and related plagues. They are infamous for their extraordinary resistance to destruction.
Taking advantage of the prions' durability, the scientists assembled the molecules into fibers by spinning them in centrifuges. By controlling the rate of spin, the concentration of protein in solution or by pounding the fibers with ultrasound vibrations, the researchers found they could make 10-nanometer-wide strands assemble themselves into exact lengths. Moreover, these fibers were incredibly stable; not breaking down when exposed to near-boiling heat, freezing cold, acid, salt, or the organic solvent ethanol.
"They're really durable and they self-assemble really efficiently," Lindquist said.
Lindquist and colleagues genetically engineered the protein -- which is not infectious to humans -- so that its surface was studded with the amino acid cysteine, which spontaneously bonds with gold. Cysteine is a protein building block chemically much like alcohol, although cysteine possesses a sulfur atom where its intoxicating counterpart has an oxygen atom.
When the fibers were placed between electrodes, they absorbed gold nanoparticles sprinkled near them. The gold then helped draw silver ions out from the surrounding fluid, for a silver-and-gold-plated wire "as conductive as if you had a solid gold wire," Lindquist said.
"This is really a very clever report," nanoscientist Charles Martin at the University of Florida at Gainesville said of the findings. "They have very good chemical stability, which is good because in chemical processing, you want to make sure the protein can withstand whatever chemistry is necessary. You might need an acid bath to plate metal with."
He explained one of the key functions of proteins in nature is to bind to other molecules with extreme specificity. One could imagine selectively binding these strands with other proteins that would act like glue to arrange the wires in a number of different kinds of junctions in space. "You can build up circuit elements this way," Martin said.
"The beauty of a protein system is the possibility of getting this one to interact with other protein systems as a possible way to develop self-assembling electronic circuits," Martin said. "That is one of the nirvanas of molecular electronics."
(Reported by Charles Choi, UPI Science News, in New York)
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