While biomedical, electronics, and other branches of research are marching steadily into the realm of the smaller-than-small nanometer scale, building needed materials at this scale has been problematic.
Recently, however, a team from The Scripps Research Institute unveiled a novel approach to the problem that yields a material with novel properties, which some might find reminiscent of Flubber. The material is produced using naturally occurring proteins as templates for uniform, self-assembled, nano-scale construction.
The material, an organic polymer described in today's early, online edition of the Proceedings of the National Academy of Sciences (PNAS), could one day find application in everything from screening for disease to microelectronics.
Using Nature's Tricks
Nature is replete with examples of molecules such as DNA that self-assemble with uniform patterns on the nanoscale, but until now researchers have had limited success duplicating such processes. The new study, however, provides one synthetic method that has effectively mimicked the templating strategy used in nature for nanoscale construction in the lab.
To create the new material, the Scripps Research team, led by Scripps Research President Richard Lerner and Assistant Professor Tobin Dickerson, began with a natural nanoscale product, a bacterial virus or phage. A nanometer is one billionth of a meter, or the width of a few average atoms.
Specifically, the product the team worked with was a phage known as M13. If scaled up, the phage is proportionally equivalent to a 4-foot-long pencil, with the tip and eraser roughly representing the active parts of the phage that infect bacteria. Other proteins that are biologically inert and analogous to the wood body of the pencil provide the filamentous phage's structure.
Having worked with phage extensively in other applications, the team decided to explore the possibility of using those structural proteins as a potential template for nanoscale construction. To do so, the team chemically modified molecular protrusions on the proteins so they would attract and bind with the components needed to form strands of polyacrylamide, a common polymer used to make laboratory gels.
The resulting polymer-phage combination, which twists into helices like DNA and RNA molecules, takes on the shape of a comb with the polymers as the teeth. These teeth in turn interlock to form a strangely resilient, rubbery solid.
Once the new material, known as a protein-polymer bioconjugate, was created, the group was shocked to find that it was almost impossible to break a sample apart. It could be sliced, but no matter how hard researchers compressed or squeezed it, it always bounced back to its original state, because the stable phage proteins act like rebar in concrete to provide strength.
Further analyses uncovered additional important characteristics. The combs do not grow completely uniformly—some combs grow more teeth than others, for instance, before interlocking with a nearby comb. But the combs can only be a prescribed distance apart for the chemical interlocking to occur, which leads to uniform size for the pores between the comb teeth. The pores proved to be about 4 nanometers wide and greater than 1 micrometer (one millionth of a meter) in length.
This uniformity is in stark contrast with experiments which showed that simply mixing the phages with polymers without the templating procedure produces a chaotic hodge podge at the molecular level.
Interestingly, after creating the material, the group discovered that a British scientist had done theoretical calculations about how flexible rods—like the phages—would pack together using the least amount of energy.
"It was extremely gratifying to see that the mathematics had exactly predicted what we were observing," says Dickerson
Putting It to Use
Although the work was intended mainly as a proof of concept for using the phages as templates, the researchers have a number of potential applications in mind. The phage can be produced easily and cheaply in very large quantities, and the polymer components are readily available and simple to combine to create the final material. These characteristics would make commercial application possible.
"In essence, bacteriophage-derived materials are a renewable resource," says Dickerson.
The defined channels established by the pores in the material might be adapted as pathways for electrons for use in microelectronics. Or, the pores might be used as a filter for certain molecules, for example to test blood samples for proteins whose presence is tied to particular diseases. More complex potential uses might include altering the biologically active portions of the phage to attract specific molecules, forcing them into the polymers' pores, or to block others.
"These tools can be visualized like Tinkertoys ® or Legos ®," says Dickerson of the possibilities. "You can think about this really in engineering terms using macroscopic analogs such as baskets, or lids, or holes."
To add to the list of potential applications, the team has already begun exploring additional materials that might be created using the basic phage construction scheme.
In addition to Dickerson and Lerner, authors on the paper, entitled "Biologically templated organic polymers with nanoscale order," were Bert Willis, Lisa Eubanks, Malcolm Wood, and Kim Janda, all of Scripps Research. See http://www.pnas.org/cgi/content/abstract/0711308105v1.
The work was supported by the Skaggs Institute for Chemical Biology, Worm Institute for Research and Medicine, and a National Institutes of Health Kirschstein National Research Service Award.
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