New nanotubes self-assemble on command.

Microscopic tubes just a few nanometers wide have recently jumped to the forefront of materials research.

These so-called nanotubes now come in two main classes: concentric ring carbon nanotubes, which look like little straws, and zeolite nanotubes, porous structures that serve as molecular sieves with industrial applications.

Now, chemists report fabricating an entirely new class of nanotube built out of peptides, or protein fragments, with potential biological applications.

M. Reza Ghadiri, a chemist at the Scripps Research Institute in La Jolla, Calif., and his colleagues describe the design, synthesis, and characterization of this new class of orgnanic nanotube in the Nov. 25 NATURE.

"For the first time, tubular structures on the molecular scale can be used in biological settings," Ghadiri says. "These nanotubes may be able to form molecular channels, self-assembling inside cell membranes and acting like junctions for transferring molecules into and out of, or between, cells."

"From another point of view, these nanotubes may also be useful for [delivering] cell-specific cytotoxins" -- for example, to destroy cancer cells or even viruses, he adds. "Since different kinds of cells have unique membrane properties, we think it would be interesting to look at cell-specific targets."

Given that these nanotubes are made entirely of molecules commonly found in biological systems, their ability to self-assemble is both unusual and intriguing, Ghadiri says. When tripped off chemically -- using a method called "proton triggering" -- a loos conglomeration of disk-shaped peptide rings will suddenly stack themselves together, forming regular tubes hundreds together, forming regular tubes hundreds of nanometers long and only 7 to 8 angstroms in diameter.

Moreover, the dimensions of these hollow, open-ended rods lie completely within the chemist's control -- a feature that makes them particularly appealing for certain kinds of applications.

"They're like little test tubes in which we can peform reactions or confine the growth of a material placed inside it sort of like a cast or mold," says Ghadiri. "We can rigorously control the tubes' sizes and shapes, their internal dimensions, and fine-tune their surface properties. This control gives us many interesting options."

For biological systems, the researchers see membrane channels as a strong posibility. "So are novel transport devices and drug delivery systems." Ghadiri says. In the area of materials research, "We'd like to try to build semiconducting or copper naowires with interesting optical and electrical properties inside these confined structures," he adds.

New types of molecular devices and catalysts could also emerge from these biologically based nanotubes, according to Juan R. Granja, a Scripps chemist and report coauthor. The key to finding such new uses will come from tinkering with the amino acid sequences and adjusting tube dimensions to get the best fit for particular molecules to be placed inside.

Chemists may even attempt to grow crystals within these new nanometer-size molds -- given that the tubes' inner dimensions, which would serve as the template, can be controlled to within a feq angstroms. "This has been very difficult to do so far because of a lack of uniformly shaped tube devices," says Ghadiri.