Peptide nanotube acts as tunnel for ions.

Say the word "nanotube," and everyone assumes you're talking about a cylindrical molecule of carbon. However, some researchers have turned their attention to a different kind of nanotube--one made from stacks of ringshaped protein fragments called peptides.

Investigators at the Scripps Research Institute in La Jolla, Calif., report that peptide nanotubes can act as channels, allowing small ions and molecules to pass from one side of a membrane to the other. This property, which is analogous to the function of numerous naturally occurring channel molecules in cell membranes, could make these nanotubes useful as sensors or even, by poking holes in cell walls, as antibiotics, says study coauthor M. Reza Ghadiri.

Ghadiri and his colleagues Thomas D. Clark and Lukas K. Buehler let ringshaped peptides diffuse into a membrane made of a double layer of lipid molecules, where the peptides arrange themselves into tubes by forming hydrogen bonds with each other. Each ring, called a cyclic [[Beta].sup.3]-peptide, consists of four amino acids that differ from their natural counterparts by the addition of a carbon atom. Previously, the group had synthesized peptide nanotubes from a different class of amino acids joined in rings of eight.

The researchers determined the ion conductance of individual [[Beta].sup.3]-peptide nanotubes--"not a trivial measurement," says Samuel H. Gellman of the University of Wisconsin-Madison. They discovered that the new nanotubes conducted ions as effectively as the eight-ringed ones and more efficiently than gramicidin A, a natural peptide channel with antibiotic properties. The group's findings appear in the Feb. 4 Journal of the American Chemical Society.

The cyclic [[Beta].sup.3]-peptide nanotubes, first synthesized in solid form last year by Dieter Seebach and his colleagues at the Federal Institute of Technology (ETH) in Zurich, complement the eight-ringed ones and could have certain advantages over them, Ghadiri says. The [[Beta].sup.3]-peptide rings orient themselves in such a way as to give the tubes a slight positive charge at one end and a negative charge at the other--in other words, a dipole. "We're interested in what the dipole buys for you," Ghadiri says. It could influence which ions pass through the channels or the direction in which the ions travel.

The channels' unusual structure helps them resist enzymes that normally degrade proteins in a cell. This trait would be beneficial if the nanotubes were used as antibiotics, killing cells by damaging their walls, Ghadiri says. The group is exploring further the biophysical characteristics and biological activity of the nanotubes.

"It's an important step forward to show that these structures will conduct ions," says Gellman. Simply adding one atom to the amino acid building block of the peptides might be considered only 11 a baby step away from proteins," he says, but "it's not as simple as it may seem." Structures like these could mimic protein function and perhaps improve on it.