Undercover tactics

Ever since the discovery of carbon nanotubes in the early 1990s, scientists and engineers have been fascinated by the possibilities for these little tubes made of organic materials in the fields of microelectronics, substance separation, and biomedicine. Freiburg researchers have now produced novel nanotube hybrids from peptides and polymers: nanotubes made of cyclic peptides are coated with a soft polymeric plastic shell.

Cyclic peptides are small molecules whose amino acid chains form a ring. The amino and acid groups, as well as the hydrogen atom can be arranged in two ways around the first carbon atom (known as the "alpha C-atom") of an amino acid.

This allows the molecule to have either a "left" or a "right" configuration. While nature uses almost exclusively "left" amino acids in proteins, the team headed by Markus Bieslaski at Germany’s IMTEK (Institute of Microsystem Technology) are building up cyclic peptides according to the "one right, one left" scheme, a technique that has been pioneered by Reza Ghadiri of Scripps Institute, San Diego.

Such peptide rings organise themselves into a tiny tubular structure. All of the peptide side chains stick out of the tube, leaving a cavity inside. The dimensions of the tube are determined by the number of amino acid building blocks in the peptide rings.

The special trick in this case is that some of the side chains selected are of a type that can act as starting points for the growth of artificial polymer chains. They can form a strongly bound shell of soft plastic around

the relatively hard peptide nanotube. In their initial experiments, the researchers used N-isopropylacrylamide as the molecular building block for the polymer. Images obtained with an atomic force microscope revealed solvent-free ("dry") individual rod-shaped objects about 80nm long and 12nm high.

The plastic used is not toxic and has interesting physical properties. In a certain temperature range, the polymer matrix collapses. This property could be useful in biomedicine, for drug transport as an enclosed drug could be released at a specific target in the body. Numerous other applications can also be imagined for these hybrid materials.

"By varying the type of polymer, the density of attachment points, and the chain length, we are able to produce hybrid nanotubes with tuneable properties," says Biesalski. "We are now carrying out systematic studies to this end in our laboratory."