Nanocavities, sized to order 8034scicon.color copy By carefully adjusting the placement of amide bonds between residues in a series of oligomers, chemists have created hollow helices of tunable size. The resulting nanocavities might be useful in catalysis, drug delivery, or ion-carrier applications, according to Bing Gong, associate professor of chemistry at the State University of New York, Buffalo, and coworkers (Proc. Natl. Acad. Sci. USA, published online Aug. 12, http://www.pnas.org/cgi/reprint/162277099v1). The backbone of the oligomers consists of benzene rings linked by amide groups. Three-center hydrogen bonding between the amide hydrogens and alkoxy substituents on the aromatic rings forces the oligomers into curved conformations. For example, a nine-unit oligomer with its amide moieties all placed meta to each other folds into a helix with a cavity about 10 in diameter. Replacing some of the meta linkages with amide groups placed para to one another changes the curvature of the backbone, the researchers report. Thus, a 21-unit oligomer with alternating meta and para connections folds into a helix with a diameter greater than 30 (shown).