It is discovered that for an n-gonal ice nanotube built from stacking a single type of n-gonal rings of water, the unit cell consists of two stacked n-gonal rings. In one ring the O–H bonds of water molecules line up clockwise whereas in the other ring the O-H bonds line up counterclockwise. Among the n-gonal ice nanotubes examined, the pentagonal or hexagonal ice nanotube appears to be the most stable.

The assembly of well-defined protein secondary structures, leads to a bewildering array of tertiary structures. 1 As the first step toward developing artificial oligomers and polymers that fold like biomacromolecules, there is currently an intense interest in designing unnatural building blocks that adopt well-defined secondary structures. 2,3 Here we report a new class of oligoamides with backbones that adopt well-defined, crescent conformations.Our design is based on diaryl amide oligomers, shown as 1. The presence of the three-center hydrogen bonding system consisting of the S(5) and S(6) type 4 intramolecularly hydrogen bonded rings should lead to rigidification of the amide linkage. Oligoamides containing such amide linkages should have a rigid backbone. With the two amide linkages on the same benzene ring being meta to each other, the resulting oligomer should have a crescent conformation. 5 Ab initio molecular orbital calculations (in vacuo) were performed on amide 2. 6 Conformations 2a-b are constrained to be planar. The relative energy of each conformation is shown in parentheses. The computational results indicated significant differences in the relative energies of the four conformations: 2 was overwhelmingly favored over the alternative conformations 2a,b. The desired conformation, 2, was planar and had two strong intramolecular hydrogen bonds with O‚‚‚H ) 1.87 Å (S(6)) and 2.14 Å (S(5)), respectively.

A general strategy for creating nanocavities with tunable sizes based on the folding of unnatural oligomers is presented. The backbones of these oligomers are rigidified by localized, three-center intramolecular hydrogen bonds, which lead to well-defined hollow helical conformations. Changing the curvature of the oligomer backbone leads to the adjustment of the interior cavity size. Helices with interior cavities of 10 Å to >30 Å across, the largest thus far formed by the folding of unnatural foldamers, are generated. Cavities of these sizes are usually seen at the tertiary and quaternary structural levels of proteins. The ability to tune molecular dimensions without altering the underlying topology is seen in few natural and unnatural foldamer systems