Cover Picture

Abstract

The cover picture shows a perspective view of the crystal structure of a double helix formed from two oligomeric molecular strands of pyridine heterocycles connected through carboxamide functions. The color coded strands are shown in stick representations with solvent accessible surfaces. The folding of these oligomers into single helical conformations and their association into double helices held by hydrogen bonds and stacking interactions are described in detail by Lehn and Huc et al. on p. 2810 ff.(We thank C. Caroli from the Visual Design Softwareentwicklungs GmbH, Karlsruhe, for assistance in the preparation of the cover picture.)

Molecular Apple Peels

“…[14][15][16][17][18] Helices derived from aromatic oligoamides (AOAs) may prove particularly suitable for the purpose of molecular recognition because their diameter can be tuned at will according to the size of the monomers and the orientation of the amine and acid groups on each aromatic ring. [16][17][18][19][20][21] Thus, hollows as large as 3 nm [19] and as low as 0.5 nm [17,18] have been reported for AOAs. A refinement of this concept is the design of an oligomeric sequence comprising both monomers that code for helical segments with a hollow and monomers that code for no hollow at all.…”

“…The hollow defined by helices of oligoamides of 2,6-diaminopyridine and 2,6-pyridinedicarboxylic acid is highly polar and can bind water in the solid state [18] and in solution. [17] However, oligoamides of 8-amino-2-quinolinecarboxylic acid form very stable helices with a hollow too small to accommodate any guest. [21] Oligomer 1 combines a central trimeric segment of pyridine monomers flanked by two short dimeric segments of quinoline monomers.…”

“…The pitch equals the thickness of one aromatic ring (about 3.45 ), as found in previous structures of pyridine and quinoline oligomers. [17,18,21] The CPK views show that the terminal quinoline units do cap the hollow defined by the pyridine segment-the inner diameter of the helix is reduced at both ends. Most importantly, the helices all contain one molecule of water despite the low water content of the unpolar crystallization medium.…”

“…[1e] The behavior of 1 in solution was studied by NMR spectroscopy to test this hypothesis. The 1 H NMR spectrum of 1 in CDCl 3 possesses all the characteristic features of folded helices formed from oligomers with only pyridine [17,18] or only quinoline [21] units: sharp lines, spreading of the signals over a wide range of chemical shifts, downfield shifts of the signals of the amide protons involved in intramolecular hydrogen bonding, shielding of the protons involved in intramolecular Angewandte Chemie p-p stacking (Figures 3 and 4). No diastereotopic patterns of the signals of the methylene protons of the side chains are observed at room temperature, thus indicating that the equilibrium between the right-handed and the left-handed helices is fast on the NMR timescale, and that partial unfolding of the helix does occur.…”

“…Such a slow exchange of the water molecule at low temperature is in sharp contrast with the behavior of oligoamides of diaminopyridine and pyridinedicarboxylic acid described previously and which do not bear quinoline end caps. [17] These helical oligomers were also shown to bind water in solution in their hollow, [17,18] but the exchange of water remains fast on the NMR timescale, even at temperatures as low as À55 8C. The quinoline end caps in oligomer 1 seem to be critical in slowing down the binding and release of the water molecule.…”