Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44
Molecular Apple Peel
Molecular capsule: helical ribbon with closed ends takes up guest molecules
Children often find it amusing when we peel an apple in one continuous strip; the apple peel curls over our hand in a spiral ribbon and can even be wrapped back around the apple. French researchers have now made such an "apple peel" on the molecular scale. It can "wrap up" a little water molecule so tightly that it is completely isolated from the surrounding medium as though in a capsule.
Tiny molecular "containers" that can take up other molecules as "guests" are of particular interest for technology and science, as catalysts, micro-reaction-chambers, transport containers for pharmaceutical agents, or protective covers for unstable molecules. Various strategies have now been established for building such miniature capsules. With their "apple peel" Ivan Huc and Joachim Garric (European Institute of Chemistry and Biology, Pessac), as well as Jean-Michel Léger (Laboratoire de Pharmacochimie, Bordeaux) have now developed a novel approach.
This is how the apple peel approach works: the French chemists synthesized a strand-like molecule from aromatic amine building blocks—nitrogen-containing carbon rings. The building blocks are chosen so that the ribbon curls into a helix. The crucial trick is that the helix is not even: it has a significantly larger diameter in the middle than at the ends—just like a normal apple peel. The researchers can precisely control the inner diameter by the selection of individual building blocks and the precise arrangement of the nitrogen atoms within the ring system. They thus select correspondingly different building blocks for the middle and end sections of the spiral ribbon. This results in a helix with a real bubble in the middle and ends without a cavity, which close off the bubble. The capsule is thus complete.
"Our capsules are constructed so that they take up a single water molecule," says Huc. "They enclose it completely and shield it from surrounding organic solvents." And how does the water molecule get into the capsule? Nuclear magnetic resonance studies support the theory that the helices partially unravel at one end, let the water molecule slip in, and then close again.
The researchers now want to expand their highly promising concept. They are thus working on larger capsules that could take up larger or multiple molecules.