A rotaxane in which a large part of the 'axle' component adopts a well-defined helical structure offers a new twist in the study of molecular machines.
Molecules comprising two or more components held together by being mechanically interlocked — rather than covalently bonded with one another — have the potential to exhibit machine-like properties. Particularly promising in this regard are compounds known as rotaxanes, which in their simplest form consist of a macrocyclic 'wheel' trapped on an 'axle' that has large blocking groups at each end. Although the components of most rotaxanes are somewhat flexible, the wheel can usually move along the axle in a similar fashion to how a bead slides from side to side on an abacus.
Now, a team of researchers in Italy and the UK led by Alessandro Moretto have made1 a switchable rotaxane in which the shuttling motion of the wheel is thought to be more complicated than this simple model. Moretto and co-workers used amino acids to build up a significant portion of the axle in their rotaxane, giving it a well-defined helical structure stabilized by intramolecular hydrogen bonding.
The outer diameter of the whole helix is larger than the opening in the wheel, but the wheel can still move from one end of the axle to the other under the appropriate conditions. It is thought that the helix does not unwind and the wheel proceeds along the axle as if sliding down a corkscrew while itself rotating.
Moretto, A. et al. A rigid helical peptide axle for a rotaxane molecular machine. Angew. Chem. Int. Ed. 10.1002/anie.200904749 (2009)
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Cantrill, S. A helter-skelter shuttle. Nature Chem (2009). https://doi.org/10.1038/nchem.469