Angew. Chem. Int. Ed. https://doi.org/c7h9 (2019)
Rotaxanes are molecules made up of two components: a ring and an axle. The ring threads the axle and cannot escape due to the presence of stopper groups. The axle can be made to contain two recognition groups, or stations, so that the ring shuttles between them in response to a chemical signal. Biagini and colleagues now show a rotaxane molecular machine in which one of the two stations is a catalyst and where catalytic activity is switched on and off as a result of a transient change in pH, induced by the addition of a fuel.
The researchers install a thiourea and an amine station along the axle. Under acidic conditions, the amine gets protonated and the axle prefers to bind to the ammonium group. Under basic conditions, interaction with the thiourea is preferred instead. When the axle vacates the thiourea station, the catalytic function of the rotaxane is switched on. More precisely, thiourea catalyses the reduction of a nitrostyrene by a Hantzsch ester. To induce acidic conditions, the researchers add trichloroacetic acid — a compound that decomposes over time into chloroform and carbon dioxide. The trichloroacetic acid is akin to a fuel: the machine only operates when it is available. The researchers show that the system is robust to a handful of consecutive additions of the fuel.
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Moscatelli, A. The catalyst and the machine. Nat. Nanotechnol. 14, 638 (2019). https://doi.org/10.1038/s41565-019-0507-x