In a new episode of Making It Move we take a look at the Michael addition Molecular Walker as conceived by Araceli Campaña et al in Angewandte (DOI). Bioinspired by the motor proteins kinesin and dynein, this surprisingly simple two-legged creature is a-methylene-4-nitrostyrene and its platform a string of polyethyleneimine.
Walking is possible by virtue of a series of reversible intramolecular Michael and retro-Michael additions. The motion is not unidirectional: a bunch of walkers will continuously move around, each on its own track and when after some time chemical equilibrium sets in there is no longer any apparent change as monitored by proton NMR.
In DMSO the half-life for a single step is 1.5 hours. When walker-free amine is added, after 3 days only a fraction of walkers have detached from their own track as monitored by mass spectroscopy. When the central amine groups are replaced by methylene groups no walking takes place at all meaning that the walkers are unable to cheat by moving from start to finish in one go.
In one configuration, an anthracene unit was added to the opposite end of the walker, known to quench nitrostyrene fluorescence. Lo and behold, the fluorescence in DMSO decreased by 50% over a period of 5 hours.