A lock for a nanoscopic bicycle

Researchers in South Korea have created a molecular host that can be switched between affinities for homologous and heterologous pairs of guests and demonstrated its usefulness by building a nanoscale bicycle lock.

Kimoon Kim and colleagues at Pohang University studied the host-guest chemistry of the ring-shaped molecule cucurbit[8]uril, which can accommodate charge transfer stabilised pairs of molecules such as methyl viologen and 2,6-dihydroxynaphthalene in its hydrophobic cavity to form a stable ternary complex. The team found that by offering the complex a second molecule of methyl viologen under reducing conditions the naphthalene was dislodged from the cavity. UV/Vis spectra indicate that the resulting complex hosts an identical pair of reduced methyl viologen radicals. The reaction was reversed by admitting oxygen to the solution.

To create a molecular machine where the exchange of guest molecules makes something happen, Kim’s team linked the two different guest molecules into one molecular construct containing first a bulky stopper, then methyl viologen, then a hydrocarbon chain, and finally the naphthalene derivative. Thus, the complex with the hetero guest pair now presents itself as a lock with a molecular loop. Adding methylviologen as a key under reducing conditions opens the molecular bicycle lock, while re-oxidation locks it again and ejects the key.

Michael Gross