Walk on by: The light at the end of the track

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  • Published: May 15, 2012
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Walk on by: The light at the end of the track

Stepping-stones

British researchers have created a molecular track to emulate this process and added a small molecule that can shuffle back and forth like a courier on the track.  

Transport processes occur between individual cells in our bodies on a permanent basis over distances as short as a few nanometres to several millimetres. The cellular "cargo carriers" involved in these processes - kinesin, dynein, and some myosin proteins - have various ways of operating but one of them exploits molecular motors to "walk" along the filaments of cytoskeleton. Now, British researchers have created a molecular track to emulate this process and added a small molecule that can shuffle back and forth like a courier on the track.

David Leigh and colleagues at the University of Edinburgh and the University of Manchester, UK, laid down their track from an oligoethylenimine. This filamentous structure contains amino groups that act as stepping-stones across which the molecular walker - alpha-methylene-4-nitrostyrene - can skip. The molecular walker resembles a stick figure with a six-membered aromatic ring for its torso, a nitro group head, and two stubby hydrocarbon legs.

In the initial state, the molecular walker stands on one leg on the first stepping stone. A ring-closing rearrangement reaction, specifically an intramolecular Michael reaction, then takes place which puts the second leg on the next stepping-stone. Then, another reaction acts to lift the first leg in a second, ring-opening rearrangement reaction (a retro-Michael reaction), followed by the rearrangement to the next stepping-stone and so on, allowing the molecular walker to move along the track step-by-step. The team used NMR spectroscopy and other techniques in their investigations of the stepping processes with the spectra revealing the precise position of the walker on short tracks.

Two steps forward, one step back

If it were not for one physical obstacle blocking the path of the molecular walker, the process could be used without further intervention to model the cellular transport systems. That obstacle is, of course, the fact that all of these rearrangement reactions are equilibrium reactions; they are chemically equivalent. As the walker steps forward, it is almost as likely that a backwards step would be taken next, and then perhaps two hops forward and another back or two reverse steps and one forward...

The walker's movement, in other words has no direction, it hops back and forth until it falls off the track. However, the team has demonstrated that they have seen the walker complete an average of 530 steps forward before falling off the track. This, they point out, is a much higher number of steps than is taken in the natural systems such as the kinesin motor proteins in the cell.

Turn out that light!

Aside from the fancy footwork, the miniature walker can also do a job. By attaching an anthracene group to the end of a track with five stepping-stones, the team observed that while the walker hops back and forth at the start of the track, the anthracene unit fluoresces. But, when the walker reaches the anthracene end of the track, an electronic interaction causes the fluorescence to be extinguished. The walker dims the light at the end of the track. The team explains that the initial dimming reduces the fluorescence to about half its starting value and it gradually gets weaker after about 6.5 hours as equilibrium is reached.

The next step is to stoke up the walker to give it the fuel necessary to march in a specific direction across the stepping-stones, perhaps even carrying a payload and opting for predetermined directions along branched tracks.

Related Links

Angew Chem, 2012, online: "A Small Molecule that Walks Non-Directionally Along a Track Without External Intervention"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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