Molecular machine's movement turns out to be a dishonest gambling game
Researchers from the Foundation for Fundamental Research on Matter (FOM) and the University of Amsterdam (UVA) have succeeded in studying the movement of a molecular piston in detail. The behaviour of this engine, consisting of just one molecule, was found to be completely different from an engine or piston of a conventional size. The movements are completely unpredictable and can be accurately described using a theory from the seventeenth century that was originally devised for gambling games. However, the molecular motor does bend the rules a bit.... The results of the research were published in Science on 4 June 2010.
Nowadays, molecules can be made that function as minuscule machines. Researchers can make engines, lifts and revolving doors that consist of just a single molecule. The potential applications of such molecular machines are virtually unlimited, and vary from molecular computers to smart surfaces with switchable properties. On the outside, molecular machines look exactly the same as their relatively 'giant sized' counterparts in our everyday lives. But do they also behave in the same manner?
Light as a fuel
To find out, researchers from FOM and UvA investigated so-called rotaxanes. These are molecules consisting of a ring that can move across a relatively long thread (several nanometres, billionths of a metre) of carbon atoms. On the carbon thread, there are two different stations (anchoring points) to which the ring can attach itself.
To get the ring moving from one station to the other, the researchers shoot the rotaxanes with an ultraviolet laser pulse. The ultraviolet light effectively acts as a fuel for the molecular engine. The movement of the ring is observed using an infrared pulse that is slightly delayed compared to the ultraviolet starting shot. The infrared light is absorbed differently depending upon where the ring is located on the thread. That is how a series of measurements with an increasing delay of the infrared pulse provide an accurate picture of both the departure and arrival of the moving ring.
Drunkard
The measurements reveal that the ring staggers like a drunkard between the starting and finishing positions; it lurches backwards and forwards in an unpredictable manner. This is therefore completely different from the supple movements of a piston in a car engine. Eventually the ring stops at its final position, yet at the moment of departure it is completely uncertain how long it will take an individual molecular machine to complete the distance.
The mathematics that describe this random process are exactly the same as those for a gambling game in which two players continually toss for heads or tails until one of the players goes broke. In the case of heads, the ring takes a step forwards and in the case of tails, a step backwards. The formulas for this were derived back in the seventeenth century by the Dutch scientist Christiaan Huygens. However, in the case of the molecular machine the coin has been fiddled with. The chance of a forward or backward movement is not exactly the same, even though the difference is just a few percent. Huygens had already allowed for this possibility in his mathematics and with this adjustment the measurements can be accurately described by the head or tails game. Indeed this cheating by the molecular machines might ultimately make it possible for them to be brought under control...
Reference
'Operation mechanism of a molecular machine revealed using time-resolved vibrational spectroscopy'
Matthijs R. Panman, Pavol Bodis, Daniel J. Shaw, Bert H. Bakker, Arthur C. Newton, Euan R. Kay, Albert M. Brouwer, Wybren Jan Buma, David A. Leigh and Sander Woutersen
Science, 4 June 2010.
Information
For further information and a preprint of the article please contact Dr Sander Woutersen, University of Amsterdam, +31 (0)20 525 70 91
or Matthijs Panman, University of Amsterdam, +31 (0)20 525 69 84.
More general information about this research can be found on the research group's website, http://www.science.uva.nl/research/molphot/trvs