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Researchers from the University of Amsterdam (UvA) and the Foundation for Fundamental Research on Matter (FOM) have managed to study the movements of a molecular piston in detail.

Researchers from the University of Amsterdam (UvA) and the Foundation for Fundamental Research on Matter (FOM) have managed to study the movements of a molecular piston in detail. As it turns out, the behaviour of this engine - consisting of a single molecule - differs greatly from that of its life-size counterparts. The piston's movements are extremely irregular, but can be effectively described using a 17th century model developed to predict games of chance. However, the molecular engine doesn't exactly play by the rules. The results of the study were published in the journal ‘Science' on Friday, 4 June.

It is currently possible to build molecules that work like miniscule machines. Researchers can now build engines, lifts and revolving doors consisting of a single molecule. The potential applications of such molecular machines are almost limitless, varying from molecular computers to surfaces with variable characteristics. At first sight, these molecular machines look like exact copies of the (comparatively huge) machines we see around us in everyday life. But does their behavior also mirror that of their life-size counterparts?

Fuelled by light

In order to find the answer to this question, researchers from the UvA and FOM studied rotaxane. These molecules consist of a ring capable of moving along a relatively long thread (several nanometres, billionths of a metre) of carbon atoms. The carbon thread has two anchors (stations) to which the ring can attach itself.

In order to make the ring move from one station to another, the researchers bombarded the rotaxanes with an ultraviolet laser pulse. The ultraviolet light basically fuels the molecular engine. The ring's movements are detected by means of an infrared pulse, which has been slightly delayed in comparison with the ultraviolet start signal. The way in which infrared light is absorbed differs depending on the ring's position on the thread. The moving ring's time of departure and arrival can thus be accurately determined by conducting a series of measurements in which the infrared pulse is delayed at increasing intervals.

Drunken movements

As the measurements demonstrated, the ring fluctuates drunkenly between its initial and final position; steps back and forth alternate at unpredictable intervals. Entirely different, in other words, than the smooth movements of a piston in a combustion engine. The ring eventually comes to rest in its final position, but the amount of time each individual molecular machine will need to do so is entirely unpredictable at the start of the process.

The mathematics used to describe this process of chance are exactly the same as the formula used to predict a gambling game in which two players bet whether a coin will land head or tails until one of the two has lost all his money. If the outcome is heads, the ring moves one step forward. If the outcome is tails, the ring will move one step back. The formulas used to describe this process were developed in the 17th century by Dutch scientist Christiaan Huygens. In this case, however, it turns out that the coin has been tampered with. The two probabilities (a step forward or a step back) are not exactly the same, even though the difference is only a few percent. Huygens had factored this deviation into his mathematical formula, and this adjustment allows for accurate measurements on the basis of the head or tails gambling model. A little cheating may be the key to getting these molecular machines under control.

Publication details

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: 'Operation mechanism of a molecular machine revealed using time-resolved vibrational spectroscopy', Science, 4 June 2010.

For further information or the full article, please contact Dr Sander Woutersen (UvA), email: s.woutersen@uva.nl.