Molecular motor’s ‘power stroke’ in focus

Chemists of the University of Groningen under the leadership of professor Ben L. Feringa have published new research on the molecular motor in Nature Chemistry. They worked in collaboration with a research group from the University of East Anglia led by Steve Meech. Their goal was to gain a better understanding of the molecule’s ultrafast dynamics during the rotation of the rotor.

In 1999, the University of Groningen chemist Ben Feringa demonstrated a world-changing invention at the nanoscale when he revealed a molecular rotary motor powered by light.

The article in Nature described a rotor that revolved in four steps, alternately driven by light or heat energy. However, a detailed picture of the molecular dynamics was lacking – before today. This knowledge is required to be able to develop new generations of molecular motors. The results of the research published in Nature Chemistry provide detailed insights into the molecular motor’s 'power stroke'.

Rapid improvement

The first version of the nanomotor produced in 1999 was far from a practical device; it took several hours of work in the laboratory to induce each of the four steps in a single revolution. However, things improved rapidly from there. In 2002, Feringa was able to report that the speed of the second generation of molecular motors had improved considerably; in 2005 the increase in speed of the third generation was described as ‘dramatic’ and in 2007 the rotor even reached a MHz frequency at room temperature (one million revolutions per second). This was achieved by improving those steps that took place under the influence of heat; the slowest part of the rotation.

Molecular structure

In the new study the rapid, light-driven stages of the rotation were placed under the microscope. Of course it was no ordinary microscope; the researchers used 'ultrafast fluorescence up-conversion spectroscopy’. This technique allows changes in molecular structure to be studied that occur within in a timeframe 50 femtoseconds (1 femtosecond is 10^-15 seconds).

The molecular motors used in the research performed a unidirectional rotation under the influence of light and heat. This means the rotor always rotates in the same direction relative to the stator (the fixed anchor point). If the molecule absorbs a photon, it enters into an ‘excited’ state. The rigid double bond of the rotational axis is then briefly broken allowing the rotation to take place. The research revealed that this occurs within a timeframe of 0.9 to 1.5 picoseconds (10^-12 seconds).

Oscillation

An oscillation was also observed during the fluorescence measurements, which indicates that some of the vibrations in the molecule are activated when it absorbs a photon. The researchers used calculations and Raman spectroscopy to discover exactly which vibrations were concerned. They suspect that a deformation of the rotational axis is the cause, but they could not prove this with certainty.

Optimization

‘The results of this research will help us to better understand and optimize the rotation of molecular motors,’ Feringa explains. ‘We can increase the efficiency of the motor with further research into the electronic structure of the rotational axis, so that it can be adjusted. Moreover, the fact that vibrations are activated when the molecule enters the excited state could provide an opportunity to control the rotation of the motor with specifically targeted laser pulses.’

Note for the press

More information: Prof. B.L. Feringa

Reference: Ultrafast dynamics in the power stroke of a molecular motor. Jamie Conyard, Kiri Addison, Ismael A. Heisler, Arjen Cnossen, Wesley R. Browne, Ben L. Feringa and Stephen R. Meech. Nature Chemistry, advanced online publication.
DOI: http://dx.doi.org/10.1038/NCHEM.1343