High-performance molecular motor with minimal design

Chemists at Linköping University took their inspiration from nature when designing a new molecular motor to convert light energy into motion. The motor has a minimal structure, but even so satisfies several important functional criteria.

3D illustration of the moleculeMolecular motor designed by the LiU researchers Bo Durbeej and Jun Wang (see a 3D simulation below the article).“Molecular motors” may sound exotic, but the fact is that your body contains huge numbers of them. One example is a light-sensitive protein, rhodopsin, found in the eye. When light falls on the rhodopsin molecule, the energy of the light is used to power a chemical reaction that causes the protein to change its three-dimensional structure. This change of structure starts a complicated chain reaction that ends in the transmission of a nerve signal to the brain, and you see what you are looking at. Thus, rhodopsin converts light energy to the energy of motion: it is, in other words, a motor.

The natural world has many examples of such tiny protein-based machines that make motion possible. Another example is myosin, a protein that forms the basis for the ability of our muscles to contract. The protein-based machines found in nature have stimulated the interest of researchers in developing artificial variants that may be used in, for example, medicine and technology.

Inspired by nature

Theoretical chemists working at Linköping University have now designed a new molecular motor, which so far exists only in the computer. They have created the design, recently presented in the scientific journal ChemistryOpen, using advanced quantum mechanical calculations in the supercomputers at the National Supercomputer Centre at LiU. And the researchers have been inspired by the rhodopsin molecule.Supercomputer at the National Supercomputer Centre at LiU. Photo credit Thor Balkhed

“The chemical reaction in rhodopsin is extremely rapid. The speed of the reaction means that we receive a visual impression of the scene almost immediately when light enters the eye. It would have been a real problem if it took several seconds before we could interpret the sight of a lion rushing towards us on the savannah... We became curious about whether we could use the principle used in the rhodopsin molecule to build a synthetic molecular motor”, says Bo Durbeej, associate professor in the Department of Physics, Chemistry and Biology.

The molecular motor designed by the LiU researchers mimics the principles used by rhodopsin in the eye, but is much smaller. It has the form of a small molecule, one half of which rotates around the chemical bond in the middle (illustrated in a video made by the researchers, below the article text). It is possible to determine the direction of rotation of the motor, clockwise or anticlockwise, as if it was a tiny drill. There are, indeed, other examples of molecular motors designed by researchers to drill tiny holes in the cell membrane of living cells, which may be possible to use for transporting and delivering drugs in the future.

“Many people believe that molecular motors have a huge potential in future applications”, says Bo Durbeej.

Simplest possible design

Molecular motors should satisfy three criteria: they should be fast, efficient and driven by light of the right type.

“Many people who develop molecular motors focus on optimising one of these factors. What our design demonstrates is that it is possible to achieve all three at the same time”, says Bo Durbeej.

Most synthetic molecular motors are driven by energy from ultraviolet light, but this is not suitable for biological applications, since ultraviolet light damages tissue. In such applications, visible light is much better. Visible light, however, contains less energy than ultraviolet light, which makes it more challenging for researchers to design molecular motors driven by visible light. The LiU researchers have designed a motor that can be driven by visible light, while at the same time maintaining very rapid rotation and high efficiency.

Bo Durbeej’s principal driving force is curiosity – how can we build really good molecular motors? Together with his colleague Jun Wang, he has attempted to find the simplest possible design.

“The simplicity of the design is truly remarkable. We suggest that this principle can be used for future molecular motors, and that we and other researchers can continue working with our design as a basis”, says Bo Durbeej.

From supercomputer to lab bench

The researchers have not only carried out theoretical predictions of the function of the molecule: they have also suggested a method for building it in the lab. They are collaborating with researchers in Hungary who are working to synthesise the molecule. The researchers intend to test how to functionalise the motor, by attaching it to a surface, among other techniques.

“Fixing the motors to a surface is a key step in being able to continue development toward future applications. However, the performance of the motors is nearly always impaired when they are attached to a support. We want to find out how to preserve the excellent properties of the motors, and how to scale them up. A single molecular motor can’t do very much, but just imagine what we could do if we could harness the power from a large number of them”, says Bo Durbeej.

The article: Toward fast and efficient visible-light-driven molecular motors: a minimal design“, Jun Wang, Bo Durbeej, ChemistryOpen 2018, 7, 583-589, doi: 10.1002/open.201800089

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Just how fast is the molecular motor?

The LiU researchers have designed a molecular motor driven by light with an energy lower than 3 eV, which corresponds to visible light.
Their calculations show that it has high efficiency: as much as 75% of the energy in the light is used to drive the rotation.
And the motor is fast: it can rotate 2,000 billion times per second (2 THz).

Simulation of the motor
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Minimal design

A simulation of the researchers' 3D model of the molecular motor (turn on captions for subtitling).
The simplicity of the design is truly remarkable. We suggest that this principle can be used for future molecular motors, and that we and other researchers can continue working with our design as a basis”, says Bo Durbeej.

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