Fluorescence obtained in molecular motors driven by light

Rotating molecular motors were first created in 1999, in the laboratory of Ben Feringa, professor of organic chemistry at the University of Groningen. These motors are driven by light. For many reasons, it would be good to be able to make these motor molecules visible. The best way to do this is to make them fluorescent. However, combining two light-mediated functions in a single molecule is quite difficult. Feringa Lab has now managed to do this, in two different ways. These two types of fluorescent light rotary motors have been described in Nature Communication (September 30) and Scientists progress (November 4).

“After the successful design of molecular motors over the past decades, another important goal was to control various functions and properties using these motors,” says Feringa, who shared the Nobel Prize in Chemistry in 2016. “As these are powered by light rotating motors, it is particularly difficult to design a system that would have any other function controlled by light energy, in addition to rotational motion.”

Feringa and his team were particularly interested in fluorescence since it is a technique of choice widely used for detection, for example in biomedical imaging. Usually two such photochemical events are incompatible in the same molecule; either the light-driven motor is running and there is no fluorescence, or there is fluorescence and the motor is not running. Feringa says: “We have now demonstrated that both functions can exist in parallel in the same molecular system, which is quite unique.

Ryojun Toyoda, a postdoctoral researcher in the Feringa group, who now holds a professorship at Tohoku University in Japan, added a fluorescent dye to a classic Feringa rotary motor. “The trick was to prevent these two features from blocking each other,” says Toyoda. He managed to stifle the direct interactions between the dye and the engine. This was done by positioning the dye perpendicular to the top of the engine to which it was attached. “It limits interaction,” says Toyoda.

different colors

In this way, the fluorescence and the rotary function of the motor can coexist. Moreover, it turned out that changing the solvent allows him to adjust the system: “By varying the polarity of the solvent, the balance between the two functions can be modified. This means that the engine has become sensitive to its environment, which could pave the way for future applications.

Co-author Shirin Faraji, professor of theoretical chemistry at the University of Groningen, helped explain how this happens. Kiana Moghaddam, a postdoc in her group, performed extensive quantum mechanical calculations and demonstrated how the key energetics governing photo-excited dynamics are highly dependent on solvent polarity.

Another useful property of this fluorescent motor molecule is that different dyes can be attached to it as long as they have a similar structure. “That way, it’s relatively easy to create engines that glow in different colors,” says Toyoda.

Antenna

A second fluorescent engine was built by Lukas Pfeifer, also while working as a postdoctoral researcher in the Feringa group. He has since joined the École Polytechnique Fédérale in Lausanne, Switzerland: “My solution was based on a motor molecule that I had already made, which is driven by two low-energy near-infrared photons.” Engines powered by near-infrared light are useful in biological systems because this light penetrates deeper into tissue than visible light and is less harmful to tissue than UV light.

“I added an antenna to the motor molecule that collects energy from two infrared photons and transfers it to the motor. Working on this, we discovered that with some modification, the antenna could also cause fluorescence” , explains Pfeifer. It turned out that the molecule can have two different excited states: in one state, energy is transferred to the motor part and drives the rotation, while the other state causes the molecule to fluoresce.

Power

“In the case of this second motor, the entire molecule becomes fluorescent,” explains Prof. Maxim Pshenichnikov, who performed a spectroscopic analysis of both types of fluorescent motors and co-authored both papers. “This engine is a chemical entity on which the wave function is not localized and, depending on the energy level, can have two different effects. By modifying the wavelength of light, and therefore the energy molecule receives, you get either rotation or fluorescence.” Faraji adds, “Our synergistic approach in principle and in practice highlights the interplay between theoretical and experimental studies, and illustrates the power of these combined efforts.”

Now that the team has combined motion and fluorescence in the same molecule, a next step would be to show motility and simultaneously detect the location of the molecule by tracing fluorescence. Feringa says, “It’s very powerful and we could apply it to show how these motors might cross a cell membrane or move inside a cell, because fluorescence is a widely used technique to show where molecules in cells. We could also use it to plot the motion induced by the light-powered motor, for example on a nanoscale trajectory or perhaps plot motor-induced transport at the nanoscale. This is all part of follow-up research.