Uses of a molecular machine enclosed in a nano cage

Scientists have successfully enclosed a supramolecular rotor in a cube-shaped porphyrin nano cage molecule.

Machines enclosed in a nano cage or casing could display interesting properties. For example, they can convert their energy input into programmed functions; the mechanical gyroscope is one such system. It is an intriguing toy with the ability to rotate continuously. Some practical applications of gyroscopes include aircraft and satellite navigation systems as well as wireless computer mice.

“In addition to the rotor, another advantage of gyroscopes is their casing, which aligns the rotor in a certain direction and protects it from obstacles,” described Lars Schäfer.

Biological nanomachines

At the molecular level, many proteins act as biological nanomachines. They are found in every biological cell and perform precise and programmed actions or functions within a confined environment. These machines can be controlled by external stimuli.

“In the lab, the synthesis and characterisation of such complex structures and functions in an artificial molecular system presents a huge challenge,” explained Schäfer.

In collaboration with a team led by Professor Kimoon Kim, at the Institute for Basic Science in Pohang, South Korea, the researchers have succeeded in enclosing a supramolecular rotor in a cube-shaped porphyrin cage molecule. Typically, fitting a completed rotor into such cages is complicated by the limited size of the cage windows.

Modifications to nano cage

In an effort to overcome these limitations, the synthetic chemists in South Korea developed a new strategy that first introduced a linear axis into the nano cage, which was then modified with a side arm to construct a rotor.

“It’s reminiscent of building a ship in a bottle,” added Chandan Das, who, together with Lars Schäfer, performed molecular dynamics computer simulations to describe the rotational motion of the rotor in the cage in atomic detail.

“Our collaboration partners made the intriguing observation that the movement of the rotor in the cage could be set in motion and also switched off again by light as an external stimulus, just like with a remote control,” described Schäfer. The researchers accomplished this by using light in the UV and visible range to dock a photo-responsive molecule to the cage from the outside and detach it again.

Molecular gyroscope movement

But how does it work, and what movements does the molecular gyroscope perform after it’s switched on in this manner?

“Molecular dynamics computer simulations show that the rotor molecule in the cage exhibits stochastic dynamics, characterised by random 90-degree jumps of the rotor side arm from one side of the cube to an adjacent side,” Chandan Das explained.

The researchers expect that the concept of encasing molecular nanomachines in a molecular cage and remotely controlling their functions will contribute to the understanding of how biological nanomachines work and to the development of smart molecular tools.

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