Boxing up molecular machines

Date:
January 18, 2022
Source:
Institute for Basic Science
Summary:
Boxing up molecular machines. The construction of a molecular
rotor inside a nanostructured cage allows for the development of
a tunable molecular device.



FULL STORY ========================================================================== Machines that are confined inside a cage or casing exhibit interesting properties by converting input energy into programmed functions. One
such system is the mechanical gyroscope or gyrotop, a fascinating
toy that amuses everyone with its incessant rotation. Gyroscopes also
have practical applications in the internal navigation systems used in airplanes and satellites, virtual reality headsets, and wireless computer pointing devices.

What makes these gyroscopes so beneficial is not only the rotor part but
also the frame that aligns the rotor in a particular direction, which
restores the momentum of the rotor and protects it against obstacles.


==========================================================================
In addition, everyday machines like table fans or electric pumps also
enclose rotors inside cage-like frameworks to isolate them from the
outside environment. At the molecular level, the biological machines that
are present in every living thing also operate within the confinement
of cells and exhibit precise and programmed motions and actions. These
machines are controlled remotely by external stimuli. Synthesizing such
complex design and functions in an artificial molecular system is very challenging.

Now, a team led by Director KIM Kimoon at the Center for Self-assembly
and Complexity within the Institute for Basic Science in Pohang, South
Korea successfully constructed a remotely controllable supramolecular
rotor inside a hollow cube-shaped zinc(II)-metallated porphyrinic cage
(Zn-PB) molecule. In general, the direct installation of a rotor inside molecular cages can be very challenging due to the limited size of
the cage windows. Therefore, researchers followed bottom-up strategies
to prepare these rotor-containing cages. In order to circumvent these challenges, Kim and colleagues devised a new strategy in which a linear
axle was first inserted inside Zn-PB and then modified with a sidearm
to construct a rotor.

While the rotor alone shows no motion, the addition of a chemical
stimulant initiates both rotary motion (rotation of the rotor arm around
the axle) and tumbling motion (rotation of the axle) with rotation
speeds of 4000 Hz and 1 Hz, respectively. "We hypothesized that by using
a catalyst-free and facile inverse electron demand Diels-Alder (IEDDA) reaction, we can easily append different functionalities with the axle
without harming the cage. Furthermore, by choosing a suitable arm for
the brake, a controlled rotor can be designed that can be started or
stopped by external stimuli," explains Avinash Dhamija, the first author
of this study.

Previously, the same group has constructed 3D superstructures by
connecting Zn- PBs through bridging ligands and fullerenes from outside
the box cavity. These results inspired them to venture a step further
to explore the inner cavity of Zn-PB. Zn-PB has 6 Zn coordination sites
that can capture multidentate molecules inside the structure. A bidentate linear axle was therefore fixed inside the cage and then post-assembly modification was performed to construct a controllable rotor.

The authors also designed a pyridine-based photoresponsive molecule that
can connect and disconnect with Zn-PB when exposed to UV and visible
light, respectively. This allows reversible control of the dual mechanical motions of the rotor. Controlling the rotor in this way is like a game
of tug-of-war - - pyridine derivatives pull the Zn centers from outside
the cavity and break the internal rotor connections, which initiate both
90DEG jump like rotary and tumbling motions in a stochastic manner.

The present concept of confining molecular machines inside a molecular
cage and remotely controlling their functions will be beneficial for understanding the operation of natural molecular machinery as well as
for the development of smart and tunable molecular devices.

========================================================================== Story Source: Materials provided by Institute_for_Basic_Science. Note:
Content may be edited for style and length.


========================================================================== Related Multimedia:
* Design,_synthesis,_and_functioning_of_the_rotor_within_Zn-PB ========================================================================== Journal Reference:
1. Avinash Dhamija, Chandan K. Das, Young Ho Ko, Younghoon Kim,
Rahul Dev
Mukhopadhyay, Anilkumar Gunnam, Xiujun Yu, In-Chul Hwang, Lars V.

Scha"fer, Kimoon Kim. Remotely controllable supramolecular
rotor mounted inside a porphyrinic cage. Chem, 2022; DOI:
10.1016/j.chempr.2021.12.008 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220118111408.htm

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