Loops make nanocapsules inseparable

First mechanically induced, reversible bond breakage of a single molecule succeeded

Drawing of oligomeric calixarene catenanes with a scanning probe and the associated force-strain curves, which have a typical sawtooth pattern. © Andreas Janshoff
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Mainz scientists have produced a molecule that breaks apart when overstretched, but then returns to its original shape. In the current issue of the journal Nature Nanotechnology, they present the first mechanically induced, reversible bond breaking of a single molecule.

"It's kind of like pulling two capsules apart until they split in the middle. When we let go, the two halves can come back

Joining together, "explains Professor Andreas Janshoff from the University of Mainz on the nanotechnology work. The reversibility of the process is achieved by a loop connection between the two halves of the molecule.

The experimental object of the nano-scientists is a so-called calixarene dimer, a molecule that consists of two parts - hence the name dimer - and which belongs with a size of a few nanometers to the comparatively large molecules.

Looped calixarene dimer prepared

Calixarenes are used in analytics because they are able to selectively include guest compounds. Using a special molecular design, the team of physicochemists and organic chemists succeeded in producing a twisted calixarene dimer in which the two parts are held together by hydrogen bonds. If the nanocapsules are now mechanically stretched with the aid of an extremely fine measuring tip, they tear apart. display

"The entanglement does not allow the halves to diverge infinitely, but they are held back by the molecular loops, " explains Janshoff. Together with the professors Jürgen Gauß and Gregor Diezemann from the Institute of Physical Chemistry and Volker Böhmer and young scientist Yuliya Rudzevich from the Institute of Organic Chemistry and Piotr Marszalek from Duke University, he has the work in the frame before moving to the Georg-August-University Göttingen of the Collaborative Research Center 625 in Mainz.

"The length of the loops, for example, allows us to limit exactly how far apart the two parts of the molecule can be pulled apart."

Function of the hydrogen-bond network revealed

The single-molecule experiments give scientists a better understanding of how the molecular complex is held together by an external force and how the hydrogen-bond network works. In addition, the experimental tests also provide the basis for many theories of current statistical mechanics: whether the molecules in the complex form are present with both halves or whether they are present with the separated H can be adjusted by the loop length and thus the transition from an equilibrium to a non-equilibrium state by speed-dependent tensile experiments is carried out in a targeted manner.

As a result, theories that want to reconstruct free energy from the non-equilibrium state can be tested. In the future, the scientists want to turn to this topic more intensively.

(idw - University of Mainz, 16.02.2009 - DLO)