Rings of rings of molecules: how they stretch and twist, and what they can be used for
L. Tubiana, F. Ferrari, E. Orlandini “Circular polycatenanes: Supramolecular structures with topologically tunable properties“, Phys. Rev. Lett (2022).
Press-release: Italian / English
The polymers that make up our bodies, such as proteins and DNA, are molecules made up by small repeating units, held together by chemical bonds. Recently, chemistry research has developed techniques that make it possible to assemble supramolecular compounds held together not by chemical bonds, but by mechanical bonds, like the rings of a chain. Among these are, for example, polycatenanes, compounds whose chemical and physical properties exceed those of polymers. Polycatenanes had already attracted some attention in 2016, when Jean-Pierre Sauvage, Sir J. Fraser Stoddart and Bernard L. Feringa were awarded the Nobel Prize for Chemistry for their studies on molecular machines. The scientific interest for the subject is growing, but so far scientists focused only on linear polycatenanes and almost no one had yet thought of closing the chains into rings of rings and observe their behaviour.
In this article we studied the consequences of circularising linear polycatenanes such that they topologically trap different amount of twist. Our results show that circular polycatenanes have physical and geometrical properties very similar to those of double stranded DNA rings. In particular, we demonstrate that one version of the Calugareanu-White-Fuller’s theorem on the connection of local and global properties holds for these structures too. In other words, what emerges from our theoretical study is that even in supramolecular structures, as in the physics of DNA, there is a connection between what occurs in a part of the structure and in its whole. The amount of twist of the polycatenanes and the degree of writhe – a measure of how much a curve entangles with itself – are closely connected to each other depending on the way in which the molecular chains are assembled. Something very similar occurs in double–stranded DNA rings where the sum of twist and writhe equals the number of times one strand is wrapped around the other.