Source: © 2022 Lorna Binks et al

X-ray crystallography structure of the rotaxane system

A new class of synthetic molecular pump has been created that can rapidly and selectively prepare oligorotaxanes with a specific number and sequence of macrocyclic rings.

Molecular pumps are a type of molecular machine which drive a system away from equilibrium by moving molecules against an energy gradient. In nature, this movement is called active transport and is a crucial part of many life-sustaining processes, relying on complex protein architecture within cells to shuttle molecules to where they are needed.

Advances within the last 10 years have led to the development of synthetic pumps, with a much simpler screw-and-bolt structure. Composed of a series of macrocycles threaded onto a collection axle, the pump pulls macrocycles from a low energy environment in bulk solution, to a higher energy environment on a molecular thread. Usually, favourable interactions between the two components drive this artificial active transport process but unstable intermediates mean that there is a risk of random dethreading so the structure of the product rotaxane can’t be efficiently controlled.

Researchers at the University of Manchester have developed a new class of pump, driven by a transamidation reaction sequence, which relies on transition state energy differences rather than specific interactions. This alternative pump mechanism avoids unstable intermediates, eliminating the risk of dethreading and enabling the team to control the precise number and sequence of macrocycles on the collection axle.

This iterative process provides convenient access to oligorotaxanes with carefully controlled sequence isomerism which have widespread applications in molecular nanotechnology.