In order to function, living organisms use an amazing number of molecular machines, each one dedicated to a specific mechanical task such as muscle contraction or intracellular transport of the products of biosynthesis. These machines are capable of rectifying random thermal Brownian motions to generate directional forces.  But what is a molecular machine?

It is a molecular complex composed of two parts each of which can move with respect to another in response to a stimulus (light, energy supply, a concentration gradient,…), the net result being a mechanical work. This intramolecular movement is capable of generating a considerable directional force which can manifest itself at either the nano and microscopic scale (intracellular transport), or the macroscopic scale when the machines work collectively (muscle contraction).

These molecular biological machines have inspired chemists who have, for some years now, been synthesising nano-machines capable of mimicking the natural world by producing a mechanical work that can be detected on a microscopic or macroscopic scale, such as the transport of a droplet up an inclined surface against gravity (1) or the rotation of submilimetric objects on a substrate (2).

The exploration of these systems on a molecular scale and the understanding of the mechanical processes has been made possible thanks to the recent development of single molecule manipulation techniques (optical tweezers, magnetic tweezers, and atomic force microscopy-AFM). The Nanochemistry and Molecular Systems research unit of the Department of Chemistry of the  University of Liège is specialised in the development of methods derived from AFM to study, manipulate and modify matter on the scale of single molecules.

The researchers in this laboratory have succeeded in exploring sub-molecular movements in a single synthetic molecular machine which is less than 5 nanometres long. In this way they have measured the forces generated and the mechanical work performed by this nanomachine.

Rotaxanes are prototypical synthetic molecular machines. This complex molecule is made of a molecular ring threaded onto a molecular axle. The thread bears one or more recognition sites called stations, onto which the ring can bind through intra-molecular bonds according to its affinities. The chemical environment forces the ring to preferentially remain on one of the stations (where it can make the strongest bonds. This site is thermodynamically favoured).

(1) Nature Materials 2005, 4, 704
(2) Nature, 2006, 440, 163