Monday, October 10, 2016

The 2016 Nobel Prize for Chemistry, Awarded for: “The Design and Synthesis of Molecular Machines.”

The following commentary will appear in the next volume of the journal Science Progress of which I am an editor. Meanwhile, here is a preview of it.

Jean-Pierre Sauvage, Sir J. Fraser Stoddart and Bernard L. Feringa share the 2016 Nobel Prize for Chemistry1, awarded jointly to them "for the design and synthesis of molecular machines". A molecular machine, or nanomachine, is any discrete number of molecular components that produce quasi-mechanical movements (output) in response to specific stimuli (input).2 This was demonstrated3 in 1983, when Jean-Paul Sauvage managed to synthesise a catenane (Figure 1), which is formed by linking together two ring-shaped molecules by a mechanical bond, a recently coined term to describe the connection between the components of a mechanically-interlocked  molecular architecture such as a catenane or a rotaxane. In order that the molecular machine can perform a specific task, its components must be able to move in relation to one another, as is the case for the two interlocked rings in the catenane.

It was Fraser Stoddart, who in 1991 synthesised a rotaxane4, which has a molecular axle threaded through a molecular ring (Figure 2a,b), and demonstrated that the ring could move up and down the axle, leading to such devices as a molecular elevator, a molecular muscle and a molecule-based computer chip. In 1999, Bernard Feringa managed to demonstrate a molecular motor5, in which the rotor blade spins continually in the same direction. Using a molecular motor, he managed to rotate a glass cylinder that was 10,000 times bigger than the motor itself. The concept of a “nanocar” has emerged, a version of which was developed at Rice University by the research group of James Tour6, and consisted of a molecule with an H-shaped 'chassis' with fullerene groups attached at the four corners to act as wheels (Figure 3). However, since the original device did not have a molecular motor, it might not be regarded as an actual “car”. Feringa and his co-workers have synthesised a molecule with four motorized "wheels" , which they deposited onto a copper surface and used electrons from a scanning tunnelling microscope (STM) to provide sufficient energy that they could drive some of the molecules in a specific direction, in similar fashion to steering a car. As a result of inelastic electron tunnelling, conformational changes are induced in the rotors which propels the molecule over the surface. Since it is possible to change, individually, the direction of the rotary motion in the motor units, either random or preferentially linear trajectories can be attained for the self-propelling molecular 'four-wheel' device. It is believed that it might be possible to produce more sophisticated molecular “cars”, in which a more complete control of the direction of motion can be achieved.7

Jean-Francois Morin et al.8 are working on a nanocar of the future, fitted with carborane wheels and a light powered helicene molecular motor. However, although a unidirectional rotation was observed for the motor in solution, it has not yet proved possible to drive it on a surface by means of light-energy. A nanocar race event, initially scheduled for October 2016 and described as “The First Ever Race of Molecule-Cars”, has been postponed9 “in order to give enough time for the teams to prepare and for the microscope to be optimized. This postponement is essential to make the event a true « sports-science » challenge.”

As yet, the real future for molecular machines is unknown and probably unknowable, but we may note the following, taken from the 2016 Nobel Prize for Chemistry website10.
The groundbreaking steps taken by Jean-Pierre Sauvage, Fraser Stoddart and Ben Feringa in developing molecular machinery have resulted in a toolbox of chemical structures that are used by researchers around the world to build increasingly advanced creations. One of the most striking examples is a molecular robot that can grasp and connect amino acids. This was built in 2013 with a rotaxane as its foundation.

Other researchers have connected molecular motors to long polymers, so they form an intricate web. When the molecular motors are exposed to light, they wind the polymers up into a messy bundle. In this way, light energy is stored in the molecules and, if researchers find a technique for retrieving this energy, a new kind of battery could be developed. The material also shrinks when the motors tangle the polymers, which could be used to develop sensors that react to light.” Thus, the promise of real-world applications surely glisters.

(2) Ballardini R. et al (2001) Acc. Chem. Res. 34, 445.
(3) Dietrich-Buchecker, C. O., Sauvage, J. P. and Kintzinger, J. P. (1983) Tet. Lett. 24, 5095.
(4) Anelli, P. L.; Spencer, N.; Stoddart, J. F. (1991)  J. Am. Chem. Soc., 113, 5131.
(5) Feringa, B. L. et al. (1999) Nature. 401, 152.
(6) Shirai, Y. et al. (2005) Nano Lett. 5, 2330.
(7) Kudernac, T. et al. (2011) Nature. 479, 208.
(8) Morin, J-F., Shirai, Y. and Tour,  J. M. (2006). Org. Lett. 8, 1713.

Captions to figures.
Figure 1. Picture of a catenane, generated from crystal structure data reported by M. Cesario, C. O. Dietrich-Buchecker, J. Guilhem, C. Pascard and J. P. Sauvage in the Journal of the Chemical Society, Chemical Communications, Year 1985, Pages 244-247. Credit: M Stone.
Figure 2. (a) Graphical representation of a rotaxane Credit: M Stone. (b) Crystal structure of rotaxane with a cyclobis(paraquat-p-phenylene) macrocycle. This  picture was generated from crystal structure data reported by Jose A. Bravo, Francisco M. Raymo, J. Fraser Stoddart, Andrew J. P. White, and David J. Williams in the European Journal of Organic Chemistry 1998, 2565-2571. It shows a rotaxane with a cyclobis(paraquat-p-phenylene) macrocyle. Credit: M Stone.
Figure 3. Chemical structure of a nanocar, in which the “wheels” are C60 fullerene molecules.