By Alberto Credi, University of Bologna
Imagine if we were able to make mechanical machines so exceedingly small that we could target individual protein or DNA molecules inside a living being. A similar scenario was the subject of the 1966 film Fantastic Voyage, in which a submarine and its crew are miniaturized and injected in a patient to remove a blood clot in the brain. Just a few years before, in 1959, the famous physicist and Nobel prize winner Richard Feynman had mentioned, for the first time, the possibility of making machines at the extreme level of miniaturization: machines the size of molecules.
Molecules are, in fact, the smallest material objects that possess a specific non-spherical shape. Their size is on the order of one nanometre — one billionth of a metre. To give you an idea, consider that a human hair has a thickness of about 100,000 nanometres. But is it possible to make mechanical machines of molecular size? And why should we do it? The most convincing answers come from nature: indeed, many important functions that sustain life are performed by machines made of biomolecules.
The mechanics of molecular machines are fundamentally different from those of macroscopic ones because the phenomena that govern motion at the molecular level are different from those ruling motion in the macroscopic world. For example, because of the tiny mass of molecules, gravity and inertial movements are negligible at the nanoscale, where the effect of inter-molecular forces and collisions prevails. An important feature that molecular machines have in common with their macroscopic counterparts is that they need an energy input to operate. Depending on the design of the machine, the energy can be supplied in the form of chemical fuels (as for biomolecular machines), electrical potential, or light irradiation. The use of light to power molecular machines is interesting in many aspects, one of which is the fact that the energy can be transmitted to molecules without physically connecting them to the source.
The construction of a molecular machine is not an easy task and implies three challenging steps: the design of the molecule, its construction by chemical synthesis, and the monitoring of its operation by appropriate techniques (as it is obviously too small to be seen). In the mid-1990s, progress in synthetic chemistry enabled the realization of species called rotaxanes. A rotaxane (from Latin rota for wheel and axis for axle) is made of a linear molecule threaded by a ring-shaped molecule (see figure). Rotaxanes, in which the ring can freely move along the axle, are often called molecular shuttles. To control the shuttling motion, however, suitable docking sites (stations) have to be introduced in specific positions along the axle.
The minimal design for a controllable molecular shuttle is based on two different stations. The ring preferentially encircles the station exerting the most intense attraction on it until a suitable chemical reaction, properly activated, switches off the ability of such a station to attract the ring. As a result, the ring shuttles towards the second station. A successive reaction can restore the originally preferred station and the ring shuttles back to its initial position. Under appropriate conditions this cycle can be repeated indefinitely.
I was doing my PhD in photochemistry at the University of Bologna when the first examples of controllable molecular shuttles were reported. The idea then arose in our laboratory to power the shuttling by using light. After long experimental work, plus collaboration with a group in the USA (pioneers in rotaxane synthesis), we developed a quite complex rotaxane and were able to show that it uses visible light to displace the ring along the axle. Briefly, as shown in the figure, the absorption of green light causes an electron-transfer process within the molecule, which eventually switches off the primary station (1). Once the ring has moved to the secondary station (2), another electron-transfer process restores the original situation (3), and the ring shuttles back to the initial site (4). At the end of these ‘four strokes’, the rotaxane is ready to absorb another photon of light and start the cycle over again. Although the efficiency of this machine is low and its motion is not (yet) used to perform a function, we learned many fundamental things about how to make and operate molecular shuttles, and we demonstrated the validity of our bottom-up design.
Molecular shuttles have recently been used for controlling chemical reactions, making materials for mechanical actuation, operating drug delivery systems, and storing data on the nanometre scale. The road to real-world applications is still long, but these achievements indicate that useful materials and devices based on molecular machines could be made in a not too distant future. Last, but not least, molecular machines that harness solar energy in the form of visible light are of high interest because it has become clear that in the years ahead humankind will have to rely on renewable energy sources.
Alberto Credi
University of Bologna
www.atomiumculture.eu
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