3.1. Some things to know about the nanometric world

The observation of natural systems reveals that molecular ma-chines are not simply miniaturized versions of macroscopic machines. In fact, the reasoning in which one claims to shrink the size of objects down to the nanometric scale, without considering the corresponding change in the properties and behavior of the matter, leads to totally wrong conclusions (Jones 2004)1Jones, R. A. L. (2004) Soft machines: nanotechnology and life, Oxford: Oxford University Press..

Although the physical laws that regulate matter are always the same, their practical consequences depend on the dimensional scale of observation. For example, macroscopic machines are typically built with rigid materials and their operation can take advantage of temperature differences with the environment, as happens in thermal machines such as combustion engines. Conversely, molecular machines are made up of “soft” and flexible parts, and must operate at a constant temperature (determined by the environment in which they are located), because the heat flows very rapidly on the nanometer scale. Due to the tiny mass of molecules, the effects of gravity and inertia, which are so important in the mechanics of macroscopic bodies, are irrelevant in the nanometric world. This realm is dominated by intermolecular interactions, which instead are often negligible in the macroscopic world.

The main characteristic of movement in the nanometric world, however, is the fact that objects of this size are subjected to the random and incessant motion determined by thermal agitation – in other words, Brownian motion. The second law of thermodynamics states that it is not possible to extract work from Brownian motion. It cannot be eliminated, unless it is at absolute zero, and its intensity is proportional to temperature. At room temperature the Brownian motion has a disruptive effect on the movement of very small objects; it is estimated that the thermal agitation to which a molecule is subjected corresponds to a power tremendously higher than that supplied by ATP hydrolysis in a biomolecular machine (Astumian 2002)2Astumian, R. D., Hanggi, P. (2002) Brownian motors, Physics Today, 55(11): 33-39..

In short, for a molecule, using energy to move in a controlled manner following a precise direction is like trying to ride a bike during an intense earthquake. Since the latter cannot be seized, the only way to move forward is to take advantage of the shocks in the right direction. Natural molecular machines do exactly this: they use energy (ATP) to rectify the disordered thermal motion, so that the movement in a certain direction becomes more probable than that in the other directions. In other words, it is thermal agitation that drives the molecular machines; in order that this thrust not be limited to producing random effects (which cannot be used to perform work), an external energy source is required.

Understandably, obtaining controlled and directional movements in a molecular system is very difficult, and the design must take into account very different aspects in comparison with the design of a macroscopic device.