Are two-station biased random walkers always potential molecular motors?

The short answer to the title question is no. Despite their tremendous complexity, many nanomachines are simply one-dimensional systems undergoing a biased, that is, unidirectional, walk on a two-minima potential energy curve. The initially prepared state, or station, is higher in energy than the final equilibrium state that is reached after overcoming an energy barrier. All chemical reactions comply with this scheme, which does not necessarily imply that a generic chemical reaction is a potential molecular motor. If the barrier is low, the system may walk back and the motion will have a large purely Brownian component. Alternatively, a large distance from the barrier of either of the two stations may introduce a Brownian component. Starting from a general inequality that leverages on the idea that the amount of heat dissipated along the potential energy curve is a good indication of the effectiveness of the biased walk, we provide guidelines for the selection of the features of artificial molecular motors.