On the way to rotaxane-based molecular motors:: Studies in molecular mobility and topological chirality

ATP synthase represents a machine at the molecular level which couples the rotation of an axle in a wheel with the endergonic production of ATP, the general source of chemical energy in the cell. The natural system prototypically bears all features of a macroscopic motor: a rotor within a stator held by a membrane and fueled by a difference in chemical potential in the form of a proton gradient combined with a machine for ATP production. The assembly of axle and wheel to a rotor device reminds one very much of a rotaxane. In this Account, we discuss some important features of motors and their (potential) realization in simpler artificial model systems, that is, the molecular mobility of mechanically bound molecules, the importance of chirality for unidirectional motion, the sources of energy for driving the rotation, and the potential of using membranes and surfaces for ordering a large number of devices to achieve macroscopic effects.