Water-Controlled Switching in Rotaxanes

In biological and abiological molecular machines, water is generally considered to act as a lubricant. But, does water only act as a lubricant? Here, a [2]rotaxane composed of a dibenzo[24]-crown-8 and an H-shaped axle was investigated at the atomic level using molecular dynamics. At low pH, the rotaxane behaves as a molecular shuttle but becomes a molecular rotor at high pH. The free-energy profiles describing the shuttling and rotary motions in pure acetonitrile and in an acetonitrile-water mixture reveal that water plays different roles in these two movements. In the electrostatically controlled shuttling, water in small amount acts as a lubricant, decreasing the free-energy barrier. Conversely, in the rotary movement controlled by hydrophobic interactions, water causes an increase of the free-energy barrier, and' thus, plays a damping role. The effect of water on rotaxane motion, therefore, differs as a function of not only the nature of the driving force at play but also the aqueous content of the environment. The microscopic mechanism of water lubrication and damping revealed in the present work paves the way for strategies designed to control motion in molecular machines and opens the way to novel, multifunctional, smart materials by regulating the aqueous fraction of the solvent.