Motor proteins are nature's solution for directing movement at the molecular level. The field of artificial molecular motors takes inspiration from these tiny but powerful machines. Although directional motion on the nanoscale performed by synthetic molecular machines is a relatively new development, significant advances have been made. In this review an overview is given of the principal designs of artificial molecular motors and their modes of operation. Although synthetic molecular motors have also found widespread application as (multistate) switches, we focus on the control of directional movement, both at the molecular scale and at larger magnitudes. We identify some key challenges remaining in the field.

We find that a bulky pyridine-based catalyst promotes carbonate-forming reactions that ratchet the displacement of the macrocycle away from the reactive sites on the track. Under reaction conditions where both attachment and cleavage of the Fmoc groups occur through different processes, and the cleavage reaction occurs at a rate independent of macrocycle location, net directional rotation of the molecular motor continues for as long as unreacted fuel remains. We anticipate that autonomous chemically-fuelled molecular motors will find application as engines for molecular nanotechnology. 2, 19,20

Acid fuels the motion of a threaded ring A central goal in the construction of molecular-scale machines is the efficient achievement of one-way motion. Erbas-Cakmak et al. developed a class of machines that transmit pH changes into the two-stage guided motion of molecular rings threaded on a linear or cyclic axle. The design relies on temporary blocking groups and landing sites along the axle that toggle between active and passive states in response to acid or base. Trichloroacetic acid initiates the first stage of motion until it is decomposed by base in the solution, spurring the second phase. Science , this issue p. 340

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These chiral “synthetic lectins” are the first to discriminate between carbohydrate enantiomers, and also show unprecedented affinities for monosaccharide substrates.