Figure 4.

A ratchet motor. The left bead of the dimer is immobilized (schematically shown by a link to the grey box). The filament (green) is mobile and can slide. (a) Initially, the dimer is in the expanded state without a ligand. (b) When a substrate arrives, the right bead of the dimer forms a connection to the filament and holds it. (c) As the dimer contracts, it moves the sliding filament to the left. (d) Once the product is formed and immediately released, the connection between the dimer and the filament disappears, and the dimer freely expands. (a*) After one machine cycle, the dimer returns to its initial configuration, but the filament becomes displaced to the left by the distance Δl = l₀ − l₁. To visualize the displacement, a mark is attached to the filament. See also the electronic supplementary material, video S1. (Online version in colour.)