Molecular muscle. Molecular model of the two states of an acid-base molecular muscle. Each leg of the muscle (grey carbons) contains two notches (or recognition sites), at different distances. The first notch incorporates a dialkylammonium (NH2+, blue) centre, while the second notch contains pyridinium (PY+, blue) units. An acid-base reaction provides the energy for the dibenzo ring (pink carbons) to move between these two notches, a distance of a few nanometres. The 2016 Nobel Prize in Chemistry was awarded to the chemists Jean-Pierre Sauvage, J. Fraser Stoddart, and Bernard Feringa for their work on the design and synthesis of molecular machines.