Walking Molecule Follows The Light

Small molecules that lift their “legs” and “walk” are a recent invention of researchers who wish to mimic protein machines that stroll through our cells. A team led by David A. Leigh of the University of Edinburgh, in Scotland, has now given these walking molecules a new skill—the ability to controllably change direction when zapped by light (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201004779). The Edinburgh researchers’ molecular walker (red) uses a previously reported scaffold consisting of one hydrazide foot and one sulfur foot, which step one at a time along a track with alternating benz­aldehyde (green) and benzylic disulfide (blue) footholds. In acidic conditions, the sulfur foot is locked in place, but the hydrazide foot is free to take a step. In basic conditions, the hydrazide foot is stuck in place and the sulfur foot can make a move. The ability to change directions comes from using light to switch the double-bond geometry of the stilbene units in the bottom level of the track. When the bond switches from Z to E geometry, ring strain directs the walker to step off, with the direction of travel dependent on which of the two feet is induced by pH to take the next step.