Credit: © 2009 ACS

Molecular photoswitches use light to swap between states that can have different colours or absorption/emission properties. Diarylethylene switches in particular have been used to control chemical and biochemical reactivity, but have not previously been used in vivo, for example, as drug-delivery agents.

Now, Neil Branda and colleagues at Simon Fraser University in Canada have shown1 that a molecular photoswitch can be taken up by — and reversibly control paralysis in — nematode worms. The transparent C. elegans were incubated in a mixture of the bipyridinium dithienylethene switch and a buffer containing 10% dimethylsulfoxide (DMSO). One group of worms was exposed to the ring-open form, another to the ring-closed and a control group merely to the buffer–DMSO solution.

The group that was fed the ring-closed form showed the characteristic blue colour of the photoswitch, demonstrating its absorption into the worms. Although those worms fed the ring-open form did not, a 2 minutes exposure to ultraviolet (UV) light flipped the photoswitch and the worms changed colour. Worms that had been fed the ring-closed form for 60 minutes appeared immobile, and this paralysis could be turned on and off several times by alternating their exposure to UV and visible light. The control group showed no paralysis and were not affected by either UV or visible light, so the paralysis must have been caused by the photoswitch.