As described in a paper in the Journal of the American Chemical Society (2009, 131 (46), pp 16644–16645), Neil Branda and colleagues at Simon Fraser University (Burnaby, BC, Canada) have demonstrated an on-off "switch" that, when exposed to ultraviolet light, holds animals in paralysis even when the light is turned off. Exposure to ordinary light, though, flips the switch and turns the paralysis off.
Transparent C. elegans worms, raised on a diet of light-sensitive dithienylethene, turned blue and became paralyzed when exposed to light of 300 to 400 nm wavelength. When exposed to visible light (wavelengths greater than 490 nm), the dithienylethene became colorless again and the paralysis ended. Many of the worms lived through the cycle, and scientists say the photoreactions tend to proceed with a high degree of efficiency and with minimal degradation. While it remains unclear just how the switch causes paralysis, the study demonstrates potential for photoswitches in photodynamic therapy and for other applications in medicine and research.
Use of the reversible photoreactions of diarylethenes for applications such as in vivo drug delivery or for unmasking therapeutic agents demand consideration of issues such as whether the photoresponsive systems can be absorbed by a living organism, whether they retain their reversible photoactivity, and if the two forms of molecular switch have unique effects on the function of the living organism.