Light-Driven Transmembrane Ion Transport by Spiropyran−Crown Ether Supramolecular Assemblies

Spiropyrans undergo reversible photoisomerization between their ring-closed (spiro) and ring-open (merocyanine) forms when exposed alternately to ultraviolet and visible light. Rates of K+ ion leakage from phosphatidylcholine unilamellar vesicles containing a K+-selective crown ether attached to an amphiphilic spiropyran increased markedly when the vesicle assembly was illuminated with UV light. This effect was fully reversible, with K+ leak rates returning to their basal levels upon illumination with visible light. In contrast, UV-induced photoisomerization of a similar spiropyran−crown construct that did not strongly coordinate K+ was not accompanied by enhancement of K+ leakage. Transient spectrophotometry revealed that, immediately following photoisomerization, the merocyanine form of the dye was in a moderately polar environment, consistent with its location in the glycerophosphate backbone region of the vesicle. Within several milliseconds, the polarity of the environment increased, as indicated by ...