Stir it up, little dendrimer

Nanofibers of a zinc porphyrin dendrimer become aligned in a stirred liquid. Switching the direction of stirring reverses the optical activity of the solution. Takuzo Aida and colleagues at the University of Tokyo, Japan, suggest that this phenomenon may have a factor in the breaking of nature's symmetry in probiotic systems. The highly branched zinc-containing molecules aggregate in solution to form long nanofibers, the researchers explain. Left unstirred, the solution optically inactive, stirred clockwise induces optical activity in one direction and stirred counterclockwise switches the direction of the rotation of polarized light shining through the liquid. This phenomenon does not stem, as first thought, from the twisting of individual nanofibers. It is evidently caused by a special macroscopic spatial arrangement of the fibers within the sample cuvette.